# Re: velocity units

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

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
>
>
>
>
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