DIRECT NUMERICAL SIMULATIONS OF ROUND JETS - VORTEX INDUCTION AND SIDE JETS

In this paper, a numerical investigation of three-dimensional round je ts subjected to streamwise and azimuthal perturbations is reported. Th e main objective of the study is to give a consistent scenario for the breaking of rotational symmetry in such flows which may ultimately le ad to the production of intense side jets. In particular it is shown t hat the development of the Widnall instability on the primary vortex r ings and the evolution of the Bernal and Roshko [J. Fluid Mech. 170, 4 99 (1986)] streamwise vortices generated by the instability of the bra id could be deeply intertwined. A comprehensive discussion of the vort ex induction mechanisms leading to the reorientation of the initial vo rticity both in the ring and braid regions and to the deformation of t he rings is presented. The recent analysis by Monkewitz and Pfizenmaie r [Phys. Fluids A 3, 1356 (1991)] is confirmed in the sense that stron g radial ejection of fluid is not directly linked to the deformation o f the vortex rings but rather to the occurrence of coherent streamwise vortex pairs. However, the final relative position of the streamwise vortex pairs with respect to the deformations of the vortex rings diff ers slightly from Monkewitz and Pfizenmaier's proposition.