Search Results (1 - 12 of 12 Results)

Sort By  
Sort Dir
 
Results per page  

Williams, Charles PLow Pressure Turbine Flow Control with Vortex Generator Jets
MS, University of Cincinnati, 2016, Engineering and Applied Science: Aerospace Engineering
In an aircraft engine at high altitude, the low-pressure turbine (LPT) section can experience low-Reynolds number (Re) flows making the turbine blades susceptible to large separation losses. These losses are detrimental to the performance of the turbine and lead to a roadblock for “higher-lift” blade designs. Accurate prediction of the separation characteristics and an understanding of mitigation techniques are of the utmost importance. The current study conducts simulations of flow control techniques for the Air Force Research Laboratory (AFRL) L2A turbine blade at low-Re of 10,000 based on inlet velocity and blade axial chord. This blade was selected for its “high-lift” characteristics coupled with massive separation on the blade at low-Re which provides an excellent test blade for flow control techniques. Flow control techniques involved various configurations of vortex generator jets (VGJs) using momentum injection (i.e. jet blowing). All computations were executed on dual-topology, multi-block, structured meshes and incorporated the use of a parallel computing platform using the message passing interface (MPI) communications. A high-order implicit large eddy simulation (ILES) approach was used in the simulations allowing for a seamless transition between laminar, transitional, and turbulent flow without changing flow solver parameters. A validation study was conducted involving an AFRL L1A turbine blade which showed good agreement with experimental trends for cases which controlled separation in the experiments. The same cases showed good agreement between different grid sizes. The differences between experimental and numerical results are largely attributed to differences in the setup. That is, the simulation did not include freestream turbulence or wind-tunnel wall effects. The flow control study conducted for the L2A blade showed a small degree of separation control for jets placed just downstream (DS) of the separation point. A limited study was conducted with jets moved upstream (US) of the natural separation point which showed an increase in effectiveness for one of the VGJs. This indicates a sensitivity of VGJ location relative to the point of separation. For the DS VGJs, separation control, increased as blowing ratio (BR) was increased and jet blowing frequency (F+) decreased. The increase in jet efficacy with decreasing F+ was unexpected and is mostly attributed to the jets being downstream of the separation location and having a low duty cycle (10%). Turbulent kinetic energy frequency spectra also show the presence of jet harmonics in the flow downstream of the best performing VGJs which dramatically increased in power when the VGJ was moved upstream. The most effective jet found in this study had BR=3.0, F+=3.02, and was located at x/Cx=0.53. This VGJ provided a 42.1% reduction in normalized integrated wake loss. One follow-on simulation was conducted taking the most effective VGJ and increasing the blowing ratio from BR=3.0 to 8.0. This provided a decrease in the amount of separation, nearly eliminating separation with only a small separation bubble remaining. This VGJ was able to provide a 42.8% reduction in normalized integrated wake loss. This work was conducted in coordination with the AFRL and has been approved for public release, case number: 88ABW-2016-1657.

Committee:

Kirti Ghia, Ph.D. (Committee Chair); Rolf Sondergaard, Ph.D. (Committee Member); Shaaban Abdallah, Ph.D. (Committee Member); Urmila Ghia, Ph.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Low Pressure Turbine;Vortex Generator Jet;Active Flow Control;Implicit Large Eddy Simulation;L2A;Low Reynolds Number Flow

Memory, Curtis L.Turbulent Transition Behavior in a Low Pressure Turbine Subjected to Separated and Attached-Flow Conditions
Doctor of Philosophy, The Ohio State University, 2010, Mechanical Engineering
Various time accurate numerical simulations were conducted on the aft-loaded L1A low pressure turbine airfoil operating at Reynolds numbers presenting with fully-stalled, non-reattaching laminar separation. The numerical solver TURBO was modified from its annular gas turbine simulation configuration to conduct simulations based on a linear cascade wind tunnel facility. Simulation results for the fully separated flow fields revealed various turbulent decay mechanisms. Separated shear layer decay, in the form of vortices forming between the shear layer and the blade wall, was shown to agree with experimental particle image velocimetry (PIV) data in terms of decay vortex size and core vorticity levels. These vortical structures eventually mix into a large recirculation zone which dominates the blade wake. Turbulent wake extent and time-averaged velocity distributions agreed with PIV data. Steady-blowing vortex generating jet (VGJ) flow control was then applied to the flow fields. VGJ-induced streamwise vorticity was only present at blowing ratios above 1.5. VGJs actuated at the point of flow separation on the blade wall were more effective than those actuated downstream, within the separation zone. Pulsed-blowing VGJs at the upstream blade wall position were then actuated at various pulsing frequencies, duty cycles, and blowing ratios. These condition variations yielded differing levels of separation zone mitigation. Pulsed VGJs were shown to be more effective than steady blowing VGJs at conditions of high blowing ratio, high frequency, or high duty cycle, where blowing ratio had the highest level of influence on pulsed jet efficacy. The characteristic "calm zone" following the end of a given VGJ pulse was observed in simulations exhibiting high levels of separation zone mitigation. Numerical velocity fields near the blade wall during this calm zone was shown to be similar to velocity fields observed in PIV data. Instantaneous numerical vorticity fields indicated that the elimination of the separation zone directly downstream of the VGJ hole is a primary indicator of pulsed VGJ efficacy. This indicator was confirmed by numerical time-averaged velocity magnitude rms data in the same region.

Committee:

JenPing Chen, PhD (Advisor); Jeffrey P. Bons, PhD (Committee Member); James W. Gregory, PhD (Committee Member); Mei Zhuang, PhD (Committee Member)

Subjects:

Aerospace Engineering; Mechanical Engineering

Keywords:

vortex generating jet; low pressure turbine; direct numeric simulation; steady blowing; pulsed blowing; particle image velocimetry; vortex

AYYALASOMAYAJULA, HARITHAHIGHER-ORDER ACCURATE SOLUTION FOR FLOW THROUGH A TURBINE LINEAR CASCADE
MS, University of Cincinnati, 2003, Engineering : Mechanical Engineering
Low-pressure turbines (LPT) in aircraft engines undergo tremendous losses at cruise conditions. The flow Reynolds number at cruise is lower than the take-off Reynolds number by a factor of almost two. At low Reynolds numbers, the flow is largely laminar, and tends to separate easily on the suction surface of the turbine blade when an adverse pressure gradient is encountered. Therefore, accurate prediction of flow separation is crucial for an effective design of LPT blade; and is achieved in the present work using a high-order accurate numerical solution procedure. The three-dimensional, unsteady, full Navier-Stokes equations are solved to analyze the flow. A MPI-based higher-order, parallel, chimera version of the FDL3DI flow solver, is extended for use with this turbomachinery application. A sixth-order accurate compact-difference scheme is used for the spatial discretization, along with second-order accurate temporal discretization. Tenth-order filtering is used to minimize the numerical oscillations in the flow solution and maintain numerical stability. The objective of the present study is to show the ability of higher-order accurate compact-difference scheme to predict the flow separation that occurs inside an LPT cascade at Re C = 25,000 (based on axial chord and inlet velocity). A new set of subsonic inflow/outflow boundary conditions that account for upstream influence (BC2) are derived by specifying stagnation quantities at the inlet, and a static quantity at the exit of the flow domain, and maintaining the inflow angle constant. For inflow/outflow boundary conditions that do not account for upstream influence, fixed inflow with extrapolated outflow (BC1) has been utilized. The effect of the two different sets of inflow/outflow boundary conditions on the flow solution is studied, for second-order, fourth-order and sixth-order accurate schemes. The computed Cp distribution for the LPT flow shows good agreement with the existing experimental data. The location of the onset of separation matches with an available LES simulation result and with the available experimental data. The performance of high-order compact difference schemes has been assessed via simulation of laminar flow over a circular cylinder at Re D = 250 (based on free-stream velocity and cylinder diameter). The sixth-order accurate compact difference scheme with tenth-order filtering on a coarser mesh preserves the vortex structure better than possible with the second-order accurate scheme on a finer mesh. This demonstrates the efficiency of the higher-order accurate compact difference scheme.

Committee:

Dr. Urmila Ghia (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

low-pressure turbine; high-order accurate method; filtering; flow separation; Navier-stokes equations

KASLIWAL, AMITFLOW SEPARATION CONTROL FOR CYLINDER FLOW AND CASCADE FLOW USING GENERATOR JETS
MS, University of Cincinnati, 2006, Engineering : Aerospace Engineering
Many attempts have been made by researchers, worldwide, to comprehend the physics of separated flows. Study of flow separation is vital as it is encountered in many engineering applications, and is generally detrimental. One such example is flow through a low pressure turbine (LPT) cascade, at relatively low-Re values, where flow separates on the suction surface of the LPT blade, and adversely affects the efficiency of the aircraft engine. Contemporary research is focused on understanding the physics of the separated flow, and identifying control strategies to delay or, if possible at all, prevent the flow separation phenomenon. The main objective of the present research is to study a model separated flow, and identify a control strategy, which can subsequently be applied to manage the flow in the LPT cascade. To achieve this, a model problem of flow past a circular cylinder is considered, as the geometry for this flow is simple and facilitates a focus on the flow itself. Despite of its simple geometry, the flow past a circular cylinder exhibits a variety of complex flow features which make this a challenging problem to solve. As a validation study, the flow for Re = 3,900 is simulated, and the results obtained are compared with the numerical and experimental data available in the literature. For the flow control study, a baseline solution for flow past a circular cylinder at Re = 13,400 is obtained as a first step towards implementation of flow separation control for preventing or delaying the flow separation. The Re value of 13,400 ensures laminar separation and serves to approximate the flow conditions prevailing in a LPT cascade. Later, flow control is incorporated by employing vortex generator jets (VGJs) on the upper surface of the cylinder at about 750 from the stagnation point. The jets are issued into the flow with a blowing ratio of 2.0 and are pitched and skewed by 300 and 700, respectively. A non-dimensional pulsation frequency F+ of 1.0 is used, along with 50% duty cycle. With this understanding, VGJs are then incorporated for the LPT cascade flow. VGJs are placed in a range of 63.5% to 67% Cax. All the jet parameters, i.e., blowing ratio, pitch angle, skew angle and duty cycle ratio, are kept the same as for the cylinder case, while the F+ value of 2.33 is employed for the LPT cascade problem. The three-dimensional, unsteady, full Navier-Stokes equations are solved to obtain accurate prediction of unsteady separated flows governed by the Navier-Stokes (N-S) equations. A fourth-order accurate, compact-difference scheme is used for spatial discretization, with sixth-order filtering to minimize the oscillations in the flow solution. For the cylinder, a multi-block structured grid generated using the grid generation software, GRIDGEN, is used for the present numerical analysis. The grid contains approximately 3.9M grid points, and approximately 70% of the total grid points are concentrated in the wake region to capture the small scales that are expected to exist in this region. A MPI-based higher-order, Chimera version of the FDL3DI flow solver developed by the Air Force Research Laboratory at Wright Patterson Air Force base is used for the numerical computations. PEGSUS a NASA Ames research code is used for storing the connectivity data at the block interfaces. The baseline case for the cylinder flow at Re = 13,400 displays a wide range of vortical structures in the wake region. The separating shear layers are subject to spanwise instability which leads to the formation of an unsteady and three-dimensional wake, with the characteristic features of typical turbulent flow. It is observed that after the jets are being turned on, the pressure on the surface of the cylinder redistributes in a way so as to reduce the pressure drag significantly. The total pressure loss coefficient and momentum thickness are calculated in the wake at x/D = 3.0 and x/D = 5.0, and are found to reduce by 10% and 30%, respectively. The flow control simulation for the LPT cascade flow reveals 27% reduction in total pressure loss coefficient, along with the total elimination of separation upon application of VGJs.

Committee:

Dr. Kirti Ghia (Advisor)

Keywords:

Flow Separation; Low Pressure Turbine Cascade; LPT Cascade; Flow over Cylinder; Vortex Generator Jets; VGJ; Multiblock Grid

Benton, Stuart IraCapitalizing on Convective Instabilities in a Streamwise Vortex-Wall Interaction
Doctor of Philosophy, The Ohio State University, 2015, Aero/Astro Engineering
Secondary flows in turbomachinery and similar engineering applications are often dominated by a single streamwise vortex structure. Investigations into the control of these flows using periodic forcing have shown a discrete range of forcing frequency where the vortex is particularly receptive. Forcing in this frequency range results in increased movement of the vortex and decreased total pressure losses. Based on the hypothesis that this occurs due to a linear instability associated with the Crow instability, a fundamental study of instabilities in streamwise vortex-wall interactions is performed. In the first part of this study a three-dimensional vortex-wall interaction is computed and analyzed for the presence of convective instabilities. It is shown that the Crow instability and a range of elliptic instabilities exist in a similar form as to what has been studied in counter-rotating vortex pairs. The Crow instability is particularly affected by the presence of a solid no-slip wall. Differences in the amplification rate, oscillation angle, Reynolds number sensitivity, and transient growth are each discussed. The spatial development of the vortex-wall interaction is shown to have a further stabilizing effect on the Crow instability due to a “lift-off” behavior. Despite these discoveries, it is still shown that amplitude growth on the order of 20% is possible and transient growth mechanisms might result in an order-of-magnitude of further growth if properly initiated. With these results in mind, an experiment is developed to isolate the streamwise vortex-wall interaction. Through the use of a vortex generating wing section and a suspended splitter plate, a stable interaction is created that agrees favorably in structure to the three-dimensional computations. A small synthetic jet actuator is mounted on the splitter plate below the vortex. Phase-locked stereo-PIV velocity data and surface pressure taps both show spatial amplification of the disturbance in a frequency range which agrees well with the prediction for the Crow instability. An analysis of the vortex response shows a primarily horizontal oscillation of the vortex column which strongly interacts with the secondary vortex structure that develops in the boundary layer.

Committee:

Jeffrey Bons, Ph.D. (Advisor); Mohammad Samimy, Ph.D. (Committee Member); James Gregory, Ph.D. (Committee Member); Jen-Ping Chen, Ph.D. (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics

Keywords:

active flow control; vortex; linear stability; synthetic jet actuator; wind tunnel; particle image velocimetry; turbomachinery; low pressure turbine;

MUTNURI, PAVAN KUMARSIMULATION OF FLOW THROUGH LOW-PRESSURE LINEAR TURBINE CASCADE, USING MULTI-BLOCK STRUCTURED GRID
MS, University of Cincinnati, 2003, Engineering : Mechanical Engineering
The Reynolds number for the flow through LPT at cruise conditions is much lower than that at the take-off conditions. This low-Re flow has a great tendency to undergo separation on the suction surface of the turbine blade when an adverse pressure gradient is encountered. This prevailing flow separation is detrimental to the performance of the LPT. Hence, low-pressure turbine (LPT) stage in aircraft engines undergo significant losses during cruise conditions. Therefore, accurate prediction of flow separation is crucial for an effective design of LPT blade, and is achieved in the present work using a high-order accurate numerical solution procedure. The accurate prediction of flow separation is necessary for implementing flow control techniques, passive or active, to possibly delay or prevent the occurrence of flow separation in the low-pressure turbine stage in an aircraft engine. A multi-block, periodic, structured grid of multiple topologies generated by the grid generation software, GRIDPRO, is used for the present numerical analysis. The three-dimensional, unsteady, full Navier-Stokes equations are solved to analyze the flow. A MPI-based higher-order, parallel, chimera Large-Eddy Simulation (LES), version of the FDL3DI flow solver, developed by the Air Force Research Laboratory at Wright Patterson Air Force Base, is extended for the present turbo-machinery application. A sixth-order accurate compact-difference scheme is usual for the spatial discretization, coupled with tenth-order filtering to minimize the numerical oscillations in the flow solution and maintain numerical stability, along with second-order accurate temporal discretization. Also examined is the effect of grid density and the location of the upstream inflow boundary location on the flow solution. Four different grids were used in this study, and it was observed that, as the grid density and the location of the upstream inflow boundary are increased, the oscillations in the predicted Cp distribution reduced significantly. Also, the physical, simulation-turn around time was reduced significantly with the multi-block and parallelization approach used in the present study through the parallelized code. Along with the two-dimensional study, the effect of the third spatial dimension on the location of the onset of separation and the transition process was studied, using a coarse-grid three-dimensional simulation with an Implicit Large-Eddy Simulation (ILES) echnique. Finally, baseline simulation results were generated for a simplified geometry of flow over a circular cylinder at a ReD = 13,400 as a starting step to implement flow control for preventing or delaying the flow separation. Two different turbine blade geometries are considered during the course of this numerical study. A high-pressure (HPT) turbine blade geometry is considered as a test case, and a low-pressure turbine (LPT) blade is studied as the main application. For the HPT blade geometry, it was found necessary to account for upstream influence in implementing the inflow/outflow boundary conditions in order for the leading-edge stagnation point to occur at the appropriate location and, hence, for the correct location of the onset of separation on the suction side of the blade. The computed Cp distribution for the LPT flow shows good agreement with the available experimental data and with the LES simulation result. The 3-D simulation showed significant effect in the growth of spanwise instabilities which thereby weaken the coherent vortical structures and break down in spanwise direction, thereby predicting the separation process more realistically and accurately. Finally, the baseline simulation study of flow over a circular cylinder at ReD = 13,400 is performed as a starting step for the future study of implementation of flow control techniques for preventing or delaying the flow separation. The Reynolds number for the flow through LPT at cruise conditions is much lower than that at the take-off conditions. This low-Re flow has a great tendency to undergo separation on the suction surface of the turbine blade when an adverse pressure gradient is encountered. This prevailing flow separation is detrimental to the performance of the LPT. Hence, low-pressure turbine (LPT) stage in aircraft engines undergo significant losses during cruise conditions. Therefore, accurate prediction of flow separation is crucial for an effective design of LPT blade, and is achieved in the present work using a high-order accurate numerical solution procedure. The accurate prediction of flow separation is necessary for implementing flow control techniques, passive or active, to possibly delay or prevent the occurrence of flow separation in the low-pressure turbine stage in an aircraft engine. A multi-block, periodic, structured grid of multiple topologies generated by the grid generation software, GRIDPRO, is used for the present numerical analysis. The three-dimensional, unsteady, full Navier-Stokes equations are solved to analyze the flow. A MPI-based higher-order, parallel, chimera Large-Eddy Simulation (LES), version of the FDL3DI flow solver, developed by the Air Force Research Laboratory at Wright Patterson Air Force Base, is extended for the present turbo-machinery application. A sixth-order accurate compact-difference scheme is usual for the spatial discretization, coupled with tenth-order filtering to minimize the numerical oscillations in the flow solution and maintain numerical stability, along with second-order accurate temporal discretization. Also examined is the effect of grid density and the location of the upstream inflow boundary location on the flow solution. Four different grids were used in this study, and it was observed that, as the grid density and the location of the upstream inflow boundary are increased, the oscillations in the predicted Cp distribution reduced significantly. Also, the physical, simulation-turn around time was reduced significantly with the multi-block and parallelization approach used in the present study through the parallelized code. Along with the two-dimensional study, the effect of the third spatial dimension on the location of the onset of separation and the transition process was studied, using a coarse-grid three-dimensional simulation with an Implicit Large-Eddy Simulation (ILES) echnique. Finally, baseline simulation results were generated for a simplified geometry of flow over a circular cylinder at a ReD = 13,400 as a starting step to implement flow control for preventing or delaying the flow separation. Two different turbine blade geometries are considered during the course of this numerical study. A high-pressure (HPT) turbine blade geometry is considered as a test case, and a low-pressure turbine (LPT) blade is studied as the main application. For the HPT blade geometry, it was found necessary to account for upstream influence in implementing the inflow/outflow boundary conditions in order for the leading-edge stagnation point to occur at the appropriate location and, hence, for the correct location of the onset of separation on the suction side of the blade. The computed Cp distribution for the LPT flow shows good agreement with the available experimental data and with the LES simulation result. The 3-D simulation showed significant effect in the growth of spanwise instabilities which thereby weaken the coherent vortical structures and break down in spanwise direction, thereby predicting the separation process more realistically and accurately. Finally, the baseline simulation study of flow over a circular cylinder at ReD = 13,400 is performed as a starting step for the future study of implementation of flow control techniques for preventing or delaying the flow separation.

Committee:

Dr. Urmila Ghia (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

low-pressure turbine; multi-block structured grid; higher-order schemes; flow separation

McQuilling, Mark W.DESIGN AND VALIDATION OF A HIGH-LIFT LOW-PRESSURE TURBINE BLADE
Doctor of Philosophy (PhD), Wright State University, 2007, Engineering PhD
This dissertation is a design and validation study of the high-lift low-pressure turbine (LPT) blade designated L2F. High-lift LPTs offer the promise of reducing the blade count in modern gas turbine engines. Decreasing the blade count can reduce development and maintenance costs and the weight of the engine, but care must be taken in order to maintain turbine section performance with fewer blades. For an equivalent amount of work extracted, lower blade counts increase blade loading in the LPT section. The high-lift LPT presented herein allows 38% fewer blades with a Zweifel loading coefficient of 1.59 and maintains the same inlet and outlet blade metal angles of conventional geometries in service today while providing an improved low-Reynolds number characteristic. The computational design method utilizes the Turbine Design and Analysis System (TDAAS) developed by John Clark of the Air Force Research Laboratory. TDAAS integrates several government-funded design utilities including airfoil and grid generation capability with a Reynolds-Averaged Navier-Stokes flow solver into a single, menu-driven, Matlab-based system. Transition modeling is achieved with the recently developed model of Praisner and Clark, and this study validates the use of the model for design purposes outside of the Pratt & Whitney (P&W) design system where they were created. Turbulence modeling is achieved with the Baldwin and Lomax zero-equation model. The experimental validation consists of testing the front-loaded L2F along with a previously designed, mid-loaded blade (L1M) in a linear turbine cascade in a low-speed wind tunnel over a range of Reynolds numbers at 3.3% freestream turbulence. Hot-wire anemometry and pressure measurements elucidate these comparisons, while a shear and stress sensitive film (S3F) also helps describe the flow in areas of interest. S3F can provide all 3 components of stress on a surface in a single measurement, and these tests extend the operational envelope of the technique to low speed air environments where small dynamic pressures and curved surfaces preclude the use of more traditional global measurement methods. Results are compared between the L1M and L2F geometries along with previous data taken in the same wind tunnel at identical flow conditions for the P&W Pack B geometry.

Committee:

Mitch Wolff (Advisor)

Keywords:

low-pressure turbine; high lift; L2F; L1M; transition; zweifel; surface stress; shear; S3F

Woods, Nathan MichaelPHASE-LOCKED PIV INVESTIGATION OF THE EFFECTS OF THE BLOWING RATIO OF A PULSED VORTEX GENERATOR JET IN A LOW-PRESSURE TURBINE
Master of Science in Engineering (MSEgr), Wright State University, 2007, Mechanical Engineering
At very high altitudes the Reynolds number flow through the low pressure turbine section of the gas turbine engine can drop below 25,000. At these low Reynolds numbers the flow is laminar and extremely susceptible to separation which can lead to increased losses and reduced lift. Small jets of air injected through the suction surface of the airfoil, called Vortex Generator Jets (VGJs), have been shown successful in suppressing separation and maintaining attached flow. Pulsing of these jets has been shown to be as effective as steady jets while reducing the amount of mass flow needed. An experiment using Particle Image Velocimetry (PIV) was set up to study the interaction of the VGJ flow with the main flow. A cascade of Pratt and Whitney Pack-B turbine blades were mounted in the test section of a low speed wind tunnel at Wright Patterson Air Force Base. On the middle six blades were rows of 1mm VGJ holes. The VGJ holes were oriented with a 30o pitch angle and 90o skew angle. The pitch angle is the angle the jet makes with the surface of the turbine blade while the skew angle is the angle the jet makes with the cross-flow. Blowing ratios, a ratio of the jet velocity to the cross-flow velocity, of 0.5, 1, and 2 were examined. These three blowing ratios were selected because they represent when the cross-flow momentum dominates the fluid interaction (B=0.5); when the momentums of the jet flow and cross-flow are equal (B=1); and when the momentum of the jet flow dominates the interaction. Blowing ratios of 0.5 and 1 were studied for pulsing frequencies of 10Hz and 0.4Hz while the blow ratio of 2 was studied only with 10Hz pulsing. A duty cycle of 50% was used for both pulsing frequencies. The two pulsing frequencies allowed data to be taken to show how the pulsed VGJ maintains attached flow (10Hz) and how the pulsed VGJ suppresses the separation bubble (0.4Hz). Results show that jets interacting with separated flow are able to suppress the separation bubble almost immediately for a blowing ratio of 1 and 0.5. The results for suppression and separation growth show the response of the crossflow is very similar in magnitude and timing between the two blowing ratios. The results for the 10Hz pulsing frequency show blowing ratios of 0.5, 1, and 2 are effective. A blowing ratio of 2 is undesirable because it carries more momentum than is needed and would therefore use more massflow than the B=1 or 0.5 case. Results from the B=0.5 case suggest that a blowing ratio of 0.5 is near the minimum effective blowing ratio.

Committee:

Mitch Wolff (Advisor)

Keywords:

Turbine; Pack-B; Pak-B; separation; Low Pressure Turbine; Vortex Generator Jets; PIV

Gompertz, Kyle AdlerSeparation Flow Control with Vortex Generator Jets Employed in an Aft-Loaded Low-Pressure Turbine Cascade with Simulated Upstream Wakes
Master of Science, The Ohio State University, 2009, Aeronautical and Astronautical Engineering
Detailed pressure and velocity measurements were acquired at Rec = 20,000 with 3% inlet free stream turbulence intensity to study the effects of position, phase and forcing frequency of vortex generator jets employed on an aft-loaded low-pressure turbine blade in the presence of impinging wakes. The L1A blade has a design Zweifel coefficient of 1.34 and a suction peak at 58% axial chord, making it an aft-loaded pressure distribution. At this Reynolds number, the blade exhibits a non-reattaching separation region beginning at 60% axial chord under steady flow conditions without upstream wakes. Wakes shed by an upstream vane row are simulated with a moving row of cylindrical bars at a flow coefficient of 0.91. Impinging wakes thin the separation zone and delay separation by triggering transition in the separated shear layer, although the flow does not reattach. Instead, at sufficiently high forcing frequencies, a new time-mean separated shear layer position is established which begins at approximately 72%Cx. Reductions in area-averaged wake total pressure loss of more than 75% were documented. One objective of this study was to compare pulsed flow control using two rows of discrete vortex generator jets (VGJs). The VGJs are located at 59%Cx, approximately the peak Cp location, and at 72%Cx. Effective separation control was achieved at both locations. In both cases, wake total pressure loss decreased 35% from the wake only level and the shape of the Cp distribution indicates that the cascade recovers its high Reynolds number (attached flow) performance. The most effective separation control was achieved when actuating at 59%Cx where the VGJ disturbance dominates the dynamics of the separated shear layer, with the wake disturbance assuming a secondary role only. On the other hand, when actuating at 72%Cx, the efficacy of VGJ actuation is derived from the relative mean shear layer position and jet penetration. When the pulsed jet actuation (25% duty cycle) was initiated at the 72%Cx location, synchronization with the wake passing frequency (8.7Hz) was critical to produce the most effective separation control. A 20% improvement in effectiveness over the wake-only level was obtained by aligning the jet actuation between wake events. A range of blowing ratios was investigated at both locations to maximize separation reduction with minimal mass flow. The optimal control parameter set for VGJ actuation at 72%Cx does not represent a reduction in required mass flow compared to the optimal parameter set for actuation at 59%Cx. Differences in the fundamental physics of the jet interaction with the separated shear layer are discussed and implications for the application of flow control in a full engine demonstrator are reviewed. Evidence suggests that flow control using VGJs will be effective in the highly unsteady LPT environment of an operating gas turbine, provided the VGJ location and amplitude are adapted for the specific blade profile.

Committee:

Jeffrey Bons, PhD (Advisor); James Gregory, PhD (Committee Member)

Subjects:

Aerospace Materials; Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

flow control; vortex generator jets; negative jet; low pressure turbine aerodynamics; simulating wakes

SINGH, NAVTEJA STUDY OF SEPARATED FLOW THROUGH A LOW-PRESSURE TURBINE CASCADE
MS, University of Cincinnati, 2005, Engineering : Mechanical Engineering
Low-pressure turbines (LPT) experience large changes in chord Reynolds number as the turbine engine operates from take-off to cruise conditions. Due to prevailing conditions at high altitude cruise, the Reynolds number reduces drastically. At low Reynolds numbers, the flow is largely laminar and tends to separate easily on the suction surface of the blade, and this laminar separation in particular leads to significant degradation of engine performance due to large re-circulation zones. Therefore, a better understanding of low-Reynolds number flow transition and separation is very critical for an effective design of LPT blade, and in exploring various possibilities for implementing flow control techniques, passive or active, to prevent or delay the flow separation in the low-pressure turbine. The objective of the present study is to understand the three-dimensional flow separation that occurs inside an LPT cascade at very low Reynolds numbers, and a high-order accurate numerical solution procedure is used to attain the same. A multi-block, periodic, structured grid generated by the grid generation software, GRIDPRO, is used to represent the flow domain. A MPI-based higher-order, parallel, chimera version of the FDL3DI flow solver, developed by the Air Force Research Laboratory at Wright Patterson Air Force Base, is extended for the present turbomachinery application. A sixth-order accurate compact-difference scheme is used for the spatial discretization, along with second-order accurate temporal discretization. Up to tenth-order filtering has been applied to minimize the numerical oscillations, and maintain numerical stability. Simulations have been performed for Reynolds numbers (based on inlet velocity and axial chord) 10,000 and 25,000. The effect of these low-Reynolds numbers on the flow physics for a low-pressure turbine cascade has been studied in detail. At Re = 10,000, the flow undergoes more separation than at Re = 25,000 as expected and the separation remains significant over the entire blade for both the Reynolds number. The location of the onset of separation matches with an available LES simulation and with the available experimental data. In addition to the above simulations, another study was carried out to understand the effect of two different sets of inflow/outflow boundary conditions on the flow solution. The two sets of boundary conditions include static inflow with extrapolated outflow (BC1), and dynamic inflow (BC2) that accounts for upstream influence in the subsonic flow. The computed Cp distribution for the LPT flow shows good agreement with the available experimental data. Application of BC2 boundary condition predicted a bounded region of separation, while BC1 boundary condition predicted significant separation over the entire blade of an LPT.

Committee:

Dr. Urmila Ghia (Advisor)

Keywords:

Low-Pressure Turbine; Flow Separation; Filtering; Compact Difference Schemes

Nessler, Chase A.Characterization of Internal Wake Generator at Low Reynolds Number with a Linear Cascade of Low Pressure Turbine Blades
Master of Science in Engineering (MSEgr), Wright State University, 2010, Mechanical Engineering

Unsteady flow and its effects on the boundary layer of a low pressure turbine blade is complex in nature. The flow encountered in a low pressure turbine contains unstructured free-stream turbulence, as well as structured periodic perturbations caused by upstream vane row wake shedding. Researchers have shown that these conditions strongly influence turbine blade performance and boundary layer separation, especially at low Reynolds numbers. In order to simulate these realistic engine conditions and to study the effects of periodic unsteadiness, a moving bar wake generator has been designed and characterized for use in the Air Force Research Labs low speed wind tunnel. The layout is similar to other traditional squirrel cage designs, however, the entire wake generator is enclosed inside the wind tunnel, up-stream of a linear cascade. The wake shed from the wake generator was characterized by its momentum deficit, wake width, and peak velocity deficit. It is shown that the wakes produce a periodic unsteadiness that is consistent with other wake generator designs.

The effect of the periodic disturbances on turbine blade performance has been investigated at low Reynolds number using the highly loaded, AFRL designed L1A low pressure turbine profile. Wake loss measurements, pressure coefficient distribution, and particle image velocimetry was used to quantify the L1A blade performance with unsteady wakes at a Reynolds number of 25,000 with 0.5% and 3.4% free-stream turbulence. Wake loss data showed that the inclusion of periodic wakes reduced the profile losses by 56% compared to steady flow losses. Previous pressure coefficient distributions showed that the L1A blade profile, under steady flow conditions, has non-reattaching separated flow along the suction surface. With the inclusion of the periodic wakes, the pressure coefficient profile revealed that the flow separation had been dramatically reduced to a small separation bubble.

The wake passing event was split into six phases and captured using two-dimensional planer PIV. The interaction between the passing wakes and the separation bubble was noted. The bubble was observed to grow in size between passing wakes, but was only able to achieve a fraction of the original level of separation. The streamlines through the unrestricted blade passage were able to better follow the blade profile, indicating an improved exit flow angle with lower losses. The data shows that the wake generator was successfully implemented into the wind tunnel and is able to properly simulate blade row interactions.

Committee:

Mitch Wolff, PhD (Committee Chair); James Menart, PhD (Committee Member); Haibo Dong, PhD (Committee Member); Rolf Sondergaard, PhD (Committee Member); George Huang, PhD (Other); John Bantle, PhD (Other)

Subjects:

Engineering; Fluid Dynamics; Mechanical Engineering; Technology

Keywords:

low pressure turbine; unsteady flow; wake generator; unsteady wakes; particle image velocimetry; L1A;

Pluim, Jonathon DouglasDESIGN OF A HIGH FIDELITY WAKE SIMULATOR FOR RESEARCH USING LINEAR CASCADES
Master of Science, The Ohio State University, 2009, Aeronautical and Astronautical Engineering
Owing to the extensive use of wake generators in the study of turbine and compressor airfoils in linear cascades, a study was undertaken to determine the most accurate model for the wakes generated by upstream blade rows. Velocity (PIV) measurements were taken to compare wake properties of several bluff bodies with different cross sections to the wake of an ultra high lift low pressure turbine profile (L1A). These measurements were taken at two Reynolds numbers, a low and a high one, to simulate a separated and attached wake, respectively, for both the blade and two of the shape configurations. The L1A turbine blade profile was determined to shed a wake typical of high lift turbine blade profiles. It is shown that the wake of the turbine blade is highly dependent on Reynolds number. In order to make an appropriate comparison, all bluff body data were extracted along a plane parallel to the equivalent inlet plane of a rotor stage in the stationary frame of reference. It was found that no single rod shape matched all of the blade wake characteristics. From the shapes used in this study, a 30° isosceles wedge placed 6 diameters upstream of the cascade inlet in the axial direction and skewed 15° from the rod relative flow was found to yield the closest match for the low Re case due to the asymmetry in the velocity and Reynolds shear stresses in this wake compared with the wake of the low pressure turbine blade. The same configuration placed 10 diameters upstream yielded the best comparison to a higher Re, more attached L1A wake. The boundary layer response of the L1A to the wake shed from a cylindrical wake was compared to the response from a wedge shaped wake skewed 15° to the rod relative flow. A hot-film anemometer was placed on a blade follower device to determine velocity, rms, and intermittency data. It was found that the wake shed from the wedge acts to suppress the separation more than the wake from a cylinder placed the same distance upstream from the cascade leading edge. The shear layer and point of transition occur further downstream on the blade surface when using a wedge shaped rod and the calm zone has a greater duration.

Committee:

Jeffrey Bons (Advisor); James Gregory (Committee Member)

Keywords:

Wake Simulation; Low pressure turbine; wake generator