Effects of initial conditions on the development of a plane turbulent jet



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The effects of initial conditions on the development of a plane turbulent air jet have been investigated experimentally. Mean and turbulent velocity distributions in the jet-lip boundary layer, due to different upstream configurations and exit velocities, characterize the initial conditions. An open-circuit jet flow facility was designed and built employing some novel construction techniques. The three upstream configurations are: a) a 44sl contraction nozzle, b) the nozzle with a 12-inch straight extension, and c) fully developed turbulent channel flow, all with the identical exit cross section of 1-1/4" x 55" (aspect ratio 44:1). Hot-wire measurements were performed with jet exit velocities of 50 and 125 fps over the longitudinal length of 40 slot widths. Longitudinal variations of mass, mean momentum, total mean momentum, and kinetic energy fluxes, jet widths and entrainment rates have been determined from the data. The initial conditions show significant effect on the development of mean and turbulent quantities. Longitudinal turbulence intensity grows to much higher values when the initial boundary layer is laminar (than when turbulent) before decaying down to the self-preserving values. The non- dimensional entrainment rate d(Q/Qo)/d(x/H) varies from 0.07 to 0.32 in the near field depending on the initial conditions, but approaches an asymptotic constant value of 0.06 in the self-preserving region. [...]