Surface Photon Monte Carlo (SPMC) Radiation Model Reference
The Surface Photon Monte Carlo method (SPMC) uses probability distributions to model photon transport in a way equivalent to solving the radiative transfer equation (RTE). Given its statistical nature, accuracy and computational cost both increase with the number of photon bundles used in the simulation. However, if the high cost can be paid, the SPMC method can give high accuracy in cases where other methods cannot.
Like the Surface-to-Surface Radiation model, this model applies to regions without participating media, which do not absorb, emit, or scatter radiation. Only surfaces have radiation properties: emissivity, reflectivity, transmissivity, and radiation temperature. This model also handles refractive effects.
The SPMC model solves for the RTE directly by providing a statistical representation of the radiative processes of emission, absorption, reflection, and transmission for the participating surfaces. In this model, photon bundles ( representative samples of radiative energy) are traced according to the probability density functions of the radiative processes, and statistics are computed to obtain the radiative heat exchange between surfaces. To obtain meaningful statistics, a large number of photon bundles are usually required, so the associated computational costs can be high. Furthermore, for combined-mode simulations where temperature fields are changing, the SPMC computations need to be done at every iteration, which can make SPMC computationally expensive.
To alleviate these issues, the SPMC model provides the following options:
- SPMC model uses smart sampling and efficient boundary interaction to minimize the required number of rays to be traced.
- The SPMC solver has an update frequency option, so as to not invoke the SPMC calculations at every iteration. The temperature fields often undergo very small change from iteration to iteration, so it is then computationally efficient to perform SPMC computations only after every five or ten iterations. The default value is 1, that is, at every iteration.
- The SPMC model also allows for aggregating the statistics over iterations via the use of the statistical sampling factor in the SPMC solver. This ensures that you do not have to use a large number of rays in the SPMC computations, and the statistics for the radiative quantities of interest can be aggregated over several iterations. See this link for more information on the statistical sampling factor. See Statistical Sampling Factor .
The Surface Photon Monte Carlo model can apply to a multi-part object only when used with the Gray Thermal Radiation model. Inputs must be specified per-boundary.
| Theory | See 后处理表面辐射交换. | ||
| Provided By | |||
| Example Node Path | |||
| Requires |
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| Activates | Boundary Inputs | See Surface Photon Monte Carlo Boundary Settings. | |
| Region Inputs | See Surface Photon Monte Carlo Region Settings. | ||
| Solvers | Photon Monte Carlo. See Photon Monte Carlo Solvers. | ||
| Field Functions | Boundary Irradiation, Boundary Irradiation on External Side, Boundary Mean Radiant Temperature, Boundary Radiation Heat Flux, Boundary Radiation Heat Flux on External Side, Boundary Radiosity, Boundary Radiosity on External Side, Radiation Patch Id, Radiation Patch Id on External Side, Total Emissivity, Voxel partition. | ||
The Surface Photon Monte Carlo model is not compatible with the Axisymmetric or Two Dimensional models.
Surface Photon Monte Carlo Boundary Settings
- Free Stream, Pressure Outlet, Wall, and Inflow Boundaries
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- Custom Patch Angularity Specification
- Patch angularity defines the maximum allowed angle between the normals to contiguous boundary faces for these faces to belong to the same patch. It provides a control over the patch planarity and is used to detect sharp angles in the geometry, such as would happen at a corner, and to prevent patches from bridging such features. The angularity is set at 150 degrees by default and would require changing only in cases where specific geometric features must be captured.
- Custom Patch Specification
- The number of patches and patch
angularity can be further refined or customized at individual
boundaries under the Physics Conditions
node. Specifies whether the number of patches is set to
Use Region
Values or custom values. When you
choose either Proportion of
Faces or Total, the
corresponding value node appears for the boundary, and you can
then enable Specify by Part
Subgroup.
注 If a region-level patching specification has Specify by Part Subgroup on, the Use Region Values option cannot be used for interface boundaries corresponding to boundary-mode interfaces in that region. Boundaries for contact-mode interfaces do support this option. See Interface Options. - Radiation Flux Option
- This option allows you to model
diffuse or directed radiation sources at boundaries according to
your own specification. See Diffuse Radiation Flux and Directional Radiation Flux.
Radiation Flux Option Corresponding Physics Value Nodes No flux None Diffuse radiation flux - Diffuse Radiation Flux
- The diffuse radiation flux is specified as a scalar profile. Requires the Multiband Thermal Radiation or Gray Thermal Radiation model. See Diffuse Radiation Flux.
Directional radiation flux - Directional Flux Orientation
- The direct radiation flux is specified by its direction (default [0.0, 0.0, 1.0]) and the divergence angle of the beam (default zero).
- Directional Flux Power Distribution
- The power of the directional flux is specified as a scalar profile. In addition to standard profile methods, a Gaussian method is also available. Requires the Gray or Multiband Thermal Radiation model.
Surface Photon Monte Carlo Region Settings
- Patch Angularity
- Patch angularity defines the maximum allowed angle between the normals to contiguous boundary faces for these faces to belong to the same patch. It provides a control over the patch planarity and is used to detect sharp angles in the geometry, such as would happen at a corner, and to prevent patches from bridging such features. The angularity is set at 150 degrees by default and would require changing only in cases where specific geometric features must be captured.
- Patch Specification
- Overall description of this condition node.
- Radiation Transfer Option
- Specifies that a region uses S2S radiation. Either a baffle or porous baffle must be placed between any two regions where only one region has this option activated. This baffle ensures that rays do not pass into regions where S2S is deactivated from regions where it is activated.
Photon Monte Carlo Solver Properties
- Verbosity
- Provides additional output
during the Photon Monte Carlo calculations.
- None
- No output.
- Low
- Displays ray tracing progress.
- High
- Displays additional progress and resource usage information.
- 冻结求解器
- 开启时，求解器在迭代过程中不更新任何物理量。该选项默认情况下关闭。这是一个调试选项，由于缺少储存，它可能导致不可恢复的错误和错误的求解。有关详细信息，请参见有限体积求解器参考。
- Child Nodes
-
- Surface Ray Tracing Parameters
-
- Rays Per Patch
- The average number of rays (photon bundles) traced from each patch. The default is 10.
- Maximum Polygons Per Voxel
- Controls the resolution of the voxel mesh that is used in ray tracing. Lowering the value increases the resolution of the voxel mesh. This property generally does not need to be changed from the default value of 50. Increasing the number slightly decreases memory requirements, but can also slightly increase the calculation time.
- Max Ray Bounces
- The number of times a ray can bounce off a surface when being traced. The default is 1000.
- Environment Load Parameters
-
- Rays Per Patch
- The number of rays traced from each patch for computing environment loads. The default is 128.
- Statistical Sampling Factor
-
- Sampling Factor
- A constant or user-defined field function
specifying what fraction of the latest PMC
computation (at a given iteration) contributes to
the aggregated solution. At a value of 1, the
aggregated solution is set to the latest PMC ray
tracing solution, whereas a value of 0 means that
the aggregated solution stays unchanged. For a value
of 0.5, the aggregated solution is updated by taking
half of the latest solution and half of the previous
aggregated solution. See Statistical Sampling
Factor.
Use high sampling factors at the start of the simulation or during any simulation transients, since that will lead to faster feedback of radiative solutions into the energy equation and, consequently, faster evolution of the temperature fields. This is similar to the use of under-relaxation factors for flow and energy, where using low factors while the solution is evolving negatively impacts the solution convergence. A small value of sampling factor behaves likewise; it leads to radiative source terms that are not truly indicative of the temperature fields, which slows down the further evolution of the solution.
- Update Frequency
-
- Iteration Update Frequency
- Specifies how often the Photon Monte Carlo calculations are solved. The default is 1 (once every iteration for steady runs, once every time-step for unsteady runs). A value of 10 would mean once every ten iterations for steady simulations, every ten time-steps for unsteady simulations.