Experimental investigations on the drag reduction over riblets in a turbulent boundary layer under adverse pressure gradients

Abstract

Experimental investigations on the drag reduction phenomenon over riblets have been made in turbulent boundary layer under adverse pressure gradients. The local wall shear stress was determined by a direct measurement device, and mean velocity and turbulence intensity profiles were obtained by a single hot-wire probe and a constant temperature anemometer. The error due to the pressure gradients on the wall shear stress measurement is successfully eliminated from analysis of pressure distribution on the lip of the floating element. Under adverse pressure gradients of the order of β ≅ 0.4, the wall shear stress reduction rate of the trapezoidal grooves riblets with dimensions 9.5 < s⁺ < 11.6 agrees with the reduction rate under zero pressure gradients. The logarithmic mean velocity profile can be recognized over riblets and Kármán constant 𝜅 keeps almost the same value for smooth and riblets surface. However, the additive constant is increased by the wall shear stress reduction over riblets. In the streamwise turbulence intensity profile normalized with the wall units, the maximum value is the almost same for smooth and riblets, but the position of the maximum value y⁺ is slightly shifted upwards from the wall to the far side for riblets.