Abstract
On the axisymmetric water jet experimental apparatus, the flow field structures in entrainment boundary layers are measured using Single Frame Long Exposure image method, in the range of Reynolds number (Re) 1849~2509. It is found that engulfing and nibbling entrainment model occur intermittently with time, in the region of L=2~3.5d streamwise and H=1~1.25d radial direction. It concludes that the occurrence probability of engulfing increases with Reynolds number when Re>1915, the influence of Reynolds number on the occurrence probability of this structures decreases when Re>2311; the occurrence frequency of this coherent structures obtained by fast Fourier transform is between 10 and 19Hz; special vortex structures were observed in the flow field during the occurrence of engulfing. The jet flow field is measured using Moving Single Frame Long Exposure image method in Lagrangian coordinate system, and it is found that the vortex structures generally exist near the interface of turbulent regions and non-turbulent regions.
Keywords: Entrainment layer, Jet, Move single frame long Exposure, Vortex structures, Single frame single exposure.
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