After the koi pond rebuild, I noticed that the air stream from a single aerated BD seem to bounch around from side to side. With some periodic "burbing", just assumed that it was an irregurlar air output.
One day, I had the pump off that served the BD/TPR circuit. The air stream was now a uniform, straight column, even with a burp here and there.
I turned the pump back and and really watched the air column for a awhile. I was memorized as I watched the air column rotate like the tornado in the Wizard of OZ, around the center point of the BD, at about a 10 degree lean.
Every several minutes or so, it made a complete rotation.
Surface rotation around the perimeter of the pond was aways noticable. But apparently the TPRs were doing want I wanted, good circulation through much of the water depth, not just in the lower depths.
At least I thought it was cool.