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13.3.12 Setting Up the S2S Model

When you select the Surface to Surface (S2S) model, the Radiation Model panel will expand to show additional parameters (see Figure  13.3.16). In this section of the panel, you will compute the view factors for your problem or read previously computed view factors into FLUENT.

Figure 13.3.16: The Radiation Model Panel (S2S Model)
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The S2S radiation model is computationally very expensive when there are a large number of radiating surfaces. To reduce the memory requirement for the calculation, the number of radiating surfaces is reduced by creating surface clusters. The surface cluster information (coordinates and connectivity of the nodes, surface cluster IDs) is used by FLUENT to compute the view factors for the surface clusters.

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You should recreate the surface cluster information whenever you do anything that changes the grid, such as:

  • change the type of a boundary zone

  • reorder the grid

  • scale the grid

Note that you do not need to recalculate view factors after shell conduction at any wall has been enabled or disabled. See Section  7.13.1 for more information about shell conduction.

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FLUENT will warn you to recreate the cluster/viewfactor file if a boundary zone has been changed from a wall to an internal wall (or visa versa), or if a boundary zone has been merged, separated, or fused.



Computing View Factors


FLUENT can compute the view factors for your problem in the current session and save them to a file for use in the current session and future sessions. Alternatively, you can save the surface cluster information and view factor parameters to a file, calculate the view factors outside FLUENT, and then read the view factors into FLUENT. These methods for computing view factors are described below.

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For large meshes or complex models, it is recommended that you calculate the view factors outside FLUENT and then read them into FLUENT before starting your simulation.

Computing View Factors Inside FLUENT

To compute view factors in your current FLUENT session, you must first set the parameters for the view factor calculation in the View Factor and Cluster Parameters panel (see below for details). When you have set the view factor and surface cluster parameters, click Compute/Write... under Methods in the Radiation Model panel. A Select File dialog box will open, prompting you for the name of the file in which FLUENT should save the surface cluster information and the view factors. After you have specified the file name, FLUENT will write the surface cluster information to the file. FLUENT will use the surface cluster information to compute the view factors, save the view factors to the same file, and then automatically read the view factors. The FLUENT console window will report the status of the view factor calculation. For example:

Completed 25% calculation of viewfactors

Completed 50% calculation of viewfactors

Completed 75% calculation of viewfactors

Completed 100% calculation of viewfactors

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The view factor file format for this version of FLUENT is known as the compressed row format (CRF) which is a more efficient way of writing view factors than in prior versions of FLUENT. In the CRF format, only non-zero view factors with their associated cluster IDs are stored to the file. This reduces the size of the .s2s file, and reduces the time it takes to read the file into FLUENT. While the CRF file format is the default, you can still use the older file format if necessary. Contact your support engineer for more information.

Computing View Factors Outside FLUENT

To compute view factors outside FLUENT, you must save the surface cluster information and view factor parameters to a file.

File $\rightarrow$ Write $\rightarrow$ Surface Clusters...

FLUENT will open the View Factor and Cluster Parameters panel, where you will set the view factor and surface cluster parameters (see below for details). When you click OK in the View Factor and Cluster Parameters panel, a Select File dialog box will open, prompting you for the name of the file in which FLUENT should save the surface cluster information and view factor parameters. After you have specified the file name, FLUENT will write the surface cluster information and view factor parameters to the file. If the specified Filename ends in .gz or .Z, appropriate file compression will be performed.

To calculate the view factors outside FLUENT, enter one of the following commands:



Reading View Factors into FLUENT


If the view factors for your problem have already been computed (either inside or outside FLUENT) and saved to a file, you can read them into FLUENT. To read in the view factors, click Read... under Methods in the Radiation Model panel. A Select File dialog box will open where you can specify the name of the file containing the view factors. You can also manually specify the view factors file, using the File/Read/View Factors... menu item.

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While the previous .s2s view factor file format can still be read seamlessly into FLUENT, there is now a more efficient compressed row format (CRF) that can be read into FLUENT (see the section on Computing View Factors Inside FLUENT). You can take advantage of the reduced size of the CRF file and thus the reduced time it takes to read the file into FLUENT, by converting the existing old file format to the new format (without having to recompute the view factors) using the following command at the command prompt in your working directory:

utility viewfac -c1 -o new.s2s.gz old.s2s.gz

where new.s2s.gz is the CRF format to which you want the old file format ( old.s2s.gz) converted.



Setting View Factor and Surface Cluster Parameters


You can use the View Factor and Cluster Parameters panel (Figure  13.3.17) to set view factor and cluster parameters for the S2S model. To open this panel, click Set... under Parameters in the Radiation Model panel (Figure  13.3.16) or use the File/Write/Surface Clusters... menu item.

Figure 13.3.17: The View Factor and Cluster Parameters Panel
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Controlling the Clusters

Your input for Faces Per Surface Cluster will control the number of radiating surfaces. By default, it is set to $1$, so the number of surface clusters (radiating surfaces) will be equal to the number of boundary faces. For small 2D problems, this is an acceptable number. For larger problems, you may want to reduce the number of surface clusters to reduce both the size of the view factor file and the memory requirement. Such a reduction in the number of clusters, however, comes at the cost of some accuracy. (See Section  13.3.7 for details about clustering.)

There are certain applications that will require most or all wall boundary zones to have the same Faces Per Surface Cluster parameter. In typical underhood simulations, for example, there can be hundreds of walls that you want to apply the same Face Per Surface Cluster parameter to. To avoid visiting each Wall boundary condition panel, you can instead click the Apply to All Walls checkbox in the View Factor and Cluster Parameters panel (Figure  13.3.17). Once you click OK, the Faces Per Surface Cluster value you specify will be copied to all wall zones that are adjacent to fluid zones in your model. You can then visit only the walls you want to define different settings for and set those parameters individually.

The Faces Per Surface Cluster can be designated for a particular wall in the Wall boundary condition panel under the Radiation tab (Figure  13.3.18). Under the Radiation tab, you can also choose to exclude a particular wall from the radiosity calculations by deselecting Participates in S2S Radiation. Note that if the surface clusters are written with this feature turned off, then the view factors will not be computed at all for that particular wall. If you are unsure whether a wall is radiating or not ahead of time, then you should keep the Participates in S2S Radiation enabled and have the view factors computed. You can always toggle the switch at a later stage to include or exclude the particular wall for radiosity calculations.

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The Faces Per Surface Cluster and Participates in S2S Radiation controls will not be visible in the GUI on wall boundary zones that are attached to a solid.

Figure 13.3.18: The Wall Panel
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In some cases, you may wish to modify the cutoff or "split'' angle between adjacent face normals for the purpose of controlling surface clustering. The split angle sets the limit for which adjacent surfaces are clustered. A smaller split angle allows for a better representation of the view factor. By default, no surface cluster will contain any face that has a face normal greater than 20 $^{\circ}$. To modify the value of this parameter, you can use the split-angle text command:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ s2s-parameters $\rightarrow$ split-angle

or

file $\rightarrow$ write-surface-clusters $\rightarrow$ split-angle

Specifying the Orientation of Surface Pairs

View factor calculations depend on the geometric orientations of surface pairs with respect to each other. Two situations may be encountered when examining surface pairs:

For cases with blocking surfaces, select Blocking under Surfaces in the View Factor and Cluster Parameters panel. For cases with non-blocking surfaces, you can choose either Blocking or Nonblocking without affecting the accuracy. However, it is better to choose Nonblocking for such cases, as it takes less time to compute.

Selecting the Method for Smoothing

In order to enforce reciprocity and conservation (see Section  13.3.7), smoothing can be performed on the view factor matrix. To use the least-squares method for smoothing of the view factor matrix, select Least Squares under Smoothing in the View Factor and Cluster Parameters panel. If you do not wish to smooth the view factor matrix, select None under Smoothing.

Selecting the Method for Computing View Factors

FLUENT provides two methods for computing view factors: the hemicube method and the adaptive method. The hemicube method is available only for 3D cases.

The adaptive method calculates the view factors on a pair-by-pair basis using a variety of algorithms (analytic or Gauss quadrature) that are chosen adaptively depending on the proximity of the surfaces. To maintain accuracy, the order of the quadrature increases the closer the faces are together. For surfaces that are very close to each other, the analytic method is used. FLUENT determines the method to use by performing a visibility calculation. The Gaussian quadrature method is used if none of the rays from a surface are blocked by the other surface. If some of the rays are blocked by the other surface, then either a Monte Carlo integration method or a quasi-Monte Carlo integration method is used.

To use the adaptive method to compute the view factors, select Adaptive in the View Factor and Cluster Parameters panel. It is recommended that you use the adaptive method for simple models, because it is faster than the hemicube method for these types of models.

The hemicube method uses a differential area-to-area method and calculates the view factors on a row-by-row basis. The view factors calculated from the differential areas are summed to provide the view factor for the whole surface. This method originated from the use of the radiosity approach in the field of computer graphics [ 64].

To use the hemicube method to compute the view factors, select Hemicube in the View Factor and Cluster Parameters panel. It is recommended that you use the hemicube method for large complex models, because it is faster than the adaptive method for these types of models.

The hemicube method is based upon three assumptions about the geometry of the surfaces: aliasing, visibility, and proximity. To validate these assumptions, you can specify three different hemicube parameters, which can help you obtain better accuracy in calculating view factors. In most cases, however, the default settings will be sufficient.


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