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 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

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Inlet flow distortion is a non-uniform total pressure, total temperature, or swirl (flow angularity) condition at an aircraft engine inlet. Inlet distortion is a critical consideration in modern fan and compressor design. This is especially true as the industry continues to increase the efficiency and operating range of air breathing gas turbine engines. The focus of this paper is to evaluate the Computational Fluid Dynamics (CFD) Harmonic Balance (HB) solver in STAR-CCM+ as a reduced order method for capturing inlet distortion as well as the associated distortion transfer and generation. New methods for quantitatively describing and analyzing distortion transfer and generation are investigated. The geometry used is the rotor 4 fan geometry, consisting of one rotor and one stator. The inlet boundary condition is a 90-degree sector total pressure distortion profile with total pressure and swirl held constant. Multiple HB simulations with varying mode combinations and distortion intensities are analyzed and compared against full annulus Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations. Best practices and recommendations for the implementation of the HB solver are given. The pre-existing Society of Automotive Engineers Aerospace Recommended Practice (SAE-ARP) 1420b descriptors are demonstrated to be inadequate for the purposes of analyzing distortion transfer and generation on a stage-to-stage basis. New implementations of Fourier methods are presented as an alternative to the SAE-ARP 1420b descriptors. These Fourier descriptors are shown to describe distortion transfer and generation to a higher degree of fidelity than the SAE-ARP 1420b descriptors. These new descriptors are demonstrated on the analysis of full annulus URANS and HB simulations. The HB solver is shown to be capable of capturing distortion transfer, generation and performance degradation. Recommendations for the optimal implementation of the HB method are given.