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From: Michele Trenti <trenti_at_stsci.edu>

Date: Sun, 29 Oct 2006 22:45:31 -0500 (EST)

Hi Melanie,

I am using Grafic-1 / Grafic-2 to generate the IC and the conversion

using sqrt(a) seems to work nicely for me.

I have tested the procedure rather extensively, including checking in a

couple of runs the power spectrum of density fluctuations at z=0 against

public snapshots from the VIRGO consortium.

I remember however to have read a message similar to yours in the Gadget

list a while ago... so it would be good to clarify the situation once for

all.

If you like to run a test, I can send you my routine to convert a Grafic

snapshot into a Gadget one, so you can check if the result is the same as

using your program.

Cheers,

Michele

Michele Trenti

Space Telescope Science Institute

3700 San Martin Drive Phone: +1 410 338 4987

Baltimore MD 21218 U.S. Fax: +1 410 338 4767

" We shall not cease from exploration

And the end of all our exploring

Will be to arrive where we started

And know the place for the first time. "

T. S. Eliot

On Sun, 29 Oct 2006, Melanie Jo Clarke wrote:

*>
*

*> I have been attempting to use Grafic-1 initial conditions to run small scale
*

*> Gadget2 simulations and have run into the following problem with velocity
*

*> units.
*

*>
*

*> The manual suggests to divide by sqrt(a) to convert from physical velocity to
*

*> Gadget units. When I do this, I find that the Gadget simulation does not
*

*> evolve far enough compared to a P3M simulation run with the same initial
*

*> conditions.
*

*>
*

*> As I understand it, the internal units for velocity are dx/d(ln a), so to
*

*> convert from physical velocity, dx/dt, to these units, I should divide by d(ln
*

*> a)/dt. This factor is a*sqrt(omega_m/a + omega_v*a^2 + 1 - omega_m - omega_v),
*

*> where omega_m is the matter density ratio and omega_v is the vacuum energy
*

*> density ratio. This factor reduces to sqrt(a) in the case that omega_m=1 and
*

*> omega_v=0. When I use this factor to convert physical velocity to Gadget
*

*> units, I get good agreement between P3M and Gadget simulations.
*

*>
*

*> Is my interpretation correct? If so, can the Gadget2 manual be corrected so
*

*> that future users don't run into this problem?
*

*>
*

*> Thank you,
*

*> Melanie Clarke Dosaj
*

*>
*

*>
*

*>
*

*>
*

*> -----------------------------------------------------------
*

*>
*

*> If you wish to unsubscribe from this mailing, send mail to
*

*> minimalist_at_MPA-Garching.MPG.de with a subject of: unsubscribe gadget-list
*

*> A web-archive of this mailing list is available here:
*

*> http://www.mpa-garching.mpg.de/gadget/gadget-list
*

*>
*

Received on 2006-10-30 04:45:38

Date: Sun, 29 Oct 2006 22:45:31 -0500 (EST)

Hi Melanie,

I am using Grafic-1 / Grafic-2 to generate the IC and the conversion

using sqrt(a) seems to work nicely for me.

I have tested the procedure rather extensively, including checking in a

couple of runs the power spectrum of density fluctuations at z=0 against

public snapshots from the VIRGO consortium.

I remember however to have read a message similar to yours in the Gadget

list a while ago... so it would be good to clarify the situation once for

all.

If you like to run a test, I can send you my routine to convert a Grafic

snapshot into a Gadget one, so you can check if the result is the same as

using your program.

Cheers,

Michele

Michele Trenti

Space Telescope Science Institute

3700 San Martin Drive Phone: +1 410 338 4987

Baltimore MD 21218 U.S. Fax: +1 410 338 4767

" We shall not cease from exploration

And the end of all our exploring

Will be to arrive where we started

And know the place for the first time. "

T. S. Eliot

On Sun, 29 Oct 2006, Melanie Jo Clarke wrote:

Received on 2006-10-30 04:45:38

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