Drag characteristics of V-groove and transverse curvature riblets

The paper examines the drag characteristics of longitudinally ribbed surfaces where the rib dimensions are the order of the initial turbulent wall burst region near the wall. Direct drag measurements are presented for symmetric and unsymmetric V-groove, and (larger wavelength) transverse curvature riblets. The direct drag data indicate net drag reductions as large as 7% for certain V-groove riblets with heights in terms of law of the wall coordinates h+ less than 30. For one of the riblet models, the drag reduction is confirmed by reductions in the momentum thickness, turbulent velocity fluctuations, and Reynolds stress profiles. Although the direct drag data for the transverse curvature riblets showed drag increases, certain models did show that the drag increase diminished with increasing velocity. This trend is opposite to the normal one found for rough surfaces and should be investigated further.