Advanced Fluid Research On Drag reduction In Turbulence Experiments –AFRODITE–

Abstract

Abstract. A hot topic in today's debate on global warming is drag reduction in aeronautics. The most beneficial concept for drag reduction is to maintain the major portion of the airfoil laminar. Estimations show that the potential drag reduction can be as much as 15%, which would give a significant reduction of NOx and CO emissions in the atmosphere considering that the number of aircraft take offs, only in the EU, is over 19 million per year. An important element for successful flow control, which can lead to a reduced aerodynamic drag, is enhanced physical understanding of the transition to turbulence process. AFRODITE in briefAFRODITE is a recently funded research programme by the European Reserach Council, and stands for Advanced Fluid Research On Drag reduction In Turbulence Experiments.In previous tuned wind tunnel measurements it has been shown that roughness elements can be used to sensibly delay transition to turbulence (cf. Fransson et al., 2006). The result is revolutionary, since the common belief has been that surface roughness causes earlier transition and in turn increases the drag, and is a proof of concept of the passive control method per se. The beauty with a passive control technique is that no external energy has to be added to the flow system in order to perform the control, instead one uses the existing energy in the flow.Within the research programme -AFRODITE-we will take this passive control method to the next level by making it twofold , more persistent and more robust. Transition prevention is the goal rather than transition delay and the method will be extended to simultaneously control separation, which is another unwanted flow phenomenon especially during airplane take offs. AFRODITE will be a catalyst for innovative research, which will lead to a cleaner sky. BackgroundToday, it is well known that the boundary layer can transition to turbulence via different routes depending on surrounding parameters, such as surface imperfections, free-stream turbulence (FST) and background acoustic noise (Kachanov, 1994). For a clean base flow, i.e. a flow with low background disturbance levels typically encountered in free flight, and a hydraulically smooth surface the classical transition scenario takes place with exponentially growing disturbance modes (Tollmien, 1929;Schlichting, 1933;Schubauer & Skramstad, 1947). The least stable mode, denoted Tollmien-Schlichting (TS) wave, starts to grow at some critical Reynolds number. Another modal scenario takes place on swept leading edges with favorable pressure gradient, where the primary exponential disturbance is denoted the cross-flow mode, which can both be stationary and/or traveling depending on the surrounding parameters (Saric et al., 2003). However, for an increasing level of FST disturbances will be induced from the boundary layer edge into the boundary layer, which give rise to streamwise oriented structures of low and high speed fluid (Kendall, 1985;Westin, 1997;Jacobs & Durbin, 2001;Matsubara & Alfredsson, 2001;Brandt & Henningson, 20...