Tollmien-Schlichting Wave Suppression and Transition Delay Using Stationary Transient Disturbances
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Numerical simulations show that stationary, spanwise-periodic transient disturbances are capable of suppressing the growth of unstable Tollmien-Schlichting (TS) waves in boundary layers. Experimental studies have verified this prediction. A natural implication is that the deliberate introduction of transient disturbances may delay TS-dominated transition. This study investigates this possibility by introducing controlled TS waves and stationary transient disturbances into a Blasius boundary layer and measuring the resulting amplitudes of the TS waves and transient disturbances as well as the distribution of turbulent intermittencies associated with the TS-dominated breakdown. TS waves are created using a combination of acoustic forcing and a 2D roughness strip while transient disturbances are created using a spanwise array of cylindrical roughness elements. In the presence of the transient disturbances, the growth rate of the TS-like disturbances is reduced and the disturbances develop a strong spanwise-periodic modulation. It is found that the receptivity of these disturbances to acoustic forcing is reduced by the presence of the transient disturbances. Despite the fact that the initial disturbances are well-controlled, breakdown to turbulence does not occur with a clear pattern that can be tied to specific spanwise locations. The introduction of largeamplitude transient disturbances does move the breakdown location downstream. However, this delaymay be more strongly associated with the reduced receptivity than with the reduced growth rate. It is found that the amplitudes of the unsteady disturbances at breakdown are also reduced when transient disturbances are present but this may simply be a consequence of the more-downstream breakdown location. Therefore, it is not clear if the reduction of the unsteady disturbances' growth by transient disturbances does indeed delay transition. 2005 by the authors.
