Mechanics of the Turbulent-Nonturbulent Interface of a Jet

Abstract

We report the results of an experimental investigation of the mechanics and transport processes at the bounding interface between the turbulent and nonturbulent regions of flow in a turbulent jet, which shows the existence of a finite jump in the tangential velocity at the interface. This is associated with small-scale eddying motion at the outward propagating interface (nibbling) by which irrotational fluid becomes turbulent, and this implies that large-scale engulfment is not the dominant entrainment process. Interpretation of the jump as a singular structure yields an essential and significant contribution to the mean shear in the jet mixing region. Finally, our observations provide a justification for Prandtl's original hypothesis of a constant eddy viscosity in the nonturbulent outer jet region.

Detection of turbulent/non-turbulent interface for an axisymmetric turbulent jet: evaluation of known criteria and proposal of a new criterion

“…Note that the Schmidt number of the dye is much greater than unity in these studies. One important observation of their study is that nibbling by the small eddies contributes substantially to the total fluid entrained, as suggested by the earlier studies of Mathew and Basu (2002), Agrawal andPrasad (2004), andWesterweel et al (2005).…”

“…Note that this criterion has been employed earlier, for example by Prasad and Sreenivasan (1989) and Westerweel et al (2002Westerweel et al ( , 2005. Whereas the earlier results are experimental, the results presented in this section are numerical.…”

“…A quantitative comparison of results from different criteria is difficult to find in the literature (Holzner et al 2006). The results are compared against those of Westerweel et al (2005). This comparison will be useful while evaluating the new criteria proposed in Sect.…”

“…Note that such a criterion can be used with DNS data because of the availability of all three components of vorticity, which is usually not the case with experimental data. Westerweel et al (2002Westerweel et al ( , 2005 and Holzner et al (2006) employed a novel method of performing PIV and flow visualization (LIF) concurrently on an axisymmetric turbulent-free jet and oscillating grid, respectively, to overcome this difficulty. While a threshold in concentration provided by the LIF data was used for interface detection, the PIV data were used to obtain conditionally averaged quantities across the interface.…”

High-frequency forcing of a turbulent axisymmetric wake

“…The superlayer is characterised by an approximate balance of turbulent advection of energy and its viscous diffusion, the equivalent physical processes having been described, respectively, as "engulfment" (Turner 1986) and "nibbling" (Corrsin & Kistler 1955). Modern experimental techniques have enabled study of the relative importance of these fundamental processes involved in vorticity transport (Westerweel et al 2005;Philip & Marusic 2012): they are key in controlling the growth rates of free shear layers and therefore the reactive forces on the solid bodies producing them. Control of entrainment can therefore make many fluid-based systems more efficient: for example, its reduction in the wake of a bluff body will reduce the pressure (or "form") drag of that body.…”