Experimental study of the stabilisation of Tollmien-Schlichting waves by finite amplitude streaks

It has recently been found by using temporal and spatial numerical simulations that steady optimal streaks of moderate amplitude, i.e. sufficiently large but not exceeding the critical amplitude for the inflectional instability, are able to reduce the growth of Tollmien-Schlichting (TS) waves up to their complete suppression. This investigation aims at experimentally verifying this stabilising effect by generating stable and symmetric, close to sinusoidal, streaks of moderate amplitudes (about 12% of the free stream velocity) by means of a spanwise array of cylindrical roughness elements. The three-dimensional (3D) streaky base flow is then subjected to a secondary instability generated through a spanwise slot in the plate by means of regulated blowing and suction. In this study the stabilising role of the streaks on TS waves is unambiguously confirmed and by increasing the height of the roughness elements, thus inducing larger amplitude streaks, we are also able to show that the stabilising action on the TS waves increases with the streak amplitude. These results are the first to confirm the numerical predictions reported in earlier works. The full cross-stream plane has been measured at different downstream positions allowing a complete evaluation and comparison of the different amplitude measures used in previous experimental works. Furthermore, theoretical impulse response analysis and stability calculations are applied to the present experimental streaky base flow enabling a qualitative comparison of the 3D modulated TS wave distribution.