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The present paper deals with the wake of a 2D body equipped with a drag reduction device. The device is a 3D trailing edge consisting of alternate segments of blunt base and spanwise cavity. The aerodynamic mechanisms acting on the near wake are studied in a water tunnel from schlieren observations by thermally marking large scale structures. The results show that the efficiency of the device is directly related to the presence of longitudinal vortices. An optimization of the shapes in subsonic compressible flow had led to a decrease of more than 40% of the total drag of the profile.

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Abstract. The aim of this paper is to calculate the motion of small dilute particles carried by a given flow with the help of two models of behaviour, and then to compare this motion to that measured by a method of multiple exposure holography. The results lead to the specification of the field of validity of each model. Also they provide information about the ability of the holographic method to be used as a velocimetry technique in fluid mechanics. List of symbols