Screen Boundary Condition

The word “screen” refers to a barrier with distributed perforations such as a wire gauze, grille, or perforated plate. The screen is assumed to have a width, which is small compared to the resolved length-scales of the flow field and can thus be modeled as an edge (in 2D) or surface (in 3D). This idea permits an economic implementation of the screen, where the details of the barrier need not be resolved. A limitation to this type of modeling may be observed when there is an appreciable velocity component parallel to the screen. In such cases the pressure may become discontinuous across streamlines, which may lead to convergence problems. If this occurs, a better approach is to model the screen as a thin domain with distributed momentum-loss terms.

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,

(3-45)

(3-46)

(3-47)

(3-48)

and, depending on the turbulence model in use, either,

(3-49)

or,

(3-50)

− and + refer to the upstream and downstream side of the screen, respectively. 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,

(3-51)

is enforced when the v2-f turbulence model is used.

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 an algebraic turbulence model, such as the Algebraic yPlus or L-VEL model, is used, the turbulence is only implicitly affected by the device through the change in the local Reynolds number.

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

(3-52)

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 following correlation is applied,

(3-53)

and for a perforated plate,

(3-54)

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,

(3-55)


	See Screen for the node settings. Also see Theory for the Nonisothermal Screen Boundary Condition for the nonisothermal version of these physics interfaces.