Influence of wave height on round jet in regular waves: Direct numerical simulations

Abstract

The influence of wave height on the dynamics of a turbulent round jet is analyzed using direct numerical simulation. The vortex dynamics associated with the interaction of the jet with waves of different heights is compared using the swirling strength criterion and the contours of vorticity. An analysis of the vortex roll-up frequency reveals that it increases with increasing wave height. With an increase in the wave height, the deflection of the jet elevates, and therefore, the span of the vortex rings in the near-field reduces in the direction of wave propagation. Further, an analysis of the braid region reveals that the number of lateral jets reduces to three with increasing wave height despite the number of counter-rotating vortex pairs remaining as four. The longer time evolution suggests that as the jet moves from the crest to the zero down-crossing, the fluid in the far-field gets detached from the main jet flow for the stronger waves. The evaluation of the preferred mode reveals that it remains helical for all wave heights. The proper orthogonal decomposition (POD) establishes that the highest energy is captured by the modes of the highest wave height. Also, as the wave height increases, a fewer number of modes capture 90% of the energy. Further, contours of POD modes represent the jet oscillations as well as the kinetic energy gain in the shear layer and the far-field. The time-averaged quantities quantitatively demonstrate an increase in mixing and entrainment in the jet as the wave height increases.