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Re: [ESPResSo-devel] [ESPResSo-users] Simulation of fluid using LB-metho

 From: Ulf Schiller Subject: Re: [ESPResSo-devel] [ESPResSo-users] Simulation of fluid using LB-method Date: Mon, 27 Jan 2014 18:35:42 +0100 User-agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:17.0) Gecko/20130107 Thunderbird/17.0.2

```Hi,

```
can anybody explain the rationale of the following lines to me? I'd have expected an exponent of either -1 or 2 for the grid spacing, depending on whether ext_force is a force or a force density. In lattice units there is no difference.
```
#ifdef EXTERNAL_FORCES
// unit conversion: force density
```
lbfields[index].force[0] = lbpar.ext_force[0]*pow(lbpar.agrid,4)*tau*tau; lbfields[index].force[1] = lbpar.ext_force[1]*pow(lbpar.agrid,4)*tau*tau; lbfields[index].force[2] = lbpar.ext_force[2]*pow(lbpar.agrid,4)*tau*tau;
```#else

Thanks and best wishes,
Ulf

On 01/27/2014 06:05 PM, Wink, Markus wrote:
```
```Hello,

thanks a lot for the fast reply and your help. I will test the script
with the new parameters and will give a feedback of the performance.

Anyway, I still have two questions left:

1) You wrote, that the dynamic viscosity nu should be one. In the user’s
guide it is written (page 158), that one inserts the kinematic viscosity
nu/rho for the lbfluid command. Sure, since rho equals unity in MD
units, that makes no difference, I am just asking to make that point clear.

2) How do you get such an “odd” number for the force density? In SI
units, it is 64.000kg/m^2/s^2. If, as you proposed, [m] = 10^-15kg,
[x]=10^-6m and [t]=10^-6s, shouldn’t the force density be 6.4*10^-5 in
MD units? Sorry, for being so confused.

3) When talking about force in the lbfluid command, one is really
talking about the force density and not about the force on every node of
the LB-grid. That is at least, what I understood while reading the user
guide. Am I right with that point?

Thanks again!

Greets,

Markus

*Im Auftrag von *Georg Rempfer
*Gesendet:* Montag, 27. Januar 2014 17:07
*Betreff:* Re: [ESPResSo-users] Simulation of fluid using LB-method

Forgot to attach the file. Here you go.

2014-01-27 Georg Rempfer <address@hidden

Sorry, I had missed the attachment in the first email. A remark
unrelated to the performance issue:

I would not feel comfortable using such extreme parameters
(density~10^10 and viscosity~10^-3) with lattice-Boltzmann. LB only
produced proper hydrodynamic behavior in a limited parameter range. By
choosing values around 1, you should be on the safe side. Also I got a
force density of 52896 kg/m^2/s^2 using your units. Attached you find a
derivation of a more suitable unit system. In that system, your force
density would be 5.29e-5. Independently of those performance issues, I
think that should speed up your calculations considerably (looking at
the flow in calculation time makes it seem much lighter and more viscous
now, so it will reach its stationary state much faster).

2014-01-27 Wink, Markus <address@hidden

>
> Hello everyone,
>
>
>
> First of all: thanks for your support.
>
>
>
> I thought that I sent the *.tcl script with my first post. Anyway.
Attached you will find the scripts and the config.log.
>
>
>
> Greetings,
>
>
>
> Markus
>
>
>
>
>
> Von:
[mailto:espressomd-users-bounces+markus.wink
<mailto:address@hidden>] Im Auftrag von Olaf Lenz
> Gesendet: Montag, 27. Januar 2014 15:05
> An: Georg Rempfer
>
>
> Betreff: Re: [ESPResSo-users] Simulation of fluid using LB-method
>
>
>
> Maybe you can also send your config.log? Then we can see how the
program is compiled.
>
>
>
> Olaf
>
>
>
> 2014-01-27 Georg Rempfer <address@hidden
>
> Hello Markus,
>
>
>
> the friction parameter won't have an influence on your simulation as
long as you don't use any explicit particles. Can you provide your
simulation script? If you give it to me, I will run and time it here.
>
>
>
> Greetings,
>
> Georg
>
>
>
> 2014-01-27 Ulf Schiller <address@hidden
>
>
>
> On 01/27/2014 02:07 PM, Wink, Markus wrote:
>
> Hello,
>
> I hope I understood your point correctly. I have run the script with
two calls of "integrate 1000", having a box of 50^3 and an agrid of 1,
resulting in 125*10^6 site updates.
> It took 22 min 2s for the simulation resulting in roughly 19k SUPS.
This is far below your expected 10M. I have no clue what the reason for
that is.
>
> The binary of Espresso I have created only had "LB", "LB_BOUNDARIES"
and "EXTERNAL_FORCES" activated.  Running the testscript "lb_planar.tcl"
(box of length 12, total integrate of 5000) takes about 14 sec,
resulting in ~0.6M SUPS.
>
>
>
> Hi,
>
> well, Espresso has of course some overhead since it's not a pure LB
code, but ~200k (that's what you get I think) resp. 600k seems pretty
slow. What CPU are you running on?
>
> Does anyone else have an estimate of how many site updates per second
they get?
>
> Cheers,
> Ulf
>
>
>
> -----Ursprüngliche Nachricht-----
> Von:
[mailto:espressomd-users-bounces+markus.wink
<mailto:address@hidden>] Im Auftrag von Ulf Schiller
> Gesendet: Montag, 27. Januar 2014 12:16
> Betreff: Re: [ESPResSo-users] Simulation of fluid using LB-method
>
> On 01/27/2014 11:54 AM, Wink, Markus wrote:
>
> 3) Even though, the velocity profile saturates, it takes pretty long
> (time scale of days). The saturation sets in at about 350 times the
> "integration 1000" command. This corresponds to a time scale of ms in Si
> units. Besides using OpenMpi or the GPU impementation, is there anyway
> to speeden up the simulation? For example by adjusting the parameter tau
> in the lbfluid command?
>
>
> Hi,
>
> that sounds a bit strange to me. If your system is 50^3 and you
> integrate 1000 steps, that gives 125*10^6 site updates. On a reasonably
> modern CPU, a reasonable LB code should get to around 10M SUPS (million
> site updates per second), that is, one call of "integrate 1000" should
> not take much more than 10 seconds, so 350 calls roughly one hour not
> days. Can you check how many site updates per second you get on your
system?
>
> Cheers,
> Ulf
>
>
>
> --
> Dr. Ulf D. Schiller                        Building 04.16, Room 3006
> Institute of Complex Systems (ICS-2)       Phone: +49 2461 61-6144
<tel:%2B49%202461%2061-6144>
> Forschungszentrum Jülich, Germany          Fax: +49 2461 61-3180
<tel:%2B49%202461%2061-3180>
>
>
>
>
>
>
>
> --
>
> Dr. rer. nat. Olaf Lenz
>
> Institut für Computerphysik, Allmandring 3, D-70569 Stuttgart
>
> Phone: +49-711-685-63607 <tel:%2B49-711-685-63607>

```
```

--
Dr. Ulf D. Schiller                        Building 04.16, Room 3006
Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
Forschungszentrum Jülich, Germany          Fax:     +49 2461 61-3180

```

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