Discussion Forum

In COMSOL Multiphysics 5.4, I had run two simulations using the Time-dependent Study. In them, the Iterative solver was used. According to the Complete Reports that were written from these simulations, there were three additional Segregated Study steps done in Simulation #2 that were not done in Simulation #1. These additional steps, as they are described in the Complete Report that was written from Simulation #2, is attached to this post as a JPG image ("AdditionalSegregatedStudySteps.JPG").

May you explain how the execution of these three additional Segregated Study steps in the Time-dependent Study affects the results in terms of their accuracy?

For answering this question, the following information may be useful:

A description of the scenarios that were being simulated:

The most basic scenario is that a collimated beam of light rays is incident upon one lens. As the rays travel through the lens, their energy gets absorbed progressively by the lens's material. This absorbed energy is a source of heat within the lens. Thereby, the temperature of the lens increases.

In Simulation #1, there was one lens, as described exactly in the paragraph immediately above.

In Simulation #2, there were two lenses, where the second one was placed immediately after the first one, i.e., with no gap in between them. That means that they were in physical contact with each other, although I do not know if this physical contact was perfect. The second lens was made by using the "Array" subnode in the Geometry node. The value I inputted into the "Displacement" setting in the "Array" subnode was the length of the first lens, which had 14 decimal places in 15 significant figures (1.26582278481013 mm). The use of the "Array" subnode means that these lenses are identical to each other.

To give you an idea of what the physical contact between the lenses looks like, please see the JPG image attached to this post named "TrianglesInContact.JPG". Then extrude these triangles into the screen/page such that there are now two triangular prisms in contact with each other at one of their edges. Along the direction of the extrusion, there are 13 linearly-spaced rays that travel into the prisms in the direction shown in that JPG image.

In the Geometry of the two lenses, there are 114 such edges of physical contact.

In both simulations, I was trying to calculate the spatial distribution of the surface temperature of the lens(es) in the scenario where the light rays were a heat source. I used the Time-Dependent Study and calculated this distribution at a time t at which I knew that the maximum and minimum surface temperatures were at steady-state. These calculations were done successfully.

The maximum and minimum surface temperatures calculated in two lenses (Simulation #2) were lower than those calculated in one lens (Simulation #1). This is a point of concern. May this difference be explained? Please note that the heat transfer physics that were considered were:

• The progressive absorption of light rays by the lens material as the rays travelled through the material. This is the origin of the heat in the simulation.

• Thermal conduction within the lens material.

• Emission of thermal radiation from the lenses, i.e., surface emissivity.

Please note that there was no medium surrounding the lenses - i.e., vacuum surrounded the lenses.

In both simulations, the same settings were used, except that, in Simulation #2, there were around 2 times the number of Mesh elements that were in Simulation #1. The Mesh Statistics from both simulations are attached to this post ("MeshStatistics_Simulation1.JPG" and "MeshStatistics_Simulation2.JPG"). This is because of the extra lens in Simulation #2. Also, by "same settings", I mean that the same values were used for all parameters in all Physics modules, with the same subnodes in the Physics modules, and with the same Multiphysics node activated. The Physics modules that were used in both simulations were the "Geometrical Optics" and the "Heat Transfer in Solids" modules. These were coupled via the Ray Heat Source Multiphysics node.

In the "Geometrical Optics" module, the subnodes that were activated were: Medium Properties; Material Discontinuity; Ray Properties; Release from Grid; Deposited Ray Power (domain); Ray Termination, which defined a rectangular prism that contained all the lenses. The beam that was simulated was monochromatic.

In the "Heat Transfer in Solids" module, the subnodes that were activated were: Solid; Initial Values; Thermal Insulation; Surface-to-Ambient Radiation. For all boundaries except two, the "Surface-to-Ambient Radiation" subnode overrode the "Thermal Insulation" subnode. These two boundaries were flat and had surface normals perpendicular to the travel direction of the rays.

I am not sure if, with only the physics settings mentioned above, thermal conduction works in the software such that it is possible for heat to flow from one domain to another domain through edges that are in contact with each other. May you confirm whether or not this is possible?

Please note that, in both simulations, a Swept Mesh was used. The values of the all the parameters of the Swept Mesh were kept the same over both simulations.

Please also note that the monochromatic light had a photon energy of 23 keV (2 significant figures), i.e., an energy in the x-ray energy range. Therefore, the refraction done by the lenses is very small to the extent that it is reasonable to approximate the path of the light ray shown in "TrianglesInContact.JPG" as a straight line.