View All Items

Title: Thermomechanical Behavior of High-Temperature Shape Memory Alloy NiTiPdPt Actuators.
Creator: Nicholson, Douglas, Vaidyanathan, Rajan, Kumar, Ranganathan, Chen, Ruey-Hung, University of Central Florida
Abstract / Description: To date the commercial use of shape memory alloys (SMAs) has been mostly limited to binary NiTi alloys with transformation temperatures approximately in the -100 to 100 (&)#186;C range. In an ongoing effort to develop high-temperature shape memory alloys (HTSMAs), ternary and quaternary additions are being made to binary NiTi to form NiTi-X (e.g., X: Pd, Pt, Au and Hf) alloys. Stability and repeatability can be further increased at these higher temperatures by limiting the stress, but the...
Show moreTo date the commercial use of shape memory alloys (SMAs) has been mostly limited to binary NiTi alloys with transformation temperatures approximately in the -100 to 100 (&)#186;C range. In an ongoing effort to develop high-temperature shape memory alloys (HTSMAs), ternary and quaternary additions are being made to binary NiTi to form NiTi-X (e.g., X: Pd, Pt, Au and Hf) alloys. Stability and repeatability can be further increased at these higher temperatures by limiting the stress, but the tradeoff is reduced work output and stroke. However, HTSMAs operating at decreased stresses can still be used effectively in actuator applications that require large strokes when used in the form of springs. The overall objective of this work is to facilitate the development of HTSMAs for use as high-force actuators in active/adaptive aerospace structures.A modular test setup was assembled with the objective of acquiring stroke, stress, temperature and moment data in real time during joule heating and forced convective cooling of Ni19.5Ti50.5Pd25Pt5 HTSMA springs. The spring actuators were evaluated under both monotonic axial loading and thermomechanical cycling. The role of rotational constraints (i.e., by restricting rotation or allowing for free rotation at the ends of the springs) on stroke performance was also assessed. Recognizing that evolution in the material microstructure results in changes in geometry and vice versa in HTSMA springs, the objective of the present study also included assessing the contributions from the material microstructural evolution, by eliminating contributions from changes in geometry, to overall HTSMA spring performance. The finite element method (FEM) was used to support the analytical analyses and provided further insight into the behavior and heterogeneous stress states that exist in these spring actuators.Furthermore, with the goal of improving dimensional stability there is a need to better understand the microstructural evolution in HTSMAs that contributes to irrecoverable strains. Towards this goal, available Ni29.5Ti50.5Pd20 neutron diffraction data (from a comparable HTMSA alloy without the solid solution strengthening offered by the Pt addition) were analyzed. The data was obtained from in situ neutron diffraction experiments performed on Ni29.5Ti50.5Pd20 during compressive loading while heating/cooling, using the Spectrometer for Materials Research at Temperature and Stress (SMARTS) at Los Alamos National Laboratory. Specifically, in this work emphasis was placed on neutron diffraction data analysis via Rietveld refinement and capturing the texture evolution through inverse pole figures. Such analyses provided quantitative information on the evolution of lattice strain, phase volume fraction (including retained martensite that exists above the austenite finish temperature) and texture (martensite variant reorientation and detwinning) under temperature and stress. Financial support for this work from NASA's Fundamental Aeronautics Program Supersonics Project (NNX08AB51A), Subsonic Fixed Wing Program (NNX11AI57A) and the Florida Center for Advanced Aero-Propulsion (FCAAP) is gratefully acknowledged. It benefited additionally from the use of the Lujan Neutron Scattering Center at Los Alamos National Laboratory, which is funded by the Office of Basic Energy Sciences (Department of Energy) and is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396.
Show less
Date Issued: 2011
Identifier: CFE0004147, ucf:49059
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004147

Title: An experimental investigation on the dynamics of bubbles utilizing refrigerant R134a under pressurized flow boiling conditions.
Creator: Vereen, Keon, Kumar, Ranganathan, Chow, Louis, Deng, Weiwei, University of Central Florida
Abstract / Description: Flow boiling heat transfer allows for the dissipation of large amounts of heat. In this work, the effect of heat flux and pressure on flow boiling of liquid refrigerant R-134a is studied in a vertical thin channel. The experimental setup mimics a refrigeration cycle and specifically looks at the effect of pressure and wall heat flux on the departure size and bubble generation rate. The experimental setup consists of a closed loop which includes a vertical narrow rectangular channel and two...
Show moreFlow boiling heat transfer allows for the dissipation of large amounts of heat. In this work, the effect of heat flux and pressure on flow boiling of liquid refrigerant R-134a is studied in a vertical thin channel. The experimental setup mimics a refrigeration cycle and specifically looks at the effect of pressure and wall heat flux on the departure size and bubble generation rate. The experimental setup consists of a closed loop which includes a vertical narrow rectangular channel and two synchronized high speed cameras for optical measurements at either sides of the channel. The setup is built to employ an accurate measurement technique to define wall temperatures of the representative flow boiling process. Instead of using thermocouples on the surface channel, the thermochromic liquid crystallography (TLC) technique is used to determine non-invasively the heater surface temperature at high temporal and spatial resolution. The TLC interval range is 30-50(&)deg;C. The TLC is attached to a Fecralloy heating section. The high speed Prosilica cameras simultaneously capture, colored TLC images as well as bubble nucleation and departure at very high frame rates. Experiments on subcooled flow boiling heat transfer have been conducted with refrigerant R-134a under a mass flux range of 484.838 kg/m2s to 1212.1 kg/m2s. With the low mass flux, the wall heat flux ranged from 167.2 to 672.1 kW/m2, the inlet subcooling ranged from 0.35(&)deg;C to 16.55 (&)deg;C, the system pressure ranged from 621 kPa to 1034 kPa. At high mass flux, the wall heat flux ranged from 329.8 kW/m2 to 744 kW/m2, the inlet subcooling from 0.16(&)deg;C to 17.21 (&)deg;C, and the system pressure from 621 kPa to 1034 kPa. A parametric study was done by maintaining various input parameters constant.From the high speed images, bubble parameters such as size and frequency are calculated. Temperature contours are utilized to determine the surface wall temperature at specific points. Sequential wall temperatures are traced over a short period of time to understand the cooling effects. The bubble propagation and coalescence are also visualized. Results show that bubble size and frequency increased with heat flux at any particular pressure. At higher pressure, the trend would be for the bubble size to decrease; however, the inlet subcooling and heat flux also affect bubble size. The bubble frequency is also seen to be affected by the inlet subcooling and the heat flux. Even though the inlet subcooling is maintained approximately constant, any slight decrease in subcooling increased bubble growth rate. Another trend that is observed is that at higher the heat flux, the bubble generation frequency is faster; however no specific trend is observed for wall superheat. With an increase in heat flux, the wall superheats are expected to increase; however, the localized nature of the nucleation activity sites is seen to affect the results. The variables are non-dimensionalized to note trends in parameters. In summary, the data analysis demonstrates that both heat flux and pressure significantly influence the bubble generation rate, size, propagation and coalescence.
Show less
Date Issued: 2011
Identifier: CFE0004175, ucf:49077
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004175

Title: Electrospray and Superlens Effect of Microdroplets for Laser-Assisted Nanomanufacturing.
Creator: Castillo Orozco, Eduardo, Kumar, Ranganathan, Mansy, Hansen, Peles, Yoav, University of Central Florida
Abstract / Description: Nanoparticles of various materials are known to exhibit excellent mechanical, chemical, electrical, and optical properties. However, it is difficult to deposit and transform nanoparticles into large two-dimensional and three-dimensional structures, such as thin films and discrete arrays. Electrospray technology and laser heating enable the deposition of these nanoparticles through the dual role of microdroplets as nanoparticle carriers and superlenses. The main goals of this dissertation are...
Show moreNanoparticles of various materials are known to exhibit excellent mechanical, chemical, electrical, and optical properties. However, it is difficult to deposit and transform nanoparticles into large two-dimensional and three-dimensional structures, such as thin films and discrete arrays. Electrospray technology and laser heating enable the deposition of these nanoparticles through the dual role of microdroplets as nanoparticle carriers and superlenses. The main goals of this dissertation are to delineate the electrospray modes, to achieve subwavelength focusing, and to enable a process for the deposition of nanoparticles into microlayers and discrete nanodots (a nanodot is a cluster of nanoparticles) on rigid and flexible substrates. This additive manufacturing process is based on the electrospray generation of water microdroplets that carry nanoparticles onto a substrate and the laser sintering of these nanoparticles. The process involves injecting nanoparticles (contained inside electric field-driven water microdroplets) into a hollow laser beam. The laser beam heats the droplets, causing the water to evaporate and the nanoparticles to sinter and form deposit of material on the substrate.The electrohydrodynamic inkjet printing of nanoparticle suspensions has been accomplished by the operation of an electrospray in microdripping mode and it allows the deposition of monodisperse microdroplets containing nanoparticles into discrete nanodot arrays, narrow lines, and thin films. For flow rates with low Reynolds number, the mode changes from dripping to microdripping mode, and then to a planar oscillating microdripping mode as the electric capillary number, Cae increases. The microdripping mode which is important for depositing discrete array of nanodots is found to occur in a narrow range, 2 ? Cae ? 2.5. The effect of the physical properties on the droplet size and frequency of droplet formation is more precisely described by the relative influence of the electric, gravity, viscous, and capillary forces. A scaling analysis is derived from a fundamental force balance and has yielded a parameter based on the electric capillary number, capillary number, and Bond number. Results for different nanoparticle suspensions with a wide range of physical properties show that the normalized radius of droplet, can be correlated using this parameter in both dripping and microdripping modes. The same parameter also correlates the normalized frequency of droplet formation, Nd* as an increasing function in the microdripping mode. Viscosity affects the shape of the cone by resisting its deformation and thus promoting a stable microdripping mode. Reduction in surface tension decreases the droplet size in the electrospray modes. However, the capillary size and electrical conductivity have minimal effect on the size of the ejected droplets. Electrical conductivity affects the transition between microdripping and oscillating microdripping modes. Based on this analysis, it is possible to design the electrospray to produce uniform monodisperse droplets by manipulating the voltage at the electrode, for any desired nanoparticle concentration of a suspension to be sintered on a substrate. For the fabrication of nanodots, a laser beam of wavelength ? = 1064 nm was focused to a diameter smaller than its wavelength. When the microdroplets did not carry nanoparticles, the subwavelength focusing of the laser yielded nanoholes smaller than its wavelength. Results show that tiny features with high resolution can be created by loading microdroplets with nanoparticles and squeezing the laser beam to subwavelength regions. Nanodots of silicon and germanium with diameters between 100 - 500 nm have been deposited on a silicon substrate. This study demonstrates an interdisciplinary mechanism to achieve subwavelength focusing in a laser process. In this process, the microdroplets serve as both a nanoparticle carrier and a superlens that focuses a laser beam to subwavelength diameters up to ? /10, thus overcoming the diffraction limit. The microdroplets are generated from a suspension of nanoparticles using an electrospray technique and the superlens characteristic of these microdroplets is attributed to three optical phenomena such as Maxwell's fish eye lens or L(&)#252;neberg lens, evanescent waves by laser scattering, and evanescent waves by the total internal reflection principle. A microfluidic cooling effect can also contribute to creating subwavelength features. In summary, this work describes a new laser-assisted additive manufacturing process for the fabrication of nanodots and microlayers using nanoparticles of different materials. In this process, microdroplets from an electrospray are used as nanoparticle carriers and superlenses to focus the laser to a diameter smaller than its wavelength. While this process is demonstrated to produce subwavelength holes and nanodots, the process is scalable to produce narrow lines and thin films of semiconductor materials by an additive manufacturing technique. This process extends the application of infrared lasers to the production of nanostructures and nanofeatures, and, therefore, provides a novel technology for nanomanufacturing.
Show less
Date Issued: 2018
Identifier: CFE0007563, ucf:52579
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007563

Title: Numerical Study of Interfacial flow using Algebraic Coupled Level Set-Volume of Fluid (A-CLSVOF) Method.
Creator: Haghshenas, Majid, Kumar, Ranganathan, Das, Tuhin, Ahmed, Kareem, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: Solving interfacial flows numerically has been a challenge due to the lack of sharpness and the presence of spurious currents at the interface. Two methods, Algebraic Coupled Level Set-Volume of Fluid (A-CLSVOF) method and Ghost Fluid Method (GFM) have been developed in the finite volume framework and employed in several interfacial flows such as Rayleigh-Taylor instability, rising bubble, impinging droplet and cross-flow oil plume. In the static droplet simulation, A-CLSVOF substantially...
Show moreSolving interfacial flows numerically has been a challenge due to the lack of sharpness and the presence of spurious currents at the interface. Two methods, Algebraic Coupled Level Set-Volume of Fluid (A-CLSVOF) method and Ghost Fluid Method (GFM) have been developed in the finite volume framework and employed in several interfacial flows such as Rayleigh-Taylor instability, rising bubble, impinging droplet and cross-flow oil plume. In the static droplet simulation, A-CLSVOF substantially reduces the spurious currents. The capillary wave relaxation shows that this method delivers results comparable to those of more rigorous methods such as Front Tracking methods for fine grids. The results for the other interfacial flows also compared well with the experimental results. Next, interfacial forces are implemented by enlisting the finite volume discretization of Ghost Fluid Method. To assess the A-CLSVOF/GFM performance, four cases are studied. In the case of the static droplet in suspension, the combined A-CLSVOF/GFM produces a sharp and accurate pressure jump compared to the traditional CSF (continuum surface force) implementation. For the linear two-layer shear flow, GFM sharp treatment of the viscosity captured the velocity gradient across the interface. For a gaseous bubble rising in a viscous fluid, GFM outperforms CSF by almost 10%. Also, a Decoupled Pressure A-CLSVOF/GFM method (DPM) has been developed which separates pressure into two pressure components, one accounting for interfacial forces such as surface tension and another representing the rest of flow pressure. It is proven that the DPM implementation results in more efficiency in PISO (Pressure Implicit with Splitting of Operators) loop. A two-phase solver is used to study buoyant oil discharge in quiescent and cross-flow ambient. Different modes of breakup including dripping, jetting (axisymmetric and asymmetric) and atomization for cross-flow oil jet are captured.
Show less
Date Issued: 2018
Identifier: CFE0007570, ucf:52582
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007570

Title: moedling phase change heat transfer of liquid/vapor systems in free/porous media.
Creator: Wilson, James, Kumar, Ranganathan, Kar, Aravinda, Chow, Louis, University of Central Florida
Abstract / Description: Effective solvent extraction incorporating electromagnetic heating is a relatively new concept that relies on Radio Frequency heating and solvents to replace steam in current thermal processes for the purpose of extracting bitumen from oil rich sands. The work presented here will further the understanding of the near wellbore flow of this two phase system in order to better predict solvent vaporization dynamics and heat rates delivered to the pay zone. This numerical study details the aspects...
Show moreEffective solvent extraction incorporating electromagnetic heating is a relatively new concept that relies on Radio Frequency heating and solvents to replace steam in current thermal processes for the purpose of extracting bitumen from oil rich sands. The work presented here will further the understanding of the near wellbore flow of this two phase system in order to better predict solvent vaporization dynamics and heat rates delivered to the pay zone. This numerical study details the aspects of phase change of immiscible, two component, liquid/vapor systems confined in porous media heated by electromagnetic radiation, approximated by a spatially dependent volumetric heat source term in the energy equation.The objective of this work is to utilize the numerical methodology presented herein to predict maximum solvent delivery rates to a heated isotropic porous matrix to avoid the over-saturation of the heated pay zone. The total liquid mass content and mean temperature in the domain are monitored to assess whether the liquid phase is fully vaporized prior to flowing across the numerical domain boundary. The distribution of the volumetric heat generation rate used to emulate the physics of electromagnetic heating in the domain decays away from the well bore. Some of the heat generated acts to superheat the already vaporized solvent away from the interface, requiring heat delivery rates that are many times greater than the energy required to turn the liquid solvent to vapor determined by an energy balance. Results of the parametric study from the pay zone simulations demonstrate the importance of the Darcian flow resistance forces added by the porous media to stabilize the flow being pulled away from the wellbore in the presence of gravity. For all cases involving an increase in solvent delivery rate with a constant heat rate, the permeability range required for full vaporization must decrease in order to balance the gravitational forces pulling the solvent from the heated region. For all conditions of permeability and solvent delivery rates, sufficiently increasing the heat rate results in complete vaporization of the liquid solvent. For the case of decreasing solvent delivery rate, a wider range of higher permeabilities for a given heat rate can be utilized while achieving full vaporization. A three dimensional surface outlining the transition from partially vaporized to fully vaporized regimes is constructed relating the solvent delivery rate, the permeability of the porous near wellbore zone and the heat rate supplied to the domain. For the range of permeabilities ~3000mD observed in these types of well bores, low solvent delivery rates and high heat rates must be utilized in order to achieve full vaporization.
Show less
Date Issued: 2015
Identifier: CFE0006018, ucf:50997
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006018

Title: ZrB2-SiC BASED ULTRA HIGH TEMPERATURE CERAMIC COMPOSITES: MECHANICAL PERFORMANCE AND MEASUREMENT AND DESIGN OF THERMAL RESIDUAL STRESSES FOR HYPERSONIC VEHICLE APPLICATIONS.
Creator: Stadelmann, Richard, Orlovskaya, Nina, Kumar, Ranganathan, Raghavan, Seetha, University of Central Florida
Abstract / Description: Ultra-high temperature ceramics (UHTCs), such as ZrB2-based ceramic composites, have been identified as next generation candidate materials for leading edges and nose cones in hypersonic air breathing vehicles. Mechanical performance of ceramic composites play an important role in the ultra-high temperature applications, therefore SiC is added to ZrB2 as a strengthening phase to enhance its mechanical performance. The high melting temperatures of both ZrB2 and SiC, as well as the ability of...
Show moreUltra-high temperature ceramics (UHTCs), such as ZrB2-based ceramic composites, have been identified as next generation candidate materials for leading edges and nose cones in hypersonic air breathing vehicles. Mechanical performance of ceramic composites play an important role in the ultra-high temperature applications, therefore SiC is added to ZrB2 as a strengthening phase to enhance its mechanical performance. The high melting temperatures of both ZrB2 and SiC, as well as the ability of SiC to form SiO2 refractory oxide layers upon oxidation make ZrB2-SiC ceramics very suitable for aerospace applications. Thermal residual stresses appearing during processing are unavoidable in sintered ZrB2-SiC ceramic composites. Residual microstresses appear at the microstructural level (intergranular microstresses) or at the crystal structure level (intragranular microstresses). These microstresses are of enormous importance for the failure mechanisms in ZrB2-SiC ceramics, such as ratio of the trans- and intergranular fracture; crack branching or bridging, microcracking, subcritical crack growth and others, as they govern crack propagation(-)induced energy dissipation and affect the toughness and strength of the ceramic material. Therefore, understanding the evolution of residual stress state in processed ZrB2-SiC ceramic composites and accurate measurements of these stresses are of high priority. In the present research the ZrB2-17vol%SiC, ZrB2-32vol%SiC, and ZrB2-45vol%SiC ultra-high temperature particulate ceramic composites were sintered using both Hot Pressing (HP) and Spark Plasma Sintering (SPS) techniques. The mechanical performance of the ZrB2-SiC composites was investigated using 3- and 4-point bending techniques for measurements of instantaneous fracture strength and fracture toughness. Resonant Ultrasound Spectroscopy was used for measurement of Young's, shear, and bulk moduli as well as Poisson's ratio of the composites. The distribution of thermal residual stresses and the effect of the applied external load on their re-distribution was studied using micro-Raman spectroscopy. Piezospectroscopic coefficients were determined for all compositions of ZrB2-SiC ceramic under study and their experimentally obtained values were compared with the piezospectroscopic coefficients both published in the literature and calculated using theoretical approach. Finally, the novel ZrB2-IrB2-SiC ceramic composites were also produced using Spark Plasma Sintering (SPS), where IrB2 powder was synthesized using mechanochemical route. It is expected that the IrB2 additive phase might contribute to the improved overall oxidation resistance of ZrB2 based ultra-high temperature ceramic composites.
Show less
Date Issued: 2015
Identifier: CFE0006008, ucf:51004
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006008

Title: Theoretical And Experimental Investigation Of The Cascading Nature Of Pressure-Swirl Atomization.
Creator: Choudhury, Pretam, Kumar, Ranganathan, Deng, Weiwei, Mansy, Hansen, University of Central Florida
Abstract / Description: Pressure swirl atomizers are commonly used in IC, aero-engines, and liquid propellant rocket combustion. Understanding the atomization process is important in order to enhance vaporization, mitigate soot formation, design of combustion chambers, and improve overall combustion efficiency. This work utilizes non-invasive techniques such as ultra -speed imaging, and Phase Doppler Particle Anemometry (PDPA) in order to investigate the cascade atomization process of pressure-swirl atomizers by...
Show morePressure swirl atomizers are commonly used in IC, aero-engines, and liquid propellant rocket combustion. Understanding the atomization process is important in order to enhance vaporization, mitigate soot formation, design of combustion chambers, and improve overall combustion efficiency. This work utilizes non-invasive techniques such as ultra -speed imaging, and Phase Doppler Particle Anemometry (PDPA) in order to investigate the cascade atomization process of pressure-swirl atomizers by examining swirling liquid film dynamics and the localized droplet characteristics of the resulting hollow cone spray. Specifically, experiments were conducted to examine these effects for three different nozzles with orifice diameters .3mm, .5mm, and .97mm. The ultra-speed imaging allowed for both visualization and interface tracking of the swirling conical film which emanated from each nozzle. Moreover, this allowed for the measurement of the radial fluctuations, film length, cone angle and maximum wavelength. Radial fluctuations are found to be maximum near the breakup or rupture of a swirling film. Film length decreases as Reynolds number increases. Cone angle increases until a critical Reynolds number is reached, beyond which it remains constant. A new approach to analyze the temporally unstable waves was developed and compared with the measured maximum wavelengths. The new approach incorporates the attenuation of a film thickness, as the radius of a conical film expands, with the classical dispersion relationship for an inviscid moving liquid film. This approach produces a new long wave solution which accurately matches the measured maximum wavelength swirling conical films generated from nozzles with the smallest orifice diameter. For the nozzle with the largest orifice diameter, the new long wave solution provides the upper bound limit, while the long wave solution for a constant film thickness provides the lower bound limit. These results indicate that temporal instability is the dominating mechanism which generates long Kelvin Helmholtz waves on the surface of a swirling liquid film. The PDPA was used to measure droplet size and velocity in both the near field and far field of the spray. For a constant Reynolds number, an increase in orifice diameter is shown to increase the overall diameter distribution of the spray. In addition, it was found that the probability of breakup, near the axis, decreases for the largest orifice diameter. This is in agreement with the cascading nature of atomization.
Show less
Date Issued: 2015
Identifier: CFE0006030, ucf:51012
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006030

Title: Multi-axial Thermomechanical Characterization of Shape Memory Alloys for Improved Stability.
Creator: Nicholson, Douglas, Vaidyanathan, Raj, Kumar, Ranganathan, Chen, Ruey-Hung, University of Central Florida
Abstract / Description: Shape recovery in shape memory alloys (SMAs) occurs against external stress by means of a reversible thermoelastic solid state phase transformation typically between so-called austenite, martensite and R-phases. The ability to do work enables their use as high-force actuators in automotive and aerospace applications while superelastic NiTi is of interest in biomedical devices such as stents. Both R-phase and martensite can detwin, reorient and undergo a thermal or stress induced...
Show moreShape recovery in shape memory alloys (SMAs) occurs against external stress by means of a reversible thermoelastic solid state phase transformation typically between so-called austenite, martensite and R-phases. The ability to do work enables their use as high-force actuators in automotive and aerospace applications while superelastic NiTi is of interest in biomedical devices such as stents. Both R-phase and martensite can detwin, reorient and undergo a thermal or stress induced transformation. For these reasons, it is difficult from ordinary macroscopic measurements to decouple elastic and inelastic contributions (from their respective phases) from the overall deformation. In situ neutron diffraction is ideally suited to probing these microstructural and micromechanical changes while they occur under external stress fields. Despite SMAs typically operating under multi-axial stress states in applications, most previous in situ neutron diffraction based investigations on SMAs have been limited to homogenous stress states as a result of uniaxial loading. The current investigation spatially maps thermoelastic deformation mechanisms during heating and uniaxial/torsional loading of shape memory and superelastic NiTi by recourse to in situ neutron diffraction, performed at Oak Ridge and Los Alamos National Laboratories. SMA spring actuators were also used to experimentally validate the ability of a recently developed model to predict the evolutionary deformation response under multi-axial loading conditions.By recourse to in situ neutron diffraction, martensite variants were tracked during isothermal, isobaric, and isostrain loading in shape memory NiTi. Results show variants were equivalent for the corresponding strain and more importantly, the reversibility and equivalency was immediately evident in variants that were first selected isobarically but then reoriented to a near random self-accommodated structure by isothermal deformation. Variants selected isothermally were not significantly affected by a subsequent thermal cycle under constant strain. During uniaxial/torsional loading and heating, thermoelastic deformation mechanisms in non-uniform states of stress in superelastic NiTi were spatially mapped. The preferred selection of R-phase variants by reorientation and detwinning processes were equivalent for the corresponding strain (in tension and compression) and was reversed by isothermal loading. The variants selected were consistent between uniaxial and torsional loading when the principal stress directions of the stress state were considered (for the crystallographic directions considered here). The similarity in general behavior between uniaxial and torsional loading, in spite of the implicit heterogeneous stress state associated with torsional loading, pointed to the ability of the reversible thermoelastic transformation to accommodate both stress and strain mismatch associated with deformation.Overall, various thermomechanical combinations of heating and loading sequences yielded the same final texture (preferred selection of variants), which highlighted the ability to take different paths yet still obtain the desired response while minimizing irrecoverable deformation mechanisms. These paths have implications for minimizing the number of cycles required to train an SMA, which limits the amount of work required for stabilizing their evolutionary response thereby increasing the fatigue life and overall durability of the SMA. This finding is valuable to the aerospace and medical device industries where SMAs find current application.
Show less
Date Issued: 2017
Identifier: CFE0006952, ucf:51676
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006952

Title: Droplet impact on deep liquid pools: secondary droplets formation from Rayleigh jet break-up and crown splash.
Creator: Castillo Orozco, Eduardo, Kumar, Ranganathan, Mansy, Hansen, Peles, Yoav, University of Central Florida
Abstract / Description: This work aims to study the impact of a droplet on liquid pools of the same fluid to understand the formation of secondary drops from the central jet and crown splash that occur after the impact. The impact of droplets on a deep pool has applications in cleaning up oil spill, spray cooling, painting, inkjet printing and forensic analysis, relying on the changes in properties such as viscosity, interfacial tension and density. Despite the exhaustive research on different aspects of droplet...
Show moreThis work aims to study the impact of a droplet on liquid pools of the same fluid to understand the formation of secondary drops from the central jet and crown splash that occur after the impact. The impact of droplets on a deep pool has applications in cleaning up oil spill, spray cooling, painting, inkjet printing and forensic analysis, relying on the changes in properties such as viscosity, interfacial tension and density. Despite the exhaustive research on different aspects of droplet impact, it is not clear how liquid properties can affect the instabilities leading to the Rayleigh jet breakup and the number of secondary drops formed after it pinches off. In this work, through systematic experiments, the droplet impact phenomena is investigated by varying viscosity and surface tension of liquids as well as impact speeds. Further, using a Volume-of-Fluid (VOF) method, it is shown that Rayleigh-Plateau instability is influenced by these parameters, and capillary timescale is the appropriate scale to normalize the breakup time. Increase in impact velocity increases the height of the thin column of fluid that emerges from the liquid pool. Under certain fluid conditions, the dissipation of this extra kinetic energy along with the surface tension forces produces instabilities at the neck of the jet. This could result in jet breakup and formation of secondary drops. In other words, both the formation of the jet and its breakup require a balance between viscous, capillary and surface tension forces. Based on Ohnesorge number (Oh) and impact Weber number (We), a regime map for no breakup, Rayleigh jet breakup, and crown splash is suggested for 0.0033 ? Oh ? 0.136. For Weber numbers beyond the critical value and Oh ? 0.091 the jet breakup occurs (Rayleigh jet breakup regime). While for Oh (>) 0.091, the jet breakup is suppressed regardless of the Weber number. In addition, high impact velocity initiates the crown formation and if further intensified it can disintegrate it into numerous secondary drops (crown splash) and it is observed to occur at all Ohnesorge numbers and high enough Weber numbers, however, at high Oh, a large portion of kinetic energy is dissipated, thus Rayleigh jet breakup is suppressed regardless of the magnitude of the impact velocity. Moreover, a correlation is proposed for normalized time with respect to the normalized maximum height of jet.
Show less
Date Issued: 2015
Identifier: CFE0006278, ucf:51593
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006278

Title: Mechanical Properties and Thermal Residual Stresses of ZrB2-SiC Ceramic Composites for Hypersonic Vehicle Applications.
Creator: Stadelmann, Richard, Orlovskaya, Nina, Kumar, Ranganathan, Raghavan, Seetha, University of Central Florida
Abstract / Description: Ultra High Temperature Ceramic (UHTC) ZrB2- 10, 20, 30wt%SiC composites are of high interest for use in hypersonic air-breathing vehicles. In this work, ZrB2- 10, 20, 30wt%SiC UHTC composites were produced by the Spark Plasma Sintering (SPS) technique. After sintering, almost dense ceramics with ~ 5-8% porosity were produced. Their mechanical properties, such as Young's, shear, and bulk moduli, along with Poisson's ratio, 4-point bending strength, and single edge V-notched beam (SEVNB)...
Show moreUltra High Temperature Ceramic (UHTC) ZrB2- 10, 20, 30wt%SiC composites are of high interest for use in hypersonic air-breathing vehicles. In this work, ZrB2- 10, 20, 30wt%SiC UHTC composites were produced by the Spark Plasma Sintering (SPS) technique. After sintering, almost dense ceramics with ~ 5-8% porosity were produced. Their mechanical properties, such as Young's, shear, and bulk moduli, along with Poisson's ratio, 4-point bending strength, and single edge V-notched beam (SEVNB) fracture toughness were measured. In addition, in-situ bending experiments under a Raman microscope were performed to determine the piezo-spectroscopic coefficients of SiC Raman active peaks for calculation of thermal residual stresses. The results show that these materials are possible candidates for hypersonic air-breathing vehicles due to their high Young's modulus, ability to withstand high temperatures, and relatively low densities.
Show less
Date Issued: 2013
Identifier: CFE0005060, ucf:49945
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005060

Title: Meshless Direct Numerical Simulation of Turbulent Incompressible Flows.
Creator: Vidal Urbina, Andres, Kassab, Alain, Kumar, Ranganathan, Ilegbusi, Olusegun, Divo, Eduardo, University of Central Florida
Abstract / Description: A meshless direct pressure-velocity coupling procedure is presented to perform Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) of turbulent incompressible flows in regular and irregular geometries. The proposed method is a combination of several efficient techniques found in different Computational Fluid Dynamic (CFD) procedures and it is a major improvement of the algorithm published in 2007 by this author. This new procedure has very low numerical diffusion and some...
Show moreA meshless direct pressure-velocity coupling procedure is presented to perform Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) of turbulent incompressible flows in regular and irregular geometries. The proposed method is a combination of several efficient techniques found in different Computational Fluid Dynamic (CFD) procedures and it is a major improvement of the algorithm published in 2007 by this author. This new procedure has very low numerical diffusion and some preliminary calculations with 2D steady state flows show that viscous effects become negligible faster that ever predicted numerically.The fundamental idea of this proposal lays on several important inconsistencies found in three of the most popular techniques used in CFD, segregated procedures, streamline-vorticity formulation for 2D viscous flows and the fractional-step method, very popular in DNS/LES.The inconsistencies found become important in elliptic flows and they might lead to some wrong solutions if coarse grids are used. In all methods studied, the mathematical basement was found to be correct in most cases, but inconsistencies were found when writing the boundary conditions. In all methods analyzed, it was found that it is basically impossible to satisfy the exact set of boundary conditions and all formulations use a reduced set, valid for parabolic flows only.For example, for segregated methods, boundary condition of normal derivative for pressure zero is valid only in parabolic flows. Additionally, the complete proposal for mass balance correction is right exclusively for parabolic flows.In the streamline-vorticity formulation, the boundary conditions normally used for the streamline function, violates the no-slip condition for viscous flow. Finally, in the fractional-step method, the boundary condition for pseudo-velocity implies a zero normal derivative for pressure in the wall (correct in parabolic flows only) and, when the flows reaches steady state, the procedure does not guarantee mass balance.The proposed procedure is validated in two cases of 2D flow in steady state, backward-facing step and lid-driven cavity. Comparisons are performed with experiments and excellent agreement was obtained in the solutions that were free from numerical instabilities.A study on grid usage is done. It was found that if the discretized equations are written in terms of a local Reynolds number, a strong criterion can be developed to determine, in advance, the grid requirements for any fluid flow calculation.The 2D-DNS on parallel plates is presented to study the basic features present in the simulation of any turbulent flow. Calculations were performed on a short geometry, using a uniform and very fine grid to avoid any numerical instability. Inflow conditions were white noise and high frequency oscillations. Results suggest that, if no numerical instability is present, inflow conditions alone are not enough to sustain permanently the turbulent regime.Finally, the 2D-DNS on a backward-facing step is studied. Expansion ratios of 1.14 and 1.40 are used and calculations are performed in the transitional regime. Inflow conditions were white noise and high frequency oscillations. In general, good agreement is found on most variables when comparing with experimental data.
Show less
Date Issued: 2015
Identifier: CFE0005733, ucf:50148
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005733

Title: Study of Transport Phenomena in Carbon-Based Materials.
Creator: Aboelsoud, Walid, Chow, Louis, Kumar, Ranganathan, Deng, Weiwei, Kar, Aravinda, University of Central Florida
Abstract / Description: In air-cooled heat exchangers, air-side thermal resistance is usually the largest compared to conduction and liquid-side thermal resistances. Thus, reducing the air-side thermal resistance with fin-like structures can greatly improve overall cooling performance. The performance of these structures is usually characterized by the rate of heat which can be transferred and the pumping power required. One promising solution is to use a high-thermal-conductivity material with a large surface per...
Show moreIn air-cooled heat exchangers, air-side thermal resistance is usually the largest compared to conduction and liquid-side thermal resistances. Thus, reducing the air-side thermal resistance with fin-like structures can greatly improve overall cooling performance. The performance of these structures is usually characterized by the rate of heat which can be transferred and the pumping power required. One promising solution is to use a high-thermal-conductivity material with a large surface per unit volume such as carbon foam. This study presents a method of utilizing V-shape corrugated carbon foam. The air-side heat transfer coefficient and the pressure drop across the foam have been investigated using different V-shape foam geometrical configurations obtained by varying its length and height. Based on design considerations and availability, the foam length has been chosen to be 25.4, 38.1 and 52.1 mm while its height is 4.4, 6.8 and 11.7 mm, resulting in nine different test pieces of foam with different heights and lengths.A total number of 81 experiments were carried out and results show that of the nine V-shape configurations, the foam with the shortest length and tallest height gives the best performance. Experimental results are also compared with the results of prior work using different carbon foam geometries. It is shown that V-shape corrugated carbon foam provides higher heat transfer coefficient and better overall performance.Numerical method is performed next. The effect of the foam length and height on thermal and hydraulic performance is demonstrated and discussed. There is excellent agreement between numerical and experimental results. An analysis is also made to better understand the transport phenomena that occur within the porous matrix. For laminar flow of air, one of the findings is the high heat transfer effectiveness of the foam which means a foam thickness of 1 mm or less is sufficient for heat transfer enhancement for air speed of up to 4 m/s. To demonstrate the feasibility of using carbon foam, an analytical case study of carbon foam heat exchanger was performed and compared to traditional heat exchanger with the same heat load. Results show that a volume saving of up to 55% can be obtained by using carbon foam instead of traditional aluminum fins.Another attractive carbon-based material is the highly oriented pyrolytic graphite (HOPG) which has an in-plane thermal conductivity of about 1700 W/m.K and an out-of-plane k of about 8 W/m.K at room temperature. HOPG is a graphite material with a high degree of preferred crystallographic orientation. HOPG can be very useful in thermal applications when axial conduction is critical and needed to be minimized as in recuperators used in cryocoolers and compact power generation. Also, an analysis of HOPG for micro-channel applications shows that the high in-plane thermal conductivity of HOPG, which is far greater than that of copper and aluminum, allows a taller height for the micro-channel. This translates to an increase in the heat flux removal rate by two to three times.
Show less
Date Issued: 2013
Identifier: CFE0005081, ucf:50732
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005081

Title: Energy-optimal Guidance of an AUV Under Flow Uncertainty and Fluid-Particle Interaction.
Creator: De Zoysa Abeysiriwardena, Demuni Singith, Das, Tuhin, Kumar, Ranganathan, Elgohary, Tarek, Behal, Aman, University of Central Florida
Abstract / Description: The work presented gives an energy-optimal solution to the guidance problem of an AUV. The presented guidance methods are for lower level control of AUV paths, facilitating existing global planning methods to be carried out comparatively more efficiently. The underlying concept is to use the energy of fluid flow fields the AUVs are navigating to extend the duration of missions. This allows gathering of comparatively more data with higher spatio-temporal resolution. The problem is formulated...
Show moreThe work presented gives an energy-optimal solution to the guidance problem of an AUV. The presented guidance methods are for lower level control of AUV paths, facilitating existing global planning methods to be carried out comparatively more efficiently. The underlying concept is to use the energy of fluid flow fields the AUVs are navigating to extend the duration of missions. This allows gathering of comparatively more data with higher spatio-temporal resolution. The problem is formulated for a generalized two dimensional uniform flow field given a fixed final time andfree end states. This allows the AUVs to navigate to certain spatial positions while maintaining the required temporal resolution of each segment of its mission. The simplistic way the problem is posed allows an analytical closed form solution of the Euler-Lagrange equations. Two dimensional thrust vectors are obtained as optimal control inputs. The control inputs are then incorporated into afeedback structure, allowing the particle to navigate in the presence of disturbance in the flow field. Further, the work also explores the influence of fluid-particle interaction on the control cost and behavior of the particle. The concept of changing the cost weights of the optimal cost formulation in situ has been introduced. Potential applications of the present concept are explored through anobstacle avoidance scenario. The optimal guidance methods are then adapted to non-uniform flow fields with quadratic and discontinuous spatial variation being the primary focus.
Show less
Date Issued: 2018
Identifier: CFE0007169, ucf:52282
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007169

Title: Numerical Simulation of Conventional Fuels and Biofuels Dispersion and Vaporization Process in Co-flow and Cross-flow Premixers.
Creator: Gu, Xin, Kumar, Ranganathan, Basu, Saptarshi, Kapat, Jayanta, Chow, Louis, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: In order to follow increasingly strict regulation of pollutant emissions, a new concept of Lean Premixed pre-vaporized (LPP) combustion has been proposed for turbines. In LPP combustion, controlled atomization, dispersion and vaporization of different types of liquid fuel in the pre-mixer are the key factors required to stabilize the combustion process and improve the efficiency. A numerical study is conducted for the fundamental understanding of the liquid fuel dispersion and vaporization...
Show moreIn order to follow increasingly strict regulation of pollutant emissions, a new concept of Lean Premixed pre-vaporized (LPP) combustion has been proposed for turbines. In LPP combustion, controlled atomization, dispersion and vaporization of different types of liquid fuel in the pre-mixer are the key factors required to stabilize the combustion process and improve the efficiency. A numerical study is conducted for the fundamental understanding of the liquid fuel dispersion and vaporization process in pre-mixers using both cross-flow and co-flow injection methods. First, the vaporization model is validated by comparing the numerical data to existing experiments of single droplet vaporization under both low and high convective air temperatures. Next, the dispersion and vaporization process for biofuels and conventional fuels injected transversely into a typical simplified version of rectangular pre-mixer are simulated and results are analyzed with respect to vaporization performance, degree of mixedness and homogeneity. Finally, collision model has been incorporated to predict more realistic vaporization performance. Four fuels, Ethanol, Rapeseed Methyl Esters (RME), gasoline and jet-A have been investigated. For mono-disperse spray with no collision model, the droplet diameter reduction and surface temperature rise were found to be strongly dependent on the fuel properties. The diameter histogram near the pre-mixer exit showed a wide droplet diameter distribution for all the fuels. In general, pre-heating of the fuels before injection improved the vaporization performance. An improvement in the drag model with Stefan flow correction showed that a low speed injection and high cone angle improved performance. All fuels achieved complete vaporization under a spray cone angle of 140(&)deg;. In general, it was found that cross-flow injection achieved better vaporization performance than co-flow injection. A correlation is derived for jet-A's total vaporization performance as a function of non-dimensional inlet air temperature and fuel/air momentum flux ratio. This is achieved by curve-fitting the simulated results for a broad range of inlet air temperatures and fuel/air momentum flux ratios. The collision model, based on no-time-counter method (NTC) proposed by Schmidt and Rutland, was implemented to replace O'Rourke's collision algorithm to improve the results such that the unphysical numerical artifact in a Cartesian grid was removed and the results were found to be grid-independent. The dispersion and vaporization processes for liquid fuel sprays were simulated in a cylindrical pre-mixer using co-flow injection method. Results for jet-A and Rapeseed Methyl Esters (RME) showed acceptable grid independence. At relatively low spray cone angle and injection velocity, it was found that the collision effect on the average droplet size and the vaporization performance were very high due to relatively high coalescence rate induced by droplet collisions. It was also found that the vaporization performance and the level of homogeneity of fuel-air mixture could be significantly improved when the dispersion level is high, which can be achieved by increasing the spray cone angle and injection velocity. In order to compare the performance between co-flow and cross-flow injection methods, the fuels were injected at an angle of 40(&)deg; with respect to the stream wise direction to avoid impacting on the wall. The cross-flow injection achieved similar vaporization performance as co-flow because a higher coalescence rate induced by droplet collisions cancelled off its higher heat transfer efficiency between two phases for cross-flow injections.
Show less
Date Issued: 2012
Identifier: CFE0004192, ucf:49004
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004192

Title: Evaporation, Precipitation Dynamics and Instability of Acoustically Levitated Functional Droplets.
Creator: Saha, Abhishek, Kumar, Ranganathan, Basu, Saptarshi, Kapat, Jayanta, Deng, Weiwei, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: Evaporation of pure and binary liquid droplets is of interest in thermal sprays and spray drying of food, ceramics and pharmaceutical products. Understanding the rate of heat and mass transfer in any drying process is important not only to enhance evaporation rate or vapor-gas mixing, but also to predict and control the final morphology and microstructure of the precipitates. Acoustic levitation is an alternative method to study micron-sized droplets without wall effects, which eliminates...
Show moreEvaporation of pure and binary liquid droplets is of interest in thermal sprays and spray drying of food, ceramics and pharmaceutical products. Understanding the rate of heat and mass transfer in any drying process is important not only to enhance evaporation rate or vapor-gas mixing, but also to predict and control the final morphology and microstructure of the precipitates. Acoustic levitation is an alternative method to study micron-sized droplets without wall effects, which eliminates chemical and thermal contamination with surfaces. This work uses an ultrasonic levitation technique to investigate the vaporization dynamics under radiative heating, with focus on evaporation characteristics, precipitation kinetics, particle agglomeration, structure formation and droplet stability. Timescale and temperature scales are developed to compare convective heating in actual sprays and radiative heating in the current experiments. These relationships show that simple experiments can be conducted in a levitator to extrapolate information in realistic convective environments in spray drying. The effect of acoustic streaming, droplet size and liquid properties on internal flow is important to understand as the heat and mass transfer and particle motion within the droplet is significantly controlled by internal motion. Therefore, the droplet internal flow is characterized by Particle Image Velocimetry for different dropsize and viscosity. Nanosuspension droplets suspended under levitation show preferential accumulation and agglomeration kinetics. Under certain conditions, they form bowl shaped structures upon complete evaporation. At higher concentrations, this initial bowl shaped structure morphs into a ring structure. Nanoparticle migration due to internal recirculation forms a density stratification, the location of which depends on initial particle concentration. The time scale of density stratification is similar to that of perikinetic-driven agglomeration of particle flocculation. The density stratification ultimately leads to force imbalance leading to a unique bowl-shaped structure. Chemically active precursor droplet under acoustic levitation shows events such as vaporization, precipitation and chemical reaction leading to nanoceria formation with a porous morphology. The cerium nitrate droplet undergoes phase and shape changes throughout the vaporization process followed by formation of precipitate. Ex-situ analyses using TEM and SEM reveal highly porous morphology with trapped gas pockets and nanoceria crystalline structures at 70 degree C. Inhomogeneity in acoustic pressure around the heated droplet can induce thermal instability. Short wavelength (Kelvin-Helmholtz) instability for diesel and bio-diesel droplets triggers this secondary atomization, which occurs due to relative velocity between liquid and gas phase at the droplet equator. On the other hand, liquids such as Kerosene and FC43 show uncontrollable stretching followed by a catastrophic break-up due to reduction in surface tension and viscosity coupled with inhomogeneity of pressure around the droplet. Finally, a scaling analysis has been established between vaporizing droplets in a convective and radiative environment. The transient temperature normalized by the respective scales exhibits a unified profile for both modes of heating. The analysis allows for the prediction of required laser flux in the levitator experiments to show its equivalence in a corresponding heated gas stream. The theoretical equivalence shows good agreement with experiments for a range of droplet sizes.
Show less
Date Issued: 2012
Identifier: CFE0004436, ucf:49346
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004436

Title: Transient CFD analysis of autorotation using hybrid LES and adaptive mesh morphing techniques.
Creator: Coronado Domenge, Patricia, Das, Tuhin, Kassab, Alain, Kumar, Ranganathan, Leishman, J., Bhattacharya, Aniket, University of Central Florida
Abstract / Description: Large-Eddy Simulation (LES) based turbulence modeling is a developing area of research in Fluid-Structure Interaction (FSI). There is considerable scope for further scientific research in this field and this dissertation aims to extend it to the study of flow-induced motion. The emphasis of this work is on autorotation, an important category of flow-induced motion that is commonly seen in energy applications such as wind turbines and in aviation applications such as the autogyro. In contrast...
Show moreLarge-Eddy Simulation (LES) based turbulence modeling is a developing area of research in Fluid-Structure Interaction (FSI). There is considerable scope for further scientific research in this field and this dissertation aims to extend it to the study of flow-induced motion. The emphasis of this work is on autorotation, an important category of flow-induced motion that is commonly seen in energy applications such as wind turbines and in aviation applications such as the autogyro. In contrast to existing works on FSI that typically assume prescribed motion of structures in a flow field, this research develops LES based FSI studies for large-scale flow-induced motions as seen in autorotation. The uniqueness of the formulation and modeling approach lies in the development of a numerically stable computational scheme that incorporates a moving and morphing mesh structure. The method is first demonstrated for the autorotation of a square flat plate and then extended to a rotor structure similar to that of a helicopter.In order to simulate an autorotating square flat plate, a coupled Computational Fluid Dynamics (CFD) - Rigid Body Dynamics (RBD) model is proposed, employing the delayed-detached-eddy simulation (DDES) and the Smagorinsky turbulence models to resolve subgrid-scale stresses (SGS). The plate is allowed to spin freely about its center of mass. Computational results are compared to experimental measurements and Reynolds Average Navier-Stokes (RANS) simulations found in the literature. When compared to RANS, the results from the LES models provide better predictions of the pressure coefficient. Moreover, LES accurately captures the transient behavior of the plate, and close correspondence is found between the predicted and measured moment coefficients. The qualitative prediction of vortex structures and the quantitative computation of pressure coefficients are in good agreement with experimental results. Hybrid models, such as improved Delayed-Detached-Eddy Simulation (iDDES), are shown to provide very similar results to those of pure LES. Therefore hybrid models are found to be a good alternative to use for the simulation of FSI in autorotation, saving valuable computational time . The iDDES method combines both RANS and LES, dividing the flow domain into LES far away from a solid wall and RANS near a solid wall, overcoming the computational costs of pure LES.Encouraging results from this effort prompted the extension to a realistic scenario, namely the autorotation of a flapping-blade rotor in a prevailing wind field. A coupled CFD - Multi Body Dynamics (MBD) model is developed to study the complex FSI of an autorotating 3-blade rotor, similar to that of a helicopter, employing the iDDES turbulence model. In addition to the rotor being allowed to spin freely about its axis, each of the individual blades is free to rotate about hinges at the root. This adds degrees of freedom to the kinematics of the rotor and necessitates localized mesh morphing around the blades to capture the FSI with accuracy. The model is validated against experimental data and shows excellent agreement. The experimental apparatus consists of a flapping blade rotor and a fixture used to mount it at different angles of incidence with respect to the wind field. The rotor is instrumented with a DC motor that is operated in generator mode. The setup is dual-purpose, providing speed measurement using the motor's back-emf and regenerative braking by varying the current draw. Overall, the presented research can help obtain accurate values of aerodynamic parameters at a high spatial resolution that would be otherwise difficult to acquire in experiments. Ultimately this approach can be a cost effective means of aerodynamic modeling in applications involving large scale FSI.
Show less
Date Issued: 2016
Identifier: CFE0006088, ucf:50952
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006088

Title: Thermally induced motion, collision and mixing of levitated droplets.
Creator: Davanlou, Ashkan, Kumar, Ranganathan, Cho, Hyoung Jin, Deng, Weiwei, Mansy, Hansen, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: This dissertation investigates the motion of a levitated droplet experimentally and analytically against the Marangoni flow in an immiscible outer fluid at higher speeds than is possible currently. Based on our earlier experiments, when a droplet is released from a height of 1.5 (-) 4 times its diameter from the liquid surface, it can overcome the impact and stay levitated at the liquid-air interface due to the existence of an air gap between the droplet and the liquid film. In order to...
Show moreThis dissertation investigates the motion of a levitated droplet experimentally and analytically against the Marangoni flow in an immiscible outer fluid at higher speeds than is possible currently. Based on our earlier experiments, when a droplet is released from a height of 1.5 (-) 4 times its diameter from the liquid surface, it can overcome the impact and stay levitated at the liquid-air interface due to the existence of an air gap between the droplet and the liquid film. In order to explain this behavior of droplet traveling against the counter-current motion, we propose a simple approach: first, the Marangoni convection inside the thin film is considered without the droplet floating on the surface. By using a level-set method and solving the Navier-Stokes equation, the free surface velocity and deformation are calculated. Then, these quantities are used to solve for droplet velocity and drag coefficient simultaneously using a force balance. In order to compare the simulation results, experiments with levitated water droplets on an immiscible carrier liquid, FC-43, were conducted for various temperature gradients, and droplet velocities were measured at different locations using high-speed imaging. The experimental results are in good agreement with the developed theoretical model. For a Reynolds number range of 2-32, it is shown that the drag coefficients are up to 66% higher than those for the fully immersed sphere at the same Reynolds numbers. A correlation is proposed to calculate the drag coefficient of levitated droplets for various temperature drops across the channel.For the first time, it is shown that it is possible to realize the natural coalescence of droplets through Marangoni effect without any external stimulation, and deliver the coalesced droplet to a certain destination through the use of surface tension gradients. The effects of the various shapes and sizes upon collision are studied. Regions of coalescence and stretching separation of colliding droplets are delineated based on Weber number and impact number. The existence of the transition line between coalescence and stretching separation in this passive mode of transport is similar to what was observed in the literature for forced coalescence at significantly higher Weber numbers. It is also found that a thermocapillary environment improves the mixing process. In order to illustrate and quantify the mixing phenomenon, the dispensed droplets were made of potassium hydroxide and phenolphthalein which is used as a pH indicator. The experiments show the possibility to reach mixing rates as high as 74% within 120 ms. This study offers new insight to thermo-coalescence and demonstrates how natural coalescence could be used to transport, mix and collect biochemical assays more efficiently. The results of this research can be engineered to enhance the performance of self-cleaning surfaces and micro-total analysis systems ((&)#181;TAS), where sample transport, filtration, chemical reactions, separation and detection are of great interest.
Show less
Date Issued: 2015
Identifier: CFE0006213, ucf:51106
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006213

Title: Experimental Investigation of Breakup and Coalescence Characteristics of a Hollow Cone Swirling Spray.
Creator: Lee, Joshua, Kumar, Ranganathan, Deng, Weiwei, Kapat, Jayanta, Basu, Saptarshi, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: Atomization can be achieved by discharging liquid at relative high velocities into a slow moving environment (hydraulic nozzles) or by discharging liquid at low velocities into a fast moving gas flow (air-blast nozzles). These two types of injector nozzles are featured in majority of the industry applications such as power generation, food or pharmaceutical powder formation, spray painting, petroleum refining, and thermal sprays. The most common atomizer used in combustion engines is the...
Show moreAtomization can be achieved by discharging liquid at relative high velocities into a slow moving environment (hydraulic nozzles) or by discharging liquid at low velocities into a fast moving gas flow (air-blast nozzles). These two types of injector nozzles are featured in majority of the industry applications such as power generation, food or pharmaceutical powder formation, spray painting, petroleum refining, and thermal sprays. The most common atomizer used in combustion engines is the pressure-swirl nozzle (Simplex nozzle) to obtain a homogenous mixture at different equivalence ratios. The experimental studies performed with pressure-swirl nozzles have reported contradictory results over the last few years. Thus, the fundamentals of spray dynamics, such as spray formation, liquid breakup length, droplet breakup regimes, and coalescence still need to be understood for a pressure-swirl nozzle.An experimental study of the breakup characteristics of various liquids and fuels with different thermal physical properties emanating from hollow cone hydraulic injector nozzles induced by pressure-swirling was investigated. The experiments were conducted using two nozzles with different orifice diameters 0.3mm and 0.5mm and injection pressures (0.3-4MPa) which correspond to Rep = 7,000-31,000 depending on the liquids being tested. Three laser-based techniques, i.e., Shadowgraph, Particle Image Velocimetry (PIV) and Phase Doppler Particle Anemometry (PDPA) were utilized in this study. Although each technique had its limitation in different flow regimes, the results were cross-validated, and generally showed correct trends in axial and radial measurements of velocity and diameter for different nozzles, Weber and Reynolds numbers.The spatial variation of diameter and velocity arises principally due to primary breakup of liquid films and subsequent secondary breakup of large droplets due to aerodynamic shear. Downstream of the nozzle, coalescence of droplets due to collision is also found to be significant. Different types of liquid film break up was considered and found to match well with the theory. The spray is subdivided into three zones: near the nozzle, a zone consisting of film and ligament regime, where primary breakup and some secondary breakup take place; a second zone where the secondary breakup process continues, but weakens, and the centrifugal dispersion becomes dominant, and a third zone away from the spray where coalescence is dominant. Each regime has been analyzed in detail to understand the effect of surface tension and viscosity. Surface tension and viscosity were engineered to mimic fuels, which were then compared with real fuels such as Ethanol, Jet-A and Kerosene. Results show similarity in the diameter in the beginning stages of breakup but in the coalescence regime, the values deviate from each other, indicating that the vapor pressure also plays a major role in this regime.
Show less
Date Issued: 2013
Identifier: CFE0005021, ucf:50014
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005021

Title: Agglomeration, Evaporation and Morphological Changes in Droplets with Nanosilica and Nanoalumina Suspensions in an Acoustic Field.
Creator: Tijerino Campollo, Erick, Kumar, Ranganathan, Deng, Weiwei, Chow, Louis, Basu, Saptarshi, University of Central Florida
Abstract / Description: Acoustic levitation permits the study of droplet dynamics without the effects of surface interactions present in other techniques such as pendant droplet methods. Despite the complexities of the interactions of the acoustic field with the suspended droplet, acoustic levitation provides distinct advantages of controlling morphology of droplets with nanosuspensions post precipitation. Droplet morphology is controlled by vaporization, deformation and agglomeration of nanoparticles, and therefore...
Show moreAcoustic levitation permits the study of droplet dynamics without the effects of surface interactions present in other techniques such as pendant droplet methods. Despite the complexities of the interactions of the acoustic field with the suspended droplet, acoustic levitation provides distinct advantages of controlling morphology of droplets with nanosuspensions post precipitation. Droplet morphology is controlled by vaporization, deformation and agglomeration of nanoparticles, and therefore their respective timescales are important to control the final shape. The balance of forces acting on the droplet, such as the acoustic pressure and surface tension, determine the geometry of the levitated droplet. Thus, the morphology of the resultant structure can be controlled by manipulating the amplitude of the levitator and the fluid properties of the precursor nanosuspensions. The interface area in colloidal nanosuspensions is very large even at low particle concentrations. The effects of the presence of this interface have large influence in the properties of the solution even at low concentrations.This thesis focuses on the dynamics of particle agglomeration in acoustically levitated evaporating nanofluid droplets leading to shell structure formation. These experiments were performed by suspending 500(&)#181;m droplets in a pressure node of a standing acoustic wave in a levitator and heating them using a carbon dioxide laser. These radiatively heated functional droplets exhibit three distinct stages, namely, pure evaporation, agglomeration and structure formation. The temporal history of the droplet surface temperature shows two inflection points. Morphology and final precipitation structures of levitated droplets are due to competing mechanisms of particle agglomeration, evaporation and shape deformation. This thesis provides a detailed analysis for each process and proposes two important timescales for evaporation and agglomeration that determine the final diameter of the structure formed. It is seen that both agglomeration and evaporation timescales are similar functions of acoustic amplitude (sound pressure level), droplet size, viscosity and density. However it is shown that while the agglomeration timescale decreases with initial particle concentration, the evaporation timescale shows the opposite trend. The final normalized diameter hence can be shown to be dependent solely on the ratio of agglomeration to evaporation timescales for all concentrations and acoustic amplitudes. The experiments were conducted with 10nm silica, 20nm silica, 20nm alumina and 50nm alumina solutions. The structures exhibit various aspect ratios (bowls, rings, spheroids) which depend on the ratio of the deformation timescale (tdef) and the agglomeration timescale (tg).
Show less
Date Issued: 2012
Identifier: CFE0004610, ucf:49914
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004610

Title: Three-phase contact line phenomena in droplets on solid and liquid surfaces: electrocapillary, pinning, wetting line velocity effect, and free liquid surface deformation.
Creator: Shabani, Roxana, Cho, Hyoung, Kumar, Ranganathan, Kapat, Jayanta, Chow, Louis, Zhai, Lei, University of Central Florida
Abstract / Description: In this dissertation, physical phenomena relevant to (i) an interface formed between two fluids and a solid phase (wettingline) and (ii) an interface between three fluids (triple contact line) were investigated. In the former case, the wetting line (WL)phenomena, which encompass the wetting line energy (WLE), the wetting line velocity (WLV), and the contact anglehysteresis, were studied using a micropump based on electrowetting on dielectric (EWOD). In the latter case, the air filmlubrication...
Show moreIn this dissertation, physical phenomena relevant to (i) an interface formed between two fluids and a solid phase (wettingline) and (ii) an interface between three fluids (triple contact line) were investigated. In the former case, the wetting line (WL)phenomena, which encompass the wetting line energy (WLE), the wetting line velocity (WLV), and the contact anglehysteresis, were studied using a micropump based on electrowetting on dielectric (EWOD). In the latter case, the air filmlubrication effect and the liquid free surface deformation were taken into account to explain the dual equilibrium states ofwater droplets on liquid free surfaces. A micropump based on droplet/meniscus pressure gradient generated by EWOD was designed and fabricated. By alteringthe contact angle between liquid and solid using an electric field a pressure gradient was induced and a small droplet waspumped into the channel. The flow rate in the channel was found to be constant in spite of the changes in the droplet'sradius. The WL phenomena were studied to unravel the physical concept behind the micropump constant flow rate. Theobservation and measurement reveal that the shrinking input droplet changes its shape in two modes in time sequence: (i)its contact angle decreases, while its wetting area remains constant, and (ii) its WL starts to move while its contact anglechanges. Contact angles were measured for the advancing and receding WLs at different velocities to capture a full pictureof contact angle behavior. The effects of the WLE on the static contact angle and the WLV on the dynamic contact angle inthe pump operation were investigated. Also the effect of EWOD voltage on the magnitude and uniformity of the micropumpflow rate was studied. Dynamic contact angles were used to accurately calculate the pressure gradient between the dropletand the meniscus, and estimate the flow rate. It was shown that neglecting either of these effects not only results in aconsiderable gap between the predicted and the measured flow rates but also in an unphysical instability in the flow rateanalysis. However, when the WLE and WLV effects were fully taken into account, an excellent agreement between thepredicted and the experimental flow rates was obtained.For the study of the TCL between three fluids, aqueous droplets were formed at oil-air interface and two stableconfigurations of (i) non-coalescent droplet and (ii) cap/bead droplet were observed. General solutions for energy and forceanalysis were obtained and were shown to be in good agreement with the experimental observations. Further the energybarrier obtained for transition from configuration (i) to (ii) was correlated to the droplet release height and the probability ofnon-coalescent droplet formation. Droplets formed on the solid surfaces and on the free surface of immiscible liquids have various applications indroplet-based microfluidic devices. This research provides an insight into their formation and manipulation.
Show less
Date Issued: 2013
Identifier: CFE0005253, ucf:50598
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005253

Microfluidics (3)	+ -
Nanofluid (3)	+ -
Acoustic Levitation (2)	+ -
Droplet (2)	+ -
Fluid flow (2)	+ -

Heat transfer (2)	+ -
Molecular Dynamics (2)	+ -
Nanoparticle (2)	+ -
NiTi (2)	+ -
ZrB2-SiC (2)	+ -
agglomeration (2)	+ -
concentration (2)	+ -
critical heat flux (2)	+ -
pool boiling (2)	+ -
AUV (1)	+ -
Acoustic Streaming (1)	+ -
Acoustically Levitated Droplet (1)	+ -
Advancing and receding wetting-lines (1)	+ -
Agglomeration (1)	+ -
Air-Water Flow (1)	+ -
Alumina (1)	+ -
Analytical method (1)	+ -
Atomic Scale Simulation (1)	+ -
Atomization (1)	+ -
Autorotation (1)	+ -
Biofluids (1)	+ -
Biofuel (1)	+ -
Biofuels (1)	+ -
Boiling (1)	+ -
Boltzmann (1)	+ -

ETD Collection (39)	+ -
Honors Collection (1)	+ -

University of Central Florida (40)	+ -
Kumar, Ranganathan (39)	+ -
Deng, Weiwei (7)	+ -
Chow, Louis (6)	+ -
Shivamoggi, Bhimsen (6)	+ -

Basu, Saptarshi (4)	+ -
Kapat, Jayanta (4)	+ -
Mansy, Hansen (4)	+ -
Das, Tuhin (3)	+ -
Kassab, Alain (3)	+ -
Castillo Orozco, Eduardo (2)	+ -
Chen, Ruey-Hung (2)	+ -
Gu, Xin (2)	+ -
Gupta, Amit (2)	+ -
Kar, Aravinda (2)	+ -
Lee, Joshua (2)	+ -
Nicholson, Douglas (2)	+ -
Orlovskaya, Nina (2)	+ -
Peles, Yoav (2)	+ -
Raghavan, Seetha (2)	+ -
Sachdeva, Parveen (2)	+ -
Saha, Abhishek (2)	+ -
Stadelmann, Richard (2)	+ -
Wilson, James (2)	+ -
Aboelsoud, Walid (1)	+ -
Ahmed, Kareem (1)	+ -
Behal, Aman (1)	+ -
Bhattacharya, Aniket (1)	+ -
Cho, Hyoung (1)	+ -
Cho, Hyoung Jin (1)	+ -

Search In

Pages

Pages

Sort Results By:

Narrow results by:

Topical Subject

Collection

Type of Resource

Language

Creator

Format

Owner