What "boundary conditions" can make a rectangle "look" like a circle?

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I posted the question below in Stackoverflow but then realized that it perhaps would find a better audience here.

I am solving a fourth order non-linear partial differential equation in time and space (t, x) on a square domain with periodic or free boundary conditions with MATHEMATICA.

Without using conformal mapping, what boundary conditions at the edge or corner could I use to make the square domain "seem" like a circular domain for my non-linear partial differential equation which is cartesian?

The options I would not like to use are:

  • Conformal mapping
  • changing my equation to polar/cylindrical coordinates?

This is something I am pursuing purely out of interest just in case someone screams bloody murder if misconstrued as a homework problem! :P

Edit:

If I have a result from solving a PDE in cartesian coordinates, how do I transfer these results or view them in polar coordinates?