[Users] Resolving two BH

[rahul kashyap]

Thanks for the reply. My apologies that I'm not completely familiar many of
the physical and numerical issues while simulating bbh. I have experience
in AMR astrophysical simulations.

When I do the bbh simulation of ratio around 1, it evolves perfectly fine.
With higher mass ratio, the smaller BH just becomes bigger and blows up.
I'm assuming this as a problem of refinement.

My naive picture was that the grid is created adaptively at every n time
step(a parameter) based on lapse values. After locating the horizon,
dynamic variables are updated in the outside region only. So, the problem
remains with locating the horizon often enough and refining to the
appropriate level so that at least 4-5 cells remains across a BH.
Please correct me, if I'm off by any big misunderstanding.

Thanks again,
-rahul

On Mon, Jun 6, 2016 at 4:22 PM, Frank Loeffler <knarf at cct.lsu.edu> wrote:

> On Mon, Jun 06, 2016 at 12:41:35PM -0400, rahul kashyap wrote:
> > I'm working on binary black hole mergers with large mass ratio (20:1 or
> > more). By using higher refinement level, I have been able to resolve the
> > smaller BH, but it doesn't remain resolved even for a single time step.
>
> Hi,
>
> Thanks for contacting us. I am not that familiar with that particular
> type of simulation (bbh with that mass ratio), but I try to give a best
> guess. Please everyone: if you disagree, say so. :)
>
> It would be interesting to know what exactly you mean with 'doesn't
> remain resolved'. Assuming that the resolution is high enough, one thing
> I would expect a simulation like this to use is some kind of
> position-dependent gauge condition. The reason is that one thing the gauge
> mostly does is (loosely speaking) to "prevent the grid points from
> falling into the black hole, or to come out of it". In your simulation
> you see this as a black hole that gets 'larger or smaller' without
> changing its mass a lot. The best parameter values that determine how
> much the grid points are "pushed out" depend on the black hole mass.
> Since you mass ratio is quite large, I would expect the optimal
> parameter to be quite different for the two black holes.
>
> There are groups that did evolve BBHs with even higher mass ratios than
> this - not using the exact same code, but the same methods. They should
> have much more experience. Anyone - you know whom I mean ...?
>
> Frank
>
>
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