Non-equilibrium Scaling of the Turbulent-Nonturbulent Interface Speed in Planar Jets

The length scale which, combined with the fluid's kinematic viscosity ν, defines the local average speed of the turbulent-nonturbulent interface has been postulated to be the smallest (Kolmogorov) length scale η of the turbulence Corrsin and Kistler, [NACA Report No. 1244, 1955, p. 1033.]. This is indeed the case when the turbulence dissipation rate obeys the Kolmogorov equilibrium cascade scaling, but in the presence of the nonequilibrium turbulence dissipation scaling the average local turbulent-nonturbulent interface 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.