Boštjan Zajec, Marko Matkovič, Nejc Kosanič, Jure Oder, Blaž Mikuž, Jan Kren and Iztok Tiselj from the Reactor Engineering Division at Jožef Stefan Institute published the article “Turbulent Flow over Confined Backward-Facing Step: PIV vs. DNS” in the Applied Sciences.

The paper compares the existing Direct Numerical Simulation (DNS) of the the flow over the backward-facing step with newly performed Particle Image Velocimetry measurements in the same flow configuration exhibiting a three-dimensional nature. This is the first time that our group performed PIV measurements while we gained many experiences with high-fidelity simulations in the last decades or so.

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. The experiment was performed in transparent test section, 1.2 m long, with 20 × 45 mm² cross-section upstream and 45 × 45 mm² downstream, while a three-times shorter domain was used in the DNS. Reynolds number is 7100 based on the hydraulic diameter of inlet part of the section. 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 analysed in the experiment are time-averaged fluid velocities, velocity RMS fluctuations and two components of the Reynolds stress tensor. Comparison of the similarities, and especially the differences between the two approaches, PIV and DNS, elucidates uncertainties of both studies and answers the question on what kind of agreement is expected when two very accurate approaches are compared.

Figure 1: Left: Test loop configuration. Top-right: Schematic of the test-section, coordinate system and measurement planes z = 0 (green) and y = 0 (blue). In parentheses: dimensionless lengths. X-length of the measurement planes is around 25 cm. Bottom-right: raw image for PIV analysis in plane z = 0.

Figure 2: Time-averaged velocity field of in plane z = 0: components u and v (top pair), u_RMS,v_RMS, u’v’ (bottom triplet). PIV experiment: color-scale and blue contours. DNS—black contours.