Effects of offset height on the turbulent characteristics of a surface attaching jet

摘要

The effects of offset height ratio on the turbulent characteristics and evolution of coherent structures in a surface attaching jet issuing from a square orifice nozzle are investigated. The experiments were conducted at four offset height ratios, h/d = 1 to 4 and at Reynolds number of 5500 using a particle image velocimetry system. Mean velocities, second to fourth order turbulent moments, as well as the production terms in the transport equations of the Reynolds stresses were used to characterize the flow field. The results showed that reducing the offset height ratio significantly decreases the decay rate in the far field and attenuates the Reynolds stresses and their production terms, especially in the upper shear layer of the jet. However, the surface mean velocity and Reynolds stresses substantially increased beyond the attachment point as offset height ratio decreases. Galilean decomposition, swirling strength and linear stochastic estimation of the velocity fields revealed that the free surface suppresses the growth of the spanwise vortex cores that are generated from the shear layer instability and aligned with the edge of the upper shear layer. Two-point correlations of swirling strength and of velocity fluctuations and proper orthogonal decomposition (POD) were used to examine the influence of offset height ratio on the turbulent structures in the surface attaching jet.