Jet engines operate by ingesting incoming air, adding momentum to it, and exhausting it through a nozzle to produce thrust. Because of their reliance on an inlet stream, jet engines are very sensitive to inlet flow nonuniformities. This makes the study of the effects of inlet nonuniformities essential to improving jet engine performance. Swirl distortion is the presence of flow angle nonuniformity in the inlet stream of a jet engine. Although several attempts have been made to accurately reproduce swirl distortion profiles in a testing environment, there has yet to be a proven method to do so. A new method capable of recreating any arbitrary swirl distortion profile is needed in order to expand the capabilities of inlet distortion testing. This will allow designers to explore how an engine would react to a particular engine airframe combination as well as methods for creating swirl distortion tolerant engines. The following material will present such a method as well as experimental validation of its effectiveness. / Master of Science

Inlet distortion research has become increasingly important over the past several years as demands for aircraft flight efficiency and performance has increased. To accommodate these demands, research progression has shifted the emphasis onto airframe-engine integration and improved understanding of engine operability in less than ideal conditions. Swirl distortion, which is considered a type of non-uniform inflow inlet distortion, is characterized by the presence of swirling flow in an inlet. The presence of swirling flow entering an engine can affect the compression systems performance and operability, therefore it is an area of current research. A swirl distortion generation device created by Virginia Tech, identified as the StreamVane, has the ability to produce various swirl distortion flow profiles. In its current state, the StreamVane methodology generates a design swirl distortion at the trailing edge of the device. However, in many applications the plane at which the researcher wants a desired distortion is downstream of the StreamVane trailing edge. After the distortion is discharged from the StreamVane it develops as it moves downstream. Therefore, to more accurately replicate a desired swirl distortion at a given downstream plane, distortion development downstream of the StreamVane must be considered. Currently Virginia Tech utilizes a numerical modeling design tool, designated StreamFlow, that generates predictions of how a StreamVane-generated distortion propagates downstream. However, due to the non-linear physics of the flow problem, StreamFlow cannot directly calculate an accurate inverse solution that can predict upstream conditions from a downstream boundary, as needed to design a StreamVane. To solve this problem, in this research, an efficient estimation process has been created, combining the use of the StreamFlow model with a Markov Chain Monte Carlo (MCMC) parameter estimation tool to estimate upstream flow profiles that will produce the desired downstream profiles. The process is designated the StreamFlow-MC2 Estimation Process. The process was tested on four fundamental types of swirl distortions. The desired downstream distortion was input into the estimation process to predict an upstream profile that would create the desired downstream distortion. Using the estimated design profiles, 6-inch diameter StreamVanes were designed then wind tunnel tested to verify the distortion downstream. Analysis and experimental results show that using this method, the upstream distortion needed to create the desired distortion was estimated with excellent accuracy. Based on those results, the StreamFlow-MC2 Estimation Process was validated. / Master of Science

An increased understanding of how inlet flow distortion affects transonic fans enables improved fan design and performance prediction. Inlet distortion refers to non-uniformities in the incoming flow properties. Complex inlet ducts in high performance aircraft result in distorted flow at the fan inlet. In this thesis, two studies were performed using Unsteady Reynolds-Averaged Navier Stokes (URANS) simulations. The first study focused on understanding how the transition abruptness between the clean and distorted sector in the inlet Pt profile as well as the circumferential extent of the distorted sector affect distortion transfer and generation through a transonic fan. Simulations on two main distortion sector sizes were carried out. For each sector size, variants with decreasing levels of transition abruptness were applied to the inlet of fan. Simulations were conducted at various operating points, ranging from choke to near-stall. Fourier-based distortion descriptors were used to quantify levels of distortion transfer and generation at various axial locations. It is shown that variations in rotor incidence occur as a result of the applied Pt distortion at the inlet. A less abrupt transition diminishes the local extrema in rotor incidence, which in turn reduces the amount of distortion transfer and generation through the rotor. The near-stall condition is affected most of all operating points considered, with a 23.4% average reduction in the amount of distortion transfer at any span. The size the inlet distorted sector affects the amount of distortion transfer and generation, particularly at the near-stall operating point. This is shown to be due to the dynamic response of the fan. The second study compared the mechanisms of stall inception for cases of both clean and distorted inlet flow. In each instance, the mechanism of stall inception is shown to be interactions between the detached bow shock and the tip clearance vortex. These interactions result in the formation of two vortices within the blade passage. The location and strength of these vortices affect the LE spillage in the adjacent blade rows. Stall inception occurs when the bow shock has moved far enough upstream to allow the resultant vortices from shock/tip clearance vortex interaction to pass in front of the leading edge. When inlet distortion is present, mass redistribution upstream of the fan results in variations in rotor incidence. Within the high incidence region, the bow shock is detached 3.9%-8.1% chord more than the clean inlet case, making LE spillage more severe. The rotating stall cell grows out of the stalled passages present at the near-stall operating point and ultimately extends 180° circumferentially and 18.7% span radially. Understanding the effects of distortion on the mechanisms of stall inception will allow appropriate steps to be taken to extend the stable operating range of modern commercial and high performance fans.

Inlet distortion is an important consideration in fan performance. Distortion can be caused through flight conditions and airframe-engine interfaces. The focus of this paper is a series of high-fidelity time accurate Computational Fluid Dynamics (CFD) simulations of a multistage fan. These investigate distortion transfer and generation as well as the underlying flow physics of these phenomena under different operating conditions. The simulations are performed on the full annulus of a 3 stage fan. The code used to carry out these simulations is a modified version of OVERFLOW 2.2 developed as part of the Computational Research and Engineering Acquisition Tools and Environment (CREATE) program. Several modifications made to the code are described within this thesis. The inlet boundary condition is specified as a 1/rev total pressure distortion. Simulations at choke, design, and near stall points are analyzed and compared to experimental data. Analysis includes the phase and amplitude of total temperature and pressure distortion through each stage of the fan and blade loading plots. An understanding of the flow physics associated with distorted flows will help designers account for unsteady flow physics at design and off-design operating conditions and build more robust fans with a greater stability margin.

Inlet total pressure non-uniformities in axial flow fans and compressors can contribute to the loss of component structural integrity through high cycle fatigue (HCF) induced by the excitation of blade vibratory modes. As previous research has shown total pressure distortion to be the dominant HCF driver in aero engines [Manwaring et al, 1997], an understanding of its transfer through, and impact on, subsequent turbomachine stages and engine components is an important topic for assessment. Since current modeling techniques allow for total pressure distortion magnitudes to be directly related to blade vibratory response, the prediction of downstream distortion patterns from an upstream measurement would allow for the inference of the vibratory response of downstream blade rows to an inlet total pressure distortion. Nonlinear Volterra theory can be used to model any periodic nonlinear system as an infinite sum of multidimensional convolution integrals. A semi-empirical model has been developed using this theory by assuming that a distortion waveform is a periodic signal that is being presented to a nonlinear system, the compressor being the system. The use of Volterra theory in nonlinear system modeling relies on the proper identification of the Volterra kernels, which make up the transfer function that defines the system's impulse response characteristics. Once the kernels of a system are properly identified, the system's response can be calculated for any arbitrary input. This model extracts these kernels from upstream and downstream total pressure distortion measurements of a transonic rotor of modern design. The resulting transfer function is then applied to predict distortion transfer at new operating points on the same rotor and compared with the measured data. The judicious choice of distortion measurement data allows predictions of the downstream distortion content based on a measured non-uniform inlet flow at conditions different from those at which the transfer function was derived. This allows for the determination of downstream total pressure distortion that has the potential to excite blade vibratory modes that could lead to HCF under operating conditions other than those at which the data was taken, such as varying inlet distortion patterns, mass flow settings, rotational speeds, and inlet geometry. This report presents the creation of a Volterra model in order to predict distortion transfer in axial flow fans and compressors. This model, in three variations, is applied to a variety of distortions and compressor operating conditions as measured in the ADLARF tests at the Compressor Research Facility. Predictions are compared with data from the test and final results are also compared with two previous studies conducted at Virginia Tech using the same experimental data. Using the Volterra model it is shown that, with appropriate limitations, distortion transfer can be predicted for flow conditions different from those used for calibration. The model is considered useful for both performance and HCF investigations. / Master of Science

Next generation aircraft design focuses on highly integrated airframe/engine architectures that exploit advantages in system level efficiency and performance. One such design concept incorporates boundary layer ingestion which locates the turbofan engine inlet near enough to the lifting surface of the aircraft skin that the boundary layer is ingested and reenergized. This process reduces overall aircraft drag and associated required thrust, resulting in fuel savings and decreased emissions; however, boundary layer ingestion also creates unique challenges for the turbofan engines operating in less than optimal inlet flow conditions. The engine inlet flow profiles predicted from boundary layer ingesting aircraft architectures contain complex distortions that affect the engine operability, durability, efficiency, and performance. One component of these complex distortion profiles is off-axial secondary flow, commonly referred to as swirl. As a means to investigate the interactions of swirl distortion with turbofan engines, an experiment was designed to measure distorted flow profiles in an operating turbofan research engine. Three-dimensional flow properties were measured at discrete planes immediately upstream and immediately downstream of the fan rotor, isolating the component for analysis. Constant speed tests were conducted under clean and distorted test conditions. For clean tests, a straight cylindrical inlet duct was attached to the fan case; for distorted tests, a StreamVane swirl distortion generator was inserted into the inlet duct. The StreamVane was designed to induce a swirl distortion matching results of computation fluid dynamics models of a conceptual blended wing body aircraft at a plane upstream of the fan. The swirl distortion was then free to develop naturally within the inlet duct before being ingested by the engine. Results from the investigation revealed that the generated swirl profile developed, mixed, and dissipated in the inlet duct upstream of the fan. Measurements immediately upstream of the fan rotor leading edge revealed 50% reduction in measured flow angle magnitudes along with evidence of fanwise vortex convection when compared to the StreamVane design profile. The upstream measurements also indicated large amounts of secondary flow entered the fan rotor. Measurements immediately downstream of the fan rotor trailing edge demonstrated that the fan processed the distortion and further reduced the intensity of the swirl; however, non-uniform secondary flow persisted at this plane. The downstream measurements confirmed that off-design conditions entered the fan exit guide vanes, likely contributing to cascading performance deficiencies in downstream components and reducing the performance of the propulsor system. / Master of Science

Boundary Layer Ingestion (BLI) is currently being researched as a potential method to improve efficiency and decrease emissions for the next generation of commercial aircraft.  While re-energizing the boundary layer formed over the fuselage of an aircraft has many system level benefits, ingesting the low velocity boundary layer flow through a serpentine inlet into a turbofan engine adversely affects the performance of the engine.  The available literature has only yielded studies of the effects of this specific type of inlet distortion on engine performance in the form of numerical simulations.  This work seeks to provide an experimental analysis of the effects of BLI-type distortion on a turbofan engine's performance.  A modified JT15D-1 turbofan engine was investigated in this study.  Inlet flow distortion was created by a layered wire mesh distortion screen designed to create a total pressure distortion profile at the aerodynamic interface plane (AIP) similar to NASA's Inlet A boundary layer ingesting inlet flow profile.  Results of this investigation showed a 15.5% decrease in stream thrust and a 14% increase in TSFC in the presence of BLI-type distortion. Flow measurements at the AIP and the bypass nozzle exit plane provided information about the losses throughout the fan flow path.  The presence of the distortion screen resulted in a 24% increase in mass-averaged entropy production along the entire fan flow path compared to the non-distorted test.  A mass-averaged fan flow path efficiency was also calculated assuming an isentropic process as ideal.  The non-distorted fan flow path efficiency was computed to be 60%, while the distorted fan flow path efficiency was computed to be 50.5%, a reduction in efficiency of 9.5%.  The entropy generation between ambient conditions and the AIP was compared to the entropy production along the entire fan flow path.  It was found that the majority of entropy generation occurred between the AIP and bypass nozzle exit.  Based on flow measurements at the bypass nozzle exit plane, it was concluded that inlet flow distortion should be located away from the tip region of the fan in order to minimize losses in a very lossy region.  It was also determined that the fan and bypass duct process the different regions of the total pressure distortion in different ways.  In some regions the entropy production decreased for the distorted test compared to the clean test, while in other regions the entropy production increased for the distorted test compared to the clean test.  Finally, it was found that small improvements in total pressure and total temperature variation at the bypass nozzle exit plane will greatly improve the fan flow path efficiency and entropy generation, thereby decreasing performance losses. / Master of Science

Accurately predicting fan performance, including bounds of operation, is an important function of any Computational Fluid Dynamics (CFD) package. The presented research uses a CFD code developed as part of the Computational Research and Engineering Acquisition Tools and Environment (CREATE), known as Kestrel, to evaluate a single stage compressor at various operating conditions. Steady-state, single-passage simulations are carried out to validate capabilities recently added to Kestrel. The analysis includes generating speedlines of total pressure ratio and efficiency, as well as radial total temperature and total pressure profiles at two axial locations in the compressor at various operating conditions and fan speeds, and simulation data from the single-passage runs is compared to experimental data. Time-accurate, full annulus simulations are also carried out to capture and analyze the processes leading to stall inception for both uniform and distorted inlet conditions. The distortion profile used contains a 90 degree sector of lower total pressure at the inlet. The observed fan behavior at stall inception is compared to previous research, and it is concluded that the inlet distortion significantly changes the behavior of the part-span stall cells that develop after stall inception. Understanding the physical processes that lead to stall inception allows fan designers to design more robust fans that can safely take advantage of the better performance associated with operating closer to stall.

Significant effort in the current technological development of aircraft is aimed at improving engine efficiency, while reducing fuel burn, emissions, and noise levels. One way to achieve these is to better integrate airframe and propulsion system. Tighter integration, however, may also cause adverse effects to the flow entering the engines, such as total pressure, total temperature, and swirl distortions. Swirl distortions are angular non-uniformities in the flow that may alter the functioning of specific components of the turbomachinery systems. To investigate the physics involved in the ingestion of swirl, pre-determined swirl distortion profiles were generated through the StreamVane method in a low-speed wind tunnel and in a full-scale turbofan research engine. Stereoscopic particle image velocimetry (PIV) was used to collect three-component velocity fields at discrete planes downstream of the generation of the distortions with two main objectives in mind: identifying the physics behind the axial development of the distorted flow; and describing the generation of the distortion by the StreamVane and its impact to the flow as a distortion generating device. Analyses of the mean velocity, velocity gradients, and Reynolds stress tensor components in these flows provided significant insight into the driving physics. Comparisons between small-scale and full-scale results showed that swirl distortions are Mach number independent in the subsonic regime. Reynolds number independence was also verified for the studied cases. The mean secondary flow and flow angle profiles demonstrated that the axial development of swirl distortions is highly driven by two-dimensional vortex dynamics, when the flow is isolated from fan effects. As the engine fan is approached, the vortices are axially stretched and stabilized by the acceleration of the flow. The flow is highly turbulent immediately downstream of the StreamVane due to the presence of the device, but that vane-induced turbulence mixes with axial distance, so that the device effects are attenuated for distances greater than a diameter downstream, which is further confirmed by the turbulent length scales of the flow. These results provide valuable insight into the generation and development of swirl distortion for ground-testing environments, and establishes PIV as a robust tool for engine inlet investigations. / Ph. D.

Current methods of creating inlet total pressure distortion for testing in gas turbine engines are only able to simulate steady-state distortion patterns. With modern military aircraft it is becoming necessary to examine the effects of transient inlet distortion on engines. One alternative being evaluated is a splitting airfoil that is essentially a wedge that can be set at different opening angles. An array of such devices would be placed in front of the engine for testing that would be capable of creating steady-state distortion patterns as well as transient distortion patterns by changing the opening angle of the airfoils. The work here analyzes the steady-state wake characteristics of some of the splitting airfoil concepts. Single-wedge tests were conducted with various opening angles in an attempt to classify the various aspects found in the wake pattern. It was found that the wake has completely different characteristics with larger opening angles. In addition, several different combinations of wedges were also examined to see if single wedge analysis could be applied to arrays of wedges. Analysis was done on combinations of wedges aligned vertically as well as combinations that were done horizontally. It was found that single wedge characteristics change considerably when different wake patterns interact with each other / Master of Science