Large-scale eddies and their role in entrainment in turbulent jets and wakes

Abstract

A large-scale or energy-containing eddy model of turbulent axisymmetric jets and wakes is developed, wherein eddies are randomly distributed in the azimuthal and convecting in the axial directions. The mean velocities and second order statistics obtained from the models agree well with the various available experimental data. There is an average inflow into the turbulent jet at the boundary, which is virtually non-existent for wakes. These eddy contributions are used to reconsider the entrainment process, which has to date been largely conceived as either an “engulfment” or “nibbling” process. Here we suggest that entrainment in turbulent jets be viewed as a three-part-process wherein non-turbulent fluid is “induced” and “engulfed” into the turbulent core due to large-scale eddies, which is converted into turbulent motion by the action of small-scale eddies via “nibbling.” However, in wakes there is no induced flow and the primary cause of entrainment is envisaged to be large-scale “engulfment” combined with small-scale “nibbling.”