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Fluid flow through a domain

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Old   December 4, 2017, 03:36
Default Fluid flow through a domain
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I am simulating the flow from these (3 inlets) 8 attached figure untill the top of the plate where my outlet is located.


I am using the following BC


Inlet - Pressure Inlet
Outlet - Mass flow outlet

I am working on inlet pressures between 6- 24 bars . Do i need to use K epsilon or k omega turbulence models currently i am running my simulations in laminar model

Due to high pressure of 6-24 bar as enough turbulence is generated do i need to select a turbulence model

Also to get accurate results how fine should my mesh or the skewness of the mesh elements be ?
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Old   December 4, 2017, 05:15
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You cannot solve such a problem to a high accuracy using some kind of recipe. The degree of turbulence does not only depend of the pressure, but to a high degree of the viscosity.

At first you need to know what accuracy you need. Do you want so see the flow pattern in principle? Or do you need some values near teh border, perhaps for heat or mass transfer? Quite different.

At first I recommend to use a high quality net without a higher degree of skewness. Your case is simpel and that can be reached easily. The experiment with finer and coarser meshes to see how the mesh influences your simulation. At last you may try the difference between RANS and LES. If you are at this point you should have a good feeling of which aspects of your model you may trust in and which are weak points.
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Old   December 4, 2017, 05:22
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Thanks Mr Uwe I basically need to see the flow pattern and see how my mass flow rate varies when i vary the pressure or the no of inlets
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Old   December 5, 2017, 07:59
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There can be no 'mass flow outlet' boundary condition. Which software are you using?
Even if its there, it sounds like you have to specify the mass flow rate at outlet but your objective is to find out the mass flow rate for different inlet pressures.
I suggest you use 'pressure outlet' boundary condition and specify a constant pressure at the outlet. Then by varying inlet pressure, for the same outlet pressure, you will get different mass flow rates. You can use any turbulence model if your concern is only about the mass flow rate at the outlet. Finally, as Mr. Uwe suggested, carry out a grid independence study to determine what grid gives you accurate results.
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Old   December 5, 2017, 10:24
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Ansys fluent but i have a small doubt as there will be a mass outflow the fluid used is air which is highly compressible so in this case density based solver should be used
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Old   December 5, 2017, 12:20
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specify total pressure and total temperature at the inlets and static pressure at the outlets
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Old   December 6, 2017, 00:53
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Quote:
Originally Posted by piu58 View Post
You cannot solve such a problem to a high accuracy using some kind of recipe. The degree of turbulence does not only depend of the pressure, but to a high degree of the viscosity.

At first you need to know what accuracy you need. Do you want so see the flow pattern in principle? Or do you need some values near teh border, perhaps for heat or mass transfer? Quite different.

At first I recommend to use a high quality net without a higher degree of skewness. Your case is simpel and that can be reached easily. The experiment with finer and coarser meshes to see how the mesh influences your simulation. At last you may try the difference between RANS and LES. If you are at this point you should have a good feeling of which aspects of your model you may trust in and which are weak points.
To add, you (the topic creator) should know what the Reynolds number and the Mach number of your flows. If you're above the expected regions of turbulent flow, you should use a turbulence model or use LES/DNS.

I highly recommend against using mass flow outlets, it isn't a good combination of boundary conditions.
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Old   December 11, 2017, 06:50
Default Outlet Pressure
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I need to check my outlet pressure in pressure outlet how can i enter gauge pressure when the pressure at outlet isnt known
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Old   December 11, 2017, 10:51
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Quote:
Originally Posted by adasgupta91 View Post
I need to check my outlet pressure in pressure outlet how can i enter gauge pressure when the pressure at outlet isnt known
what is driving the flow?
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Old   December 11, 2017, 11:04
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I am using pressure inlet BC you can refer the attached figure my inlet pressure atm is 6 bar which i can vary till 24 bar also change the no of inlets and check my flow vel , pressure and mass flow rate at outlet
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Old   December 11, 2017, 12:07
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Quote:
Originally Posted by adasgupta91 View Post
I am using pressure inlet BC you can refer the attached figure my inlet pressure atm is 6 bar which i can vary till 24 bar also change the no of inlets and check my flow vel , pressure and mass flow rate at outlet
If you use mass flow outlet as your boundary condition at the outlet then you cannot check mass flow rate at outlet, because you have enforced it there.

If you use pressure outlet boundary condition at the outlet then you cannot check the pressure at outlet, because you have enforced it there.

In my earlier question what I wanted you to understand is that in your case the flow is happening because there is a pressure difference.

I suggest you study your problem in details.
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