DNS for Turbulent Spot Formation of Wavepacket Evolution in a Blasius Boundary Layer

The spatial temporal development of the pulse-initiated wavepacket in a Blasius boundary layer was studies by DNS from linear stage up to formation of the turbulent spot and allowed us to extract more intricate details of the turbulent spot than could generally be deduced from experiments. The wavepacked is evolving in turn by the appearance of triad resonance, streaky structures, onset of mean flow distortion and rapid intensification of frequency and wavenumber spectra, and formation of the turbulent spot. The incipient turbulent spot contains low frequency, low spanwise wavenumber fluctuations in streamwise velocity and high frequency harmonics in vertical and spanwise velocities with selective frequencies up to 7 times the fundamental frequency. The vertical component of disturbance plays an important role in the onset of turbulent spots by transporting energy from the outer region into the inner region. Four-and five-wave resonances also appear to play a non-trivial role. The vortex core and wavelet analyses were used to identify vertical structures and showed the incipient turbulent spot is derived from spatial amalgamation of a multitude of small-scale vertical patches. Preliminary simulations have been conducted for non-linearly evolving wavepackets over compliant panels and indicated this can affect and suppress the nonlinear growth of wavepacket disturbances.