The Effect of Riblet Shapes on Flow Characteristics

The effect of riblets with different cross-sectional shapes (i.e. semi-circular (RS), triangular (RT), and square (RSq)) on the spatially developing shear layer in the transitional regime is studied using large eddy simulations (LES). The imposed flow condition at the inlet is given by the value of \({\text{Re}}_{\theta }\) as 360, where \({\text{Re}}_{\theta }\) is defined as Reynolds number based on momentum thickness (θ). Further, the riblets are arranged in a series across the flow field, resulting them to be two-dimensional and homogeneous in a spanwise direction. The assumed values for the riblets’ height and wavelength are \(k = \delta^{*}\) and \(\lambda = 3k\) , respectively. The introduction of surface modifications has influenced the flow characteristics, which are being compared for the three simulated cases with the help of spanwise and time-averaged statistics. An early transition is observed in the presence of wall roughness, which is being supported with the contours of shear stress and rms profiles. The rough surface with triangular riblets experiences maximum growth of disturbance and hence early transition when compared to semi-circular and square riblets. Further, the stable recirculation region within the riblets is observed in all the three cases, which allowed the flow to skim over the roughness. Moreover, the time-averaged skin friction coefficient ( \(\overline{C}_{{\text{f}}}\) ) is evaluated above the roughness crest, and the maximum and minimum values of \(\overline{C}_{{\text{f}}}\) are observed for the RT and RS, respectively.