The spread of pollutants, especially from local, concentrated sources, can be influenced strongly by the distortion resulting from passages between or around buildings or hills. This chapter describes an integral method for computing such flows and compares the results with measurements on distorted wakes, with particular emphasis on the effects of extremely rapid distortion. Various integral methods are proposed for the prediction of turbulent wake development, utilizing the nearly universal similarity in the defect velocity profile that is always found to prevail except in the immediate neighborhood of the wake generating body. The simple integral method described in this chapter using the relaxation diffusion model for the stress is adequate to describe wake flow subjected to various pressure gradients and constant pressure distortions. In the case of large pressure gradients or severe distortion, it is likely that the development of turbulent stresses can be understood in terms of rapid distortion analysis; eventually, even the mean velocity field becomes amenable to an ideal fluid treatment. It appears that inviscid development of the turbulent quantities is likely to occur earlier in the pressure gradient scale than does inviscid mean flow development.
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