This paper presents an application of large eddy simulations (LESs) using the filtered structure function model to spatially developing compressible round jets issuing from a perturbed upstream velocity profile close to a top hat. For centreline Mach number M = 0.9 and Reynolds number Re = 3600, the numerical solution compares satisfactorily against a forced-jet direct numerical simulation (DNS) and experimental data, both previously reported. High Reynolds number (Re = 36 000) 'free' jets at Mach 0.7 (case 1) and 1.4 (case 2) are studied. Here, an isotropic random white-noise perturbation is superposed on the upstream velocity. The Mach 0.7 jet has a convective Mach number of 0.35, and is weakly affected by compressibility. In this case, axisymmetric vortex rings are first shed from the nozzle and undergo alternate pairing further downstream. Then turbulence develops. The centreline velocity decay and some other statistical quantities are, in the self-similarity region, in very good agreement with previous incompressible experiments. At Mach 1.4, an impressive upstream reduction of the jet spreading rate is observed, due to an important delay of Kelvin-Helmholtz instability due to compressibility effects. Alternate pairing occurs immediately, and vortices are much more elongated in the flow direction. Further downstream, the jet becomes subsonic, develops into turbulence and spreads out again at a rate comparable with its subsonic counterpart. The potential-core length is increased by 27% from the subsonic to the supersonic case. This is in agreement with several laboratory experiments. Finally, the effects of Mach number increase upon various statistical quantities such as Reynolds stresses and radial lengthscale are studied. Results compare favourably against some experiments and temporal DNSs. From the point of view of Lumley's anisotropy invariant map evaluated on the whole physical domain, the Mach 0.7 jet is dominated by axisymmetric structures and the Mach 1.4 jet by streamwise perturbations.
Direct numerical simulations (DNSs) have been carried out for single and multiple square jets issuing normally into a cross-flow, with the primary aim of studying the flow structures and interaction mechanisms associated with the jet in cross-flow (JICF) problems. The single JICF configuration follows a similar study previously done by Sau et al. (2004, Phys. Rev. E, 69, p. 066302) and the multiple JICF configurations are arranged side-by-side in the spanwise direction with a jet-to-jet adjacent edge distance (H) for the twin-jet case and an additional third jet downstream along the centerline with a jet-to-jet adjacent edge distance (L) for the triple-jet case. Simulations are performed for two twin-jet cases with H=1D,2D, respectively, and for one triple-jet case with H=1D, L=2D, where D is the jet exit width. Flow conditions similar to Sau et al. are considered, i.e., the jet to the cross-flow velocity ratio R=2.5 and the Reynolds number 225, based on the freestream velocity and the jet exit width. For the single jet in cross-flow, the vortical structures from our DNS are in good qualitative agreement with the findings of Sau et al. For the side-by-side twin-jet configuration, results have shown that the merging process of the two initially separated counter-rotating vortex pairs (CRVPs) from each jet hole exit is strongly dependent on the jet-to-jet adjacent edge distance H with earlier merging observed for the case H=1D. Downstream, the flow is dominated by a larger CRVP structure, accompanied by a smaller inner vortex pair. The inner vortex pair is found not to survive in the far-field as it rapidly dissipates before exiting the computational domain. These observations are in good agreement with the experimental findings in the literature. Simulations of the triple-jet in cross-flow case have shown some complicated jet-jet and jet-cross-flow interactions with three vortex pairs observed downstream, significantly different from that seen in the twin-jet cases. The evidence of these flow structures and interaction characteristics could provide a valuable reference database for future in-depth flow physics studies of laboratory experimental and numerical investigations.
Direct numerical simulations have been performed in this study to visualize the flow behavior of single and multiple square jets issuing normally into a cross-flow. Three configurations are considered, a single jet located in the centre of the domain, twin jets in side-by-side (SBS) arrangement in the spanwise direction and triple jets in tandem arrangement with twin jets at the front and a third jet in downstream along the centre line. Simulation uses a jet to cross-flow velocity ratio of 2.5 and the Reynolds number 225, based on the free-stream quantities and the jet width. While the vortical structures predicted from single jet case were in good qualitatively agreement with the findings of other researchers, our results show that the process of merging between two counter-rotating vortex pairs (CRVP) in twin jets configurations is strongly dependent on the jet-to-jet edge distance. Further downstream in the far-field, results from the SBS twin jets show a most dominating larger CRVP accompanied with a smaller inner vortex pair. The observations are in good qualitative agreement with the experimental findings in the literature. The resulting flow structures of triple jets in tandem configuration have revealed, for the first time, more complicated flow interactions between individual jets and cross-flow, providing further insights of complex flow physics and its potential engineering applications.
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