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From: Aldo Alberto Batta Marquez <abatta_at_astro.unam.mx>

Date: Mon, 18 Nov 2013 21:03:55 -0600

Hi everyone,

I've been working with the collapse of rotating polytropic envelopes onto a

sink particle (BH) originally set at the centre of the spherical

distribution. I'm studying the effect of cooling and self gravity of the

accretion disk in the dynamics of the system (gas and BH).

I've noticed that BH dynamic's (whose mass is comparable to the gas mass)

is calculated using a huge time step compared to the integration time used

in the system (determined mainly by high density SPH particles lying close

to the BH), which translates on sudden kicks on the BH that mess with

momentum conservation. The large time step is due to the small

gravitational accelerations acting on the BH (because of the disk's

symmetry). Up to now I've chosen a gravitational softening for the BH

smaller than it's accretion radius and I've implemented a Paczynski-Wiita

potential for the BH.

I've tried to fix this issue by choosing a smaller softening length for the

BH but that can result in rather slow simulations when the resulting time

step is much smaller than the minimum time step for SPH particles.

Apparently this solution is not enough because linear momentum is still not

being conserved as it should. Only simulations with really thick disks

(with highly unefficient cooling), conserve momentum in a proper way. I've

noticed that increasing the accuracy on the gravitational force, by

choosing smaller node opening parameters, improves linear momentum

conservation but only up to some point. At this point I'm not sure if this

is a problem with time integration (time step) or with the gravity force

calculation.

Do you think an adaptive gravitational softening would improve things?

Has anyone implemented it in Gadget2?

Any ideas are welcomed.

I also tried to set the BH time step as the minimum time step obtained for

all particles, but apparently I was missing something and couldn't get that

to work properly.

I'm looking forward to any feedback.

Thanks

Aldo Batta

Date: Mon, 18 Nov 2013 21:03:55 -0600

Hi everyone,

I've been working with the collapse of rotating polytropic envelopes onto a

sink particle (BH) originally set at the centre of the spherical

distribution. I'm studying the effect of cooling and self gravity of the

accretion disk in the dynamics of the system (gas and BH).

I've noticed that BH dynamic's (whose mass is comparable to the gas mass)

is calculated using a huge time step compared to the integration time used

in the system (determined mainly by high density SPH particles lying close

to the BH), which translates on sudden kicks on the BH that mess with

momentum conservation. The large time step is due to the small

gravitational accelerations acting on the BH (because of the disk's

symmetry). Up to now I've chosen a gravitational softening for the BH

smaller than it's accretion radius and I've implemented a Paczynski-Wiita

potential for the BH.

I've tried to fix this issue by choosing a smaller softening length for the

BH but that can result in rather slow simulations when the resulting time

step is much smaller than the minimum time step for SPH particles.

Apparently this solution is not enough because linear momentum is still not

being conserved as it should. Only simulations with really thick disks

(with highly unefficient cooling), conserve momentum in a proper way. I've

noticed that increasing the accuracy on the gravitational force, by

choosing smaller node opening parameters, improves linear momentum

conservation but only up to some point. At this point I'm not sure if this

is a problem with time integration (time step) or with the gravity force

calculation.

Do you think an adaptive gravitational softening would improve things?

Has anyone implemented it in Gadget2?

Any ideas are welcomed.

I also tried to set the BH time step as the minimum time step obtained for

all particles, but apparently I was missing something and couldn't get that

to work properly.

I'm looking forward to any feedback.

Thanks

Aldo Batta

-- Aldo Alberto Batta Márquez Instituto de Astronomia Universidad Nacional Autonoma de Mexico (UNAM) http://www.astroscu.unam.mxReceived on 2013-11-19 08:02:35

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