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(1 - 9 of 9)
- Title
- ANALYTIC & NUMERICAL STUDY OF A VORTEX MOTION EQUATION.
- Creator
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Bueller, Daniel, Rollins, David, University of Central Florida
- Abstract / Description
-
A nonlinear second order differential equation related to vortex motion is derived. This equation is analyzed using various numerical and analytical techniques including finding approximate solutions using a perturbative approach.
- Date Issued
- 2011
- Identifier
- CFE0003720, ucf:48802
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0003720
- Title
- REDUCTION OF VORTEX-DRIVEN OSCILLATIONS IN A SOLID ROCKET MOTOR COLD FLOW SIMULATION THROUGH ACTIVE CONTROL.
- Creator
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Ward, Jami, Leonessa, Alexander, University of Central Florida
- Abstract / Description
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Control of vortex-driven instabilities was demonstrated via a scaled-down, cold-flow simulation that modeled closed-end acoustics. When vortex shedding frequencies couple with the natural acoustic modes of a choked chamber, potentially damaging low-frequency instabilities may arise. Although passive solutions can be effective, an active control solution is preferable. An experiment was performed to demonstrate an active control scheme for the reduction of vortex-driven oscillations. A non...
Show moreControl of vortex-driven instabilities was demonstrated via a scaled-down, cold-flow simulation that modeled closed-end acoustics. When vortex shedding frequencies couple with the natural acoustic modes of a choked chamber, potentially damaging low-frequency instabilities may arise. Although passive solutions can be effective, an active control solution is preferable. An experiment was performed to demonstrate an active control scheme for the reduction of vortex-driven oscillations. A non-reacting experiment using a primary flow of air, where both the duct exit and inlet are choked, simulated the closed-end acoustics. Two plates, separated by 1.27 cm, produced the vortex shedding phenomenon at the chamber's first longitudinal mode. Two active control schemes, closed-loop and open-loop, were studied via a cold-flow simulation for validating the effects of reducing vortex shedding instabilities in the system. Actuation for both control schemes was produced by using a secondary injection method. The actuation system consisted of pulsing compressed air from a modifed, 2-stroke model airplane engine, controlled and powered by a DC motor. The use of open-loop only active control was not highly effective in reducing the amplitude of the first longitudinal acoustic mode, near 93 Hz, when the secondary injection was pulsed at the same modal frequency. This was due to the uncontrolled phasing of the secondary injection system. A Pulse Width Modulated (PWM) signal was added to the open-loop control scheme to correct for improper phasing of the secondary injection flow relative to the primary flow. This addition allowed the motor speed to be intermittently increased to a higher RPM before returning to the desired open-loop control state. This proved to be effective in reducing the pressure disturbance by approximately 46%. A closed-loop control scheme was then test for its effectiveness in controlling the phase of the secondary injection. Feedback of the system's state was determined by placing a dynamic pressure transducer near the chamber exit. Closed-loop active control, using the designed secondary injection system, was proven as an effective means of reducing the problematic instabilities. A 50% reduction in the FFT RMS amplitude was realized by utilizing a Proportional-Derivative controller to modify the phase of the secondary injection.
Show less - Date Issued
- 2006
- Identifier
- CFE0000920, ucf:46728
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000920
- Title
- VORTEX TILTING AND THE ENHANCEMENT OF SPANWISE FLOW IN FLAPPING WING FLIGHT.
- Creator
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Frank, Spencer, Raghavan, Seetha, University of Central Florida
- Abstract / Description
-
The leading edge vortex has been identified as the most critical flow structure for producing lift in flapping wing flight. Its stability depends on the transport of the entrained vorticity into the wake via spanwise flow. This study proposes a hypothesis for the generation and enhancement of spanwise flow based on the chordwise vorticity that results from the tilting of the leading edge vortex and trailing edge vortex. We investigate this phenomenon using dynamically scaled robotic model...
Show moreThe leading edge vortex has been identified as the most critical flow structure for producing lift in flapping wing flight. Its stability depends on the transport of the entrained vorticity into the wake via spanwise flow. This study proposes a hypothesis for the generation and enhancement of spanwise flow based on the chordwise vorticity that results from the tilting of the leading edge vortex and trailing edge vortex. We investigate this phenomenon using dynamically scaled robotic model wings. Two different wing shapes, one rectangular and one based on Drosophila melanogaster (fruit fly), are submerged in a tank of mineral oil and driven in a flapping motion. Two separate kinematics, one of constant angular velocity and one of sinusoidal angular velocity are implemented. In order to visualize the flow structure, a novel three dimensional particle image velocimetry system is utilized. From the three dimensional information obtained the chordwise vorticity resulting from the vortex tilting is shown using isosurfaces and planar slices in the wake of the wing. It is observed that the largest spanwise flow is located in the area between the chordwise vorticity of the leading edge vortex and the chordwise vorticity of the trailing edge vortex, supporting the hypothesis that the vortex tilting enhances the spanwise flow. Additionally the LEV on the rectangular wing is found to detach at about 80% span as opposed to 60% span for the elliptical wing. Also, two distinct regions of spanwise flow, one at the base and one at the tip, are observed at the beginning of the sinusoidal kinematic, and as the velocity of the wing increases these two regions unionize into one. Lastly, the general distribution of vorticity around each wing is found to be nearly the same, indicating that different wing shapes do not greatly affect the distribution of vorticity nor stability mechanisms in flapping flight. In summary the tilting mechanism helps to explain the overall flow structure and the stability of the leading edge vortex.
Show less - Date Issued
- 2011
- Identifier
- CFH0004124, ucf:44875
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFH0004124
- Title
- Experimental and Numerical Study of Endwall Film Cooling.
- Creator
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Mahadevan, Srikrishna, Kapat, Jayanta, Verma, Shashi, Vasu Sumathi, Subith, Ahmed, Kareem, Shivamoggi, Bhimsen, University of Central Florida
- Abstract / Description
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This research work investigates the thermal performance of a film-cooled gas turbine endwall under two different mainstream flow conditions. In the first part of the research investigation, the effect of unsteady passing wakes on a film-cooled pitchwise-curved surface (representing an endwall without airfoils) was experimentally studied for heat transfer characteristics on a time-averaged basis. The temperature sensitive paint technique was used to obtain the local temperatures on the test...
Show moreThis research work investigates the thermal performance of a film-cooled gas turbine endwall under two different mainstream flow conditions. In the first part of the research investigation, the effect of unsteady passing wakes on a film-cooled pitchwise-curved surface (representing an endwall without airfoils) was experimentally studied for heat transfer characteristics on a time-averaged basis. The temperature sensitive paint technique was used to obtain the local temperatures on the test surface. The required heat flux input was provided using foil heaters. Discrete film injection was implemented on the test surface using cylindrical holes with a streamwise inclination angle of 35? and no compound angle relative to the mean approach velocity vector. The passing wakes increased the heat transfer coefficients at both the wake passing frequencies that were experimented. Due to the increasing film cooling jet turbulence and strong jet-mainstream interaction at higher blowing ratios, the heat transfer coefficients were amplified. A combination of film injection and unsteady passing wakes resulted in a maximum pitch-averaged and centerline heat transfer augmentation of ? 28% and 31.7% relative to the no wake and no film injection case. The second part of the research study involves an experimental and numerical analysis of secondary flow and coolant film interaction in a high subsonic annular cascade with a maximum isentropic throat Mach number of ? 0.68. Endwall (platform) thermal protection is provided using discrete cylindrical holes with a streamwise inclination angle of 30? and no compound angle relative to the mean approach velocity vector. The surface flow visualization on the inner endwall provided the location of the saddle point and the three-dimensional separation lines. Computational predictions showed that the leading-edge horseshoe vortex was confined to approximately 1.5% of the airfoil span for the no film injection case and intensified with low momentum film injection. At the highest blowing ratio, the film cooling jet weakened the horseshoe vortex at the leading-edge plane. The passage vortex was intensified with coolant injection at all blowing ratios. It was seen that increasing average blowing ratio improved the film effectiveness on the endwall. The discharge coefficients calculated for each film cooling hole indicated significant non-uniformity in the coolant discharge at lower blowing ratios and the strong dependence of discharge coefficients on the mainstream static pressure and the location of three-dimensional separation lines. Near the airfoil suction side, a region of coalesced film cooling jets providing close to uniform film coverage was observed, indicative of the mainstream acceleration and the influence of three-dimensional separation lines.
Show less - Date Issued
- 2015
- Identifier
- CFE0005973, ucf:50775
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005973
- Title
- Holographic optical elements for visible light applications in photo-thermo-refractive glass.
- Creator
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Kompan, Fedor, Glebov, Leonid, Schulzgen, Axel, Richardson, Kathleen, Rahman, Talat, University of Central Florida
- Abstract / Description
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This dissertation reports on design and fabrication of various optical elements in Photo-thermo-refractive (PTR) glass. An ability to produce complex holographic optical elements (HOEs) for the visible spectral region appears very beneficial for variety of applications, however, it is limited due to photosensitivity of the glass confined within the UV region. First two parts of this dissertation present two independent approaches to the problem of holographic recording using visible radiation...
Show moreThis dissertation reports on design and fabrication of various optical elements in Photo-thermo-refractive (PTR) glass. An ability to produce complex holographic optical elements (HOEs) for the visible spectral region appears very beneficial for variety of applications, however, it is limited due to photosensitivity of the glass confined within the UV region. First two parts of this dissertation present two independent approaches to the problem of holographic recording using visible radiation. The first method involves modification of the original PTR glass rendering it photosensitive to radiation in the visible spectral region and, thus, making possible the recording of holograms in PTR glass with visible radiation. The mechanism of photoionization in this case is based on an excited state absorption upconversion process in the glass when doped with Tb3+. By contrast, the second approach uses the original Ce3+ doped PTR glass and introduces a new modified technique for hologram formation that allows for holographic recording with visible light. Complex HOEs including holographic lenses and holographic curved mirrors were fabricated in PTR glass with visible light using both techniques. The third part of the dissertation takes a step in a different direction and discusses the development of the methods for fabrication of phase masks in PTR glass. A method for relatively straightforward and inexpensive fabrication of phase masks with the aid of a Digital Micromirror Device is presented. This method enabled to produce phase masks containing complex greyscale phase distributions for generation of vortex (helical) beams. A phase mask can be holographically encoded into a transmission Bragg grating where a holographic phase mask (HPM) is formed. HPM has an advantage over a regular phase mask of being capable of multi-wavelength operation. All optical elements recorded in PTR glass preserve the advantages peculiar to VBGs recorded in PTR glass such as stability to heating and illumination with high-power laser beams.
Show less - Date Issued
- 2019
- Identifier
- CFE0007665, ucf:52480
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007665
- Title
- Turbulent Flame-Vortex Dynamics of Bluff-Body Premixed Flames.
- Creator
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Geikie, Marissa, Ahmed, Kareem, Vasu Sumathi, Subith, Bhattacharya, Samik, Singh, Arvind, University of Central Florida
- Abstract / Description
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This study explores the effects of turbulence and pressure-gradient tailoring on the turbulent flame and vorticity transport mechanisms of premixed flames. A turbulent premixed flame stabilized by a bluff-body in a high-speed combustor is used for the investigation. The combustor pressure gradient is altered using a variable-geometry test section. The turbulence within the combustor is controlled using a custom-designed, supersonic variable-turbulence generator. The turbulent flame-flow field...
Show moreThis study explores the effects of turbulence and pressure-gradient tailoring on the turbulent flame and vorticity transport mechanisms of premixed flames. A turbulent premixed flame stabilized by a bluff-body in a high-speed combustor is used for the investigation. The combustor pressure gradient is altered using a variable-geometry test section. The turbulence within the combustor is controlled using a custom-designed, supersonic variable-turbulence generator. The turbulent flame-flow field is measured and characterized using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The flame-vortex dynamics of the turbulent flame are analyzed using a Lagrangian tracking methodology. Lagrangian fluid elements (LFEs) are tagged on the experimental data and are tracked as they propagate across the turbulent flame. The vorticity generation and transport mechanisms are decomposed along the Lagrangian trajectories to determine their relative balance under the various pressure gradient and turbulence conditions. It is demonstrated that the turbulence and induced pressure-gradient independently affect the relative magnitudes of dilatation, baroclinic torque, and vortex stretching mechanisms. Increasing the combustor pressure gradient augments the relative magnitudes of the vorticity mechanisms; baroclinic torque exhibits the largest gain for augmented pressure gradient relative to attenuated. The turbulence causes a in a reduction of the dilatation and baroclinic torque vorticity mechanisms, meanwhile the vortex stretching increases. When the turbulence and pressure gradient are altered simultaneously, the aforementioned effects superpose. These results confirm that alteration of the test section pressure gradient can be used to augment the vorticity mechanisms independently from turbulence. Furthermore, a change in the relative balance of the combustion-induced vorticity mechanisms and turbulence energy transport occurs as a result of increasing the turbulence and pressure gradient.
Show less - Date Issued
- 2018
- Identifier
- CFE0007180, ucf:52277
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007180
- Title
- Aerodynamic Characteristics of a Gas Turbine Exhaust Diffuser with an Accompanying Exhaust Collection System.
- Creator
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Bernier, Bryan, Kapat, Jayanta, Deng, Weiwei, Raghavan, Seetha, University of Central Florida
- Abstract / Description
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The effects of an industrial gas turbine's Exhaust Collector Box (ECB) geometry on static pressure recovery and total pressure loss were investigated in this study experimentally and computationally. This study aims to further understand how exit boundary conditions affect the performance of a diffuser system as well as the accuracy of industry standard computational models. A design of experiments approach was taken using a Box-Behnken design method for investigating three geometric...
Show moreThe effects of an industrial gas turbine's Exhaust Collector Box (ECB) geometry on static pressure recovery and total pressure loss were investigated in this study experimentally and computationally. This study aims to further understand how exit boundary conditions affect the performance of a diffuser system as well as the accuracy of industry standard computational models. A design of experiments approach was taken using a Box-Behnken design method for investigating three geometric parameters of the ECB. In this investigation, the exhaust diffuser remained constant through each test, with only the ECB being varied. A system performance analysis was conducted for each geometry using the total pressure loss and static pressure recovery from the diffuser inlet to the ECB exit. Velocity and total pressure profiles obtained with a hotwire anemometer and Kiel probe at the exit of the diffuser and at the exit of the ECB are also presented in this study. A total of 13 different ECB geometries are investigated at a Reynolds number of 60,000. Results obtained from these experimental tests are used to investigate the accuracy of a 3-dimensional RANS with realizable k-? turbulence model from the commercial software package Star-CCM+. The study confirms the existence of strong counter-rotating helical vortices within the ECB which significantly affect the flow within the diffuser. Evidence of a strong recirculation zone within the ECB was found to force separation within the exhaust diffuser which imposed a circumferentially asymmetric pressure field at the inlet of the diffuser. Increasing the ECB width proved to decrease the magnitude of this effect, increasing the diffuser protrusion reduced this effect to a lesser degree. The combined effect of increasing the ECB Length and Width increased the expansion area ratio, proving to increase the system pressure recovery by as much as 19% over the nominal case. Additionally, the realizable k-? turbulence model was able to accurately rank all 13 cases in order by performance; however the predicted magnitudes of the pressure recovery and total pressure loss were poor for the cases with strong vortices. For the large volume cases with weak vortices, the CFD was able to accurately represent the total pressure loss of the system within 5%.
Show less - Date Issued
- 2012
- Identifier
- CFE0004517, ucf:49296
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004517
- Title
- Nonlinear dispersive partial differential equations of physical relevance with applications to vortex dynamics.
- Creator
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VanGorder, Robert, Kaup, David, Vajravelu, Kuppalapalle, Nevai, Andrew, Mohapatra, Ram, Kassab, Alain, University of Central Florida
- Abstract / Description
-
Nonlinear dispersive partial differential equations occur in a variety of areas within mathematical physics and engineering. We study several classes of such equations, including scalar complex partial differential equations, vector partial differential equations, and finally non-local integro-differential equations. For physically interesting families of these equations, we demonstrate the existence (and, when possible, stability) of specific solutions which are relevant for applications....
Show moreNonlinear dispersive partial differential equations occur in a variety of areas within mathematical physics and engineering. We study several classes of such equations, including scalar complex partial differential equations, vector partial differential equations, and finally non-local integro-differential equations. For physically interesting families of these equations, we demonstrate the existence (and, when possible, stability) of specific solutions which are relevant for applications. While multiple application areas are considered, the primary application that runs through the work would be the nonlinear dynamics of vortex filaments under a variety of physical models. For instance, we are able to determine the structure and time evolution of several physical solutions, including the planar, helical, self-similar and soliton vortex filament solutions in a quantum fluid. Properties of such solutions are determined analytically and numerically through a variety of approaches. Starting with complex scalar equations (often useful for studying two-dimensional motion), we progress through more complicated models involving vector partial differential equations and non-local equations (which permit motion in three dimensions). In many of the examples considered, the qualitative analytical results are used to verify behaviors previously observed only numerically or experimentally.
Show less - Date Issued
- 2014
- Identifier
- CFE0005272, ucf:50545
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005272
- Title
- ADDITIVE LITHOGRAPHY FABRICATION AND INTEGRATION OF MICRO OPTICS.
- Creator
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Pitchumani, Mahesh, Johnson, Eric, University of Central Florida
- Abstract / Description
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Optical elements are the fundamental components in photonic systems and are used to transform an input optical beam into a desired beam profile or to couple the input beam into waveguides, fibers, or other optical systems or devices. Macroscopic optical elements are easily fabricated using grinding and polishing techniques, but few methods exist for inexpensive fabrication of micro optical elements. In this work we present an innovative technique termed Additive Lithography that makes use of...
Show moreOptical elements are the fundamental components in photonic systems and are used to transform an input optical beam into a desired beam profile or to couple the input beam into waveguides, fibers, or other optical systems or devices. Macroscopic optical elements are easily fabricated using grinding and polishing techniques, but few methods exist for inexpensive fabrication of micro optical elements. In this work we present an innovative technique termed Additive Lithography that makes use of binary masks and controlled partial exposures to sculpt photoresist into the desired optical surface relief profile. We explore various masking schemes for fabricating a variety of optical elements with unprecedented flexibility and precision. These masking schemes used in conjunction with the additive lithographic method allows us to carefully control the photoresist exposure and reflow processes for fabricating complex aspheric lens elements, including aspheric elements whose fabrication often proves highly problematic. It will be demonstrated that employing additive lithography for volume sculpting followed by controlled reflow can also allow us to fabricate refractive beam shaping elements. Finally we will discuss the dry etching techniques used to transfer these optical elements into the glass substrate. Thus the additive lithography technique will be demonstrated as an inexpensive, high throughput and efficient process in the fabrication of micro optical elements.
Show less - Date Issued
- 2006
- Identifier
- CFE0000914, ucf:46761
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000914
