Turbulent flow downstream of a large solidity perforated plate: near-field characteristics of interacting jets

Abstract

Measurements of streamwise velocity characteristics in a flow downstream of a perforated plate with 96% solidity were obtained using a single-component laser Doppler system. The channel bulk velocity was 1 m s−1, the diameter D of the holes was 5 mm and the Reynolds number of the flow through the holes was around 10 000. The results are presented in terms of profiles of the mean and rms of streamwise velocity, their decay behaviour on the jet centre line, probability density functions (PDFs) of velocity fluctuations, velocity autocorrelation functions and integral time scales. In comparison with the literature data, the mean velocity decay on the centre line was much larger than that for a free jet but smaller than that for a confined jet. There was a mean reverse flow between the jets, and instantaneous reverse flow events were found also on the centre line of the jets. These were quantified based on the PDF of velocities, and at streamwise position x/D = 12 around 5% of the flow events on the centre line of the jet had negative streamwise velocities. Flow visualizations and laser Doppler anemometer measurements showed that this reverse flow is highly turbulent, and suggest that the penetration of a turbulent fluid is larger than that of a laminar or low-turbulence fluid. Finally, the consequences of the instantaneous flow reversals for droplet injection downstream of the perforated plate were analysed, and it was estimated that a small fraction of 10 μm water droplets injected at x/D = 12 could reverse their direction of flight and deposit on the plate.