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This thesis presents the results of a study performed in the Virginia Tech low speed linear cascade wind tunnel operating at a Reynolds number of 382,000 designed to model an axial compressor rotor. To simulate the flow created by the junction of a set of inlet guide vanes and the compressor casing, vortex generators were glued to a moving end wall. In this investigation, the tip clearance was varied from 0.83% to 12.9% chord. Measurements of the midspan and the tip blade loading were made using static pressure taps. The tip loading shows that the minimum suction surface pressure coefficient increases in magnitude linearly up to a tip clearance of 7.9% chord. Unsteady pressure was measured on the pressure and suction surfaces at the tip of two cascade blades using an array of 23 microphones mounted subsurface. These measurements reveal that the unsteady pressure at the blade tip is a linear function of tip clearance height. The instantaneous pressure shows that the surface pressure at the blade tip has the same character regardless of whether or not the blade is disturbed by the inflow vortices. This suggests that the vortex generators simply stimulate and organize the existing response of the blade. Single sensor hot-wire measurements were made within the tip clearance on the suction side of the blade 1mm from the tip gap exit. These measurements show that the mass flux through the tip clearance is closely related to the pressure difference across the tip gap. / Master of Science

This dissertation presents the results from an experimental study of three-dimensional turbulent tip gap flow in a linear cascade wind tunnel with 3.3% chord tip clearance with and without moving endwall simulation. Experimental measurements have been completed in Virginia Tech low speed linear cascade wind tunnel. A 24" access laser-Doppler velocimeter (LDV) system was developed to make simultaneous three-velocity-component measurements. The overall size of the probe is 24"à 37"à 24"and measurement spatial resolution is about 100 μm. With 24" optical access distance, the LDV probe allows measurements to be taken from the side of the linear cascade tunnel instead of through the bottom of the tunnel floor. The probe has been tested in a zero-pressure gradient two-dimensional turbulent boundary layer. Experimental measurements (oil flow visualization, pressure measurement, and LDV measurement) for the stationary wall captured the major flow structures of the tip leakage flow in the linear compressor cascade, such as tip leakage vortex, tip leakage vortex separation and tip separation vortex. Large velocity gradients in the tip leakage vortex separation, tip leakage vortex, and tip separation vortex regions generate large production of the Reynolds stresses and turbulent kinetic energy. One of the most interesting features of the tip leakage flow is the bimodal velocity probability histograms of the v component due to the unsteady motion of the flow in the interaction region between the tip leakage vortex and tip leakage jet. The tip separation vortex, tip leakage vortex separation, and tip leakage vortex contain most of turbulent kinetic energy and generate the highest dissipation rate. Relative motion of the endwall significantly affects the tip gap flow structures, especially in the near wall region. Compared to the stationary wall case, velocity gradients in the near wall region for the moving wall case are much smaller and lower velocity gradients in the near wall region cause the low production of Reynolds stresses and turbulent kinetic energy. Similar to the stationary wall case, high Reynolds stresses and turbulent kinetic energy values are mainly located in the vicinity of the tip leakage vortex and tip separation vortex region. The bimodal velocity probability histograms of the v component are also found at the same locations. The tip separation vortex with most of the turbulent kinetic energy generates the highest dissipation rate. The dissipation rate in the tip leakage vortex region is reduced with the decrease of turbulent kinetic energy under the moving wall effect. / Ph. D.

This thesis presents some results from an experimental study of three-dimensional turbulent tip gap flows in the linear cascade wind tunnel, for two different tip gap clearances (t/c=1.65% and 3.3%). The experiments focus on near-wall flow field measurements for the stationary wall and moving wall, and static pressure measurement on the low end-wall for the stationary wall case. The representative flows were pressure driven, three-dimensional turbulent boundary layers in the linear cascade tunnel for the stationary wall case, and the combination of the pressure driven and shear driven flow for the moving wall case. Several experimental techniques are used in the studies: a three-orthogonal-velocity-component fiber-optic laser Doppler anemometer (3D-LDA) system, surface oil flow visualization, and a scanivalve system for static pressure measurement through pressure ports on the end-wall. From the details of the oil flow visualization pattern on the end-wall, some features of the passage flow, cross flow, and the tip leakage vortex in this cascade flow were captured. Oil flow visualization on the blade surface reveals the reattachment of the tip leakage vortex on the blade surface. The static pressure results on the lower end-wall and mid-span of the blade show huge pressure drop on the lower end-wall from the pressure side to the suction side of the blade and from mid-span to the lower end wall. The end-wall skin friction velocity is calculated from near-wall LDA data and pressure gradient data using the near-wall momentum equation. The statistics of Reynolds stresses and triple products in two-dimensional turbulent boundary layer and three-dimensional turbulent boundary layer was examined using a velocity fluctuation octant analysis in three different coordinates (the wall collateral coordinates, the mid tip gap coordinates, and the local mean flow angle coordinates). The velocity fluctuation octant analysis for the two-dimensional turbulent boundary layer reveals that ejections of the low speed streaks outward from the wall and the sweeps of high speed streaks inward toward the wall are the dominant coherent motions. The octant analysis for the three-dimensional turbulent boundary layer in the tip gap shows that the dominant octant events are partially different from those in the two-dimensional turbulent boundary layer, but ejection and sweep motions are still the dominant coherent motions. For the three-dimensional turbulent boundary layer in the moving wall flow, the near-wall shear flow reinforces the sweep motion to the moving wall and weakens the out-ward ejection motion in the shear flow dominant region. Between the passage flow and the shear flow, is the interaction region of the high speed streaks and the low speed streaks. This is the first time that the coherent structure of the three-dimensional turbulent boundary in the linear cascade tip gap has been studied. / Master of Science

This dissertation presents results from a thorough study of the tip-gap turbulent flow structure in a low-speed linear compressor cascade wind tunnel at Virginia Tech that includes a moving belt system to simulate the relative motion between the tip and the casing. The endwall pressure measurements and the surface oil flow visualizations were made on a stationary endwall to obtain the flow features and to determine the measurement profiles of interest. A custom-made miniature 3-orthogonal-velocity-component fiber-optic laser-Doppler velocimetry (LDV) system was used to measure all three components of velocity within a 50 mm spherical measurement volume within the gap between the endwall and the blade tip, mainly for the stationary wall with 1.65% and 3.30% tip gaps as well as some initial experiments for the moving wall. Since all of the vorticity in a flow originates from the surfaces under the action of strong pressure gradient, it was very important to measure the nearest-wall flow on the endwall and around the blade tip. The surface skin friction velocity was measured by using viscous sublayer velocity profiles, which verified the presence of an intense lateral shear layer that was observed from surface oil flow visualizations. All second- and third-order turbulence quantities were measured to provide detailed data for any parallel CFD efforts. The most complete data sets were acquired for 1.65% and 3.30% tip gap/chord ratios in a low-speed linear compressor cascade. This study found that tip gap flows are complex pressure-driven, unsteady three-dimensional turbulent flows. The crossflow velocity normal to the blade chord is nearly uniform in the mid tip-gap and changes substantially from the pressure to suction side. The crossflow velocity relies on the local tip pressure loading that is different from the mid-span pressure loading because of tip leakage vortex influence. The tip gap flow is highly skewed three-dimensional flow throughout the full gap. Normalized circulation within the tip gap is independent of the gap size. The tip gap flow interacts with the primary flow, separates from the endwall, and rolls up on the suction side to form the tip leakage vortex. The tip leakage vortex is unsteady from the observation of the TKE transport vector and oil flow visualizations. The reattachment of tip separation vortex on the pressure side strongly depends on the blade thickness-to-gap height ratio after the origin of tip leakage vortex but is weakly related to it before the origin of tip leakage vortex for a moderate tip gap. Other than the nearest endwall and blade tip regions, the TKE does not vary much in tip gap. The tip leakage vortex produces high turbulence intensities. The tip gap flow correlations of streamwise and wall normal velocity fluctuations decrease significantly from the leading edge to the trailing edge of the blade due to flow skewing. The tip gap flow is a strongly anisotropic turbulent flow. Rapid distortion ideas can not apply to it. A turbulence model based on stress transport equations and experimental data is necessary to reflect the tip gap flow physics. For the moving endwall, relative motion skews the inner region flow and is decorrelated with the outer layer flow. Hence, the TKE and correlations of streamwise and wall normal velocity fluctuations decrease. / Ph. D.

This report documents the experimental study for four different three-dimensional turbulent flows. The investigation focuses on near wall measurements in these flows. Several experimental techniques are used in the studies; however, the bulk of the investigation focuses on a three-orthogonal-velocity-component fiber-optic laser Doppler anemometer (3D-LDA) system. The control volume of the 3D-LDA is on the order of 50 micro-meter in size, or a y⁺ distance of around 2.3 units (using average values of U_&#964 and ν from the experiment). An auxiliary small boundary layer wind tunnel (auxiliary tunnel) and a low speed linear compressor cascade wind tunnel (cascade tunnel) are utilized in this study. One of four flow experiments is done in the auxiliary tunnel the other three are in the cascade tunnel. The first three-dimensional turbulent flow is a vortical flow created by two half-delta wing vortex generators. Near wall secondary flow features are found. The second flow is an investigation of the first quarter chord tip gap flow in the cascade tunnel. Strong three-dimensional phenomena are found. The third flow investigated is the inflow to the compressor cascade with the moving wall. The experiment records shear layer interaction between the upstream flow and moving wall. Finally the fourth flow investigated is the inflow to the compressor cascade with the moving wall with half-delta wing vortex generators attached. Phase-averaged data reveal asymmetrical vortex structures just downstream of the vortex generators. This is the first time any near wall data has been taken on any of these flows. / Master of Science