UFR 4-20 Evaluation

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

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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/UM at x/L = 0.2 (ROI2). (b) Locations of x/L = 0.2, 0.5 and 0.8. (c) U/UM at x/L = 0.5; (d) U/UM 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).

Figure 9 shows the turbulent kinetic energy profiles along two vertical lines, obtained from the PIV measurements in ROI2 and the CFD simulations. Since the 2D PIV measurements only provided the velocity components in two out of three directions (streamwise and vertical), the third component (lateral; w) was unknown. Therefore, to be able to calculate the turbulent kinetic energy, the correlation between the normal stresses in a 2D wall jet, as described by Nielsen (1990), was used:

${w_{RMS}\approx {\sqrt {0.8}}\ u_{RMS}}\qquad (7)$

At x/L = 0.2, the best agreement is obtained by the RSM model, while at x/L = 0.5 the best agreement is obtained with the SST k-ω model. However, it must be noted that all RANS models strongly underpredict the values of k in the inner region (boundary layer) of the wall jet (around y/L = 0.99 at x/L = 0.2 and around y/L = 0.94 at x/L = 0/5). Especially at x/L = 0.2 the RANS models do not seem to be able to capture the local increase in turbulent kinetic energy near the top surface (onset to more turbulent flow). Although all RANS models are low-Re number models, i.e. they solve the flow until the wall, they were not developed to model the transition from laminar to turbulent flow and this can be regarded as one of the main potential reasons for the observed discrepancies between experiments and CFD in the boundary layer.

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

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