Mean Flow Characteristics and Turbulent Structures of Turbulent Boundary Layers in Varying Pressure Gradients and Reynolds Numbers

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Virginia Tech

Turbulent boundary layers flowing over a smooth surface were studied to understand the influence of varying pressure gradients and flow Reynolds number on the boundary layer growth and mean turbulent properties. The test was conducted in the Virginia Tech Stability Wind Tunnel with a 0.914 m chord length, NACA 0012 Airfoil in the test section. This airfoil was rotated to different angles of attack to induce varying pressure gradients on the boundary layer developing on the test section walls. Mean pressure measurements, boundary layer pressure measurements, and time-resolved, wall-normal, stereoscopic particle image velocimetry (TR-PIV) measurements were made. The TR-PIV data was acquired at a chord-based Reynolds number of 1.2 million, 2 million, and 3.5 million, at a sampling rate of 1 kHz, in two different camera configurations. The boundary layer pressure measurements were acquired at different flow Reynolds numbers ranging between 0.76 million and 3.5 million. Both adverse and favorable pressure gradients of varying intensities were imposed on the boundary layer by rotating a 0.914 m chord NACA 0012 airfoil to angles of attacks between -{10}^o and {12}^o. Measurements at varying streamwise locations enabled the study of boundary layer flow development under changing pressure gradients. The pressure gradient influences were observed in the boundary layer characteristic properties, on the mean velocities, and on the Reynolds stresses present in the flow. The pressure gradient influences were found to be consistent at varying Reynolds numbers, but the intensity of their effects was influenced by the flow Reynolds number. Moreover, the influence of pressure gradients and flow Reynolds numbers was evident in both outer and inner scales. The test data acquired was also validated with previous works.

Turbulent boundary layers, Reynolds stresses, Pressure Gradients, Reynolds number Variation, TR-PIV, Boundary Layer Rake, Mean Pressure Measurements