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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

Packard, Nathan OwenActive Flow Separation Control of a Laminar Airfoil at Low Reynolds Number
Doctor of Philosophy, The Ohio State University, 2012, Aero/Astro Engineering
Detailed investigation of the NACA 643-618 is obtained at a Reynolds number of 6.4x104 and angle of attack sweep of -5° < α < 25°. The baseline flow is characterized by four distinct regimes depending on angle of attack, each exhibiting unique flow behavior. Active flow control is exploited from a row of discrete holes located at five percent chord on the upper surface of the airfoil. Steady normal blowing is employed at four representative angles; blowing ratio is optimized by maximizing the lift coefficient with minimal power requirement. The range of effectiveness of pulsed actuation with varying frequency, duty cycle and blowing ratio is explored. Pulsed blowing successfully reduces separation over a wide range of reduced frequency (0.1-1), blowing ratio (0.5–2), and duty cycle (0.6–50%). A phase-locked investigation, by way of particle image velocimetry, at ten degrees angle of attack illuminates physical mechanisms responsible for separation control of pulsed actuation at a low frequency and duty cycle. Temporal resolution of large structure formation and wake shedding is obtained, revealing a key mechanism for separation control. The Kelvin-Helmholtz instability is identified as responsible for the formation of smaller structures in the separation region which produce favorable momentum transfer, assisting in further thinning the separation region and then fully attaching the boundary layer. Closed-loop separation control of an oscillating NACA 643-618 airfoil at Re = 6.4x104 is investigated in an effort to autonomously minimize control effort while maximizing aerodynamic performance. High response sensing of unsteady flow with on-surface hot-film sensors placed at zero, twenty, and forty percent chord monitors the airfoil performance and determines the necessity of active flow control. Open-loop characterization identified the use of the forty percent sensor as the actuation trigger. Further, the sensor at twenty percent chord is used to distinguish between pre- and post- leading edge stall; this demarcation enables the utilization of optimal blowing parameters for each circumstance. The range of effectiveness of the employed control algorithm is explored, charting the practicality of the closed-loop control algorithm. To further understand the physical mechanisms inherent in the control process, the transients of the aerodynamic response to flow control are investigated. The on-surface hot-film sensor placed at the leading edge is monitored to understand the time delays and response times associated with the initialization of pulsed normal blowing. The effects of angle of attack and pitch rate on these models are investigated. Black-box models are developed to quantify this response. The sensors at twenty and forty percent chord are also monitored for a further understanding of the transient phenomena.

Committee:

Jeffrey Bons, Dr. (Advisor); Mohammad Samimy, Dr. (Committee Member); Jen-Ping Chen, Dr. (Committee Member); Andrea Seranni, Dr. (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics

Keywords:

active flow control; experimental fluid dynamics; closed-loop control

Kearney-Fischer, Martin A.The Noise Signature and Production Mechanisms of Excited High Speed Jets
Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

Following on previous works showing that jet noise has significant intermittent aspects, the present work assumes that these intermittent events are the dominant feature of jet noise. A definition and method of detection for intermittent noise events are devised and implemented. Using a large experimental database of acoustically subsonic jets with different acoustic Mach numbers (Ma = 0.5 – 0.9), nozzle exit diameters (D = 2.54, 5.08, & 7.62 cm), and jet exit temperature to ambient temperature ratios (ETR = 0.84 – 2.70), these events are extracted from the noise signals measured in the anechoic chamber of the NASA Glenn AeroAcoustic Propulsion Laboratory. It is shown that a signal containing only these events retains all of the important aspects of the acoustic spectrum for jet noise radiating to shallow angles relative to the jet axis, validating the assumption that intermittent events are the essential feature of the peak noise radiation direction. The characteristics of these noise events are analyzed showing that these events can be statistically described in terms of three parameters (the variance of the original signal, the mean width of the events, and the mean time between events) and two universal statistical distribution curves. The variation of these parameters with radiation direction, nozzle diameter, exit velocity, and temperature are discussed.

A second experimental database from the Ohio State University Gas Dynamics and Turbulence Laboratory of far-field acoustic data from an excited subsonic jet with hydrodynamic Mach number of 0.9 (Mj = 0.9) at various total temperature ratios (TTR = 1.0 - 2.5) is analyzed using the same process. In addition to the experimental acoustic database, conclusions and observations from previous works using Localized Arc Filament Plasma Actuators (LAFPAs) are leveraged to inform discussion of the statistical results and their relationship to the jet flow dynamics. Analysis of the excited jet reveals the existence of a resonance condition. When excited at the resonance condition, large amounts of noise amplification can occur – this is associated with each large-scale structure producing a noise event. Conversely, noise reduction occurs when only one noise event occurs per several large-scale structures. One of the important conclusions from these results is that there seems to be a competition for flow energy among neighboring structures that dictates if and how their dynamics will produce noise that radiates to the far-field.

Utilizing the results from both databases, several models for noise sources addressing different aspects of the results are discussed. A simple model for this kind of noise signal is used to derive a relationship between the characteristics of the noise events and the fluctuations in the integrated noise source volume. Based on the known flow-field dynamics and the acoustic results from the excited jet, a hypothetical model of the competition process is described. These various models speculate on the dynamics relating the noise sources to the signal in the far-field and, as such, the present work cannot provide a definitive description of jet noise sources, but can serve as a guide to future exploration.

Committee:

Mo Samimy, PhD (Advisor); Igor Adamovich, PhD (Committee Member); James Bridges, PhD (Committee Member); Michael Dunn, PhD (Committee Member); Walter Lempert, PhD (Committee Member)

Subjects:

Acoustics; Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

Jet Noise; Aeroacoustics; Active Flow Control; Turbulence

Metka, MatthewApplication of Fluidic Oscillator Separation Control to a Square-back Vehicle Model
Master of Science, The Ohio State University, 2015, Mechanical Engineering
Aerodynamic drag is an increasingly important factor in ground vehicle design due to its large impact on overall fuel economy. The average vehicle drag coefficient has improved significantly since the advent of the automobile, however the marginal gains possible with traditional shape optimization are beginning to decrease. There is increased need to improve the drag coefficient as a means of reducing global fossil fuel consumption, which prompts the automotive industry to investigate additional methods of drag mitigation. One method may be the use of active flow control (AFC) aimed at large scale changes in the flowfield through the introduction of energy perturbations at strategic locations on the vehicle surface. In this study, separation control with fluidic oscillators was examined on a modified square-back Ahmed vehicle model to advance the possibility of AFC application to production vehicles. A fluidic oscillator is a simple pneumatic device that converts a steady flow input into a spatially oscillating jet. This AFC actuator was selected due to its proven separation control efficiency and robustness. Studies involving the application of fluidic oscillator separation control to simplified vehicle models have been conducted by other researchers, however the large parameter space related to oscillator effectiveness yields many unanswered questions. The goal of this work was to answer more of the relevant questions needed to bridge the gap between lab and application. The majority of this experimental study was done in a scale wind tunnel facility owned and operated by a North American automaker at a Reynolds number based on model length of 1.4x10^6 or higher. A modified aft section containing boat-tail flaps and fluidic oscillators was added to the square-back Ahmed model and various parameter sensitivity trends were examined. Parameters of interest included flap angle, oscillator jet location, jet velocity, jet spacing, jet size, moving ground plane simulation, ride height, speeds changes, underbody turbulence, actuation symmetry, and model geometric scaling. Studies related to fluidic oscillator acoustics, separation control mechanism, and energy consumption were also conducted to build practical implementation knowledge. The results indicated that drag reduction was sensitive to many of the examined parameters. The character of the underbody flow and the use of symmetric actuation were shown to be of critical importance for optimal drag reduction, however exploitation of underbody flow modification may lend the most efficient use of actuator energy. Parameters such as Re (test speed), ride height, and simulated ground plane weakly affected the drag coefficient changes experienced with actuation. A model scaling study indicated that the actuator momentum requirements for a given drag reduction decreased as the model size was increased, partially because the number of oscillators required scales with base perimeter. A notional energy analysis suggested that the actuator energy consumption relative to drag reduction estimate on a full scale vehicle are within reason. The trends and sensitivity information gathered over the course of this study prompt further investigation into this flow control method.

Committee:

James Gregory, Dr. (Advisor); Jeffrey Bons, Dr. (Committee Member)

Subjects:

Aerospace Engineering; Automotive Engineering; Mechanical Engineering

Keywords:

separation control; aerodynamics; drag reduction; bluff body; Ahmed model; fluidic oscillator; bluff body wake; active flow control;

Lopera, JavierAerodynamic Control of Slender Bodies from Low to High Angles of Attack through Flow Manipulation
Doctor of Philosophy in Engineering, University of Toledo, 2007, Mechanical Engineering
This dissertation presents experimental investigations of several novel active flow control methodologies that have been implemented for aerodynamic control and maneuvering of slender bodies at low and high angles of attack through flow manipulation. For low angles of attack, a U.S. Army Smart Cargo projectile was examined. For high angles of attack a U.S. Air Force countermeasure concept projectile termed DEX (Destructive Expendable) was examined. Low angle of attack control was attempted using two novel separation control techniques: reconfigurable porosity and miniature deployable spoilers. Results show that significant aerodynamic forces are generated by implementing reconfigurable porosity and can be effectively used to steer and maneuver air vehicles. Porous patterns with a “saw-tooth” configuration seem to be the most effective in generating consistent control forces over a wide range of angles of attack. Miniature deployable spoilers successfully demonstrated their ability in producing both positive and negative pitch and yaw controls by modulating the spoiler height and length when used on the boattail and Aero Control Fins (ACFs) of a projectile. The effect of aftbody strake parameters such as shape, locations (axial and azimuthal), deployment height, and number of strakes implemented was examined on a short blunt-nose projectile. Large yaw control authority was attained for á > 40 deg. The largest yaw control authority was produced by a rectangular-shaped strake. A robust closed-loop feedback controller was successfully tested using dynamic wind tunnel experiments to control the coning motion of a projectile. The controller showed good control authority and was capable of attaining and maintaining the commanded roll angle with a tolerance of ± 10 deg. A study was conducted to gain some insights into the fluid mechanics of short blunt-nose bodies of revolution at high angles of attack. Off- and on-surface flow visualization records are collected to study the effects of two blunt noses: a hemispherical nose and an elliptical nose with 33% ellipticity. It was found that the elliptical-nose results in flow behaviors typical of a blunt-nose, while the hemispheric-nose results in behaviors that are akin to a pointed-nose. An explanation for the contrasting behaviors is provided.

Committee:

T. Ng (Advisor)

Keywords:

active flow control; aerodynamic control; slender bodies; blunt bodies; high angle of attack; forebody vortex control; coning motion; strakes; flowfield

Gan, SubhadeepActive Separation Control of High-Re Turbulent Separated Flow over a Wall-Mounted Hump using RANS, DES, and LES Turbulence Modeling Approaches
PhD, University of Cincinnati, 2010, Engineering : Mechanical Engineering
Most practical flows in engineering applications are turbulent, and exhibit separation which is generally undesirable because of its adverse effects on performance and efficiency. Therefore, control of turbulent separated flows has been a topic of significant interest as it can reduce separation losses. Often, flow control work employs passive techniques to manipulate the flow. Passive-flow control does not require any additional energy source to achieve the control, but is accompanied by additional viscous losses. It is more desirable to employ active techniques as these can be turned on and off, depending on the flow control requirement. The primary goal of the present work is to numerically investigate a high Reynolds number turbulent separated flow. It is Case 3 of the 2004 CFD Validation on Synthetic Jets and Turbulent Separation Control Workshop, http://cfdval2004.larc.nasa.gov/case3.html, conducted by NASA for the flow over a wall-mounted hump. Followed by the baseline flow simulation, i.e, without flow control, active flow control will be investigated using both steady suction jet as well as a "synthetic" jet. The present work also implements the use of two jets (steady suction and synthetic jets) as have not been previously implemented for this flow model. For the synthetic-jets case, the work also studies the effect of two jets in opposite phase. The secondary goal of this work is to bring together a variety of turbulence models and simulation approaches for one flow problem. The flow is simulated using steady and unsteady-state three-dimensional RANS equations-based turbulence models and three-dimensional time-dependent DES and LES methods. Multiple turbulence modeling approaches help to ascertain what models are most appropriate for capturing the physics of this complex separated flow. The results will help us better decide what models to choose for flows with adverse pressure gradients, flow separation and control of separated flows. For the flow over the wall-mounted hump, the simulation results agree well with experiment. Significant computational-resources savings was realized by using an analytical exit velocity profile for the active flow control jets, instead of simulating the entire flow-control manifold without sacrificing the quality of the work. Results compared with experimental values were surface pressure coefficient, skin friction coefficient, mean velocity profiles, Reynolds stresses and flow reattachment locations. Simulation results show some degree of variation with experimental results in the separated flow region. The steady-suction active control was able to reduce the reattachment length the most. The region of negative streamwise velocity was the smallest in the active flow control with steady suction. The multiple jets cases, with steady suction and synthetic jets, were able to reduce the length of separation bubble in comparison to the corresponding single jet cases. The synthetic jets case, using two jets in opposite phase, was able to achieve the most uniform velocity field in the separation bubble region. The work shows great promise in implementing active flow control, using single and multiple jets, for separated flow at high Reynolds number.

Committee:

Urmila Ghia, PhD (Committee Chair); Milind Jog, PhD (Committee Member); Kirti Ghia, PhD (Committee Member); Donald French, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

CFD;Separated flow;Active flow control;Steady Suction;Synthetic Jet

Yugulis, Kevin LeeHigh Subsonic Cavity Flow Control Using Plasma Actuators
Master of Science, The Ohio State University, 2012, Aero/Astro Engineering

Localized arc filament plasma actuators have been used to control pressure fluctuations in a cavity with a length to depth ratio of 4.86. The rear wall of the cavity is inclined 30° above the horizontal plane and the cavity length is 61.7 mm, measured from the leading edge of the cavity to the mid-plane of the ramp. Five actuators have been uniformly distributed along the span of the wind tunnel at 1 mm upstream to the cavity leading edge. Experiments were conducted at Mach 0.6 and a Reynolds number of approximately 2x105 based on cavity depth. Forcing was conducted quasi-two-dimensionally and three-dimensionally. With this Mach number and geometry, the cavity was strongly resonating at the 2nd Rossiter mode corresponding to a frequency of 2.5 kHz.

Time-resolved pressure measurements were used to assess the effectiveness of the actuators. Forcing quasi-two-dimensionally was found to be very effective, achieving a reduction in peak tone magnitude of over 20 dB and a reduction in broadband SPL of up to 5 dB. In general, the results for forcing in this manner were extremely sensitive to forcing frequency. The most effective forcing frequency was found at approximately 3300 Hz. Forcing was also conducted in several three-dimensional configurations. Overall certain three-dimensional configurations were found to be more effective than the quasi-two-dimensional forcing, and significantly less sensitive to frequency.

Particle image velocimetry was used to understand how the forcing affected the shear layer. Interesting vortex dynamics such as possible vortex merging was observed, the details of which help to understand why certain frequencies are more effective than others. It was determined that the vortices in the shear layer are significantly weaker under three-dimensional forcing compared to quasi-two-dimensional forcing. This could help to explain the overall increase in effectiveness seen with three-dimensional forcing.

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Committee:

Mo Samimy, Dr. (Committee Co-Chair); James Gregory, Dr. (Committee Co-Chair)

Subjects:

Aerospace Engineering

Keywords:

aerodynamics; plasma actuators; cavity flow; active flow control

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;

POONDRU, SHIRDISHLarge-Eddy Simulation and Active Flow Control of Low-Reynolds Number Flow through a Low-Pressure Turbine Cascade
PhD, University of Cincinnati, 2008, Engineering : Mechanical Engineering

The operating Reynolds numbers (Re) for a low-pressure turbine (LPT) in an aircraft engine can drop below 25,000 during high-altitude cruise conditions, resulting in massive separation and subsequent transition on the blade suction surface. This separation causes a significant loss in the engine efficiency. Hence, accurate prediction of the flow physics at these low-Re conditions is required to effectively implement flow control techniques which can help mitigate separation-induced losses. The present work investigated this low-Re transitional flow through a LPT cascade comprised of the generic Pratt & Whitney “PAKB” blades, using high-order accurate compact numerical schemes in conjunction with large-eddy simulation (LES), with and without subgrid-scale (SGS) models. The study examined the predictive capability of the explicit Smagorinsky and dynamic Smagorinsky SGS models, as compared to the Implicit LES (ILES) technique (LES without an explicit SGS model). The research also implemented active flow control on the LPT blades using momentum injection via surface blowing. All simulations utilized a dual-topology, multi-block, structured grid, and computations were performed on a massively parallel computing platform using MPI-based communications. The baseline cases (without control) were simulated at Re ~ 10,000, 25,000 and 50,000. The computed numerical results for all three cases showed good agreement with available experimental data. The Smagorinsky and dynamic Smagorinsky SGS model results provided no significant improvement over the ILES results because of the low level of energy in the subgrid-scales for the present low-Re flow conditions investigated, and hence, the ILES technique was used for all subsequent flow-control simulations.

Separation control of the LPT flow was implemented using synthetic normal jets, synthetic vortex-generator jets, and pulsed vortex-generator jets (VGJs) at Re ~ 10,000, for four blowing ratios ranging from 0.5 to 4.7, where the blowing ratio is defined as the ratio of the jet-exit velocity magnitude to the local free-stream velocity. All the jets were implemented by specifying an analytical boundary condition at the jet exit surface. The effectiveness of the jets was assessed in terms of the integrated wake loss coefficient values, and the modified Zweifel coefficient values. The Zweifel coefficient represents the component of the integrated blade Cp distribution contributing to the direction of rotation. Among the three types of control jets implemented, the synthetic normal jets were found to be more effective than the synthetic or pulsed VGJs. For pulsed VGJs, the effective blowing ratio was found to be 2.0 in the present study, compared to the value of 0.4 documented in the literature for control at a Re = 25,000, indicating a strong dependence of the effective blowing ratio on Re. The study also examined the flow control mechanisms of the synthetic normal jets and vortex-generator jets. It was found that the mechanism for effectiveness of synthetic jets was a combination of instability-triggered transition and free-stream momentum entrainment. Finally, the synthetic jets and synthetic VGJs were found to be more effective when the jets were located just upstream of the natural separation point.

Committee:

Urmila Ghia (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

Large-eddy simulation; Low-pressure turbines; Active flow control