Secondary Instabilities in a Laminar Separation Bubble

Summary

If a strong adverse pressure gradient is imposed on an initially laminar boundary layer, it is decelerated and eventually separates. In this case, disturbances are strongly amplified and finally transition to turbulence takes place. Thus, a strong exchange of high-momentum fluid far away from the wall and low-momentum fluid near the wall is forced and reattachment may occur. Besides this more general understanding of the physics of laminar separation bubbles (LSB), detailed knowledge of the transition to turbulence in a LSB is still missing In Direct Numerical Simulations (DNS) artificial 2-D and 3-D disturbances are excited. If the size of the bubble is sufficiently large, DNS with realistic 2-D amplitude but very low initial 3-D amplitude indicate temporal amplification of the 3-D disturbances. Depending on the spanwise wavenumber, the self excited 3-D disturbances are either subharmonic or fundamental with respect to the 2-D disturbance. Subsequently, the spatio-temporal development of the 3-D disturbances is analyzed in order to find the source of their amplification.