Hydraulic jump boundary conditions
 

Dear all,

I´m trying to simulate the hydraulic jump below ogee spillway using flow 3d. The problem is I don't know which is better to use as a downstream BC:
specified pressure (tail depth) or outflow.

I would suggest to use 'specified pressure boundary'. There can be three variation in such boundary depending on your design conditions:

1) Pressure with fluid height => if you are maintaining a tailwater downstream of jump

2) Pressure with fluid fraction=1 => if the flow is sub-critical in the downstream reach

3) Pressure with fluid fraction=1 => if the flow is super-critical in the downstream reach

When i use pressure with fluid height corresponding to downstream depth of jump, is this boundary behave like outlet (outflow) or the flow may be return back.

This will depend on hydraulic design of basin/jump. If Tailwater > Tailwater required, jump will moved towards chute; otherwise it will move towards downstream channel or might sweepout if tailwater < Min. tailwater required. The software just solve equations based on your input and model the exact situation.