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Re: [ESPResSo-users] Calibration of friction/mass
From: |
Marcello Sega |
Subject: |
Re: [ESPResSo-users] Calibration of friction/mass |
Date: |
Tue, 19 May 2015 18:06:58 +0200 |
Hi,
to obtain the friction, it should be enough to compute the force
autocorrelation at equilibrium [e.g. Kubo, Rep. Prog. Phys. 29 255
(1966) doi:10.1088/0034-4885/29/1/306 eq 8.15 ].
This way you don't need to perform out-of-equilibrium simulations of
any kind, neither fitting exponential decays -- which are anyway not
exponential when the hydrodynamic interaction is involved, nor
terminal velocity calculations.
This should bypass the 1st step problem.
Hope this helps,
M.
On Tue, May 19, 2015 at 5:51 PM, Owen Hickey <address@hidden> wrote:
> 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
>>
>> _________________________________________________________
>>
>> Fraunhofer ICT-IMM
>>
>> Gruppe Modellierung / Modeling Group
>>
>> Abteilung Zukunftstechnologien / Future Technologies Department
>>
>> Carl-Zeiss-Straße 18-20, 55129 Mainz, Germany
>>
>> Phone +49 6131 990-396 / Fax +49 6131 990-205 / Mobile +49 1577-5327027
>>
>> address@hidden / www.imm.fraunhofer.de
>>
>>
>
--
University of Vienna, Institute of Computational Physics