Near and far field laminar flow structures in an axisymmetric zero-net-mass-flux jet

Numerical simulations have been performed to study the near and far fields of an axisymmetric zero-net-mass flux (ZNMF) jet in the laminar-transitional regime, before the three-dimensional vortex breakdown that promotes transition to turbulence. The main frequencies driving the flow and their associated spatial modes have been calculated using the iterative, multidimensional higher order dynamic mode decomposition algorithm. The results show that the flow is periodic in the near field, but it is quasi-periodic in the far field, where two additional high frequency traveling modes appear. One of them is localized downstream a saddle point (a topology pattern characteristic of the suction phase defining the jet flow oscillations) and seems to be responsible for a shear layer instability. The second high frequency traveling mode, instead, extends further downstream and is associated with the overall wake of the ZNMF jet, which behaves in this sense as continuous jet. Evidence of non-linear coupling between the different spatio-temporal modes is presented in this work, which suggests the existence of a mechanism of energy transition between the periodic modes in the near field and the quasi-periodic modes in the far field previous to transition.