Nonequilibrium Scaling of the Turbulent-Nonturbulent Interface Speed in Planar Jets

The length-scale which, combined with the fluid's kinematic viscosity ν, defines average the local speed of the turbulent/non-turbulent interface (TNTI) has been postulated to be the smallest (Kolmogorov) length-scale η of the turbulence (Corrsin & Kistler 1955). This is indeed the case when the turbulence dissipation rate obeys the Kolmogorov equilibrium cascade scaling, but in the presence of the non-equilibrium turbulence dissipation scaling the average local TNTI speed scales as ν/λ, instead of ν/η, where λ is the Taylor length. We derive this theoretically and confirm it experimentally in the range of distances between 20 and 50 nozzle widths of a turbulent planar jet