Experimental study of turbulent boundary layer flows over forward facing steps with different surface conditions.
Date
2014-09-01
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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.
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Keywords
Turbulent flow, Rough forward facing step, Proper orthogonal decomposition, Particle image velocity, Two-point correlation