Re: [ESPResSo-users] Calibration of friction/mass

[Owen Hickey]

Hey Markus,

The Verlet algorithm normally works as follows:

v+=0.5*F*dt
x+=v*dt
*recalculate forces, including the LB coupling force, LB gets whole force here*
v+=0.5*F*dt //note here only half of the force is applied to the particles

The issue is that the Velocity verlet works as follows on the first step:

*recalculate the forces ignoring the LB coupling force if something
has change in the simulation setup*
v+=0.5*F*dt
x+=v*dt
*recalculate forces, including the LB coupling force, LB gets whole force here*
v+=0.5*F*dt //note here only half of the force is applied to the particles

One can avoid the first recalculation of the forces by using the
reuse_forces flag when calling integrate from Tcl. The issue is of
course that when one redefines potentials, or in your case changes the
velocity of the particles, the appropriate forces also change. In a
sense you are only really missing the first half of the first time
step, since in the second half of the Verlet step the forces will be
recalculated with the coupling force. The issue is that there is no
obvious way of applying only half of the force to the LB fluid without
calculating the coupling force twice, which would obviously be
inefficient. One could certainly due something where recalc_forces
flag is used and the LB force coupling is calculated twice at every
integration step (the bare friction would then also be off by a factor
of two).

To summarize, it is doable, the question is if it is worth the effort
of hacking in a solution...

Hope that helps,
Owen

On Tue, May 19, 2015 at 5:41 PM, Wink, Markus
<address@hidden> wrote:
> Hello everybody,
>
>
>
> I exhibit the same behavior. To be more precisely, it happens in the first
> integration step.
>
> I used the OIF Implementation and wanted to perform the mass/friction
> calibration concerning doi:10.1016/j.camwa.2012.01.062.
>
> The object was put into a fluid at rest, all surface nodes were set to an
> initial velocity and the exponential decay of the velocity with time was
> investigated.
>
>
>
> The problem seems to be that ,when performing the first integration step,
> the LB forces are not included in the particle force (concerning the warning
> message). Having a look in integrate.cpp shows, that “transfer_momentum = 0”
> in line 245. Thus we don’t have a fluid/particle-coupling there since no
> momentum transfer between fluid and particle occurs (?). Why is that the
> case?
>
>
>
> I tried to perform an integration step before setting the surface nodes to
> the initial velocity (I thought this might help). Nevertheless, the warning
> always occurs in the first integration step after setting the velocity. Is
> there any way to avoid that to get a smooth exponential decay of the
> velocity? Does one really need to bother about the particle not being
> coupled to the fluid in the first integration step?
>
>
>
> Greetings and thanks for your answer,
>
>
>
> Markus
>
>
>
>
>
>
>
> Von: address@hidden
> [mailto:address@hidden
> Im Auftrag von Holst, Lena
> Gesendet: Montag, 18. Mai 2015 17:29
> An: 'address@hidden'
> Betreff: [ESPResSo-users] Calibration of friction/mass
>
>
>
> Hello,
>
>
>
> I like to simulate a round object in a fluid.
>
> Now, I try to calibrate the relation mass/friction.
>
> I have a fixed mass of 15.004 and change the friction.
>
> The Reynoldsnumber has a value of 0,0579 and the timesteps are 0,05us.
>
> The problem is, that the velocity does not shows an exponent trend at the
> second timestep, the value is too big. For better understanding there is a
> picture in the appendix.
>
> The blue line is the computed trend, the red and green simulated trends.
>
> I have try the same at a mass of 1 and a friction of 11, there I have the
> same problem.
>
> Could you help me?
>
> Thank you.
>
>
>
> Kind regards,
>
>
>
> Lena Holst
>
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