ABSTRACT
This chapter provides formulations for determining the eddy coefficients emphasizing the zero-equation models, which are still widely in use. It also provides methods for estimating dispersion. The chapter emphasizes the zero-equation method that can be readily applied to far-field circulation and water quality simulations in estuaries. The development of formulations to describe turbulent mixing mechanistically is the closure problem, a semiempirical process necessary because Reynolds averaging introduces an extra variable into the hydrodynamic equations. In some flows, however, turbulent fluctuations and the Reynolds stresses that quantify the effects of turbulence develop differently in different directions of the flow field. Dispersion can be included in the eddy viscosity or eddy diffusivity terms when these coefficients are calibrated to field data. The turbulence closure equations provide an estimate of the eddy coefficients for use in the momentum and mass transport equations. Zero-equation closure does not include a separate transport equation for turbulent energy.
