Hello guys,
I try to simulate very small particles with diameters about 1 micrometer.
Therefore I want to change my unit system from "SI" to another system.
I looked at the "ErgunTestMPI_cgs" tutorial, but some things are unclear to me:
If I change the unit system in DEM folder (in.liggghts; units), the unit system in CFD will also change. - Is this right?
(Otherwise I cannot understand why "g" in CFD/constant/ is set to 981.)
If I change the unit system to "micro", the velocity unit is "micrometer/microseconds".
If I want a velocity of 200 micrometers per seconds, I have to write 200.e-6 to "insertion_velocity".
But then there is following message:
ERROR: Fix insert/stream (id ins0): insertion velocity projected on face normal is < 1e-3.
That means the minimal velocity is 0.001 micrometer per microseconds, which means 1000 micrometer per seconds.
How can I realise lower velocities ?
Thank you for answering :)
NTT1508 | Thu, 07/09/2015 - 00:56
See previous discussions
Hi mate,
I think units of DEM and CFD should be consistent. If you use unit of DEM as CGS, you have to change unit of OpenFoam to CGS too. (e.g. g=9.81m/s2 (SI)===> 981cm/s2 (CGS)...). There are many discussions regarding this issue previously. You can easily find them. The manual also mentions about the feasibility of unit for different scale of geometry.
Regards,
mytxii | Thu, 07/09/2015 - 10:08
Velocity minimum
Thank's for your fast answer !
Okay, it makes sense to take same unit system in CFD and DEM.
But I cannot understand, if I use "micro" as unit system,why the minimum of velocity is 1e-3 micrometers per microseconds (1000 micrometer/second).
Because small geometries will also have low velocities for particle inseration. How can I fix this problem ??