抄録
Near-wall velocity measurement has been performed at y+=2.6 in a fully developed turbulent square-duct flow in order to evaluate fluctuating wall skin friction. Turbulence structures, which induce high wall shear rates, have been identified using simultaneous flow visualization. It is shown that quasi-streamwise vortical structures play a dominant role in high-friction generation, and that they can be classified into the 'cyclonic' or the 'anticyclonic' vortex, which is tilted in the spanwise direction with its vorticity parallel or antiparallel, respectively, to the mean shear vorticity. The generation of high wall shear rates is often seen on the sweep sides of the cyclonic and anticyclonic streamwise vortices. In addition to the experiments, the long-wavelength instability of a spanwise vortex filament in a background shear flow has been investigated using the Biot-Savart law with a suitable cutoff in order to demonstrate the generation mechanism of streamwise vortices. It is found that the unstable modes lead to the generation of streamwise vorticity, and that the most unstable mode has a spanwise wavelength of the order of 100 wall units in the near-wall region.
