Torna a Manuali online

60x

005724

2024-07-11

Seleziona il manuale

Panoramica

Introduzione

Metodo di analisi

Dati di input

Calcolo

Risultati

Superfici permeabili

In RWIND 3 Pro, è possibile applicare la permeabilità a una superficie. A brief theory about the permeability can be found in Chapter Permeability. In RWIND 3 Pro, the permeability is modeled using a boundary condition, a prescribed pressure drop on defined surfaces. La caduta di pressione (gradiente di pressione) è data dall'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

Caduta di pressione

dove i coefficienti D e I sono definiti come:

I = \frac{C_{F}}{\sqrt{α}}

C_F[-]	Forchheimer parameter
α[m²]	Permeability

Coefficiente d'inerzia

D = \frac{1}{α}

α[m²]

Permeability

Coefficiente di Darcy

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 3 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). Per maggiori dettagli, consulta la guida di OpenFOAM guida. 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.

Informazioni più specifiche sul modello di permeabilità (porousBafflePressure) sono disponibili in OpenFOAM-4.1 manuale.

Permeabilità e zone

In RWIND 3 Pro, la permeabilità è assegnata alle zone selezionate come proprietà del materiale, vedi l'immagine di seguito.

Zone e Permeabilità

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

Modifica Materiale, Permeabilità

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: Calcolatore di Darcy-Forchheimer One way to obtain the coefficient and model the permeability is described in the Knowledge Base Article on the Dlubal website.

Kb | Uso della funzione delle superfici permeabili in RWIND Pro

After setting all coefficients and assigning zones to surfaces, the model with permeable surfaces is ready for calculation.

Suggerimento

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.

Importante

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.

Modello Mesh con e senza superficie permeabile

Modello di Mesh con e senza Superficie Permeabile

Importante

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.

Campo di Velocità, Superfici Permeabili

Capitolo principale

Modelli importati