Question

Hello! I have some questions related to using Ansys Fluent, I have read the instruction of Ansys Book , but I still do not understand clearly, can you help me? They are as the following:
1. When do we need to use pressure inlet for B.C for the inlet boundary and pressure outlet B.C for the outlet boundary?

2. Can we use pressure inlet B.C for the inlet boundary and outlet B.C for the outlet boundary in one fluid flow study (air flow, gas flow..)? The reason why I ask this question because I used to see on the internet written that if we use pressure inlet B.C for the inlet boundary, we are not recommended to use pressure outlet B.C for the outlet boundary, we are recommended to use velocity outlet B.C for the outlet boundary. Is it right or not?

3. Does the operating pressure in the operating condition effect the gauge pressure in the pressure outlet/ inlet ?

4. In the wall B.C, what does it mean if I choose the stationary wall or the moving wall? Sorry, I have read the instruction of ansys, but I still do not get it.

Thank you in advance! I will thumb you up!

Answer 1

1. Pressure boundary conditions are used when we are sure about pressure at that point. This usally happens in cases like 1) Outlet open to surrounding (Pressure gets fixed as ambient pressure 2) Inlet of pipe deep inside a large reservior (Pressure gets fiixed to ) 3) and points where pumps are used to fix pressure, and there are many more complex cases too.

2) Ansys fluent solves navier stokes equations for fluent flow.

In this equation pressure is 1st order derivative (as shown with red circle). This translates to that we only need one condtion in pressure to define the entire problem.

Practically too, You can never fix pressure at two points of the flow. What you can do is fix pressure at one point, run the experiment or solve theoretical model, and find out the pressure at different point.

If you have pressure at inlet and outlet, both, at your disposal to solve the system. Then you should carefully notice, only one will be operating condition. Other will change as you change the operating conditions.

3)

Only pressure difference matters in solving these equations. You can choose arbitrary operating pressure but then be careful of not making mistake while inputing gauge.

Or put operating pressure = 0, and then gauge pressure will be equal to operating pressure given.

4)

In navier stokes equations, you also need conditions for velocity as well.

One of these conditions are provided by wall. At wall interface, we assume that fluid velocity also becomes zero relative to wall.

When we say sationary wall, it means that flow is moving only because of pressure gradient. (For example: a flow inside a pipe, the pipe walls doesn't move)

In moving wall, fluid also flows because of the moment of the solid interface. (For example: if you put a metal sheet in water and move it in the direction of its plane, water also moves with it.)