Downstream Pressure outlet BC - FLUENT VOF
 

Hello,

I am trying to simulate flow through a gate using Fluent's VOF model. The bottom portion of the domain is water and the top portion is air. Flow coming in through the velocity inlet passes under a gate and travels downstream. I'd like to match the water elevation downstream as close as possible to the conditions at the actual site. To do this, I've made a downstream section the same height as the water elevation and patched it with water. I've set a pressure outlet with constant backflow volume fraction = 1 as a downstream BC. There is a downstream section of air above the water, and I've also set that as a pressure outlet for air. The solution blows up quickly at the interface between those two pressure outlets, diverging within a few iterations. Any advice on what may be happening here? I've tried changing the pressure outlet to calculate from neighboring cells, and I've tried initializing both the air and water domains with a downstream velocity but that doesn't seem to work.

Thanks in advance.

Hi Adam,
What you specify is an open channel flow. Activate the open channel option under the VOF multiphase tab.
Then when you specify the outlet BC you can specify the free surface level of water level in the outlet BC tab.
DO NOT specify two separate outlets for air and water.

Regards
Luke

Thanks Luke. When I do this, I get some recirculation back into the flow. Do you know if the open channel option corrects for tailwater elevation by sending backflow into the domain at the outlet? And if this in turn affects the computations in other parts of my domain?

Thanks,
Adam