Experimental and Numerical Study on Wall Impedance Effects on Tollmien–Schlichting Waves

This paper investigates the effects of a porous wall on the development of Tollmien–Schlichting (TS) waves propagating inside a boundary layer. An experimental study performed in a low-speed wind tunnel showed that microperforated panels over hollow chambers could promote laminar–turbulent transition. First, measurements of the streamwise evolution of the TS instabilities reveal that greater amplification occurs over perforated panels compared to a solid wall. This effect was attributed to a wall impedance condition imposed by the perforated panels over the chambers. This setup is somewhat analogous to an acoustic liner for which the wall response is represented by a complex impedance. The impedance of the experimental setup was thus measured in situ. These experimental impedance values were then used as wall boundary conditions of a linear local stability calculation to quantify the influence of the wall impedance condition on TS wave growth. The approach was applied to predict the corresponding transition positions. Comparison between experimental and numerical results shows that the use of impedance boundary conditions is a promising approach to predict transition location, particularly in the case of the panel with the lowest resistance.