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FLOW CONTROL OF TANDEM CYLINDERS USING PLASMA ACTUATORS
Title
FLOW CONTROL OF TANDEM CYLINDERS USING PLASMA ACTUATORS.
Creator
Larsen, Jonah, Bhattacharya, Samik, University of Central Florida
Abstract / Description
The flow over a set of tandem cylinders at a moderate Reynolds numbers (Re), and with different separation lengths has been studied. Two dimensional (2D) and three-dimensional (3D) plasma actuators were used to control the flow over the leading cylinder to change the vortex shedding, and subsequently the flow on the second cylinder. The 3D plasma actuator was segmented along the length of the cylinder with a spacing of ? = 4 while the 2D actuator simply ran straight down the span of the...
Show moreThe flow over a set of tandem cylinders at a moderate Reynolds numbers (Re), and with different separation lengths has been studied. Two dimensional (2D) and three-dimensional (3D) plasma actuators were used to control the flow over the leading cylinder to change the vortex shedding, and subsequently the flow on the second cylinder. The 3D plasma actuator was segmented along the length of the cylinder with a spacing of ? = 4 while the 2D actuator simply ran straight down the span of the cylinder. Particle image velocimetry (PIV) measurements were used to investigate the flow along the central plane in the wake of the cylinders. The image pairs were processed into velocity grids which were then averaged. Plots of the shear, vorticity, and turbulent kinetic energy were created. These plots are used to understand how the character of vortex shedding from the upstream cylinder changes the same from the downstream one.
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Date Issued
2018
Identifier
CFH2000425, ucf:45872
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFH2000425
Detailed Understanding of Flow, Heat Transfer, and Pressure Drop Behavior in a Square Channel With 45 Deg Ribs
Title
Detailed Understanding of Flow, Heat Transfer, and Pressure Drop Behavior in a Square Channel With 45 Deg Ribs.
Creator
Ahmed, Lumaya, Kapat, Jayanta, Gordon, Ali, Ahmed, Kareem, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description
Internal Duct Cooling (IDC) with rib turbulators is one of the common cooling techniques applied inside the turbine airfoils. It is very important for the gas turbine industry to design and develop an optimized cooling channel that maximizes the amount of heat removed, while simultaneously minimizing the pressure drop for a target overall cooling effectiveness. Angled ribs perform superior to the transverse ribs due to additional secondary flow associated with them. However, they result in a...
Show moreInternal Duct Cooling (IDC) with rib turbulators is one of the common cooling techniques applied inside the turbine airfoils. It is very important for the gas turbine industry to design and develop an optimized cooling channel that maximizes the amount of heat removed, while simultaneously minimizing the pressure drop for a target overall cooling effectiveness. Angled ribs perform superior to the transverse ribs due to additional secondary flow associated with them. However, they result in a highly non-homogenous heat transfer distribution, which is a manifestation of the complex, turbulent flow field inside the channel. It is very important to comprehend the secondary flow physics to characterize the heat transfer distribution in such angled ribbed channels. Additionally, due to the manufacturing constraint, the gas turbine industry encounters a challenge to make ribs edge sharp and results in ribs with rounded edges. The one of the main objectives of the present study is to provide a fundamental understanding of the flow physics on the heat transfer and pressure drop behavior in 45(&)deg; ribbed channels both with sharp and rounded-edge ribs. It is found that the secondary flow has a significant effect on the heat transfer behavior for both types of ribs. There is a great need of high-fidelity PIV flow field data in the inter-rib space for an angled ribbed channel which can be used for CFD validation, especially for LES. The current study provides benchmarking flow field data in the inter-rib space in a square channel with 45(&)deg; ribs using stereoscopic PIV technique. Besides the experiments, numerical studies were also conducted by using LES and different RANS models. The LES results show an excellent prediction capability for aerothermal behavior in such channels. However, the prediction capability of RANS models is found to be inconsistent for different rib configurations and flow conditions.
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Date Issued
2018
Identifier
CFE0007302, ucf:52171
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007302
Investigation of the Flow Field and Associated Heat Transfer within an Asymmetrical Leading Edge Jet Impingement Array
Title
Investigation of the Flow Field and Associated Heat Transfer within an Asymmetrical Leading Edge Jet Impingement Array.
Creator
Torres, Jorge, Kapat, Jayanta, Bhattacharya, Samik, Fernandez, Erik, University of Central Florida
Abstract / Description
This thesis investigates the turbulent flow features present in asymmetrical leading edge jet impingement and their effects from a fluid and heat transfer prospective using both numerical and experimental techniques. The jet-centerline plane flow field was quantified experimentally through the non-intrusive experimental method of Particle Image Velocimetry (PIV), while an area average heat transfer was acquired via a traditional copper block method. The numerical element served to investigate...
Show moreThis thesis investigates the turbulent flow features present in asymmetrical leading edge jet impingement and their effects from a fluid and heat transfer prospective using both numerical and experimental techniques. The jet-centerline plane flow field was quantified experimentally through the non-intrusive experimental method of Particle Image Velocimetry (PIV), while an area average heat transfer was acquired via a traditional copper block method. The numerical element served to investigate how well the Reynolds Averaged Navier-Stokes (RANS) k-? SST turbulence model predicts the flow field and heat transfer within the leading edge and further investigate the results outside of the experimental scope.Two different geometries, varied by H/d, were investigated at various Reynolds numbers ranging from 20,000 to 80,000. The geometry consisted of an array of 9 identical jets impinging on a leading edge of diameter D/d = 2, with an asymmetrical sidewall configuration to better represent the pressure side (PS) and suction side (SS) of a turbine blade. Several vortices were identified within the flow field of the leading edge geometry. These vortices were larger for the H/d = 4 configuration but did not contribute to any increased or decreased heat transfer compared to that of the H/d = 2.7 configuration. The most influential aspect to both the flow field and heat transfer was the change in crossflow velocity between the two geometries. The smaller cross sectional area of the H/d = 2.7 configuration saw an increase in crossflow velocity and jet bending, tending to also decrease the heat transfer. The numerical results also reflected these results and in both area averaged heat transfer and localized heat transfer contour plots.
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Date Issued
2019
Identifier
CFE0007734, ucf:52431
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007734
VORTEX TILTING AND THE ENHANCEMENT OF SPANWISE FLOW IN FLAPPING WING FLIGHT
Title
VORTEX TILTING AND THE ENHANCEMENT OF SPANWISE FLOW IN FLAPPING WING FLIGHT.
Creator
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.
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Date Issued
2011
Identifier
CFH0004124, ucf:44875
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFH0004124
Hydrodynamic Measurements of the Flow Structure Emanating From A Multi-Row Film Cooling Configuration
Title
Hydrodynamic Measurements of the Flow Structure Emanating From A Multi-Row Film Cooling Configuration.
Creator
Voet, Michael, Kapat, Jayanta, Vasu Sumathi, Subith, Ahmed, Kareem, University of Central Florida
Abstract / Description
The demand for more power is rapidly increasing worldwide. Attention is turned to increasingthe efficiency of modern methods for power generation. Gas turbines provide 35% of the powerdemands within the United States. Efficiency of gas turbines is defined in an ideal sense by thethermal efficiency of the Brayton Cycle. The overall efficiency of a gas turbine can be increased while simultaneously maximizing specific work output, by increasing the turbine inlet temperature. However, even with...
Show moreThe demand for more power is rapidly increasing worldwide. Attention is turned to increasingthe efficiency of modern methods for power generation. Gas turbines provide 35% of the powerdemands within the United States. Efficiency of gas turbines is defined in an ideal sense by thethermal efficiency of the Brayton Cycle. The overall efficiency of a gas turbine can be increased while simultaneously maximizing specific work output, by increasing the turbine inlet temperature. However, even with the advancements in modern materials in terms of maximum operatingtemperature, various components are already subjected to temperatures higher than their melting temperatures. An increase in inlet temperature would subject various components to even higher temperatures, such that more effective cooling would be necessary, whilst ideally using the same (or less) amount of cooling air bled from compressor. Improvements in the performance of these cooling techniques is thus required. The focus of this thesis is on one such advanced cooling technique, namely film cooling.The objective of this study is to investigate the effects of coolant density on the jet structure for different multi-row film cooling configurations. As research is performed on improving the performance of film cooling, the available conditions during testing may not reflect actual engine-like conditions. Typical operating density ratio at engine conditions are between 1.5 and 2, while it is observed that a majority of the density ratios tested in literature are between 1 and 1.5. While thesetests may be executed outside of engine-like conditions, it is important to understand how density ratio effects the flow physics and film cooling performance. The density ratio within this study is varied between 1.0 and 1.5 by alternating the injecting fluid between air and Carbon Dioxide, respectively.Both a simple cylindrical and fan-shape multi-row film cooling configuration are tested in the present study. In order to compare the results collected from these geometries, lateral and spanwise hole-to-hole spacing, metering hole diameter, hole length, and inclination angle are held constant between all testing configurations. The effect of fluid density upon injection is examined by independently holding either blowing, momentum flux, or velocity ratio constant whilst varying density ratio. Comparisons between both of the film cooling configurations are also made as similar ratios are tested between geometries. This allows the variation in flow structure and performance to be observed from alternating the film cooling hole shape.Particle Image Velocimetry (PIV) is implemented to obtain both streamwise and wall normal velocitymeasurements for the array centerline plane. This data is used to examine the interactionof the jet as it leaves the film cooling hole and the structure produced when the jet mixes with theboundary layer.Similarities in jet to jet interactions and surface attachment between density ratios are seen for the cylindrical configuration when momentum flux ratio is held constant. When observing constant blowing ratio comparisons of the cylindrical configurations, the lower density ratio is seen to begin detaching from the wall at M = 0.72 with little evidence of coolant in the near wall region. However, the higher density cylindrical injection retains its surface attachment at M = 0.74 with noticeably more coolant near the wall, because of significantly lower momentum flux ratio and lower (")jetting(") effect. The fan-shape film cooling configuration demonstrates improved performance, in terms of surface attachment, over a larger range of all ratios than that of the cylindrical cases. Additionally, the fan-shape configuration is shown to constantly retain a thicker layer of low velocity fluid in the near wall region when injected with the higher density coolant, suggesting improved performance at the higher density ratio.When tracking the jet trajectory, it is shown that the injection of CO2 through the cylindricalconfiguration yields a higher centerline wall normal height per downstream location than that of the lower density fluid. Comparing the results of the centerline tracking produced by the third and fifth rows for both the injection of air and CO2, it is confirmed that the fifth row of injection interacts with the boundary layer at a great wall normal height than that of the third row. Additionally, when observing the change in downstream trajectory between the fifth and seventh row of injection, a significant decrease in wall normal height is seen for the coolant produced by the seventh row. It is believed that the lack of a ninth row of injection allows the coolant from the seventh row of injection to remain closer to the target surface. This is further supported by the observation of the derived pressure gradient field and the path streamlines take while interacting with the recirculatory region produced by the injection of coolant into the boundary layer.Further conclusions are drawn by investigating the interaction between momentum thickness andthe influence of blowing ratio. Relatively constant downstream momentum thickness is observedfor the injection of lower density fluid for the blowing ratio range of M= 0.4 to 0.8 for the cylindrical configuration. It is suggested that a correlation exists between momentum thickness and film cooling performance, however further studies are needed to validate this hypothesis.
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Date Issued
2017
Identifier
CFE0006817, ucf:51791
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006817
FUNDAMENTAL UNDERSTANDING OF INTERACTIONS AMONG FLOW, TURBULENCE, AND HEAT TRANSFER IN JET IMPINGEMENT COOLING
Title
FUNDAMENTAL UNDERSTANDING OF INTERACTIONS AMONG FLOW, TURBULENCE, AND HEAT TRANSFER IN JET IMPINGEMENT COOLING.
Creator
Hossain, Md. Jahed, Kapat, Jayanta, Ahmed, Kareem, Gordon, Ali, Wiegand, Rudolf, University of Central Florida
Abstract / Description
The flow physics of impinging jet is very complex and is not fully understood yet. The flow field in an impingement problem comprised of three different distinct regions: a free jet with a potential core, a stagnation region where the velocity goes to zero as the jet impinges onto the wall and a creation of wall jet region where the boundary layer grows radially outward after impinging. Since impingement itself is a broad topic, effort is being made in the current study to narrow down on...
Show moreThe flow physics of impinging jet is very complex and is not fully understood yet. The flow field in an impingement problem comprised of three different distinct regions: a free jet with a potential core, a stagnation region where the velocity goes to zero as the jet impinges onto the wall and a creation of wall jet region where the boundary layer grows radially outward after impinging. Since impingement itself is a broad topic, effort is being made in the current study to narrow down on three particular geometric configurations (a narrow wall, an array impingement configuration and a curved surface impingement configuration) that shows up in a typical gas turbine impingement problem in relation to heat transfer. Impingement problems are difficult to simulate numerically using conventional RANS models. It is worth noting that the typical RANS model contains a number of calibrated constants and these have been formulated with respect to relatively simple shear flows. As a result typically these isotropic eddy viscosity models fail in predicting the correct heat transfer value and trend in impingement problem where the flow is highly anisotropic. The common RANS-based models over predict stagnation heat transfer coefficients by as much as 300% when compared to measured values. Even the best of the models, the v^2-f model, can be inaccurate by up to 30%. Even though there is myriad number of experimental and numerical work published on single jet impingement; the knowledge gathered from these works cannot be applied to real engineering impingement cooling application as the dynamics of flow changes completely. This study underlines the lack of experimental flow physics data in published literature on multiple jet impingement and the author emphasized how important it is to have experimental data to validate CFD tools and to determine the suitability of Large Eddy Simulation (LES) in industrial application. In the open literature there is not enough study where experimental heat transfer and flow physics data are combined to explain the behavior for gas turbine impingement cooling application. Often it is hard to understand the heat transfer behavior due to lack of time accurate flow physics data hence a lot of conjecture has been made to explain the phenomena. The problem is further exacerbated for array of impingement jets where the flow is much more complex than a single round jet. The experimental flow field obtained from Particle Image Velocimetry (PIV) and heat transfer data obtained from Temperature Sensitive Paint (TSP) from this work will be analyzed to understand the relationship between flow characteristics and heat transfer for the three types of novel geometry mentioned above.There has not been any effort made on implementing LES technique on array impingement problem in the published literature. Nowadays with growing computational power and resources CFD are widely used as a design tool. To support the data gathered from the experiment, LES is carried out in narrow wall impingement cooling configuration. The results will provide more accurate information on impingement flow physics phenomena where experimental techniques are limited and the typical RANS models yield erroneous resultThe objective of the current study is to provide a better understanding of impingement heat transfer in relation to flow physics associated with it. As heat transfer is basically a manifestation of the flow and most of the flow in real engineering applications is turbulent, it is very important to understand the dynamics of flow physics in an impingement problem. The work emphasis the importance of understanding mean velocities, turbulence, jet shear layer instability and its importance in heat transfer application. The present work shows detailed information of flow phenomena using Particle Image Velocimetry (PIV) in a single row narrow impingement channel. Results from the RANS and LES simulations are compared with Particle Image Velocimetry (PIV) data. The accuracy of LES in predicting the flow field and heat transfer of an impingement problem is also presented the in the current work as it is validated against experimental flow field measured through PIV.Results obtained from the PIV and LES shows excellent agreement for predicting both heat transfer and flow physics data. Some of the key findings from the study highlight the shortcomings of the typical RANS models used for the impingement heat transfer problem. It was found that the stagnation point heat transfer was over predicted by as much as 48% from RANS simulations when compared to the experimental data. A lot of conjecture has been made in the past for RANS' ability to predict the stagnation point heat transfer correctly. The length of the potential core for the first jet was found to be ~ 2D in RANS simulations as oppose to 1D in PIV and LES, confirm the possible underlying reason for this discrepancy. The jet shear layer thickness was underpredicted by ~ 40% in RANS simulations proving the model is not diffusive enough for a flow like jet impingement. Turbulence production due to shear stress was over predicted by ~130% and turbulence production due to normal stresses were underpredicted by ~40 % in RANS simulation very close to the target wall showing RANS models fail where both strain rate and shear stress plays a pivotal role in the dynamics of the flow. In the closing, turbulence is still one of the most difficult problems to solve accurately, as has been the case for about a century. A quote below from the famous mathematician, Horace Lamb (1849-1934) express the level of difficulty and frustration associated with understanding turbulence in fluid mechanics. (")I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic.(")Source: http://scienceworld.wolfram.com/biography/Lamb.htmlThis dissertation is expected to shed some light onto one specific example of turbulent flows.
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Date Issued
2016
Identifier
CFE0006463, ucf:51424
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006463