Parametric study of multi-armed jets

The paper presents the results of numerical simulations of excited incompressible jets with the transport of a passive scalar field. A forcing is obtained through an axial and helical velocity disturbance defined as sinusoidal waves changing in time and travelling in azimuthal direction along the jet border. It includes three control parameters, which determine its strength and temporal behaviour, an amplitude and two characteristic frequencies. The research is performed with the use of direct numerical simulations (DNS) and high-order numerical code based on the compact difference approximation and projection method for the pressure-velocity coupling. The obtained results show that when the axial and helical forcing terms are applied separately, their impact on the flow and passive scalar field is very limited and only quantitative. The situation changes drastically when both forcing terms are applied together. It is found that by a proper choice of the ratio of their frequencies, various type of multi-armed jets can be created, e.g., with 5, 8, or 13 branches. It is observed that the angle at which the branches disjoin from the main jet and an axial location where this phenomenon starts are related to the axial forcing frequency and also to the Reynolds number, to some extent. It is shown that the mixing efficiency analyzed through an evolution of the scalar field or entrainment can be controlled in a smooth way and in a relatively wide range.