Asymmetric Diffuser:
 

 

  

 
Obi et al [1] obtained experimental data for an asymmetric plane diffuser at a Reynolds number of 21,200 based on inflow centerline velocity and channel height, h. Figure 1 shows the overall geometry including typical velocity contours. The diffuser is 21h long and the expansion angle of 10 degrees gives an overall expansion ratio of 4.7. Corners are rounded at both ends of the diffuser with arc radius of 4.3h.

Velocity and normal stress profiles at the inlet are provided, (=15.9 m/s) and the experimental data include velocity components and Reynolds stresses at several streamwise locations. The computations were done on a 45,700 size mesh with y0.1 on both walls and plenty of grid points to capture the shear layers. Atmospheric conditions were used, with a constant temperature of 293K. Turbulence models used:  k-e, k-ℓ and SST. Figure 2 shows the location and extent of the separation bubble on the diffuser. Note that the separation and reattachment points do not coincide with any geometrical features.

While all three models predict about the same bubble, differences in the details are seen in figures 3 (a,b), showing velocity profiles at the approximate locations of incipient separation and reattachment.

The k-e model predicts the separation profile accurately whereas the SST closure exhibits early separation and poor agreement with the  data. The k-ℓ model’s prediction is almost as good as that of k-e. On the other hand the latter model predicts reattachment too early while the SST and k-ℓ closures capture it accurately. The k-ℓ model yields the best reattachment profile overall. 

Reference

 [1] S. Obi, K. Aoki, and S. Masuda, “Experimental and computational study of turbulent separating flow in an asymmetric plane diffuser,” 9th Symposium on Turbulent Shear Flows, Kyoto, Japan, 1993
              
Asymmetric diffuser velocity profiles at separation (L) and reattachment (R) locations