Discussion Forum

Hi,

I am currently doing a 2D simulation in COMSOL 5.1 of a rectangular cavity enclosed by 1D surfaces. The cavity has a length of 1 m and height 0.1 m. The top and bottom surfaces are diffusive surfaces with Temperature =353.15 K , epslon=0.5 and Temperature=293.15 K, epson 0.8 respectively.The lateral sides are assumed to be diffusive mirrors .The surface to surface radiation physics is used. I want to calculate the view factors with respect to the various surfaces and not the ambient. I have defined also 4 sets of variables (variables1, variables 2, variables 3, variables 4). Within the sets i define variables "up","down","left" and "right". In each set i define only 1 boundary and for the position it represents (lets say its the upper surface) i write under the respective variable that it is suppose to calculate rad.J for that surface (so for upper surface, the variable "up" would have rad.J as expression while maintaining the others expressed as 0). The same is done for the other surfaces. Also for each surface i define an integral coupling . intop_up, intop_down, intop_left, intop_right for the upper, lower, left and right boundaries respectively.

I then calculate the view factor F_up_down and it gives me a result of 0.91058. I used the following expression to do so (under global evaluation):

intop_down(comp1.rad.radopu(up,0))/intop_up(up)

But then I try to calculate F_down_up using the following expression:

intop_up(comp1.rad.radopd(0,down))/intop_down(down)

and i get 0. Can someone tell me whats wrong? Also how do I calculate it for the lateral walls?

Also is there a possibility to calculate a sort of view factor varying with position along a surface, considering the radiosity of a given point on a surface and not all of it?