# Monte Carlo - Number of Iterations?

 Register Blogs Members List Search Today's Posts Mark Forums Read

 January 31, 2007, 09:11 Monte Carlo - Number of Iterations? #1 tsimon Guest   Posts: n/a Hi, I am trying to use the Monte Carlo radiation model in order to calculate, at a first stage, geometric view factors for complex geometries based on radiation heat transfer values from one surface to another. From what I've read in the manuals (CFX10) and tried with some simple geometries (with known analytical solutions like two perpendicular planes), it seams to me that the results that come from the first iteration (radiation variables like Radiation Wall Heat Flux) are, probably, better (or worse depending on the geometry parameters) than those of the second ore of the nth iteration. In other words, it seams to me that more iterations don't obligatory bring to better radiation results once we are dealing with a statistical (MC) model (no rms) that has to be completely solved each iteration. Is this so? Could any one say something about this? To calculate the Geometric View Factors based on the heat exchanges between the surfaces (transparent medium), I'm assuming air absorption coef. equal to 0. Even with a great number of histories (1.000.000) the %SD stays by 100% for some cases. Does any one knows what really means this parameter and how is it calculated? Once I verify for this cases that the numerical solution is quite good (near the analytical one)? Thanks in advance. tsimon

 January 31, 2007, 16:48 Re: Monte Carlo - Number of Iterations? #2 opaque Guest   Posts: n/a Dear tsimon, The heat exchange between boundaries is recomputed every iteration.. As you said, the statistical nature of MC model will not give you the same results between iterations. You mentioned that %SD is 100%, is is for the surfaces, or fo r the volume? %SD stands for standard deviation. At each radiation surface/volume a mean value is obtained by averaging the solution of "several accumulators" (multiple calculations for the same quantity). Then, a standard deviation is computed. The maximum standard deviation among all surfaces, or volumes is then reported. Opaque..

 February 8, 2007, 09:17 Re: Monte Carlo - Number of Iterations? #4 opaque Guest   Posts: n/a Dear tsimon, My apologies I did not answer earlier.. you asked: " ..doesn't means that the results are better in the later iterations?" Assuming the temperature distribution has not changed, the MC results should be nearly as good as the previous iterations. Any RMS target used for a radiation calculation (no flow) using MC/DTM only applies to the energy equation. Again, if the k is not zero you need iterations to solve the temperature for the radiation solution to be valid. I assume that your media is absorbing radiation. For a transparent material with all temperature specified boundaries, the radiation solution is obtained in one iteration. Well, distributing the histories (photons) among the boundaries is kind of an art.. ANSYS CFX does not distribute them randomly, but it uses the emission as a weigth.. That is, if one of those 10 surfaces emitts 90% of the total emission in the domain, it will get 90 photons out of 100. Leaving the other boundaries to divide last 10 among themselves (again base on their contribution to the total emission). The main assumption is that boundaries that do not emitt enough energy are not assigned any photons (the minimum is 10 if recall correctly). Basically, the photon distribution is self-adaptive. Hope the information helps, Opaque..

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Luiz Eduardo Bittencourt Sampaio (Sampaio) OpenFOAM Mesh Utilities 41 January 17, 2013 02:43 bookie56 OpenFOAM Installation 8 August 13, 2011 04:03 Lord Kelvin OpenFOAM 6 April 12, 2011 11:24 sebastian OpenFOAM Running, Solving & CFD 13 October 21, 2010 05:25 msrinath80 OpenFOAM Running, Solving & CFD 9 July 22, 2007 02:58

All times are GMT -4. The time now is 20:33.