Transition to turbulence delay using a passive flow control strategy

Since the mid-50s, until the first proof-of-concept experiment of transition delay using circular roughness elements in 2006, there was a strong consensus within the research community that roughness elements in general promoted the transition to turbulence process. From a series of wind tunnel experiments miniature vortex generators have emerged as being a coveted passive device for transition delay and hence skin-friction drag reduction. These devices are miniature with respect to classical vortex generators typically used for separation control and fall under the appealing category passive flow control device, since it uses the existing energy in the flow and hence no energy has to be added to the control system in order to accomplish the control. The underlying physical mechanism is attributed to an additional term in the perturbation energy equation, when the boundary layer is modulated in the direction orthogonal to the base flow by the control devices, which counteracts the wall-normal production term and, hence, stabilizes the flow. The present paper briefly reviews the current state-of-art of passive flow control using physical devices for transition delay with the implication of reducing skin-friction drag. This passive control strategy has potential to lead to an unforeseen positive impact on the broad spectrum of industrial applications where reducing drag is a daily challenge. (C) 2014 The Authors. Published by Elsevier B.V.