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2023-06-19

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Overview

Introduction

Theory

Input Data

Calculation

Results

Permeable Surfaces

In RWIND 2 Pro, it is possible to apply permeability to a surface. A brief theory about the permeability can be found in Chapter Permeability. In RWIND 2 Pro, the permeability is modeled using a boundary condition, a prescribed pressure drop on defined surfaces. The pressure drop (pressure gradient) is given by Eq.:

∆ p = - (D μ U + \frac{1}{2} I ρ {|\begin{matrix} U \end{matrix}|}^{2}) \cdot L

Δp[Pa]	Pressure drop
μ[Pa.s]	Dynamic viscosity of the fluid
U[m/s]	Velocity of the fluid
D[1/m²]	Darcy coefficient
I[1/m]	Inertial coefficient
ρ[kg/m³]	Density of the fluid
L[m]	Permeable media length in the flow direction

Pressure Drop

where the coefficients D and I are defined as:

I = \frac{C_{F}}{\sqrt{α}}

C_F[-]	Forchheimer parameter
α[m²]	Permeability

Inertial Coefficient

D = \frac{1}{α}

α[m²]

Permeability

Darcy Coefficient

In the permeable media models discussed in Chapter Permeability, a source term is added on the right side of the N‑S equations in the centroid of the cells where the permeability should be solved. Since RWIND 2 Pro only solves permeable surfaces (that is, relatively thin elements), the permeability is modeled using a cyclic boundary condition (porousBafflePressure) so far, prescribing the pressure gradient on the selected elements (patch). For more details, see the OpenFOAM guide. This is a computationally simple model and interesting results can be achieved in a short computation time. However, it has its limitations, for example, using the model for high pressure drop may not lead to convergence and results.

More specific information about the permeability model (porousBafflePressure) can be found in the OpenFOAM-4.1 manual.

Permeability & Zones

In RWIND 2 Pro, the permeability is assigned to the selected zones as a material property, see the image below.

Zones & Permeability

In the "Edit Zone" dialog box, the "Material" section, click "Create new material..." or "Edit material...". A dialog box with permeability parameters appears.

Editing Material, Permeability

Here, the permeability coefficients D, I, and the permeable surface length (thickness) L have to be defined. An introduction on how to derive and obtain these coefficients was described in Chapter Permeability. More ideas and approaches to deriving the coefficients can also be found here. One way to obtain the coefficient and model the permeability is described in the Knowledge Base Article on the Dlubal website. After setting all coefficients and assigning zones to surfaces, the model with permeable surfaces is ready for calculation.

Tip

When setting the coefficients D and I, it is important to keep in mind their physical interpretation. The D coefficient affects the importance of the frictional (viscose) forces, whereas the I coefficient affects the importance of the inertial forces of velocity as the flow passes through the permeable surface.

Important

The calculation with the surface permeability can be performed on simplified models only. The shrink-wrapping mesh ensures a geometrically correct mesh without any open volumes. If the model simplification is disabled, the generated volume mesh could be of a poor quality and the results may be incorrect. Here it is important to underline that the simplified model with and without permeable surfaces differs significantly, see , the model with permeable surfaces in this case forms an open volume model, which then leads to a larger volumetric mesh then the same model without them.

Model Mesh with and Without Permeable Surface

Important

The current permeability model (OpenFOAM, porousBafflePressure) is functional for relatively simple permeable surfaces (e.g., wire meshes, louvers, barriers, etc. ), i.e., simple shapes defined by a set of equally oriented triangles.Thus, if we use permeable surfaces for the whole building (for example, the "Eiffel Tower" model from Project Manager), then the calculation will most likely be unstable, the results will be incorrect or the calculation will not work at all.

Velocity Field, Permeable Surfaces

Parent Chapter

Imported Models