Reynolds number effects in the near-field of a turbulent square jet

Abstract

High-resolution particle image velocimetry (PIV) was used to study the flow characteristics in the near-field of a turbulent square water jet issuing from a smooth contraction nozzle. Mean velocity and turbulence statistics are investigated over a range of jet Reynolds number ReD (based on jet exit velocity and equivalent nozzle diameter) varying from 10,000 to 41,400. The influence of ReD on the shear layer formation in terms of momentum thickness, Taylor length scale and the evolution of the turbulent/non-turbulent interface are also studied. The velocity measurements reveal that the jet in the near-field is dependent on Reynolds number at the lower end of the range (ReD=10,000) despite the fact that all exit jet profiles closely approximate a “top-hat” shape. It is shown that the jet shear layer grows faster at lower Reynolds number which, combined with an increase of the spanwise turbulence component (vrms) along the jet centerline, suggest rapid axis switching. Much weaker Reynolds number dependence of the mean velocity, turbulence intensities, momentum thickness and jet centerline anisotropy (urms/vrms) was found for square jets at ReD>104. The Taylor length scale calculated along the jet centerline decreases with increasing Reynolds number and asymptotes to a constant value for X/D>4. The turbulent/non-turbulent (T/NT) regions of the jet have been identified using the velocity criteria proposed in the literature. Evolution of the T/NT interface and the effect of ReD on the conditionally averaged streamwise velocity and vorticity are also investigated in the near-field of the jet. No jump of the conditionally averaged streamwise velocity and vorticity profiles was noted in the near-field of the square jet.