# Turbulent Flow over Confined Backward-Facing Step: PIV vs. DNS
###### Abstract  
Particle Image Velocimetry measurements of the liquid velocity fields in the flow over the backward-facing step were performed in the same flow configuration as in the existing Direct Numerical Simulation (DNS). The experiment and the simulation were performed in an identical cross-section geometry with step expansion rate 2.25 and the square shape of the outlet duct at the Reynolds number in an inlet part of the section 7100. The experiment was performed in transparent test section, 1.2 m long, with \(20\times 45\) mm\({}^{2}\) cross-section upstream and \(45\times 45\) mm\({}^{2}\) downstream, while a domain that was three times shorter was used in the DNS. A 2D-2C PIV system with a single high-speed camera and a pulse laser was used for a series of two-dimensional measurements of the velocity field at several cross-sections from two different perspectives. Variables analyzed in the experiment are time-averaged fluid velocities, velocity RMS fluctuations and two components of the Reynolds stress tensor. The key novelty is the comparison of two very accurate approaches, PIV and DNS, in the same cross-section geometry. Comparison of the similarities, and especially the differences between the two approaches, elucidates uncertainties of both studies and answers the question on what kind of agreement is expected when two very accurate approaches are compared. flow separation; direct numerical simulation; particle image velocimetry; uncertainty 22 September 2a consequence of BFS confinement in a spanwise direction. The temperature field of [PERSON]'s study is a passive scalar; thus, the velocity fields allow comparison with adiabatic experiments that can be performed with water. The BFS geometry in SESAME project was considered in a liquid metal experiment [2] at the same Reynolds number 7100, which included the heated section at the same position as in DNS of [PERSON] et al. [2]. Such low Reynolds numbers are relevant for liquid metal reactors [3], where low flow rates are foreseen due to the superb heat transfer characteristics of the coolants. Since the final geometry and the Reynolds number of liquid metal experiment [4] were slightly different than in DNS, the second BFS experiment related to the SESAME project was developed: adiabatic experiment performed with water and without heating. This experiment is described in the present paper. When the spanwise dimension of the BFS geometry is at least ten step heights wide, the flow behind the step can be treated as two-dimensional in time-averaged sense [5], which means that it contains another simplification comparing to the general sudden expansion geometry. Two-dimensional time-averaged BFS flows have been studied experimentally in many previous publications. Some of the early publications were written by [PERSON] and [PERSON] [6] and [PERSON] and [PERSON] [7]. More recent studies are those of [PERSON] et al. [8], [PERSON] and [PERSON] [9], and [PERSON] et al. [10]. Relatively new studies were recently performed by [PERSON] and [PERSON] [11], [PERSON] [12], [PERSON] et al. [13], [PERSON] and [PERSON] [14], and [PERSON] et al. [15]. Many of the previous studies were focused on the heat transfer behind the BFS expansion that might include natural or mixed convection cases [16]. Especially in nuclear engineering, studies are focused on heat transfer of liquid metal flows [12; 17]. An extensive review of separated flows, including the BFS flows, has been recently performed by [PERSON] [1]. The main analysis in previous paper was aimed at assessing the effect of external turbulence on the dynamics and thermal characteristics of the separated flow behind different obstacles and it has been shown that turbulence of the incoming flow has a more noticeable effect on separated flows than on the boundary layers. At the same time, the turbulence upstream the BFS is not correlated with high turbulence induced in the shear layer of the separated flow [18; 19; 20].