Can inlet become an outlet and vice vers?

Leb · December 8, 2015, 08:30

Hi,

I am currently playing around with dynamic mesh where I attempt to simulate a tube like structure, which should simulate a valveless (or impedance) pump. I get results which look reasonable by eye, but I feel that Fluent should not be happy with inlet becoming an outlet and vice versa (It does complain about reversed flow, but doesn't stop convergence).

Does anyone have experience with this? Is there something horrible and unacceptable I am doing here?

I normally set up inlet boundary (where I set certain time varying velocity which coincides with the mesh motion), and set another boundary as an outflow.

Thanks for your time.

LuckyTran · December 9, 2015, 12:15

Fluent is not happy, that's why it claims about reversed flow, but it doesn't break the simulation. Mathematically the boundary conditions can be arbitrarily specified.

When flow is entering the domain it must have some properties (pressure, velocity, turbulence, temperature, species, etc). Similarly when flow leaves the domain it should have properties taken from the domain. Generally speaking, you know what goes in, but don't know what comes out.

The problem is that all these properties are forced at an inlet. For example a velocity inlet, you specify the velocity profile (and temperature and so on), you can even specify an outward velocity. The temperatures and so on at this "inlet" are prescribed, and they are not from the domain. On the other hand, at an outlet, the temperature is taken from within the domain. If there is reversed flow then you need to specify the temperature of the reversed flow. Also when there is unexpected reversed flow, you must make some assumptions about the direction of the flow. Thus there are certain limitations at inlets & outlets caused by their implementation. Of course the way Fluent handles these inlets/outlets might not be how you want it to behave, but something is happening.

December 8, 2015, 08:30	Can inlet become an outlet and vice vers?	#1
Leb New Member V.Leb Join Date: Jun 2015 Posts: 8 Rep Power: 10	Hi, I am currently playing around with dynamic mesh where I attempt to simulate a tube like structure, which should simulate a valveless (or impedance) pump. I get results which look reasonable by eye, but I feel that Fluent should not be happy with inlet becoming an outlet and vice versa (It does complain about reversed flow, but doesn't stop convergence). Does anyone have experience with this? Is there something horrible and unacceptable I am doing here? I normally set up inlet boundary (where I set certain time varying velocity which coincides with the mesh motion), and set another boundary as an outflow. Thanks for your time.

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December 9, 2015, 12:15		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,676 Rep Power: 66	Fluent is not happy, that's why it claims about reversed flow, but it doesn't break the simulation. Mathematically the boundary conditions can be arbitrarily specified. When flow is entering the domain it must have some properties (pressure, velocity, turbulence, temperature, species, etc). Similarly when flow leaves the domain it should have properties taken from the domain. Generally speaking, you know what goes in, but don't know what comes out. The problem is that all these properties are forced at an inlet. For example a velocity inlet, you specify the velocity profile (and temperature and so on), you can even specify an outward velocity. The temperatures and so on at this "inlet" are prescribed, and they are not from the domain. On the other hand, at an outlet, the temperature is taken from within the domain. If there is reversed flow then you need to specify the temperature of the reversed flow. Also when there is unexpected reversed flow, you must make some assumptions about the direction of the flow. Thus there are certain limitations at inlets & outlets caused by their implementation. Of course the way Fluent handles these inlets/outlets might not be how you want it to behave, but something is happening.