Color contours of q/U_o show that the turbulence velocity scale, q, increases in the shear layers at the edges of the separation reaching levels above 0.3 of the inlet velocity. This turbulence then diffuses into the blade wake where the mean velocity is very low. The result is turbulence velocity variations which are bigger than the mean velocity. (Click on the picture to see it larger.)

The following picture shows turbulence intensity as contours of q over the magnitude of the local velocity. This shows not only the relatively high turbulence in the separation and wake, but also the effect of mean pressure on turbulence intensity. On the pressure side of the blade, the high pressure gives low free stream velocity so that the pressure side boundary layer is initially developing in a region with turbulence intensities in excess of 30%, i.e., in a highly unsteady flow. On the suction side, the low pressure gives high free stream velocity and the turbulence intensity at the edge of the boundary layer is reduced compared to the inlet value.

Pressure side/leading edge. A close-up of the leading edge/adverse pressure gradient region on the pressure side shows the flow stagnation, the initial laminar boundary layer in the favorable (p falling) pressure gradient region, then, in the adverse (p rising) pressure gradient region, flow separation followed by turbulent recovery.

All this is occurring in a region of very high turbulence intensity, so that when the turbulence velocity variations are imposed on the mean velocity vectors, they dominate in the separation region.
Note that the directions and relative magnitudes of u_P1 and u_P2 are consistent with DNS channel and boundary layer calculations. In the outer part of the boundary layer the larger principal velocity is pointed more towards the wall than the mean velocity vector. This is consistent with a shear stress (in the streamwise/normal coordinate system) which gives positive turbulence production. As the wall is approached the larger principal velocity becomes parallel to the mean velocity (and the wall). This is the expected streaky behavior of the near-wall turbulence and the mechanism by which shear stress varies as y_normal³.

Suction side separation. On the suction side of the blade, the flow separates after the minimum pressure location. The turbulence velocities outside the separation are smaller compared to the mean velocity, because the mean velocity has increased. Note that through the boundary layer, the direction of largest principal velocity variation is nearly aligned with the flow direction.

This can be seen better in the picture below where the largest component of u_Pi is shown in blue and the smaller ones in red. Also shown as crosses are the principal directions of the mean velocity strain-rate tensor. With negative values (flow compression) shown in light blue, this is the direction that the Boussinesq model for Reynolds stresses (used in 2-equation turbulence models) gives for the largest component of u_Pi.
With the color coding used in the picture, turbulence production is positive if the blue leg of u_Pi aligns with the blue leg of the strain rate to within 45 degrees. (Bousinnesq models, with perfect alignment always give P>=0.) In the Reynolds stress model the orientation of the turbulence is affected not only by strain rate but also by convection, diffusion and vorticity. The net result for this convex suction surface is angle differences which appear to be about 45 degrees in the boundary layer. The underlying color fill of production/dissipation shows that production is positive in this region (so the angles are < 45 degrees) with production/dissipation increasing to above 5 in the shear layer at the edge of the separation. At this Reynolds number, however, the corresponding increased turbulence is insufficient to mix out the separation and give turbulent reattachment.

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