The three-dimensional Navier-Stokes equations are used in the Boussinesq approximated non-hydrostatic form including sink terms for drag forces at vegetation elements. The pressure perturbation is removed from the equations and an auxiliary velocity field is computed. Mass conversation is satisfied by correcting the auxiliary field by an iterative solution of the Poisson-Equation and correction at the outflow boundaries. The flow is updated at given time intervals. ENVI-met also supports a real-time flow calculation which means that the flow field is treated as a normal prognostic variable and calculated each step. Due to the very small time steps needed here, this way of calculation need very powerful computers.

Advection and diffusion of temperature and humidity is calculated using the previous calculated wind field. The ground surface and vegetation is incorporated using a source/sink term in both equations, building walls are only acting as a source/sink for temperature.

The turbulence is calculated using the E-epsilon 1.5order closure (“E-epsilon” or “k-epsilon” model). Two prognostic equations for turbulent energy production (E) and its dissipation (epsilon) are used to simulate the distribution of turbulent energy. Exchange coefficients in the air are calculated using the Prandtl-Kolmogorov relation. For low wind situations, the 1st order mixing length model can be used instead of the E-epsilon model (which often fails in this situations). Improved troubleshooting has been added since version 2.5, which effectively reduces the number of problems with the turbulence model even under difficult atmospheric conditions.