DesNumNgb

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.



-- 
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  Yves Revaz
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Received on 2006-02-21 14:43:16

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