Evolution of turbulent spots in a parallel shear flow - art. no. 046307

The evolution of turbulent spots in a parallel shear flow is studied by mea ns of full three-dimensional numerical simulations. The flow is bounded by free surfaces and driven by a volume force. Three regions in the spanwise s pot cross section can be identified: a turbulent interior, an interface lay er with prominent streamwise streaks and vortices, and a laminar exterior r egion with a large scale flow induced by the presence of the spot. The lift -up of streamwise streaks that is caused by non-normal amplification is cle arly detected in the region adjacent to the spot interface. The spot can be characterized by an exponentially decaying front that moves with a speed d ifferent from that of the cross-stream outflow or the spanwise phase Veloci ty of the streamwise roll pattern. Growth of the spots seems to be intimate ly connected to the large scale outside flow, for a turbulent ribbon extend ing across the box in the downstream direction does not show the large scal e how and does not grow. Quantitatively, the large scale flow induces a lin ear instability in the neighborhood of the spot, but the associated front v elocity is too small to explain the spot spreading.