Experimental investigation of boundary layer transition by wall-mounted elements

In model tests with low Reynolds numbers (Re), wall-mounted elements are used for promoting boundary layer transition, which aims to imitate the turbulent flow around the full-scale subjects with high Reynolds numbers. Ship model test results are obtained for model-scale ship flow for Re of 105 to 106, and these results are extrapolated to full-scale ship flow for Re of 108–109. However, only global measurement data have been used to enforce a brute force approach, ignoring relaminarization, overstimulation, and parasitic drag in local flow features. To address these problems, the freestream velocity within the boundary layer around the wall-mounted elements was investigated in this study. Different size two-dimensional (2D) cylindrical rods and various configurations of an array of three-dimensional (3D) cylinders were considered. The effects of wall-mounted elements were analyzed based on the roughness Reynolds number and spanwise wavenumber, calculated based on the height in the wall normal direction and the spacing between the 3D cylinders, respectively. The range of roughness Reynolds number for promoting a transition to turbulence without overstimulation was identified as 30–118 for 2D cylindrical rods and 262–1268 for arrays of 3D cylinders.