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@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