Flow Drag Reduction with Wavy Riblets

Drag reduction of internal and external liquid flows is of immense importance in aerospace, naval, petroleum applications, to name a few. For example, in petroleum industry, reduction of viscous drag in a flow through the pipelines reduces the pressure losses and, therefore, the costs associated with gas compression. One of the known drag reduction mechanisms especially suited for use inside the pipelines, is riblets. Riblets are the wall grooves manufactured in the surface (Figure 1). Conventional (straight) riblets provide 4-11% drag reduction depending on the cross-section.

Figure 1. Schematics of riblets

In the current project, we were investigating the possibility of improving drag reduction properties of riblets by geometrical modification of their shape. Thus, we have designed quasi-three-dimensional (wavy) riblets [1], see Figure 2, in an attempt to imitate the flow conditions occurring above a spanwise-oscillating wall (another drag reduction mechanism [2]).

a) Straight riblets

b) Wavy riblets

Figure 2. From straight to wavy riblets (view of the riblet surface from above)

Flow drag consists of two components: skin friction drag and pressure drag (which is identically zero for a smooth wall and for straight riblets, but becomes non-zero for wavy riblets). We have found from Large Eddy Simulations of turbulent flow over riblets with different wave parameters that skin friction drag decreases with respect to a smooth wall for large wavelengths (lambda+~1080 and 1680 in wall units), but stays the same or increases for a smaller wavelength (lambda+~580). Pressure drag was found to scale with the square of a tangent of the maximum slope (tan^2 beta), consistent with physical considerations. As a result, total drag reduction can be improved from 11% for straight riblets to 14.5% for wavy riblets (knife-blade) with lambda+~1080 and beta~11.3 degrees. Additional information can be found in Ref. [3].

We have also developed a theoretical approach allowing us to analyze contribution of different dynamical effects into a skin friction coefficient in a case of laminar and turbulent flows over complex three-dimensional surfaces. We have used this approach to investigate the mechanism of skin friction modification by straight and wavy riblets [4].

References