CONVECTIVELY UNSTABLE WAVE-PACKETS IN THE BLASIUS BOUNDARY-LAYER

Based on the formal solution of the initial-boundary-value linear stab ility problem for the Blasius boundary layer, BREVIDO [8], the unstabl e wave packets in this flow, i.e. the packets of the Tollmien-Schlicht ing waves, are investigated numerically for a wide range of Reynolds n umbers R by using a Chebyshev collocation (pseudospectral) method. We show that the Blasius boundary layer is absolutely stable, but convect ively unstable for R(c) < R < 10(4), where R(c) is the critical Reynol ds number. A dependence of the properties of the convectively unstable wave packets, such as the speed of propagation, the growth rate, the spatial structure etc., on the Reynolds number R is investigated. In p articular, it is shown by using the e(N)-approach that transition to t urbulence can be enhanced for the Reynolds numbers far above the Reyno lds number R(m) for which the growth rate of the wave packet gamma(ig) reaches its maximum. For a fixed Reynolds number R a variation of the local spatial structure as a function of the ray velocity V is studie d. Our results compare favorably with the available experimental data.