Understanding radiation parameters

Use the options on the Radiation Parameters page of the Solver Parameters dialog box to set up global control parameters for radiation simulations. For most models, you will not have to set any of these parameters; the default setup should provide good results. However, for particular modeling situations you may be able to improve accuracy or reduce solution time by specifying more appropriate settings. You can:

• Set global parameters for the computation of black body view factors.

• Select the treatment of residual view factors.

• Strip insignificant radiative conductances from the model to reduce solution time.

View Factor Adjustment

Closure of the radiation problem requires that the black body view factors for each element sum to 1.0. Because of numerical imprecision in computing shadowed view factors, this condition is generally not satisfied. The difference is called the residual view factor; its magnitude indicates the inaccuracy in the view factor calculations for that element. Use the View Factor Adjustment option to select the method the software uses to eliminate the residual view factor:

Self

Creates a view factor (equal to the residual) to the element itself. This is equivalent to making the element slightly concave or convex. This is the recommended option for all models.

Normalized

Adjusts the computed view factors iteratively to bring the view factor sum for each element as close to 1.0 as possible. Only view factors computed with shadowing are adjusted.

Residual to Radiative Environment

Creates a view factor (equal to the residual) to the implicit radiative environmental temperature.

Small View Factors

Small View Factor option controls how the software disposes of the smallest view factors. Prior to calculating a view factor, the solver estimates its value based on the elements' size and distance. If you choose Dispose of from the Small View Factor list and the software's estimated view factor magnitude is less than the value you specified in the View Factors less than box, then the software does not retain it. This value must be significantly less than the average view factor in the model. Otherwise, important couplings will be lost.

If you choose Dispose up to from the Small View Factor list, you can dispose of all the smallest view factors in a problem, by setting what proportion of the total view factors should be ignored. With the Dispose up to option, the solver sorts all the view factors by size, then adds the smallest view factors one by one until it satisfies the proportion value you specify in the Sum of Disposed View Factors less than box. All the view factors that add up to the specified value are ignored. For greater accuracy in modeling small view factors, reduce the value of the Sum of Disposed View Factors less than; for a faster solution time, increase it.

Use Radiation Patches

This option can significantly reduce solution time on very large models with complex radiative exchange. It temporarily merges adjacent elements for the purposes of calculating radiative exchange. After the calculation of view factors for individual elements, Oppenheim elements that are coplanar within a 15 degree tolerance are treated as a single element in the radiative exchange matrix.

Before merging elements, the solver performs several checks to ensure that merging is appropriate. In addition to being adjacent and coplanar within a tolerance, the elements must have identical radiative material properties, and must not be separated by any surface that intersects their surface (such as the two arms of a “T” intersection).

Elements with emissivity of 1.0 are excluded from the radiation patches. The thermal solver creates a group in the group.unv file that contains the excluded elements. For more information, see Import solution error and warning groups.

On Screen Rendering

This option can accelerate view factor calculation when you select the Hemicube Rendering method in the Radiation simulation object. With On Screen Rendering, a small window appears in the top left hand corner of your screen during the solve, utilizing the full power of the workstation's graphics hardware. For more information, see Radiation.

On Screen Rendering only improves performance if the analysis uses the same workstation for both processing (CPU) and display (graphics card and monitor). If you run the analysis on one workstation and display the On Screen Rendering window on another, performance will not be improved.

The following limitations apply:

• You must set your display mode to True Color 24-bit or True Color 32-bit

• You cannot cover any part of the small window with any other window while it is processing.

• You must ensure that any screen saver or screen lock is disabled before processing.

Caution:

If these limitations are not respected, the view factor calculation will be incorrect.

Ignore Specular and Transparent Effects for Radiation Request Calculations

When you assign a mesh specular or transmissive thermo-optical properties and include it in a radiation request, the solver automatically enables the deterministic ray tracing Calculation Method even if you had selected Hemicube Rendering in the Radiation simulation object.

Select this option to force the solver to use Hemicube Rendering as calculation method. Since 1=α+(ρdiffuse+ρspecular)+τ the values of ρspecular and τ are added to ρdiffuse.

Clear this option to automatically enable the deterministic ray tracing Calculation Method for radiation enclosures defined with elements including specular or transmissive thermo-optical properties .

Allow all Elements to Shadow for Orbital and Solar Heating Requests

Specifies which elements participate in shadowing checks for orbital and solar heating requests.

When you select this option all elements with appropriate thermal properties are included in shadowing checks.

When you clear this option only elements selected in orbital or solar heating requests are included in shadowing checks.

This option is helpful when you want elements that are not in the enclosure defined by the request to participate in shadowing. If the enclosure contains specularly reflective or transmissive elements, this option allows other elements to receive reflected or transmitted energy. An example for when you would use this option is when solar radiation accidentally falls on a telescope aperture, passing through to the electronics inside.

Caution:

You should not use this option for the vast majority of thermal analyses. Selecting this option increases the computation time of your thermal analysis.

Use Segregated Radiation Solver

The Segregated Radiation Solver option separates the solution of radiative heat transfer from the solution of all other thermal physics. The segregated solver creates two matrix equations, one formed by the network of conductances to Oppenheim elements and a second matrix containing all other terms. The solutions of both matrices are passed back and forth to each other until the solution converges.

This option improves solver performance in some particular cases like:

• Large and dense radiation matrices.

• Ill conditioned solid matrices.

• Models with cryogenic temperatures.

Element subdivision

Element subdivision is used for computing shadowed view factors. A higher subdivision level will yield more accurate radiative conductances but will require additional computation time.

By default, the element subdivision is performed when one of the view factors seen through:

• The top viewing plane is greater than 0.03.

• The side viewing plane is greater than 0.3.

Set the ENFORCE MESH IN HEMIVIEW advanced parameter to ensure that the software always performs the element subdivision. See Define Advanced Parameters for more information.

Spectral Bands

The options in the Spectral Bands group permit selection of the spectral band discretization to improve accuracy when calculating radiative heat exchange. The solver calculates radiation exchange either using the default two band (IR and solar) model, or for greater accuracy, using multiple spectral bands.

Solar and IR

Defines two spectral bands one for solar radiation and one for infrared (IR) radiation. This option is adequate for most applications.

Based on Temperature Range

Defines the specified number of spectral bands, N, between the estimated maximum and minimum representative temperatures, Tmax and Tmin respectively, of the radiation source. Choose N such that there is a good distribution of radiative power in each band over this temperature range. The first and last breakpoint wavelengths, λ(2) and λ(N) respectively, are calculated assuming that first and last spectral bands radiate a fraction of energy equal to one over the number of bands, 1/N. The solver defines band demarcations between these two wavelengths in logarithmic intervals.

Emissive power at Tmin and Tmax	Average emissive power between Tmin and Tmax

Based on Key Temperatures

Defines the spectral bands based on a table of representative temperatures of the radiation source. The demarcations of the bands are the wavelengths at the peak of the black body spectrum for each temperature, with reference to Wien's law: λ = C3/T where λ is the wavelength, T is the temperature and C3 is the Wien's Law constant.

Equal Power Intervals

Defines the spectral bands based on the specified total number of bands, N, and the black body temperature, T. Choose a band discretization such that a black body radiating at this temperature radiates equal power into each band as shown in the following graphics.

Emissive power at T

Wavelength Range

Defines the specified number of spectral bands, N, based on a specified minimum and maximum wavelengths, λmin and λmax respectively. The lesser wavelength, λmin, is treated as the upper limit of the lower band, and the greater wavelength, λmax, is the lower limit of the upper band. The solver defines band demarcations between these two wavelengths in logarithmic intervals.

Specify Wavelength

Defines the spectral bands based on a table of wavelengths (in microns). The wavelengths specified in the table demarcate the spectral bands.

Spectral Model

If you select any option from the Bands list other than Solar and IR, the Spectral Model option determines how bands are used in the infrared and solar regions of the spectrum. The solar region is defined by the transition wavelength.

Full Multi-bands

The full band range is used for radiative heating calculations, and the bands at and above the transition wavelength are used for calculation of non-gray infrared heat transfer.

One Band for IR

The bands above the transition wavelength are condensed into one band for the calculation of gray infrared heat transfer. All bands are used for radiative heating (orbital, solar, planet, and radiative source) calculations.

Transition Wavelength

The Solar to IR Transition Wavelength field demarcates the transition between the infrared and solar spectra. In the case that multiple bands are being used and optical properties are defined in terms of "solar" or "infrared", the transition wavelength is used to decide which of the nominally solar or nominally infrared optical properties are used in a specific band.

If you select Solar and IR option from the Bands list, and optical properties are defined in terms of wavelength, the transition wavelength is used to determine the limit at which the wavelength dependent properties are integrated.

When solving with multiple bands, bands below the transition wavelength do not participate in infrared exchange. The solver lumps emissive non-gray radiative power in these lower bands into the lowest band which contains the transition wavelength.

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