[Yt-dev] boundedness change

[david collins]

> It's unclear to me that we should.  If we did, it would be a simple
> addition to the total potential, rather than anything that needed to
> be calculated through a C function.  My brain is blanking right now,
> but what is the self-binding energy of a rectangular prism of given
> mass, assuming hte mass is distributed evenly?

I'm not sure what that energy is.  The 20 minutes I've spent on it
have thoroughly choked mathematica, and spit out a bunch of log[ a +
sqrt( b + c ) ] kinds of things.  and the rest of my integration
skillz are rusty.  So I suspect that there might still be some
computation for each cell, though not n^2, so might be fast enough to
do with numpy.  How much faster is a C log than numpy?

d.

>
> On Sun, Jan 18, 2009 at 11:38 AM, david collins <antpuncher at gmail.com> wrote:
>> I have another, somewhat related question about bounded calculations--
>> Should one also include the self binding energy of each cell, in
>> computing the total gravitational binding for an object?
>>
>> d.
>>
>> On Sun, Jan 18, 2009 at 9:47 AM, Matthew Turk <matthewturk at gmail.com> wrote:
>>> (I responded to Stephen by mistake; I need to fix the reply-to on this
>>> list!  This response is now obsolete.)
>>>
>>> Well, one of the problems -- which I addressed with the rather
>>> non-portable CUDA implementation of IsBound -- is that the means of
>>> calculating the potential energy is still n^2.  I think a better means
>>> of calcualting it with the HOP halos is actually to assuming spherical
>>> shells centered on the center of the halo -- even though we all know
>>> halos are not completely spherical, I think we are more likely to have
>>> a good fit to a spherical profile than with hydro clumps.  (Or am I
>>> sorely misled here?)  In that case, what you'd probably want to do is
>>> set up log bins from some minimum radius to the max radius,
>>> caluclating an accumulating sum of mass in those shells, and then
>>> additionally an accumulating sum of kinetic energy.  Then look for
>>> where they cross to find the boundedness.
>>>
>>> But, thinking about it, I think this is actually a naive and incorrect
>>> description of what Britton does in the halo profiler when he
>>> calculates the virial radius...  Britton?
>>>
>>> (Also, Brian, feel free to chime in about DM halo boundedness.)
>>>
>>> -Matt
>>>
>>> On Sat, Jan 17, 2009 at 8:59 PM, Stephen Skory <stephenskory at yahoo.com> wrote:
>>>>> I (reluctantly?) agree with Britton that there's an error.  He's going
>>>> to fix it, unless somebody speaks now.  (Else, holding peace forever is
>>>> expected.)
>>>>
>>>> I give up none of my rights!
>>>>
>>>> IsBound interests me. What would be the best way to make this work with HOP haloes? Put a kind of wrapper in the HopGroup class that cuts out a sphere centered on .center_of_mass() with radius .maximum_radius()? I'd prefer to only operate on the DM particles, not the gas mass. Any suggestions?
>>>>
>>>> Thanks!
>>>>
>>>>  _______________________________________________________
>>>> sskory at physics.ucsd.edu           o__  Stephen Skory
>>>> http://physics.ucsd.edu/~sskory/ _.>/ _Graduate Student
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