Riblet drag reduction in mild adverse pressure gradients: A numerical investigation

• We model several differently sized scalloped riblets using LES.
• Riblets were modeled in both ZPG and mild APG and compared to each other and to a baseline (flat plate) case.
• Scalloped riblets in the mild APG reduce drag only slightly more than those in ZPG.
• Maximum values of streamwise turbulence intensities, streamwise vorticity, and TKE are proportional to riblet width.
• Primary Reynolds shear stresses and turbulence energy production scale with riblet drag reduction.

Riblet films are a passive method of turbulent boundary layer control that can reduce viscous drag. They have been studied with great detail for over 30 years. Although common riblet applications include flows with Adverse Pressure Gradients (APG), nearly all research thus far has been performed in channel flows. Recent research has provided motivation to study riblets in more complicated turbulent flows with claims that riblet drag reduction can double in mild APG common to airfoils at moderate angles of attack. Therefore, in this study, we compare drag reduction by scalloped riblet films between riblets in a zero pressure gradient and those in a mild APG using high-resolution large eddy simulations. In order to gain a fundamental understanding of the relationship between drag reduction and pressure gradient, we simulated several different riblet sizes that encompassed a broad range of s+ (riblet width in wall units), similarly to many previously published experimental studies. We found that there was only a slight improvement in drag reduction for riblets in the mild APG. We also observed that peak values of streamwise turbulence intensity, turbulent kinetic energy, and streamwise vorticity scale with riblet width. Primary Reynolds shear stresses and turbulence kinetic energy production however scale with the ability of the riblet to reduce skin-friction.