Dear steve,

As some part of this question is not related to lammps technical question, so please allow me for such post.

Dear all,

If one system have C-O-M velocity ( as the consequence of shock generation method,basically momentum mirror method ) how can I calculate global pressure, where lammps thermo variable and compute stress/atom both would be affected by com velocity in the kinetic energy contribution term. Is there any idea how to do that ?

From the previous discussions regarding that by Ray and Oscar G, I got some idea basically how to compute stress locally that by using the velocity (after subtracting of the com velocity ), it was like-

Stress_xx = [ Sum(Force_xx of i) / (LyLz) ] + [ Sum(mass_i * Vx* Vx) / (LxLyLz)]. Here my question is, as a shock phenomenon is beyond hydro-static case so there xx, yy and zz comp of the stress tensor should differ significantly, so if I do average it out, would it be safe to say that this is the pressure, locally for a region ? From there, would it make sense if I do the above computation assuming the whole system as one group ?

I’ve done the above calculation for a local pressure (assuming a region as a group ) but I doubt about the result, may be I have done some wrong in implementation within lammps, here it is what I’ve done —

compute “property/atom fx” , “compute reduce/region fx” and used “vcm()” to calculate com velocity of a region. The Lx, Ly,Ly and mass was computed for a region as readily available in lammps. Is there anything which I have meshed up ?

One thing regarding shock physics:

If I calculate P from momentum conservation i.e. P = density_unshocked * Us*Up so it shocked signify that it would be the pressure at the shock-precursor/front, is it wrong ? If so it should be well merged with the computed local pressure ( by the above calculation) just behind the shock front if I safely track the shock front by Mott-Smith density profile or by any means, is it so ?

If I wish to study the (P Vs T), or (P Vs density) profile so if I compute this variables by averaging over a fixed region, spatially averaged fixed volume /fixed no of particles bins, in a defined region whatever may be my sample length along shock, would it be correct ?

Thanks a lot

Rajdeep Behera

IOP, Bhubneswar