Breakdown in a boundary layer exposed to free-stream turbulence

The natural secondary breakdown modes of a flat plate boundary layer exposed to a free-stream turbulence level of 6.7% are characterised by experimental visualisations. The used experimental set-up is a water channel and no external triggering of the instabilities is used. The visualisations show the presence of two secondary instability modes, a sinuous (antisymmetric) and a varicose (symmetric) mode. The amplitudes of both instabilities increase, according to a non-linear profile, in upstream direction. The amplitudes also experience temporal growth. When a critical amplitude is reached, roll-up structures appear. These structures develop and finally merge, resulting in a turbulent spot. The development of the amplitude is similar in both modes. However, the amplitude of the varicose instability varies between 0.38 and 0.63 [2 pi A/ lambda] and the sinuous amplitude between 0.11 and 0.23 [2 pi A/ lambda]. The propagation velocities of the sinuous and varicose instabilities are 0.91 [u/u (blas)] and 0.87 [u/u (blas)] respectively. This shows that both modes are located in a low speed streak.