Experimental Analysis of the Effect of Suction and Step Height on Boundary-Layer Transition

Laminar flow is challenging to maintain in the presence of surface imperfections such as steps and/or gaps, which can occur at structural joints. A substantial stream-wise delay of laminar-turbulent transition, however, can be achieved with boundary-layer suction. This work examines the delay of transition that can be obtained with slit suction. Imperfections such as forward-facing steps and backward-facing steps in combination with stream-wise gaps are studied. Systematic experimental investigations were performed in the low-turbulence Cryogenic Ludwieg-Tube Gottingen for large Reynolds numbers (chord Reynolds numbers up to Re-c = 16 . 10(6) ), Mach numbers 0.35 <= M <= 0.77 and various streamwise pressure gradients. Suction proves to delay transition significantly and even overcompensates the adverse effect of surface imperfections. With suction, transition locations for backward-facing steps were measured even further downstream than for similar sized forward-facing steps.