Direct simulations of the transitional regime of a circular jet

Accurate numerical simulations of temporal evolving round jets at a low Reynolds number have revealed the same features observed in experiments and vortex filament simulations. The initial layer of azimuthal vorticity, by the Kelvin–Helmholtz instability, produces vortex rings undergoing successive pairings leading to larger rings. Axisymmetric simulations have shown that the initial roll‐up is not affected by the Reynolds number, consequently insights of practical importance on the transitional regime, can be obtained from low Reynolds number simulations affordable by numerics. The 3‐D simulations displayed the formation of longitudinal structures, and their role in the spreading of the jet is described. Streamwise rib vortices develop in the braid region and these vortices are responsible for the creation of small scales, premonitory of turbulence. In analogy to the plane mixing layer, the pairing reduces the growth of longitudinal and radial vorticity components and triggers the transition to turbulence. Finally comparisons between azimuthal vorticity and passive scalar surfaces have revealed that the latter collects in fat structures while the vorticity is found in thin regions where it is augmented by stretching.