Experimental study of turbulent boundary layer flows over forward facing steps with different surface conditions.

Abstract

This thesis is a fundamental study that was conducted experimentally to investigate the effects of different surface types on turbulent flows over forward facing steps. A particle image velocimetry technique was employed to conduct field velocity measurements at the mid-plane of the test channel at selected locations downstream to 68 step heights. Three surface conditions were investigated. A reference smooth acrylic step and two rough steps created using sandpaper 24-grits and 36-grits. Reynolds numbers based on step height and centerline mean velocities of 1200, 3600 and 4800 were employed. The results show that the mean reattachment length increases as Reynolds number increases over the smooth step. However, the mean reattachment length decreases with increasing surface roughness at a given Reynolds number. The mean velocities, Reynolds stresses, triple velocity products and production of Reynolds stresses are used to examine the effects of different surface types on the turbulent characteristics downstream. The results reveal that surface roughness reduced the turbulent quantities in the recirculation and early redevelopment regions. On the other hand, the effects on turbulent flow at downstream locations show no consistent trends. In addition, proper orthogonal decomposition (POD) was used to study the effects of roughness on the large scale structures downstream and to reconstruct flow structure. Results show that low order POD modes can capture up to 90% of the peaks of the Reynolds shear stress profiles using only the first 100 modes. Furthermore, the two-point correlation was employed to quantity the extent of larger scale structures embedded in the flow and how the turbulence is correlated. The results indicate that surface roughness generally decreased the extent of turbulence correlations in both the recirculation and redevelopment regions.