TOPOLOGY OF FINE-SCALE MOTIONS IN TURBULENT CHANNEL

An investigation of topological features of the velocity gradient fiel d of turbulent channel flow has been carried out using results from a direct numerical simulation for which the Reynolds number based on the channel half-width and the centreline velocity was 7860. Plots of the joint probability density functions of the invariants of the rate of strain and velocity gradient tensors indicated that away from the wall region, the fine-scale motions in the flow have many characteristics in common with a variety of other turbulent and transitional flows: th e intermediate principal strain rate tended to be positive at sites of high viscous dissipation of kinetic energy, while the invariants of t he velocity gradient tensor showed that a preference existed for stabl e focus/stretching and unstable node/saddle/saddle topologies. Visuali zation of regions in the flow with stable focus/stretching topologies revealed arrays of discrete downstream-leaning flow structures which o riginated near the wall and penetrated into the outer region of the fl ow. In all regions of the flow, there was a strong preference for the vorticity to be aligned with the intermediate principal strain rate di rection, with the effect increasing near the walls in response to boun dary conditions.