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From: Yves Revaz <yves.revaz_at_obspm.fr>

Date: Fri, 30 Mar 2007 11:49:25 +0200

Dear Gadget list,

Using Gadget-2 with radiative cooling,

I encounter problems in reproducing the Temperature-Density distribution

of gas particles in a LCDM model (see for example Fig. 3 of Katz et al

96 or Fig.11 of

Springel & Hernquist 2002).

The overall distribution is correct, however, I fail to reproduce the

thinness

of the horizontal branch, corresponding to high density regions

((rho/rhom)>1e4, T<1e5K).

In my simulations, the temperature dispersion of the horizontal branch

is high,

with some particles having temperature up to 1e6 K !

This problem comes from the competition between cooling (dA/dt)_rad and

viscosity heating (dA/dt)_visc, where A is the entropy.

For a particle with a density (rho/rhom)>1e4 and temperature > 1e4K,

we have:

|(dA/dt)_rad| >> |(dA/dt)_visc| => dA/dt)_tot << 0,

the cooling dominates and the temperature quickly decreases. When the

temperature of

the particle goes below 1e4K, (dA/dt)_rad drops nearly to zero (cutoff

in the cooling function),

and the entropy variation is only due to the (dA/dt)_visc therm, which,

in some cases is so high

that the particle temperature instantaneously rises up to 1e6K !!!

In summary, in the horizontal branch, instead of being more or less

constant at 1e4K (equilibrium between

viscosity heating and radiative cooling), the temperature of the

particles oscillate between 1e4 and 1e5-1e6K.

This behavior is the result of the cooling and heating time scale, much

shorter than

the time-step imposed by the currant condition. The cooling is limited

by the condition

that :

dA/dt > -0.5 A.

Imposing also

dA/dt < A,

in not sufficient to damp the temperature oscillation.

There is probably a well known solution to this problem,

but I haven't found it in the literature.

Does anyone has a solution ?

Thanks in advance.

Yves

MY PARAMETERS

---------------

The simulation test contains 2*64^3 particles in 20 Mpc^3 h^-3

I use the following parameters :

ErrTolIntAccuracy 0.025

CourantFac 0.15

MaxSizeTimestep 0.03

MinSizeTimestep 0

ErrTolTheta 0.8

TypeOfOpeningCriterion 0

ErrTolForceAcc 0.005

DesNumNgb 32

MaxNumNgbDeviation 2

ArtBulkViscConst 0.8

MinGasHsmlFractional 0.25

SofteningGas 8.

SofteningHalo 8.

Date: Fri, 30 Mar 2007 11:49:25 +0200

Dear Gadget list,

Using Gadget-2 with radiative cooling,

I encounter problems in reproducing the Temperature-Density distribution

of gas particles in a LCDM model (see for example Fig. 3 of Katz et al

96 or Fig.11 of

Springel & Hernquist 2002).

The overall distribution is correct, however, I fail to reproduce the

thinness

of the horizontal branch, corresponding to high density regions

((rho/rhom)>1e4, T<1e5K).

In my simulations, the temperature dispersion of the horizontal branch

is high,

with some particles having temperature up to 1e6 K !

This problem comes from the competition between cooling (dA/dt)_rad and

viscosity heating (dA/dt)_visc, where A is the entropy.

For a particle with a density (rho/rhom)>1e4 and temperature > 1e4K,

we have:

|(dA/dt)_rad| >> |(dA/dt)_visc| => dA/dt)_tot << 0,

the cooling dominates and the temperature quickly decreases. When the

temperature of

the particle goes below 1e4K, (dA/dt)_rad drops nearly to zero (cutoff

in the cooling function),

and the entropy variation is only due to the (dA/dt)_visc therm, which,

in some cases is so high

that the particle temperature instantaneously rises up to 1e6K !!!

In summary, in the horizontal branch, instead of being more or less

constant at 1e4K (equilibrium between

viscosity heating and radiative cooling), the temperature of the

particles oscillate between 1e4 and 1e5-1e6K.

This behavior is the result of the cooling and heating time scale, much

shorter than

the time-step imposed by the currant condition. The cooling is limited

by the condition

that :

dA/dt > -0.5 A.

Imposing also

dA/dt < A,

in not sufficient to damp the temperature oscillation.

There is probably a well known solution to this problem,

but I haven't found it in the literature.

Does anyone has a solution ?

Thanks in advance.

Yves

MY PARAMETERS

---------------

The simulation test contains 2*64^3 particles in 20 Mpc^3 h^-3

I use the following parameters :

ErrTolIntAccuracy 0.025

CourantFac 0.15

MaxSizeTimestep 0.03

MinSizeTimestep 0

ErrTolTheta 0.8

TypeOfOpeningCriterion 0

ErrTolForceAcc 0.005

DesNumNgb 32

MaxNumNgbDeviation 2

ArtBulkViscConst 0.8

MinGasHsmlFractional 0.25

SofteningGas 8.

SofteningHalo 8.

-- (o o) --------------------------------------------oOO--(_)--OOo------- Yves Revaz Lerma Batiment A Tel : ++ 33 (0) 1 40 51 20 79 Observatoire de Paris Fax : ++ 33 (0) 1 40 51 20 02 77 av Denfert-Rochereau e-mail : yves.revaz_at_obspm.fr F-75014 Paris Web : http://obswww.unige.ch/~revaz/ FRANCE ----------------------------------------------------------------Received on 2007-03-30 11:49:25

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