Re: [ESPResSo-users] writing fluid properties

[Stefan Kesselheim]

Hallo Farnoosh, hallo mailinglist,

On 10/25/2011 07:41 PM, Farnoosh Farahpoor wrote:

Dear Espresso developers

I am using Lattice Boltzmann method in Espresso. Is there any way to write fluid properties systematically in a file? Something like blockfile in MD part? I am familiar with "lbnode x y z print args" command but I want to write whole nodes' velocity or density for example in a file. 

Many thanks for your concern

Farnoush

In Espresso 3.* there is is no feature to write out the whole fluid. The main reason is, that you are rarely interested in the fluid at a certain instant but you need to average over many configurations if the fluid is thermalized, because usually the noise is much larger than the average fluid velocity.

Averageing is definitely something you need to do on the TCL level, e.g. using TCL-lists. In many cases it is however useful to average only along certain lines or planes. In the correlation framework we have written observables that allow you to average automatically, but this is still experimental.

If you want to write out the fluid so that you can stop your simulation and restart it later, you will need not only the velocities but the populations. This is currently still impossible, but on my list to be implemented.

If you are really interested in the fluid velocity at a certain instant in time you can still make a simple loop (not tested!):
set ofile [ "u.dat" "w"]
for { set i 0 } { $i < $box_l } { incr i } {
  for { set j 0 } { $i < $box_l } { incr j } {
    for { set k 0 } { $i < $box_l } { incr k } {
      puts $ofile "$i $j $k [ lbnode $i $j $k print u ]
   }
  }
}
close $ofile
 
I hope that helps.

Cheers
Stefan