Abstract

The reduction of skin friction in turbulent flows has been an active area of research for several decades. Surface geometry manipulations have been studied thoroughly as a form of passive flow control to reduce viscous drag. A few techniques, such as surface riblets and superhydrophobic surfaces have been shown to produce drag reduction.

Here we propose the use of anisotropically-porous coatings to reduce skin friction. Porous surfaces can produce an effectively slip for the overlying, turbulent flow. If the permeability of the porous layer is anisotropic, the apparent slip depends on the orientation of the flow with respect to the surface. The theoretical analyses of Luchini et al. (1991) and Jimenez (1994) predict that the drag reduction is proportional to the difference between slips. However, this proportional trend begins to degrade for a given size, beyond which the performance ceases to improve, and the surface treatment is no longer interesting from an application perspective. This degradation occurs when Kelvin—Helmholtz-like spanwise rollers begin to develop over the surface. These rollers generate additional Reynolds stresses, which are responsible for the degradation of drag.

Based on the above concepts of anisotropic slip length and onset of a Kelvin—Helmholtz-like instability, we aim to obtain estimates for the parameters that maximize the drag reduction produced by a coating with anisotropic porosity.