Copying the particle distribution function

Palabos General
#1

Hi,

I am wondering how to copy the particle distribution function from a simulation result. I used the “saveAsciiData(lattice.getDataAnalysis().getPopulations()” but some of the values I get are negative, which should not be the case. They are also small that I do not think they will add up to 1 (one), the density of the fluid.

Regards

#2

This is the right way of getting the populations. If some of them are negative, I’d suspect that your simulation is likely to be numerically unstable, or close to.

#3

Well, to verify that I was on the right track, I copied the populations for the cavity2d examples and I get very similar results…some negative values and most of them are of the order of e-6 and e-5 such that their addidtion will not be equal to 1. Thus I wonder how the output says average density =1,…while a peep at the populations does not show that the sum is equal to 1.

#4

Aaah, right, sorry for the confusion. If I am not mistaken, the value which is stored in OpenLB is not f_i, but (f_i-t_i). The purpose of this is to gain numerical accuracy, as it is explained here. Therefore, the sum of these is approximately zero, and you need to add 1 to get the density. For the actual value of the f_i, you need to add the lattice constant t_i (4/9, 1/9, or 1/36 for D2Q9) to each direction.

Hope this helps.

#5

Waow, now I need to digest this information very well. I understand it but will have to try it out because I have been stalled in my research as I could not understand why the values were outrageous.

Thanks as I wonder how I would have done with out your suggestion.
I will get back to you as soon as I try it out.

#6

Hi,

It worked add my analysis of the results showed that it was equal to 1 (one) when I accounted for the weights. However, my file is too big, close to 2.5G. I have therefore decided not to store the distribution functions in a file, but to use them directly for my analysis. There again, I faced another problem as the method of dynamic object I used, as explained in the manual and in the templates, would not work.

I have posted an inquiry in this forum and I hope I can get the solution.