# Turbulent Flow over Confined Backward-Facing Step: PIV vs. DNS
## 5 Results and Discussion
### Recirculation Zone, Planes y = Const  
Results of the measurements in planes y = const. are shown in Figures 9 and 10. Complete fields of streamwise velocity component u are shown in Figure 9, although, as already mentioned, the results in the near-wall regions of around 0.4 dimensionless units on each side should be ignored. Thus, the color fields and contours of the measurements at z \(>\) 1.4 and z \(<-\)1.4 should be disregarded in Figure 9. The profiles shown in Figure 10 have this part of the measurements already removed.  
The top four measurements in Figure 9 at y = 0.8, 0, \(-\)0.8, \(-\)1.6 are made with comparable accuracy, while the bottom image in the plane y = \(-\)1.92 is close to the wall and less accurate. Plane position 1 mm away from the wall is in the region with large gradients and is consecutively very sensitive to laser sheet positioning. Thus, reasonable accuracy (quantification in Section 5.5) is available only for the first-order statistics, i.e., mean velocities.  
Discontinuities in the merged images of each y = const. in Figure 9 are more pronounced than the discontinuities in the z = const. cases in Figure 5. This is expected because the flow field gradients in the y direction are larger than in the z direction. Consequently, measurements in y = const. planes are more sensitive to the uncertainties in the camera positions, scaling, and laser sheet position.  
The y= \(-\)1.92 measurement shown in the bottom image of Figure 9 was performed in order to verify the length and the shape of the recirculation zone downstream the step. Despite the reduced accuracy of this measurement, taken 1 mm from the wall, with a laser sheet thickness of 1 mm, the results show good agreement with the recirculation zone length obtained in the DNS. Recirculation zone length is marked with u = 0.00 contour at a position of around x = 15.  
The results in Figure 9 show another view on the slight asymmetry of the flow in the experiment, which was not observed in the DNS results. Despite the asymmetry, reasonable quantitative agreement with simulation is achieved. The seemingly large differences in the contours of the streamwise velocity u in Figure 9 are shown from a different aspect in Figure 10.  
Figure 10 gives qualitative comparison of the measured and computed profiles of turbulence statistics in selected line segments on the y = 0.8 plane (case 'y + 0.8'). Two solid curves on each graph are actually plots of the original measurement and the same measurement mirrored over the z = 0 plane. The symmetry of the measurements at z > 0 and z < 0 was used also to estimate the experimental uncertainty, which is plotted with a single uncertainty bar for each measurement and explained in Section 5.5.  
Position of the line segments, where the profiles are plotted, is shown in the top-right sketch of Figure 10. Similar graphs with similar agreement can be obtained from the database at other y = const. planes, except the plane y = \(-\)1.92, where the accuracy of the measurements is deteriorated. Seemingly, larger differences are observed in the profiles  
Figure 10: Measurement ’y + 0.8’, profiles at x = 3.2, 9.6, 16. Variables: u, w, u\({}_{\text{RMS}}\), w\({}_{\text{RMS}}\), and u’w’. Solid lines: original measurements and measurements mirrored over z = 0. DNS—dotted lines. Uncertainties are given in a single point per profile.  
of the w direction mean velocity; however, one should notice that the magnitudes of this velocity are very low: just around 1% of the mean flow velocity u and almost 10 times lower than the magnitude of the v velocity component shown in Figure 8. Agreement of the velocity RMS fluctuations is similar as the one observed in measurements in z = const. planes in Figure 8 and the agreement of the Reynolds stress component u'w' is slightly worse than the one observed in Figure 8 for the component u'v'.