Pressure Gradient Effects on the Performance of Riblets Installed on Aircraft Surfaces

A method for estimating the drag reduction performance of riblets installed on the surfaces of an aircraft was extended to take the effect of the pressure gradient over the aircraft into account. This method simply and roughly estimates the effectiveness of riblets applied to arbitrary surfaces of any aircraft by fusing wind tunnel test data and computational fluid dynamics analysis. The drag reduction rate is calculated from the relationships between friction drag reduction DR, wall-normalized riblet spacing s+, and Clauser pressure gradient parameter β. To obtain this relationship, a wind tunnel test with straight riblets in an adverse pressure gradient was carried out. The resulting shift in the value in the intercept of the log-law region of the mean velocity profile for riblet cases indicates that the drag reduction effect decreases as the Clauser parameter β increases. Based on these results, an approximate surface for the relationship between DR, s+, and β was proposed, and the drag reduction performance of riblets installed on the surfaces of a conceptual aircraft was estimated. The total drag reduction rates estimated by the extended method considering pressure gradient effects and the original method case were both about 2%, which indicates that the effects of the pressure gradient on the aircraft on overall riblet performance are small.