The influence of Reynolds number on a plane jet

Abstract

The present study systematically investigates through experiments the influence of Reynolds number on a plane jet issuing from a radially contoured, rectangular slot nozzle of large aspect ratio. Detailed velocity measurements were performed for a jet exit Reynolds number spanning the range 1500≤Reh≤16 500, where Reh ≡ Ubh/ν with Ub as the momentum-averaged exit mean velocity, ha the slot height, and ν as the kinematic viscosity. Additional centerline measurements were also performed for jets from two different nozzles in the same facility to achieve Reh = 57 500. All measurements were conducted using single hot-wire anemometry to an axial distance (x) of x≤160h. These measurements revealed a significant dependence of the exit and the downstream flows on Reh despite all exit velocity profiles closely approximating a "top-hat" shape. The effect of Reh on both the mean and turbulent fields is substantial for Reh<10 000 but becomes weaker with increasing Reh. The length of the jet's potential core, initial primary-vortex shedding frequency, and far-field rates of decay and spread all depend on Reh. The local Reynolds number, Rey0.5 ≡ 2Ucy0.5/ν, where Uc and y0.5 are the local centerline velocity and half-width, respectively, are found to scale as Rey0.5 ∼ x1/2. It is also shown that, for Reh ≥ 1500, self-preserving relations of both the turbulence dissipation rate (ε) and smallest scale (η), i.e., ε ∼ Reh3(x/h)-5/2 and η∼ Reh-3/4(x/h)5/8, become valid for x/h ≥ 20. © 2008 American Institute of Physics.