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.
