Master of Science in Mechanical Engineering (MSME), Wright State University, 2024, Mechanical Engineering

In the constant search for more efficient engines, one approach to gain performance is to reduce the weight of the low pressure turbine (LPT) module. This module can account for up to 30% of the total engine weight [1], and a reduction in LPT weight results in clear gains to engine performance and a reduction in engine cost. High lift airfoils accomplish this weight reduction by each blade extracting a larger amount of work from the flow and thus requiring fewer blades to drive the compressor when compared to conventional blades. However, high lift LPT blades, quantified by a high Zweifel loading coefficient Zw>1.15, encounter increasing loss at low Reynolds numbers. Named Reynolds lapse, this effect is problematic if the engine must operate at high altitude cruise conditions such as the case with unmanned air vehicles. The two airfoils of this study, the L2FHW and the L3FHW, were designed to be front loaded and to demonstrate favorable low Reynolds number loss characteristics. Both airfoils were tested in the Transonic Turbine Cascade (TTC) at the Air Force Research Laboratory Building 18 Test Cell 21. The TTC is capable of high Mach number and low Reynolds number flow via independent control of each. Each airfoil was tested across a broad range of Mach numbers: exit Mach 0.78 down to 0.2 and exit Reynolds numbers from 23,000 to 201,000. Across each condition an exit total pressure traverse yielded the loss coefficient of the cascade at that condition. It was found that across all design exit Mach conditions, 0.78, both airfoils experience fully attached flow and nearly flat loss behavior. This strongly aligns with the design level predictions made. At conditions beyond expected operating conditions, the L2FHW displayed resistance to un-reattaching separations at all conditions down to exit Mach 0.2 Reynolds number 23,300. The L3FHW showed un-reattaching separations at only the most extreme condition tested, exit Mach 0.2 and Reynolds number 25,300. (open full item for complete abstract)

Committee: Mitch Wolff Ph.D. (Advisor); Andrew Lethander Ph.D. (Committee Member); John Clark Ph.D. (Committee Member) Subjects: Aerospace Engineering

Doctor of Philosophy (Ph.D.), University of Dayton, 2023, Aerospace Engineering

Turbomachinery components, such as the low pressure turbine, are highly complex rotating machines, therefore, conducting fundamental fluid mechanics studies in them is exceedingly difficult. For this reason, testing is generally completed in facilities such as linear cascades, like the one present in the Low Speed Wind Tunnel Facility at AFRL, which typically utilize a low freestream turbulence intensity, when in reality, the freestream turbulence intensity in a full, rotating low pressure turbine is likely much higher. Slightly elevating the freestream turbulence intensity (e.g., 3%) typically improves the Reynolds-lapse characteristics of a blade profile by affecting the transition process, reducing the detrimental effects of laminar boundary layer separation, and shifting the knee in the loss curve. Front loaded blades are more resistant to separation, however, they can experience high losses in the endwall region due to the complex vortical structures present. Therefore, a better understanding whether high levels of freestream turbulence intensity will increase the overall losses generated in the passage is important. An intial study with a jet based active grid was completed on the L2F blade. Based of the insight gained from that study, a new mechanical agitator based active grid was implemented into a linear cascade of L3FHW-LS blades in order to more effectively study how elevated FSTI impacts the endwall flow behavior and loss production. Coefficient of pressure measurements, three planes of SPIV, two additional planes of flow visualization, and three planes of total pressure loss measurements were collected. Impacts of incoming turbulence on the endwall losses as well as the endwall flow structures were assessed.

Committee: Markus Rumpfkeil (Advisor); Christopher Marks (Committee Member); Sidaard Gunasekaran (Committee Member); John Clark (Committee Member) Subjects: Aerospace Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2020, Mechanical Engineering

Flow through the turbine section of gas turbine engines is inherently unsteady due to a variety of factors, such as the relative motion of rotors and stators. In low pressure turbines, periodic wake passing has been shown to impact boundary layer separation, blade surface pressure distribution, and loss generation. The effect of periodic disturbances on the endwall flow is less understood. Endwall flow in a low-pressure turbine occurs in the boundary layer region of the flow through the blade passage where the blade attaches to the hub in the turbine. The response of an endwall vortical structure, the passage vortex, to various upstream disturbances is considered in this investigation. The passage vortex is a three-dimensional unsteady flow feature which generates aerodynamic losses as it interacts with the flow along the blade suction surface. High-speed velocimetry and numerical simulations have shown that the vortex intermittently loses coherence and varies in strength and position over time. The intermittent loss of coherence of the passage vortex is believed to be related to the leading-edge junction flow dynamics. An array of pneumatic devices was installed upstream of a linear cascade of low-pressure turbine blades to produce periodic disturbances that impact the blade leading edge region. A small disturbance and a large disturbance were created and characterized by their maximum velocity deficit and nondimensionalized solenoid valve on time using a plane of particle image velocimetry. A plane of high-speed stereoscopic particle image velocimetry data was collected inside the blade passage to examine how the disturbances impacted the vortex. Surface-mounted hot-film data was collected near the leading edge and in passage region to help relate flow behavior in both locations. The size and frequency of the disturbances had a nonlinear impact on the vortex size and strength. Fourier analysis revealed that the actuation frequency caused a harmonic response, and a (open full item for complete abstract)

Committee: Mitch Wolff Ph.D. (Advisor); Rolf Sondergaard Ph.D., P.E. (Committee Member); Christopher Marks Ph.D. (Committee Member) Subjects: Mechanical Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2019, Mechanical Engineering

A primary source of periodic unsteadiness in low-pressure turbines is the wakes shed from upstream blade rows due to the relative motion between adjacent stators and rotors. These periodic perturbations can affect boundary layer transition, secondary flow, and loss generation. In particular, for high-lift front-loaded blades, the secondary flowfield is characterized by strong three-dimensional vortical structures. It is important to understand how these flow features respond to periodic disturbances. A novel approach was taken to generate periodic unsteadiness which captures some of the physics of turbomachinery wakes. Using stationary pneumatic devices, pulsed jets were used to generate disturbances characterized by velocity deficit, elevated turbulence, and spanwise vorticity. Prior to application in a turbine flow environment, the concept was explored in a small developmental wind tunnel using a single device. The disturbance flowfield for different input settings was measured using hot-film anemometry and Particle Image Velocimetry. Insight was also garnered on how to improve later design iterations. With an array of devices installed upstream of a linear cascade of high-lift front-loaded turbine blades, settings were found which produced similar disturbances at varying frequencies that periodically impinged upon the leading-edge region. These settings were used to conduct an in-passage secondary flow study using high-speed Stereoscopic Particle Image Velocimetry. Results demonstrated the application of the periodic unsteadiness generator but found minor changes to the passage vortex. The vortex rotational strength decreased, and migration increased with increased perturbation frequency. Fourier analyses found the PV to be responsive at the actuation frequency with phase-locked ensemble-averaged data revealing that the disturbance periodically caused the PV to lose rotational strength. However, at the tested discrete frequencies, the vortex did not become locked (open full item for complete abstract)

Committee: Mitch Wolff Ph.D. (Advisor); Rolf Sondergaard Ph.D. (Committee Member); Christopher Marks Ph.D. (Committee Member) Subjects: Aerospace Engineering; Engineering; Experiments; Fluid Dynamics; Mechanical Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2019, Mechanical Engineering

The low-pressure turbine is an important component of a gas turbine engine, powering the low-pressure spool which provides the bulk of the thrust in medium- and high-bypass engines. It is also a significant fraction of the engine weight and complexity as it can comprise up to a third of the total engine weight. One way to drastically reduce the weight of the low-pressure turbine is to utilize high lift blades. To advance high-lift technology, the Air Force Research Laboratory (AFRL) designed the L2F blade profile, which was implemented in the linear cascade at AFRL/RQT's low speed wind tunnel facility. The L2F blade has very high lift and an excellent midspan performance, however, it was previously demonstrated to generate significant losses in the endwall region. These losses are primarily driven by the complex time-dependent three-dimensional vortical structures present in the region of the junction of the blade and the endwall, dominated by the Passage Vortex (PV). Aerodynamic flow control is one way to mitigate these losses. Previously, a pulsed endwall blowing system was implemented in the endwall region of the L2F blade which produced a loss reduction. This loss reduction was dependent on the pulsing frequency. In this research, the vortical structures for the baseline flow were characterized with respect to time. The time dependent behavior of the passage vortex motion, location, and strength were found for each pulsing frequency to determine a relationship with total pressure loss reduction. The flow through the passage of the tunnel was characterized with respect to time using high-speed stereoscopic particle image velocimetry. The flow for each test condition was characterized using Q-criterion to determine the strength of the passage vortex and its time dependent behavior. It was found that the passage vortex loses and gains strength in an unsteady manner at time scales between 1.9 < ΔT+ < 6.7. The largest total pressure loss reduction was found to corres (open full item for complete abstract)

Committee: Mitch Wolff Ph.D. (Advisor); Christopher R. Marks Ph.D. (Committee Member); Rolf Sondergaard Ph.D., P.E. (Committee Member) Subjects: Mechanical Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2018, Mechanical Engineering

Various approaches have been used to shape the geometry at the junction of the endwall and the blade profile in high-lift low-pressure turbine passages in order to reduce the endwall losses. This thesis will detail the workflow to produce an optimized non-axisymmetric endwall contour design for a front-loaded high-lift research turbine profile. Validation of the workflow was performed and included a baseline planar and test contour case for a future optimization study. Endwall contours were defined using a series of Bezier curves across the passage to create a smooth surface. A parametric based approach was used to develop the test contour shape with a goal of directing incoming endwall flow at the leading edge towards the suction side of the blade. A commercial RANS flow solver was used to model the flow through the passage. The test contour performance was measured in a low-speed linear cascade wind tunnel to verify that the numerical tools adequately captured the necessary endwall flow physics. The numerical model showed excellent agreement of total pressure loss and endwall flow structure compared with experimental measurements. Utilizing the validated workflow, the grid size, mesh deformation method, and commercial RANS flow solver, previously determined to be adequate, were used to optimize the endwall and gave confidence that the optimized contour would perform well experimentally. A genetic algorithm was used to optimize the endwall and to improve the total pressure loss characteristics. Experimental measurements for the final optimized endwall were obtained in the low-speed wind tunnel. Comparisons between the planar endwall, test case endwall, and optimized endwall shapes were made to show how different shapes affect the flowfield. The test case endwall was found to reduce the losses associated with the passage vortex, while the optimized endwall reduced losses associated with the suction side corner separation vortex.

Committee: Mitch Wolff Ph.D. (Advisor); Christopher Marks Ph.D. (Committee Member); Rolf Sondergaard Ph.D. (Committee Member) Subjects: Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

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 cond (open full item for complete abstract)

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

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 frequenc (open full item for complete abstract)

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

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 si (open full item for complete abstract)

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

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 immedi (open full item for complete abstract)

Committee: Mitch Wolff (Advisor) Subjects:

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 (open full item for complete abstract)

Committee: Mitch Wolff (Advisor) Subjects:

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 w (open full item for complete abstract)

Committee: Dr. Kirti Ghia (Advisor) Subjects:

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 remai (open full item for complete abstract)

Committee: Dr. Urmila Ghia (Advisor) Subjects:

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 distributio (open full item for complete abstract)

Committee: Dr. Urmila Ghia (Advisor) Subjects: Engineering, Mechanical

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 locatio (open full item for complete abstract)

Committee: Dr. Urmila Ghia (Advisor) Subjects: Engineering, Mechanical

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 (open full item for complete abstract)

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

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 l (open full item for complete abstract)

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

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 (open full item for complete abstract)

Committee: Jeffrey Bons PhD (Advisor); James Gregory PhD (Committee Member) Subjects: Aerospace Materials; Engineering; Fluid Dynamics; Mechanical Engineering