Nonreflecting far-field (transonic)

Bren

Hi Guys, I've been experimenting with some far-field conditions for a transonic flow around an airfoil. I am using the conditions proposed by Kwak. I mentioned these in an earlier post.

I am studying the effect of the far-field conditions by using a large grid along with the traditional reflecting conditions to produce one converged solution to a steady transonic flow around an airfoil. Then I'm reducing the grid size and solving twice, once with reflecting & once with nonreflecting conditions. I'm attempting to determine whether the non-reflecting conditions are avoiding the degradation of the near-field by allowing reflected waves to leave the domain.

I am noticing, however, that when the grid is very close to my airfoil I can sometimes get a very large build-up of the potential at the edge of the domain. This is obviously non-physical but doesn't seem to have much effect on the near-field. The flow near the airfoil seems relatively unaffected by this.

I wonder if anyone else has had experience with non-reflecting boundary conditions. If so could you please advise me whether this type of phenomena is to be expected?