Direct numerical simulations of vortex breakdown in swirling jets

Vortex breakdown in swirling free jets exhibits a rich variety of flow structures in a compact subdomain of the flow field. Model equations assuming steady and rotationally symmetric motion above a conical stream surface show bi- and multi-stability (Shtern and Hussain 1996 J. Fluid Mech. 309 1-44) of several flow forms, hysteresis and negative thrust phenomena. Direct numerical simulations are used to investigate the development of the conical jet layer and its break up into three-dimensional vortex structures for moderate Reynolds numbers. Three-dimensional flows for swirl numbers at and above the threshold value observed experimentally (Billant et al 1999 J. Fluid Mech. 376 183-219) are considered.