GLOBAL DENSITY EFFECTS ON THE SELF-PRESERVATION BEHAVIOR OF TURBULENTFREE JETS

An experimental investigation was designed to test the hypothesis that all axisymmetric turbulent free jets become asymptotically independen t of the source conditions and may be described by classical similarit y analysis. Effects of initial conditions were studied by varying jet exit boundary conditions and the global density ratio. The exit veloci ty profile and turbulence level was changed by using both pipe and noz zle flow hardware. Initial density differences were imposed by using t hree gases: helium, methane, and propane. The scalar field (concentrat ion) in the momentum-dominated regime of the far field (10 to 60 jet e xit diameters downstream) of turbulent free jets was characterized usi ng Rayleigh light scattering as the diagnostic. The results show that regardless of the initial conditions axisymmetric turbulent free jets decay at the same rate, spread at the same angle, and both the mean an d r.m.s. values collapse in a form consistent with full self-preservat ion. The means and fluctuations follow a law of full self-preservation in which two virtual origins must be specified. The two displacements are required to account for the effects of a finite source of momentu m and different development of the velocity and mass distributions in the near fields of the jets. The memory of the jet is embodied in thes e two virtual origins.