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From: Yves Revaz <Yves.Revaz_at_obs.unige.ch>

Date: Tue, 21 Feb 2006 14:43:06 +0100

Dear All,

In the new implementation of sph, the smoothing length is not

constraints by the number

of neighbours but by imposing that the volume defined by the smoothing

length contains

a constant mass (eq. 6 of Springel 2005).

However, when running gadget (with DesNumNgb=50), I get the typical

following values

out of the routine "density_evaluate" :

numngb_inbox = 12 # numbers of neighbours in a box of

dimension 2h x 2h x 2h

numngb = 10 # numbers of neighbours in a sphere of

radius h

weighted_numngb = 49.936302 # weighted number of neighbours

I have been first surprised by the fact that from 10 effective

neighbours we finally obtain

49.936302 weighted number of neighbours ! However, this may be

understood from the

kernel definition and the weighted number of neighbours definition :

wk ~ 8/(pi*h3) # for r/h<<1

weighted_numngb ~= 4/3*pi*h3 *rho * / m ~= 4/3*pi*h3 *

numngb*8/(pi*h3)*m / m ~= 10.66 numngb

Thus, when setting DesNumNgb we do not really set the effective number

of neighbours,

but rather ~10 times (depending on the distribution around the particle)

the effective number

of neighbours. As a consequence, with DesNumNgb=50, only few particles

are taking into account

to compute the hydrodynamical forces (about 10 in the latter case),

while in classical sph one expect

to have at least 30 neighbours.

Could someone comment on that ?

Thanks.

Date: Tue, 21 Feb 2006 14:43:06 +0100

Dear All,

In the new implementation of sph, the smoothing length is not

constraints by the number

of neighbours but by imposing that the volume defined by the smoothing

length contains

a constant mass (eq. 6 of Springel 2005).

However, when running gadget (with DesNumNgb=50), I get the typical

following values

out of the routine "density_evaluate" :

numngb_inbox = 12 # numbers of neighbours in a box of

dimension 2h x 2h x 2h

numngb = 10 # numbers of neighbours in a sphere of

radius h

weighted_numngb = 49.936302 # weighted number of neighbours

I have been first surprised by the fact that from 10 effective

neighbours we finally obtain

49.936302 weighted number of neighbours ! However, this may be

understood from the

kernel definition and the weighted number of neighbours definition :

wk ~ 8/(pi*h3) # for r/h<<1

weighted_numngb ~= 4/3*pi*h3 *rho * / m ~= 4/3*pi*h3 *

numngb*8/(pi*h3)*m / m ~= 10.66 numngb

Thus, when setting DesNumNgb we do not really set the effective number

of neighbours,

but rather ~10 times (depending on the distribution around the particle)

the effective number

of neighbours. As a consequence, with DesNumNgb=50, only few particles

are taking into account

to compute the hydrodynamical forces (about 10 in the latter case),

while in classical sph one expect

to have at least 30 neighbours.

Could someone comment on that ?

Thanks.

-- (o o) --------------------------------------------oOO--(_)--OOo------- Yves Revaz Geneva Observatory Tel : ++ 41 22 379 23 65 51. Ch. des Maillettes Fax : ++ 41 22 379 22 05 1290 Sauverny e-mail : Yves.Revaz_at_obs.unige.ch SWITZERLAND http://obswww.unige.ch/~revaz/ ----------------------------------------------------------------Received on 2006-02-21 14:43:16

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