THE REDUCTION OF SKIN FRICTION BY RIBLETS UNDER THE INFLUENCE OF AN ADVERSE PRESSURE-GRADIENT

In this paper we address the effectiveness of riblets on skin friction reduction under the influence of an adverse pressure gradient. The me asurements were taken in a wind tunnel. Skin friction was observed wit h a drag balance which has a reproducibility of better than 1%. The ac curacy of the balance is estimated to be less than 1% for the case of zero-pressure gradient and at most 3% for a pressure gradient. The dat a on skin friction reduction at zero pressure gradient were consistent with previous results and amount to 5% at dimensionless riblet width of s+ = 13. We find that at all adverse pressure gradients the skin fr iction reduction by riblets persists. At moderate pressure gradients t he reduction increases somewhat to 7%. The velocity profile which is a lso measured, exhibits the characteristic shape for a boundary layer w ith an adverse pressure gradient and agrees well with theory. From the velocity profiles measured at two stations we estimated with the help of a momentum balance the skin friction and skin friction reduction. The results differ from the drag-balance data. Due to the poor accurac y of the momentum balance method which we estimate in our case, we con clude that the results obtained with this method are less reliable tha n those obtained with the drag balance. This throws some doubt on prev ious results on drag reduction under the influence of a pressure gradi ent which were based on the momentum balance method.