Boundary treathment in multi-body sim + radiation
Posted: 30 May 2023, 17:22
Hello again,
First of all thank you for your patient, I know I have asked so many basic questions; I hope you can guide me as before:
I am simulating a hot object, inside a box; the bot T is fixed and constant, but the air T and the object T can change.
My goal is to simulate and calculate the T and air flow (natural convection), Especially ! while considering the radiation from the hot surface, and back reflection of other surfaces.
I believe, after some time, other surfaces can also act as secondary radiators, and of course reflection of the walls can increase the T of the hot surface as well.
Usually, engineers will simplify the question and only simulate the air, but before that, I need to make sure about the reaction of boundaries.
Noslip
1 normally, when there is a fluid in contact with a solid, the surface will be nonslip. I believe that I only need to apply the noslip condition to the boundary of my fluid, but just to be sure, please let me know if I need to check the nonslip condition at the boundary of my solid as well or not?
(I believe it is not needed, since the navier-stoke is not active for the solid body, but better safe to ask than sorry)
Radiation
2 When I want to have radiation, I need external T to be defined (?)(is it only for idealized or for gray as well?)
3 When the outside of my boundary has a constant T, it is fine, but how about when there is a heat flux coming to my boundary?
4 How about when two boundaries are starting at the same initial T, but I expect one of the surfaces to get hot fast ?
General Radiation Q
5 When two boundaries do not have a fixed T or flux, but they are under effect of other elements in the simulation, how can I apply an emissivity to them, and force Elmer to calculate the reflections and secondary radiation?
6 Is the external T, in the boundary condition, like dirichlet T (meaning it will be fixed and overwrite everything through the simulation) or is it like initial T (some starting T and it can be changed after some iterations)
or maybe there is a third option....?
Redundancy Q
7 This is not a short question as it seems, but I am wrried about the redundancies of applying these 3 factors multiple time; I know it is case dependant, but :
7.1 _ nonslip : applying this condition both at the solid boundary and fluid boundary.
7.2 _ emissivity : applying the emissivity value both at the material property and boundary condition after idealized radiation.
7.3 _ temperature : when I have heatflux/constant T from one boundary/surface, entering to another, Do I need to apply it both at my hot object and as an external T in my receiving object ?
Some visualization of my question + some other scenarios are demonstrated with the following pictures:
First of all thank you for your patient, I know I have asked so many basic questions; I hope you can guide me as before:
I am simulating a hot object, inside a box; the bot T is fixed and constant, but the air T and the object T can change.
My goal is to simulate and calculate the T and air flow (natural convection), Especially ! while considering the radiation from the hot surface, and back reflection of other surfaces.
I believe, after some time, other surfaces can also act as secondary radiators, and of course reflection of the walls can increase the T of the hot surface as well.
Usually, engineers will simplify the question and only simulate the air, but before that, I need to make sure about the reaction of boundaries.
Noslip
1 normally, when there is a fluid in contact with a solid, the surface will be nonslip. I believe that I only need to apply the noslip condition to the boundary of my fluid, but just to be sure, please let me know if I need to check the nonslip condition at the boundary of my solid as well or not?
(I believe it is not needed, since the navier-stoke is not active for the solid body, but better safe to ask than sorry)
Radiation
2 When I want to have radiation, I need external T to be defined (?)(is it only for idealized or for gray as well?)
3 When the outside of my boundary has a constant T, it is fine, but how about when there is a heat flux coming to my boundary?
4 How about when two boundaries are starting at the same initial T, but I expect one of the surfaces to get hot fast ?
General Radiation Q
5 When two boundaries do not have a fixed T or flux, but they are under effect of other elements in the simulation, how can I apply an emissivity to them, and force Elmer to calculate the reflections and secondary radiation?
6 Is the external T, in the boundary condition, like dirichlet T (meaning it will be fixed and overwrite everything through the simulation) or is it like initial T (some starting T and it can be changed after some iterations)
or maybe there is a third option....?
Redundancy Q
7 This is not a short question as it seems, but I am wrried about the redundancies of applying these 3 factors multiple time; I know it is case dependant, but :
7.1 _ nonslip : applying this condition both at the solid boundary and fluid boundary.
7.2 _ emissivity : applying the emissivity value both at the material property and boundary condition after idealized radiation.
7.3 _ temperature : when I have heatflux/constant T from one boundary/surface, entering to another, Do I need to apply it both at my hot object and as an external T in my receiving object ?
Some visualization of my question + some other scenarios are demonstrated with the following pictures: