

Artikel
Linear and nonlinear development of localized disturbances in zero and adverse pressure gradient boundarylayers.
Författare: 
Bech, K.H., Henningson, D.S.H., Henkes, R.A.W.M. 
Dokumenttyp: 
Artikel 
Tillstånd: 
Publicerad 
Tidskrift: 
Phys. Fluids 
Volym: 
10
14051418 
År: 
1998 
AbstractLocalized disturbances in laminar boundarylayers at Reynolds number Red* = 950 were studied using direct numerical simulation. Instability mechanisms in both an adverse and zero pressure gradient we e investigated by introducing three different threedimensional disturbances. The first disturbance was centered around a pair of oblique waves in Fourier space, the second around a plane wave, while the third was axisymmetric. For small amplitudes, the first disturbance developed into a wavepacket of oblique waves in adverse pressure gradient and into a streaky structure with a trailing wavepacket in a zero pressure gradient. The second disturbance developed into a wavepacket centered around plane waves in both pressure gradients. The third disturbance developed into a wavepacket of plane waves in adverse pressure gradient and, due to the transient growth mechanism, into a streaky structure in a zero pressure gradient. For finiteamplitude plane wavepackets in a zero pressure gradient, a subharmonic secondary instability was observed which subsequently developed into an elongated "Lambda" structure. The secondary instability was less significant in adverse pressure gradient due to the large growth rate of the primary instability. Breakdown was observed as highfrequency oscillations on the spike over the head of the "Lambda" vortices. Providing the initial amplitude was sufficiently large, the vortex pair yielded the fastest route to turbulence. The main growth mechanism in this scenario, in addition to the exponential growth in the adverse pressure gradient case, was the nonlinear excitation of transient growth streaks by interacting oblique modes. Disturbances dominated by streaks needed substantially larger amplitudes before secondary instability or breakdown occurred. In that case, breakdown was shifted from the spike to an instability of the rear part of the streaks.

