Mixing in coaxial jets

The stirring and mixing properties of one-phase coaxial jets, with large outer (annular) to inner velocity ratio r(mu) = mu (2)/mu (1) are investigated. Mixing is contemplated according to its geometrical, statistical and spectral facets with particular attention paid to determining the relevant timescales of the evolution of, for example, the interface area generation between the streams, the emergence of its scale-dependent (fractal) properties and of the mixture composition after the mixing transition. The two key quantities are the vorticity thickness of the outer, fast stream velocity profile which determines the primary shear instability wavelength and the initial size of the lamellar structures peeled-off from the slow jet, and the elongation rate gamma = (mu (2) - ur)le constructed with the velocity difference between the streams and the gap thickness of the annular jet. The kinetics of evolution of the interface corrugations, and the rate at which the mixture evolves from the initial segregation towards uniformity is prescribed by gamma (-1). The mixing time t(s), that is the time needed to bring the initial scalar lamellae down to a transverse size where molecular diffusion becomes effective, and the corresponding dissipation scale s(t(s)) are t(s) similar to Sc(1/5)/gamma , s(t(s)) similar to eRe(-1/2)Sc(-2/5), where Re and Sc denote the gap Reynolds number and the Schmidt number, respectively. The persistence of the large-scale straining motion is also apparent from the spectra of the scalar fluctuations which exhibit a k(-1) shape on the inertial range of scales.