Friction Velocity Determination Techniques in Turbulent Boundary Layers with Miniature Vortex Generators

Miniature vortex generators (MVGs) have the potential to control turbulent boundary layers (TBLs). Analyzing the scaled turbulent structure of MVG TBLs in experiments typically requires indirect methods for accurate friction velocity U_τ U τ determination. The established methods for U_τ U τ determination need to be verified for non-generic TBLs, such as MVG TBLs, especially at near-wake stations. This study compares the performance of the five most common U_τ U τ -determination methods when applied to MVG TBLs. The methods are tuned in terms of their free parameters and fitting ranges using direct measurements of U_τ U τ from large eddy simulation data, and applied to a corresponding experiment to assess their accuracy. Based on the findings, an “inner” method, which is the Musker function with a drifting buffer layer incorporating a bump function, is recommended for MVG TBLs of the form investigated, as it provides a good estimation of U_τ U τ with uncertainty $$<3%$$ < 3 % for all streamwise stations. The method is applied to three further experimental tests with different flow conditions to study trends in U_τ U τ . The error of the drag variation is $$<6%$$ < 6 % , which indicates the method is reliable for MVG TBLs at high Reynolds numbers.