Effect of external turbulence on the entrainment and interface of axisymmetric jets

The effect of zero-mean-flow background turbulence on the topology of the scalar turbulent/turbulent interface (TTI) of a round jet is investigated and compared to the traditional turbulent/non-turbulent interface (TNTI) using planar laser-induced fluorescence. The intensity of the external turbulence, generated by a random jet array (RJA), is systematically varied by changing the distance between the RJA sheet and the jet exit. The TTI and TNTI outlines are detected by applying a low-magnitude threshold to high-Schmidt number scalar concentration fields. The thickness of the interfacial layer, as interpreted from the gradient of the conditionally averaged scalar profiles along the interface-normal coordinates, is an increasing function of the background turbulence intensity. Compared to the quiescent ambient, background turbulence leads to a ‘rougher’ interface, as is evident by its higher tortuosity, higher fractal exponent, and the lowered occurrence of zero-curvature surface elements. Lastly, the spatially averaged scalar skewness profiles indicate a net reduction in the entrainment rate into the jet in the presence of external turbulence.