Numerical and Experimental Investigation of the K-Regime of Boundary-Layer Transition

One particular case of K-type transition has been investigated using hot-wire measurements and spatial direct numerical simulation (DNS). Detailed quantitative comparisons of the results of both approaches showed very good agreement of the spatial disturbance development, the disturbance spectra, the instantaneous velocity traces, and the local frequency-spanwise-wave-number spectra. Indications for a direct generation of three-dimensional modes as higher harmonics of the fundamental modes were found. A closer look at the phase speeds of these modes, however, revealed that weak-nonlinear interactions are only initially appropriate to describe the flow, they fail when local events dominate, like, for example the formation of small-scale vortices in the boundary layer. The investigation of the later stages showed that the hot-wire ‘spike’-signals are connected with small ring-like vortices.