The structure of the velocity field in a confined flow driven by an array of opposed jets

Abstract

We investigate the confined flow in a new turbulence box configuration. Fluid is injected through two sets of 16 vertically opposed jets and outflows through two top/bottom porous planes. The resulting flow is generated by pairs of opposed round jets with backflow and their subsequent interactions. The research issue being addressed here is that of the dependence of the velocity field structure on two parameters: the injection Reynolds number based on jet diameter Reinj, which is varied between 6000 and 28000, and the flow geometry. The latter issue is addressed by investigating two kinds of flow geometries: (i) recirculating opposed jets (ROJ), for which the distance among two consecutive jet nozzles is 2.4 diameters and the nozzle-to-nozzle distance among each two opposed jets is 6 diameters, and (ii) simple opposed jets (SOJ), for which the distance among two consecutive jet nozzles is 4 diameters and the nozzle-to-nozzle distance among each two opposed jets is 10 diameters. The instantaneous aspect of the flow field is dominated by vortical structures and it is strongly dependent on the flow geometry. For both flow geometries, no coherence between each two consecutive pairs of jets is observed. All the statistics (dimensionless profiles of mean velocities and kinetic energy, derived quantities, and inner scales) do not depend on the Reynolds number and they are only the result of the flow geometry. The ROJ geometry leads to a flow which is characterized by strong interactions between opposed and neighboring jets which lead to both top-bottom and left-right instabilities in the central region. This leads to a strong energy and enstrophy injection, which imposes its signature on the two-dimensional kinetic energy spectra regime, characterized by a kxy−3 scaling, associated to the vortical structures present in the flow. The classical kxy−5/3 regime is very poorly represented, most likely because it is supposed to be present at scales smaller than the particle image velocimetry cut-off. Only a small volume of the chamber is characterized by local homogeneity and isotropy. The SOJ geometry is associated to flow which exhibits large regions that are locally homogeneous and isotropic. However, the top-bottom instabilities remain present. The mean pressure increase in the center of the chamber is of maximum 20% of the dynamic pressure at the inlet and decreases with increasing Reynolds number. The two-dimensional kinetic energy spectra clearly exhibit the classical kxy−5/3 scaling range; this is the signature of pure turbulence that finds enough space to develop. A fine-scale analysis is performed for both flow geometries in a particular point where the return flow is locally homogeneous, isotropic, and Gaussian. After an estimation of the mean energy dissipation rate according to various methods, we find that the Reynolds number based on the Taylor microscale scales as Reinj1/2, as in classical flows, and attains a maximum value of 350.