Experimental Study of Skin Friction Drag Reduction of Turbulent Boundary Layer over Shallow Dimples

This study investigates skin-friction drag reduction of a turbulent boundary layer over shallow dimples. The motivation of the present study is to explore the influence of shallow dimples on skin friction and the turbulent boundary layer profile. The behavior of flow structures downstream of the dimpled surface has also been investigated. With a view to search for the influence of dimple surface on the mentioned flow characteristics, direct force measurements have been carried out to measure the skin friction force acting on the dimpled surface and the reference flat plate. The force acting on the dimpled surface is found to be higher than that acting on the reference flat plate. From measurement of the average pressure difference between the upstream and downstream extents of the test section, it was found that the contribution of the pressure force on the total force obtained using direct force measurement is negligible for both the dimple surface and flat plate. For the dimpled surface, it is observed that the thickness of the boundary layer over the valley region is thicker than at the crest region. The boundary layer is found to be thicker for dimple surface than that of flat plate. Results obtained from oil-film interferometry measurement at the downstream region of dimple and flat plate, spanwise wavy fringe pattern which has wavelength equivalent to the dimple diameter is observed for dimple surface. Besides, skin friction coefficient measured at the valley region of spanwise wavy fringe pattern is found to be higher than that of peak region. Furthermore, average skin friction obtained using oil-film interferometry at the downstream region of dimple surface is found to be higher than that of flat plate. The results obtained from this study may help in the future drag reduction studies of turbulent boundary layer over shallow dimples