Near-Wall Turbulence and Utilisation of the Nonlinear Dynamics Towards Control of Turbulent Flow

Direct numerical simulations of plane Poiseuille flow are performed in an extended domain at transitional Reynolds numbers. In minimal domains, turbulence in this Reynolds number range displays substantial intermittency that is associated with chaotic movement of turbulent trajectories between lower (LB) and upper branch (UB) invariant solutions known as exact coherent states (ECS). We address the relationship between temporal dynamics in minimal channels and spatiotemporal dynamics in extended domains. Both temporal and spatial analyses of the turbulent velocity fields are performed. These analyses partition the flow characteristics into low-, intermediate- and high-drag classes. The temporal and spatial analysis methods, although completely independent of one another, yield very similar results for both low- and high-drag regions. The conditional mean profiles in regions of low-drag closely resemble those found in low-drag temporal intervals in the minimal channel. Finally, we compare turbulence and LB-ECS and show that both the local near-wall structure in the low-drag patches of the large domain and the conditional mean profiles in the near-wall region resemble those of an LB minimal domain ECS.;Lower and upper branch ECS from one particular family of solutions are imprinted separately on a turbulent flow in the minimal channel at transitional Reynolds numbers. Specifically, the spatial patterns of their wall shear stress are imprinted on the channel-wall that moves in the wall-normal direction only. The motion of the wall results in a travelling wave of wall deformation in streamwise or spanwise direction. When an LB-ECS is imprinted, drag-reduction is observed for most of the cases, and a maximum drag reduction of just over 15% is achieved. The resultant flow field shows characteristics of a flow with lesser drag and the flow trajectories pass through the vicinity of the imprinted LB-ECS in the state space. When a UB is imprinted on the flow, the drag increases. In such cases, the trajectories remain in the high-drag region of the state space. These results indicate that flow can be driven toward a desired state by manipulating the near-wall dynamics using information from that state. This opens many avenues for the development of new flow control strategies.