[suktan]

Can I simply use an inlet to model the flow generated by a piston? A circular piston is located somewhere on a face of a rectangular box. It vibrates back and forth in sinusoidal manner. I want to simulate the flow in the box.


Thanks.

[FMDenaro]

Originally Posted by suktan

Can I simply use an inlet to model the flow generated by a piston? A circular piston is located somewhere on a face of a rectangular box. It vibrates back and forth in sinusoidal manner. I want to simulate the flow in the box.


Thanks.

If the movement of the piston in the is small, you could use a sinusoidal normal velocity compontent on the boundary. This is like a synthetic jet condition see https://en.wikipedia.org/wiki/Synthetic_jet


But if the movement is large that can be quite approximate

[suktan]

Originally Posted by FMDenaro

But if the movement is large that can be quite approximate

Thanks for your advice.



What should I do if the movement is not large compared to the piston diameter but rapid?

[FMDenaro]

Originally Posted by suktan

Thanks for your advice.



What should I do if the movement is not large compared to the piston diameter but rapid?

You have to set a computational time-step enough small to describe the period of the piston.

[suktan]

Thanks again Filippo. Is selection of BC type: velocity, pressure and mass flow matter? Which can do a better job to simulate the flow generated by the piston?

[FMDenaro]

Originally Posted by suktan

Thanks again Filippo. Is selection of BC type: velocity, pressure and mass flow matter? Which can do a better job to simulate the flow generated by the piston?

I suppose that the piston does not allow a net mass to enter, you could prescribe e normal velocity on the boundary, a the whole period it is inflow and outflow so that the total mass vanishes.

[suktan]

From the text book, an outlet is necessary for any case with an inlet (potential mass source). If the rectangular box is essentially sealed and there is only an inlet, will I encounter numerical problem? It is said the mass is not conserved.



At the moment, air (fluid) within the box is incompressible.

[FMDenaro]

Originally Posted by suktan

From the text book, an outlet is necessary for any case with an inlet (potential mass source). If the rectangular box is essentially sealed and there is only an inlet, will I encounter numerical problem? It is said the mass is not conserved.



At the moment, air (fluid) within the box is incompressible.

For incompressible flows You need to set a further region on the boundaries to be inlet/outlet in order to conserve total mass.
I was thinking your problem was modelled by a compressible flow.

[suktan]

Originally Posted by FMDenaro

I was thinking your problem was modelled by a compressible flow.

Thanks again! Now I understand why my model is having problem with incompressible flow.