Interactions of Tollmien-Schlichting waves and stationary transient disturbances.

摘要

Numerical simulations have shown that stationary, spanwise periodic transient disturbances are capable of suppressing the growth of Tollmien-Schlichting (TS) waves in boundary layers. A natural implication is that the deliberate introduction of transient disturbances may delay TS-dominated transition. This experimental study investigates the interactions of these two types of disturbances and the breakdown of TS waves with and without transient disturbances. TS waves are created using a combination of acoustic forcing and a two-dimensional roughness strip while transient disturbances are created using a spanwise array of cylindrical roughness elements. It is found that in the presence of stationary transient disturbances, wall-normal velocity profiles of the unsteady TS-like waves are modulated and their growth rate is reduced. The modulated wall-normal velocity profiles have a characteristic M-shaped appearance and this shape appears to be mainly due to the component of the unsteady disturbances that varies on the spanwise wavelength of the stationary disturbances and higher spanwise harmonics of this wavelength. An important finding with respect to transition control applications is that the receptivity of the TS waves is reduced when the spanwise roughness array is present. The underlying mechanism responsible for this reduction is not yet known. Additionally, it has been found that unsteady disturbance amplitudes at breakdown are reduced when transient disturbances are introduced and, therefore, the breakdown location is actually moved upstream when care is taken to match initial unsteady disturbance amplitudes. It is concluded that even though TS growth is reduced, transition to turbulence is not successfully delayed.