# Turbulent Flow over Confined Backward-Facing Step: PIV vs. DNS
###### Abstract
Moreover, coherent vortex structures have been proved to exist in the separation-reatachment process downstream a backward-facing step flow with a Reynolds number of 4400 and 9000 [21]. They are well-organized in the recirculation regions with forming, developing, shedding, and redeveloping in the shear layer. Understanding the flow behavior is necessary for accurate modeling of such flows. For a detailed review of the numerical studies of BFS geometry, the reader is referred to the paper by [PERSON] et al. [2], where highly accurate numerical simulations performed with DNS and LES approaches are summarized. Here, we mention only the earliest DNS study in BFS geometry by [PERSON] et al. [22], while several recent publications are focused on more special configurations [2; 15; 23]. In this paper, we emphasize the studies which are relevant for our setup and deal with 3D time-averaged flows, where spanwise dimension of the duct with BFS is comparable with the step height. The shape of the recirculation vortices downstream the step has a complex 3D geometry, which presents an additional challenge for measurements and simulations. [PERSON] et al. [24] performed experiments on the backward-facing step flow of air through a rectangular duct with an aspect ratio of 3.3 and expansion ratio of 2.0 at a Reynolds number of 10,000. They found that the reattachment length in 3D cases is smaller than in 2D. Velocities were measured with Laser-Doppler velocimetry and hot wire anemometry. [PERSON] et al. [25] performed LDV measurements on air flowing over a BFS with an aspect ratio of 2 over a range of Reynolds numbers between 100-8000 with a special focus on reversed flow regions. [PERSON] et al. [26] performed PIV measurements on water flowing in a square duct with a BFS expansion ratio of 1.25 with Reynolds numbers between 12,000 and 55,000. They have confirmed the reattachment lengths that are smaller for 3D flows than in 2D. The key parameter of the BFS geometry is its expansion ratio, i.e., the ratio of channel cross sections downstream and upstream of the backward step. Several experimental and numerical studies in BFS geometries have been performed with different Reynolds numbers, channel shapes and expansion ratios. Most of them at expansion ratios close to 1, while the geometries with ratios around 2 are rather scarce [13; 15; 27]. The DNS data of [PERSON] et al. [2] served as a starting point for experimental part of the research described in this paper. We have not found the specific cases of the rectangular channel with expansion ratio of around 2 and with moderate Reynolds numbers, which can be accurately described with DNS (or LES) studies. Since such a numerical study was performed by [PERSON] et al. [2], we decided to build a test-section based on this simulation. The first preliminary experimental results with velocities measured in a single plane were presented by [PERSON] et al. [28] and represent the first PIV measurements performed in our laboratory. The present PIV results are obtained in a new test section at Reynolds number 7100, which corresponds to the Reynolds number of the simulation. While DNS results are often treated as perfect and are assumed to contain no errors except the statistical uncertainty of the realized turbulent flow, even DNS data are not entirely immune to systematic errors, which can only be exposed with accurate measurements. The new PIV study of the 3D BFS turbulence presents an added value in the field of turbulence studies. Comparison of PIV and DNS presents a mutual verification of both approaches and demonstrates their accuracy and their uncertainties. These data will be applicable for the assessment and development of LES and RANS models [29], which do not solve the smallest turbulent scales explicitly, but consider them with (semi)empirical turbulence models.