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layer, the turbulent mixing properties should be essentially the same for the flow over smooth and rough walls which was initially proposed by [33] and

location close to the bed generating much higher Reynolds shear stress than the smooth bed. The distinct effect of roughness for the present study can be seen penetrating deep into the flow and distinctly visible at the location as high as

6.The trend of the changes of the value of R (correlation coefficient) in the nearbed and outer layer indicating the changes of flow structure characteristics between the near-bed region and outer region. The present results clearly dispute the observation of [5] that the distribution of R is independent of the properties of the wall roughness, mean flow, and called the distribution of R is

7.The magnitude of various velocity triple products changes in the range of 200– 300% when comparing the flow over the smooth bed to the flow over rough beds. This is a clear indication that the transportation of turbulent kinetic energy and Reynolds shear stress is significantly affected by the bed roughness.

8.Turbulent activity at the near bed location also seen to be dependent on bed surface conditions. Flow over smooth bed shows the ejection type activity near bed location whereas the flow over rough beds show the sweep type activity at the location close to the bed. Interpolating this scenario to the real life stream or river flow, one can clearly note the influence of strong ejection/sweeping motion of the fluid parcels to the resuspension/transport of the bed particles.

9.Ejection type events are very evident throughout the depth of flow with the exception of the location very close to the bed with flow over smooth bed only where one can observe some sweeping type of event. Bed surface conditions clearly affect the strength of the ejection like events with distributed roughness again shows the highest strength compared to other form of bed roughness.

10.Effect of roughness is clearly visible well beyond the near-bed region and deep into the outer layer (y ≈ 0.7d) from the analysis/result of turbulent bursting events (through quadrant decomposition). For the flow over rough walls and inclusive of all turbulent events, it was noted higher magnitude of Q2 and Q4 contributions compared to the flow over smooth wall for H = 0.

11.Analysis were also carried out to investigate the contribution of the extreme turbulent events at different threshold levels (H = 2–5). The region affected over the depth of flow for active sweep (Q4) events reduces with respect to the increase of the threshold level of H but the affected region goes deep into the outer layer (y � 0.7d) for the active ejection (Q2) events even for the value of H as high as 5. Although due to the change of threshold value from 0 to 2, the number of events occurring corresponding to Q2 and Q4 reduce quite sharply but the events corresponding to H = 2 produced very large instantaneous Reynolds shear stress >5 ð Þ :5 uv , which can potentially influence the sediment transport in the stream, causing resuspension of pollutant from the bed, bed formation/changes, downstream transportation of nutrients, entrainment and the exchange of energy and momentum in the flow.

5.Effect of roughness on the Reynolds shear stress is very evident at the

Boundary Layer Flows - Theory, Applications and Numerical Methods

generalized by [34].

y/d ≈ 0.7.

universal.

76

Abdullah Faruque Civil Engineering Technology, Rochester Institute of Technology, Rochester, New York, USA

\*Address all correspondence to: aafite@rit.edu

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
