Mixing by side jets’’ in strongly forced and self‐excited round jets

The topic of this paper is an experimental investigation of entrainment and mixing in transitional, i.e., initially laminar, axisymmetric jets that are either self‐excited or strongly forced at Strouhal numbers of around 0.4, where self‐excitation is achieved by heating the jet. In the potential core region such jets have been found to spread in a nonaxisymmetric fashion at unusually large rates, a phenomenon which is due to the formation of so‐called ‘‘side jets’’ [Phys. Fluids A 1, 446 (1989)]. It has been speculated that the ejection of jet fluid into side jets, which are directed radially away from the jet axis, is related to an early appearance of lobes on the primary jet vortex rings. Quantitative LDA measurements in side jets suggest that their generation not only involves lobes in the primary vortex rings, but also pairs of longitudinal vortices in the braid regions between the vortex rings. Furthermore, it is shown that the velocity of side jets is comparable in forced cold jets and self‐excited hot jets, if the primary vortex rings are of equal strength.