A structural model for turbulent exchange in boundary layers

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Abstract
In this paper a model is presented that incorporates characteristic features of the turbulent structures as revealed by recent experimental observations. The principal characteristic features are the occurrence of periodicity and intermittency, not only at the edge of the boundary layer, but also close to the wall.By an averaging procedure, equations are derived for the large-scale part of the turbulent motion. The unknown terms, representing the small-scale turbulent stress, are assumed to be zero except in the so-called burst regions which occupy only a small fraction of the total flow field and arise from local instability of the large-scale flow field. In the model distinction is made between a thin layer near the wall where viscous forces play an important role and the remaining part of the boundary layer where inviscid equations are valid. The momentum transport takes place in three different stages with different mechanisms. First of all the fluid in the wall region is retarded by viscous forces and collected in long narrow regions (streaks). After this a rapid exchange takes place in the burst regions where the low-momentum fluid is ejected into the outer region. Finally the large-scale structures in the outer region take over the transport.It turns out that the transport properties of a turbulent boundary layer can be calculated reasonably well with this deterministic model. It can be concluded that the coherent part of the turbulent motion is very important in the transport process.