# Turbulent Flow over Confined Backward-Facing Step: PIV vs. DNS
## 6 Conclusions  
We have measured velocity field of water flowing over a confined backward-facing step with the 2D PIV system and compared the fields with the results of the DNS. Experiments and computations were performed at Reynolds number 7100. The measurements were performed in several planes from two different perspectives. The experiments presented in this paper were obtained after three years of training, gathering the experience with the PIV technique, gradual improvements of the test section, test loop, and camera and laser positioning techniques. The experiment has confirmed the predictions of the DNS about the flow behavior: shape of time-averaged vortices downstream the step, velocity fluctuations in the separation region, shape of the reattachment curve, and other properties. Comparison of PIV and DNS presents a mutual verification of both approaches. Uncertainty of the velocity measurements was estimated by comparing different 1D profiles measured in different experimental cases on the same line segments. After the analysis of profiles in around 50 line segments of various orientations, the uncertainty of the streamwise velocity was estimated to be around 3-4% of the bulk velocity in the inlet section of the channel. This is roughly two times higher than in the DNS. The main contributions to the experimental uncertainty stem from the positioning of the camera and corresponding scaling of the results and from the laser sheet position. We have shown that systematic uncertainty in laser sheet position results in negligible additional uncertainty when measurements are performed in the symmetry plane z = 0, while the same laser sheet position uncertainty becomes the dominant source of the total uncertainty, when measurements are performed closer to the walls, where gradients of the variables are large. Improvement of the camera and laser sheet positioning to below 0.1 mm accuracy could produce measurements with accuracy comparable to the DNS, where the systematic measurement errors would be reduced below the statistical uncertainties of the finite time-averaging interval. When such an accuracy is reached, measurements (and DNS) should continue with much longer averaging times, if one would require results with further reduction of statistical uncertainties. In the specific case of the BFS flow at Reynolds number 7100, the required time-averaging interval should be increased from around 5 min to a few dozen minutes to reduce 2-sigma uncertainty of mean velocities below 1%. Although the measurements are estimated to be slightly less accurate than DNS, they are accurate enough to raise some doubts about the sufficient entrance length of our experimental test section and of the DNS inlet flow boundary condition generator. Graphs in Figure 4 show small, but very consistent discrepancies of the measurement and simulation in the inlet part of the test section. Making a point-by-point wise comparison, both datasets are very close to each other's uncertainty bands. However, the measurements (or simulation points) are not distributed randomly and show a very constant behavior over the recorded length interval. This difference could be a topic of our further experimental and numerical analyses. Flow over the confined Backward-Facing Step is one of the simplest computationally and experimentally accessible geometries with the separation and reattachment of turbulent flows that can be used as a starting point for studies of the flows with sudden expansion. Such geometric changes are responsible for significant enhancement of turbulent mixing and heat and mass transfer. The measurements described in the present paper, combined with the existing DNS results by [PERSON] et al. [2], represent a high-fidelity dataset, which can be used for benchmarking of the DNS computations and especially for the validation of other, less accurate LES and RANS turbulence models.