| Author | Message | ||
     sk484@cam.ac.uk (seongmin) New member Username: seongmin Post Number: 1 Registered: 03-2006 |
Hi, can you explain the unit of emission average, tranport average and absorption average and physical meaning. And in the exmple, what's teh unit of temperature? I need 3d cylindrical rad.flow but first I need to know this things. Regards | ||
| Robert G. Nelson (rgnelson) Moderator Username: rgnelson Post Number: 566 Registered: 06-2003 |
FlexPDE does not have any built-in system of units of measure. You can use any units you want, as long as you make all your equations and parameters consistent. The transport of radiation is a complex phenomenon in which the absorption and emission characteristics of the medium depend on the frequency of the radiation. Absorption and emission lines, bound-free electron transitions, and other processes introduce a complex structure into the cross-section. In the usual case, there is a wide distribution of frequencies in any radiation field. For example, in a Hohlraum,the distribution is the Black-Body or Planck distribution B(nu;T) = 2*h*(nu^3/c^2)/(exp(h*nu/k*T)-1) In order to make the computation of radiative transfer practicable, some form of averaging, either over the entire spectrum (if the field is near equilibrium with the material) or over a selected set of frequency groups (if the system is far from equilibrium) must be used. The common averages used for this purpose are three: The emission average or Planck Mean cross-section is the average weighted by the emission spectrum B(nu;T) at the material temperature. The transport average or Rosseland Mean cross-section is the inverse of the average of mean-free-path weighted by the Temperature derivative of B. The absorption average cross-section is the average weighted by the actual frequency distribution of radiation at each specific point in the field. The subject is amenable to various approximations, depending on the range of conditions you wish to handle in your analysis. I suggest you consult a text appropriate to your problem. See for example "Foundations of Radiation Hydrodynamics" by Mihalas and Mihalas. Other texts may be more appropriate to your specific case. |