Observations on the passive control of flow oscillations over a cavity in a transonic regime by means of a spanwise cylinder

This paper deals with an experimental investigation of the passive control of a flow over a cavity. The control system is based on the introduction of a spanwise cylinder upstream of the cavity. Visualisations and pressure measurements confirm that this simple device may lead to the suppression of the self-sustained oscillations as previously shown by McGrath, S. & Shaw [2] and Staneck et col. [3], [4]. The large scale structures of the shear layer are eliminated and replaced by a single high frequency oscillation at the cylinder wake frequency. Geometric conditions under which the cylinder device becomes effective are specified and discussed. It is confirmed that the cylinder is effective in suppressing the cavity tones when it is placed in the vicinity of the upstream edge of the cavity. It is shown that the best parameter correlating our results is the ratio z_d/d where z_d denotes the distance from the bottom of the cylinder and the wall, and d is the cylinder diameter. An interval 0.5 -0.7 <z_d/d <1.6-.2 is shown to provide the best results. The length scales characterizing the incoming boundary layer and the cavity geometry have little influence. Different Interpretation elements are discussed.