A note on the effect of the separation wall in the initial mixing of coaxial jets

Experiments are carried out to characterize the mixing field of two coaxial jet configurations, having, respectively, a thick and a sharp inner duct wall. The influence of the separation wall thickness on the initial development of the "mixing transition" of the two streams is analysed as a function of the jet velocity ratio through measurements with hot- and cold-wire anemometry and by using temperature as a passive scalar. To study the mixing at the near-exit region, a new thickness measure based on the mean scalar concentration is introduced. It is shown that the presence of a sufficiently thick wall significantly increases the interpenetration between the two streams, effectively enhancing the mixing process. However, this is observed only for nearly unitary velocity ratios, which correspond to the existence of a regular alternate vortex shedding from the two sides of the inner duct wall. Conversely, for small and large velocity ratios, the difference in mixing between the two geometries greatly decreases and becomes of the order of the experimental accuracy.