The influence of wall suction and blowing on boundary-layer laminar streaks generated by free-stream vortical disturbances

The effect of mean flow wall transpiration on boundary-layer fluctuations generated by free-stream disturbances of the convected gust type is investigated numerically. The theoretical frameworks of Leib [J. Fluid Mech. 380, 169 (1999)] and Ricco [J. Fluid Mech. 638, 267 (2009)], based on the linearized unsteady boundary-region equations, are adopted. It is found that wall suction has a more significant attenuating effect on the low-frequency laminar streaks, while high-frequency disturbances are brought closer to the wall, but unaffected in magnitude. A simple asymptotic result, confirmed by the numerical calculations, shows that the characteristic peak of the laminar streaks in the core of the boundary layer may be suppressed completely by suction if this is sufficiently intense. Thought experiments of the modification induced by suction on existing wind-tunnel root-mean-square data are carried out. The findings are compared with other laboratory data with wall suction, and the reasons for discrepancy are outlined.