Learning Forum

[luca.vene]

Hello everyone,

I am seeking clarification regarding the correct boundary condition setup in ANSYS Fluent for a natural or artificial channel, in cases where both the inlet velocity and the water depth are known. I have activated the Open Channel Flow option in the VOF sub-models.

Specifically, I would like to understand:

• At the inlet, is it better to use a Mass Flow Inlet or a Pressure Inlet, and how does this choice affect the velocity field upstream in the channel?

• At the outlet, should one use a Pressure Outlet with the water depth equal to the inlet (selecting the Multiphase Open Channel Flow option), or simply set Gauge Pressure = 0 Pa without enabling the open channel option?

For context, I performed several numerical tests with the following characteristics:

• Rectangular channel, 60 m long and 2 m wide, with a water depth of 1 m;

• Rough bottom with d90=0.001 m, lateral walls set as slip;

• I tested both the case with no slope and a slope corresponding to a longitudinal acceleration of 0.039 m/s², equivalent to a geometric slope of about 0.23°;

• At the inlet, I applied both a Mass Flow Inlet of 4000 kg/s (≈ 2 m/s) and a Velocity magnitude of 2 m/s;

• At the outlet, I tried both a Pressure Outlet with the water depth equal to the inlet, and a standard Pressure Outlet with Gauge Pressure = 0.

In all cases, the velocity distributions at about 30 m downstream of the inlet were almost identical, regardless of slope, bottom inclination, or applied acceleration. This suggests that the flow is primarily determined by the inlet conditions, while the slope and Open Channel settings have a secondary effect.

I would like to understand:

• How the Open Channel Flow model in Fluent actually works, in terms of solved equations and free surface treatment;

• What are the recommended boundary conditions when both the inlet water depth and mean velocity are known, for channels with or without slope;

• The practical difference between enabling the Multiphase Open Channel option at the outlet with fixed water depth and simply using a Pressure Outlet with Gauge Pressure = 0.

Thank you in advance for any guidance or best practices, particularly for applications involving natural or artificial channels with a free surface and known slope.