What is a floating boundary condition in CFD? In what condition can I use it? I suppose such a BC exists only in few commercial packages. Are there any other BCs equivalent to a floating BC in other software?
Hi Nikesh, do you mean a so-called FREE floating boundary condition?
That means that the velocities are constant at two sides of a boundary. Holds basically for all physical parameters. Other boundaries are reflecting and periodic; these do exactly what you "expect" because of their name.
Is this answer of any help?
As alternative to having a constant velocity across the boundary, it can also mean that the velocity outside the boundary is set by keeping the velocity gradient constant across the boundary. The velocity that follows from this extrapolation is in turn is then used to set other parameters, such as density, by specifying that mass flow is conserved. This method is used to set the inflow boundary for an accelerating flow such as a stellar wind (Keeping the velocity constant across the boundary would result in an instability that stops the model from relaxing.)
Allard, you're absolutely right. In fact you highlight another point in passing: if you choose the wrong kind of boundary conditions you may screw up a computation! Philosophically speaking: you already have to understand to some extent what's going on when you specify BC's!
Manuel,
I agree, in fact I remember from when I started with computational MHD, that I was told that getting the BCs right was 90 percent of the work. Perhaps a bit exaggerated, but not exactly wrong either. The floating BC is especially troublesome. In some models, you don't just have to extrapolate the velocity gradient, but the magnetic field gradient as well.Getting that one right in multiple dimensions is a computational nightmare.
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