Partially resolved numerical simulation and RANS modeling of flow and passive scalar transport in an urban environment

The predictive capabilities of a building-resolving prognostic numerical simulation model (urbanSTREAM) for small-scale (microscale) atmospheric flows in an urban environment will be evaluated based on detailed comparisons between the predictions and measurements of various flow quantities obtained in the Joint Urban 2003 (JU2003) field experiment in Oklahoma City. The prognostic model for the wind field in a cityscape is obtained by solving the unsteady Reynolds-averaged Navier–Stokes (URANS) and the partially resolved numerical simulation (PRNS) equations. For URANS, a two-equation k– ε turbulence closure model is used. However, in contrast to URANS, PRNS attempts to provide a generalized subscale turbulence model that permits the simulation (rather than modeling) of various scales of motion associated with turbulence production, inertial energy transfer, and viscous dissipation depending on the subscale filter cut-off chosen. In this sense, PRNS provides a unified framework for the numerical simulation of turbulent flows, and includes URANS, large-eddy simulation (LES) and direct numerical simulation (DNS) as special cases, depending on the choice of the cut-off frequency of the filter used. A two-equation k– ε PRNS is applied here, in which the turbulent viscosity is modulated by a resolution control parameter function that is dependent on the cut-off wavenumber (or, equivalently, the cut-off frequency) of the filter.