Turbulence models for environmental problems

Various models for describing the turbulent transport of momentum, heat and mass in environmental flows are presented. The merits and demerits of the individual models are discussed with respect to their predictive ability, their computational economy and the ease with which they can be used in practical calculations. Examples are presented of model applications to various flow situations relevant to environmental problems, most of them being examples from the water environment. The two-equation models employing differential transport equations for the velocity and length scales of the fluctuating motion are sufficiently complex for most practical calculations. In particular, the k - epsilon model is shown to predict reasonably well a fairly large range of problems with the same empirical input. In shear-layer situations where the length scale can be specified empirically, energy one-equation models are judged to work equally well; when history and transport effects on the turbulence are unimportant, the mixing length hypothesis can also be used. The most complex models included, which employ transport equations for the individual turbulent stresses and heat or mass fluxes, are conceptually superior, but there is no evidence that the increased computational effort they require is justified for most problems.