LES of Pulsating Turbulent Flows over Smooth and Wavy Boundaries

Flows driven by a pressure gradient that oscillates periodically around a non-zero mean (pulsating flows) are found in a variety of geophysical, engineering and biomedical settings. Moreover, aside from their practical importance, they are a useful model to understand the more general problem of how unsteadiness affects the properties of a boundary layer. In this paper, we consider examples of pulsating flows over smooth and wavy surfaces studied with the aid of LES. For the smooth case, the surprising result is that the time averaged statistics are marginally if at all affected by the presence of oscillations (at least in the regime considered of current dominated flows), whereas the oscillating part is influenced by the underlying steady turbulence. Introducing waviness of sufficient amplitude to induce flow separation, at least during part of the cycle, couples the mean to oscillating component much more tightly, resulting in an increased drag felt by the mean flow. Such enhanced drag is due to the ejection of large, coherent spanwise vortices that form in the lee of the ripples, and it has a strong and non trivial dependence on the frequency of the oscillation.