Random roughness effects on the near-wall flow in the transitionally rough regime
ORAL
Abstract
Direct numerical simulation of turbulent channel flow over a random rough wall is performed at $Re_{\tau}=400$ and $600$. The rough surface corresponds to the experiments of Flack and Schultz (personal communication). The skin friction coefficient of the random-rough channel matches with the experimental results of Flack and Schultz. The roughness effects on the near-wall regions of mean velocity, Reynolds stresses, pressure fluctuations and streamwise mean momentum balance are investigated. The statistics of wall-shear stress fluctuations in the peak (above the mean height location) and valley (below the mean height location) regions are examined. The probability distribution function of wall-shear stress shows a better collapse after subtracting the mean and normalizing by the root-mean-squared value. The distribution tail is widened by the random roughness, implying that the probability of extreme events is increased. The probability of extreme events in the random-rough channel increases with increasing $Re_{\tau}$, in accordance with previous studies on smooth-wall flows.
*This work was supported by the United States Office of Naval Research (ONR) Grant N00014-17-1-2308 managed by Drs. J. Gorski, T. Fu and P. Chang. Computing resources were provided by the Minnesota Supercomputing Institute (MSI). We are grateful to Professor K. Flack at the United States Naval Academy for providing us with the scanned surface data used in the present work.
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