Space structure of the free, unsteady, round, homogeneous jet at low reynolds numbers
Abstract
Three-dimensional direct numerical simulations of unforced, incompressible, free, spatially evolving round jets are used to investigate the onset of the instability at low diametral Reynolds numbers (10 < Re < 500). Compact, coherent structures are identified by means of isosurfaces of the vorticity field. At the upper limit of the investigated range of Reynolds numbers, the present simulations are consistent the widely accepted scenario of the primary and secondary instability of the round jet. The appearance of pairs of axially counter-rotating vortex filaments is found (for the first time, to our knowledge, in unforced, spatial numerical simulations) to characterize the secondary instability. For lower Reynolds numbers, a superposition of symmetry-breaking (helical) modes is shown to prevail downstream of three nozzle diameters