European Turbulence Conference 14

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Skin-Friction Measurements in the Transitionally Rough Regime

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An important unresolved issue in fluid mechanics is accurately predicting the drag on a generic rough surface. Currently, hydrodynamic tests are required to determine an equivalent roughness height (ks) for a given roughness geometry. The equivalent height takes into account the surface texture and roughness density. This technique is employed in the widely used Moody diagram which is based on the results of Nikuradse and Colebrook. Using this diagram, a friction factor as a function of Reynolds number can be determined for a narrow range of surfaces whose equivalent roughness heights are listed. Slightly altering the roughness texture or density could result in a significantly different equivalent roughness scale. Additionally, if the specific roughness is not on the list, a hydrodynamic test needs to be performed. While the Moody diagram is accurate in the fully rough regime for a given equivalent roughness, some studies have shown discrepancies in the transitionally rough regime. Therefore, the diagram may only capture the asymptotic behavior of the skin friction in the hydraulically smooth and fully rough flow regimes. This indicates that a mapping of the transitionally rough regime for a wide range of roughness geometries, coupled with detailed measurements of roughness scales, is needed to accurately predict frictional drag in the transitionally rough regime.



Karen Flack    
United States Naval Academy
United States

Michael Schultz    
United States Naval Academy
United States


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