Mixing ventilation flow in an enclosure driven by a transitional wall jet

Confined Flows

Underlying Flow Regime 4-20

Evaluation

Comparison of CFD calculations with experiments

The results of the steady RANS CFD simulations are compared with the measurement results from the PIV measurements. Figure 7 shows the dimensionless streamwise velocities (U/U_M) along vertical lines at three locations in the vertical center plane (z/L = 0.5) of the enclosure; x/L = 0.2; x/L = 0.5 and x/L = 0.8, for a slot Reynolds number of ≈ 1,000. Note that the results at x/L = 0.2 (Fig. 7a) are provided for the smaller region of interest, i.e. ROI2, while the other results are provided for the total vertical cross-section, i.e. ROI1.

Figure 7a shows that a top-hat profile is present at x/L = 0.2 in both the measurements and simulation results, and that the agreement between the predictions of the CFD models and the experimental results is very good; only RSM starts deviating below y/L = 0.82. Please note that in contrast to Figure 5, no top-hat profile is visible in Fig. 7c, which is due to the too low measurement resolution for ROI1 to capture the large velocity gradients in the boundary layer and shear layer. Figure 7d shows that the low-Reynolds number version of the k-ε model by Chang et al. (1995) provides the best agreement with the experimental results with respect to the location of maximum velocity, and thus with respect to the location of detachment of the wall jet. The worst overall agreement is present for the RSM model and SST model, especially with respect to the prediction of the location of maximum velocity and of jet detachment.

Figure 7: (a-c) Comparison of PIV results in ROI1 with CFD simulation results for Re ≈ 1,000: (a) U/U_M at x/L = 0.2 (ROI2). (b) Locations of x/L = 0.2, 0.5 and 0.8. (c) U/U_M at x/L = 0.5; (d) U/U_M at x/L = 0.8. Figure modified from van Hooff et al. (2013).

Velocity vector fields in the vertical center plane for Re ≈ 1,000 are shown in Figure 8. The vector fields illustrate the difference in location of jet detachment from the top surface. The detachment point as predicted by the low-Reynolds number version of the k-ε model shows the best agreement with the experimentally obtained location. The location of jet detachment is predicted to be too far upstream by the SST k-ω and RSM models, as indicated in the discussion on Figure 7 above. Figure 8 also depicts the center point of the large recirculation cell, as obtained from the experiments (Fig. 8a), and as obtained from the CFD simulations (Fig. 8b-d). Again, the best agreement is shown by the low-Reynolds k-ε model.

Figure 8: Time-averaged velocity vector fields in the vertical center plane for Re ≈ 1,000. (a) PIV measurements; (b) LR k-ε; (c) SST k-ω; (d) RSM. ● = measured center of the large recirculation zone, ○ = computed center of large recirculation zone. Figure from van Hooff et al. (2013).

Contributed by: T. van Hooff — Eindhoven University of Technology