Spatially advancing coherent structures in curved channel turbulent flow

Direct numerical simulation was performed for the spatially advancing turbulent flow in a two-dimensional curved channel. The fully developed turbulence was introduced into the curved channel through a straight part, and observation was made for how the near-wall turbulence advanced along curved walls. The frictional Reynolds number, Re(tau 0), was fixed at 150, whereas the radius ratio of the curved channel, alpha, was changed at three steps; 0.8, 0.92, and 0.975. Five computational tests were performed, which include simulations with changing the spanwise length of the channel or the grid arrangement. The maximum number of grid points was 43,610,112 (=936 x 91 x 512) used in the case of alpha = 0.92. Numerical results for the curved flow were nearly free from the grid arrangement or the spanwise length of the channel, and changes of the mean velocity qualitatively agreed with the experiment by Kobayashi [Trans. Jpn. Soc. Mech. Eng., Ser. B 57, 4064 (1991)], which both supported credibility of the simulation. The spanwise pre-multiplied power spectrum and the spanwise two-point correlation captured the peaks corresponding to the micro-scale structures near the straight wall, which continuously grew along the concave wall and advanced to exhibit the wave length of organized flows. Therefore, the micro-scale structures such as quasi-stream vortices and the ejection near the outer wall of the straight channel were suggested to trigger the onset of initial seeds of the organized structure, which grew into the large scale vortices expanding almost over the channel width.(C) 2011 American Institute of Physics. [doi: 10.1063/1.3584126]