2D:DefiningTheComponentAndTheNumericalGrid

Defining the Component and the Numerical Grid

In a first step you define the geometry of the component by assembling it from rectangular elements, then you subdivide it into small numerical grid elements.

The definition of the geometry is done interactively in a graphical diagram of the component.

The transport equations cannot be solved analytically. To allow a numerical treatment, the building component must be divided into discrete grid elements. WUFI can automatically generate a general-purpose grid which will suffice for most cases (you can select one of three degrees of fineness for the grid).

In special cases (e.g. severe interstitial condensation in a narrow region) you may need to adapt the grid manually. You can easily set up and manually edit the grid by using WUFI's grid editor. All grid data are entered into a table where each line represents a layer of the component in x- or y-direction. Each layer is subdivided into two sublayers with expanding and contracting grid elements. You may define expanding or contracting grids in order to tailor the local fineness of the grid to the demands of the simulation model:

For the discretisation of the transport equations, the values of the calculated quantities are assumed to be constant across each grid element. Therefore, the size of each element should be chosen so as to make this assumption approximately valid, depeding on the local conditions. In general, this requires the use of small elements in regions where steep moisture or temperature gradients are to be expected, and allows the use of larger elements in more placid regions to cut down on calculation effort.

In general, steep gradients are to be expected close to the boundaries of a layer, i.e., close to the surfaces of the component because of the influence of climate conditions, and close to internal layer interfaces because of condensation. Less variation will occur in the middle of each layer.
The two sublayers of each layer may have different expansion factors (>1, <1) for the grid and thus allow you to achieve a smooth, not necessarily symmetrical, expansion and contraction of the grid elements which can be adapted to the requirements in each individual layer.

The choice of a suitable grid is further discussed in the topic Background, details of how to set up a manual grid are described in the help topic for the "Geometry" dialog.

Finally you specify the material data for each layer. The material data need not be entered by hand if the material in question is contained in WUFI's material database. Just select the material in the database and transfer all the data.

WUFI comes complete with a database which offers a selection of materials for instant use. (Please note, however, that due to large variations in the materials and often incomplete or inconsistent literature data, the data provided here should not be taken as divine revelations. They are just intended to give you a starting point.)
You can supplement the database with your own materials.