The figures show that the flow at speeds below 50 % of the peak is in a region close to the wall, with most of it confined to a thin bounary layer. However, there are also prominent "low-speed streaks" protruding out of the thin boundary layer. These low-speed streaks are hallmarks of wall-bounded turbulent flows. Here, we see the influence of grid resolution on the streak formation. All simulations were computed with the same spectral element mesh, which contains 1040 elements. However, the polynomial degrees are 9, 11, and 15.
The computation with elements of order 15 consists of roughly 3.5 million gridpoints for the velocity and 2.9 million points for the pressure.
Third-order timestepping is used. A filter with a parabolic transfer function is applied to the top three modes [F. & Mullen '01] , and the nonlinear terms are dealiased with the 3/2's rule.
The movie shows isosurfaces where the streamwise velocity takes on roughly 1/2 its mean value as computed using N=17. The color map represents pressure, and we can see that high-pressure regions propagate faster than the low-speed streaks (represented by folds in the velocity isosurface). These high-pressure zones result from the passage of large structures in the high-speed, center, section of the channel. The frame of reference for this computation is moving at the same speed as the velocity isosurface, so the structures appear to meander rather than convect at their laboratory speeds.
A short paper describing these results can be found here.
Description of Flow Geometry and Velocity Profiles
More views of streaks at N=11.
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Last update: March 1, 2002 (pff)