Card 6w - Monte Carlo Method Activation

This optional card activates the use of the Monte Carlo method to calculate view factor requests, radiative heating requests, orbital heating, and diurnal heating requests.

Parameters: L, TOGGLE, NRAYS, RAY_OPTION, VFTYPE, SEED, ERR, CL

L

L is the code MONTECARLO

TOGGLE

TOGGLE switches between using the Monte Carlo method and the standard deterministic subdivision method.

  • TOGGLE = ON: all following Card 6 VFS12, VFSALL, VFS1ALL, VFSENC, SOURCExxx, DIURNALx, ORBIT, ORBDEFx, EARTHx, and SOLxxx cards will be processed by the VUFAC module using the Monte Carlo method.
  • TOGGLE = OFF: all following cards will be processed with the VUFAC module using the standard deterministic subdivision method. The Monte Carlo method will not be active until it is toggled back on again.

NRAYS

NRAYS = number of rays to be launched. Depending on the RAY_OPTION flag, the number of rays to be launched to or from an element will be either NRAYS (absolute) or NRAYS per unit area. For view factor requests and radiative source requests, NRAYS specifies the number of rays launched from each element. For environmental heating requests (solar, earth, orbit, or diurnal heating requests), NRAYS specifies the number of rays launched to each element.

RAY_OPTION

RAY_OPTION determines how rays are launched in simulations.

  • : NRAYS rays per unit area are launched from or (in the case of environmental heating requests) to the element.
  • RAY_OPTION = ABSOLUTE: NRAYS rays are launched per element.
  • RAY_OPTION = ERROR: The ray density control is based on error criterion, i.e. more rays are launched until the specified error criterion ERR is satisfied to within a confidence level CL percentage.

VFTYPE

VFTYPE specifies whether the Monte Carlo method is used to compute view factors or radiative couplings and heat loads.

  • VFTYPE = VF: View factors will be computed with the Monte Carlo method. This includes blackbody view factors, ray-traced view factors, heat flux view factors, earth, albedo, and solar view factors, provided those requests follow this card.
  • VFTYPE = HTFRAD: Radiative couplings and heat loads will be computed with the Monte Carlo method. View factor requests result in the direct computation of radiative couplings. Heat flux view factor requests and environmental heating requests result in the direct computation of radiative heat loads.

SEED

SEED: This value sets the pseudo-random number generator seed for the following Monte Carlo request. The seed is the first number in a unique sequence of pseudo-random numbers, so re-using the same seed will yield identical results between runs (assuming identical TMG versions are run). The seed can be modified to study convergence of the Monte Carlo results. If the seed value is not entered, a default of zero is used.

ERR

If RAY_OPTION = ERROR, then ERR is the error criterion value.

CL

If RAY_OPTION = ERROR, then CL is the confidence level value expressed in percentage.

Code example

The first TMG radiation request is an enclosure request which computes black body view factors between the elements in group “BUS_ENC”. It computes the black body view factors using the Monte Carlo ray tracing method, by casting 10,000 rays randomly from each element in the enclosure.

The second TMG radiation request is an enclosure request which computes black body view factors between the elements of group “MIDDLE_”. It does not use the Monte Carlo method, rather, TMG’s standard deterministic subdivision method is used.


 The following is a TMG model with two different enclosure type view factor requests:
 $
 $ 1 – BUS_ENCLOSURE
 $
 MONTECARLO ON 10000 ABSOLUTE VF
 VFSENC BUS_ENC 0
 MONTECARLO OFF
 $
 $ 2 – MIDDLE_ENCLOSURE
 $
 MESH ERROR 9.000000E-01
 VFSENC MIDDLE_ 0

Notes

If toggled ON, the VUFAC module uses the Monte Carlo method to calculate view factor requests, radiative heating requests, orbital heating and diurnal heating requests which follow this Card. If toggled OFF, these view factors are calculated using the deterministic subdivision method.

The Monte Carlo method can calculate view factors, orbital view factors, heat flux view factors, radiative couplings, or heat loads. It does so by sampling rays in random directions and following the ray paths to their termination.

When the Monte Carlo option is active, ray tracing is automatically performed to all specular, diffuse, and transparent elements.

When VFTYPE = HTFRAD, and view factors are requested with Card 6a VF type Cards, then radiative couplings are computed directly from the Monte Carlo ray tracing technique by the VUFAC module. This results in a radiative conductance matrix similar to that generated by using Gebhardt’s method from view factor data.

When VFTYPE = HTFRAD, and heat flux view factors are requested with SOURCE or ORB type Cards, radiative heat loads are computed directly from the Monte Carlo ray tracing technique by the VUFAC module, and written as HTF cards into MODLF.

It is possible to mix methods, e.g. to compute radiative couplings of some radiative enclosures with the HTFRAD option, and then to the Monte Carlo method OFF, and to compute black body view factors for other enclosures. If you have selected the Oppenheim method, TMG will automatically use the Oppenheim method only for the enclosures for which black body view factors have been calculated.

The default maximum number of rays cast per element is 20,000. This can be modified with the advanced Card 9 option GPARAM4 305 NMAX.

The Monte Carlo method makes use of a pseudo-random number generator. To modify the seed value for the random number generator, use the advanced option GPARAM 4 294 N.