Mixing of swirling inclined dense jets – A numerical study

Abstract

Recent experimental investigations demonstrated the possibility that the addition of swirls at the discharge outlet can lower the terminal rise height of inclined dense jets and increase the dilution at the return point, which can potentially influence the outfall design in coastal waters. In the present study, we further examined the effect of swirls on the mixing characteristics of 45 degree inclined dense jets using numerical simulations with two different approaches. The first approach was the Large Eddy Simulation (LES) with the dynamic Smargorinsky Sub-grid Model, and the second one was the Reynolds-Averaged Navier-Stokes (RANS) with the standard k − ε turbulence closure. The comparison showed that the trajectories of the non-swirling inclined dense jet predicted by LES were closer to the experimental results, while RANS underpredicted the centreline peak height. This was consistent with the previous findings and can be attributed to the fact that the turbulence characteristics of non-swirling dense jets were highly anisotropic. Both approaches however underestimated the additional spreading in the lower half of the inclined dense jet due to the convective mixing by the buoyancy induced instability, and also the dilution in general. With swirls, the RANS predictions matched the jet trajectory better compared to LES, which was probably because the turbulence anisotropy was reduced and the distributions were more axisymmetric. The dilutions of swirling jets remained underestimated by both approaches, until the Swirl Number became sufficiently large at 0.33. Finally, strong swirls up to twice the highest value in the experiments were attempted in the numerical simulations to explore the possibility of jet breakup and disintegration at the outlet. It was found that the high swirl intensity did not lead to significant changes beyond what were observed at G = 0.33. The turbulence kinetic energy spectrum of the swirling inclined dense jet was also analysed along the trajectory based on the LES results. The spectral analysis confirmed that the effects of swirls were different depending on the magnitude, with strong swirls producing turbulence energy at the range of frequencies just before the inertial range probably due to eddy break up around the vortical layer, while extremely strong swirls increased the turbulence energy production range overall with the additional formation of large eddies.