The general influence of a screen on the flow field is a loss in the normal momentum component, a change in direction (related to a suppression of the tangential velocity component), attenuation of the turbulence kinetic energy and preservation of the turbulence length scale (Ref. 18). The conditions across the screen are expressed as

Here, ‘−’ and ‘+’ refer to the upstream and downstream side of the screen, respectively. Furthermore, K is the dimensionless resistance coefficient, which parameterizes the magnitude of the drag exerted by the screen on the flow. η is the dimensionless refraction coefficient, which parameterizes the extent to which the screen causes the flow to change direction to align parallel to the normal downstream direction from the plane of the screen. The refraction coefficient should be between 0 and 1. Isotropic turbulence is expected on the downstream side of the screen, hence,

The attenuation of the turbulence kinetic energy (Equation 3-48) is based on the suppression of the tangential velocity (Equation 3-47) and the changes in ε and ω are determined by the assumption of preservation of the turbulence length-scale across the screen.

When the transition model is active for the SST model, continuity of intermittency, γ, is applied across the screen.

The Screen feature provides three commonly used correlations for K (Ref. 19). The following correlation is valid for wire gauzes

Here σs is the solidity (ratio of blocked area to total area of the screen) and d is the diameter of the wires. For a square mesh, the correlation

holds. These correlations are based on common shapes for meshes and perforated plates encountered in engineering contexts, and it is assumed that the wire gauze, square mesh or perforated plate is thin compared to the mixing length scale up- and downstream of the screen. The following correlation for wire gauzes (Ref. 20) gives reasonable values for η for a wide range of applications and has been included in the implementation: