Experimental Investigation of the Delay of Gap- and Step-Induced Transition by Means of Suction

The influence of suction on gap- and step-induced boundary-layer transition was investigated experimentally and systematically for large chord Reynolds numbers (up to \(16 \times 10^{6}\)) and various pressure gradients at Mach number 0.6. The experiments were conducted in the low-turbulence Cryogenic Ludwieg-Tube Göttingen with a two-dimensional wind-tunnel model. Transition is detected non-intrusively by means of temperature-sensitive paint. Forward-facing steps and spanwise gaps have caused transition to occur at a more upstream location as compared to a smooth configuration at the same test conditions. Applying suction through the gap, however, was shown to significantly delay transition and even overcompensate the adverse effect of steps and gaps. Further increasing suction rate shifts transition even more downstream until a certain value of suction is reached, above which there is effectively no change in transition location even for larger suction rates.