shock compression of material

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1

Dear all,
I wish to do some shock simulation to calculate Hugoniot points and as well as EOS. From literature and lammps discussion archive I am bit used to with NEMD type shock simulation i.e. “momentum mirror” method. But as my current interest is to use reactive forcefield like reax/c to describe water as our target system, then realizing the computational cost I am trying to use MSST method with reax/c for water. Regarding this I have some questions, although Qs are not really directly related to lammps issues yet please allow me.

  1. I’m getting the values of TD state variables which represent the Hugoniot curve of water properly but from rdf analyses I’m not getting the freezing of water to iceVII which have confirmed by the past literature by using this reax and NEMD. Here my confusion is that is there any restriction of MSST that it cannot produce the structural reconstruction and/or phase transition properly or this is due to any wrong in my simulation ? I have gone through the documents of MSST but have not seen this type of restriction.

  2. By MSST as I understand that, it can achieve the steady state of the sample assuming that a shock with specified shock velocity have passed through it. So can it be true to say that it can achieve the final state as gained by NEMD simulation ?

But in case of my simulation, from NEMD it is well known that after passing the shock front it reaches a equilibrium state and after that the P and T decreases accordingly when the shock front moves further i.e. when the front travels a considerable distance. But after a msst “ns” simulation when it is showing that the sample is beyond 100nm then also T and P is not decreasing, rather keeps the equilibrium value at per Us. Can anybody make some comment to clear.

Thanks for your help and comment.

Liu.

2

Dear all,
I wish to do some shock simulation to calculate Hugoniot points and as well
as EOS. From literature and lammps discussion archive I am bit used to with
NEMD type shock simulation i.e. "momentum mirror" method. But as my current
interest is to use reactive forcefield like reax/c to describe water as our
target system, then realizing the computational cost I am trying to use MSST
method with reax/c for water. Regarding this I have some questions, although
Qs are not really directly related to lammps issues yet please allow me.

1. I'm getting the values of TD state variables which represent the Hugoniot
curve of water properly but from rdf analyses I'm not getting the freezing
of water to iceVII which have confirmed by the past literature by using this
reax and NEMD. Here my confusion is that is there any restriction of MSST
that it cannot produce the structural reconstruction and/or phase transition
properly or this is due to any wrong in my simulation ? I have gone through
the documents of MSST but have not seen this type of restriction.

I am not aware of a such limitation. It it likely your setup and MSST
parameters causing the difference: size too small, shock velocity too
high, rise time too fast, etc.

2. By MSST as I understand that, it can achieve the steady state of the
sample assuming that a shock with specified shock velocity have passed
through it. So can it be true to say that it can achieve the final state as
gained by NEMD simulation ?

NEMD, by definition, does not have a final state so you have nothing
to compare to. You can, however, compare to a slice near the
impactor. Whether you get comparable results or not depends on your
setup and MSST parameters.

But in case of my simulation, from NEMD it is well known that after passing
the shock front it reaches a equilibrium state and after that the P and T
decreases accordingly when the shock front moves further i.e. when the front

No, that is not true. If no reactions occur and P/T drops then the
prior state is not an equilibrium state. If reactions occur, then it
is also not an equilibrium state.

travels a considerable distance. But after a msst "ns" simulation when it is
showing that the sample is beyond 100nm then also T and P is not decreasing,

This means a steady state has been reached.

Ray

3

Dear Ray,

Thanks for your comments.

I was comparing the MSST with a slice of a NEMD sample. In NEMD as the shock front moves furthest edge of the sample, the T/P of that slice drops from the equilibrium state, isn’t it ? So the T/P of the slice in case of NEMD attains a maximum value of T/P when just beyond the front, but then drops and the slice cooled down when the front moves away. I wonder why the temperature and pressure are not decreasing from the shocked state i.e. equilibrium state ? Why my msst sample not cooling down although the shock front travels a long distance ~100 nm ?

I think I have meshed up equilibrium state/steady state with shocked state. Please comment.

Thanks for your help.

4

Dear Ray,
Thanks for your comments.
I was comparing the MSST with a *slice* of a NEMD sample. In NEMD as the
shock front moves furthest edge of the sample, the T/P of that slice drops

What does "moves furthest edge of the sample" mean? Are you saying
when the shockfront moves away from the impactor? Or when the
shockfront reaches the free surface on the ohter side?

from the equilibrium state, isn't it ? So the T/P of the slice in case of

No, the state achieved right behind the shock front is not an equilibrium state.

NEMD attains a maximum value of T/P when just beyond the front, but then
drops and the slice cooled down when the front moves away. I wonder why the

These max values of T/P right behind the shock front are due to shock
and are not in equilibrium.

temperature and pressure are not decreasing from the shocked state i.e.
equilibrium state ? Why my msst sample not cooling down although the shock

Shocked state != equilibrum state.

These are not LAMMPS questions but shock physics questions. It is
best if you talk to your advisor or read a good book on the topic.

Ray