Laminar flow control by suction at Mach 2

Abstract

For subsonic and transonic flight Mach numbers the feasibility and potential benefits of laminar flow control by suction have been demonstrated in several wind tunnel and flight experiments already. For supersonic boundary layers, however, such know-how is not available yet. Therefore, in the EU project SUPERTRAC (SUPERsonic TRAnsition Control) experimental and numerical studies on laminar flow control by suction for an infinite swept-wing configuration at Mach 2 were performed. The Ludwieg Tube Facility (RWG) at DLR Göttingen with a test chamber cross-section of 0.34m × 0.35m was used for these experiments. Based on comprehensive preparatory numerical studies a biconvex profile of 30 cm chord length with sharp leading edge and a relative thickness of 13 percent was chosen. The measurements were performed for sweep angles of 20 and 30 degrees. Suction was applied between 5 and 20 percent chord through a single suction chamber covered by porous sinter material. Suction panels manufactured from sinter material to our knowledge have never been used successfully before. They were selected here because our studies suggested that delay of transition would have been hardly feasible at the conditions of this RWG experiment using a standard suction panel with micro-drilled holes. The experiment showed that significant delay of transition by suction could be achieved for the geometry and flow conditions considered here. However, the efficiency of suction for delay of transition seems to decrease above a certain level of suction velocity. Subsequent analysis of the experimental results provided transition N-factor values with relative small variations independent of level of suction rate, sweep angle and unit Reynolds number. Similar values were also found for the reference configuration without suction panel. Hence, the additional disturbances introduced by the increased relative height of geometrical surface roughness of the suction panel and by the non-uniform suction distribution seemed to be negligible in the current experiments. This may suggest that, in the current experiments, the transition process is dominated by the non-stationary crossflow disturbances which are not sensitive to surface inhomogeneities.