[Ahmed Saeed Mansour]

Hello, I want to know what is the value of the reference pressure in CFX that I have to use? 1 atm or 0 atm for a centrifugal pump..
Thanks

[ghorrocks]

The reference pressure should be set to somewhere between the highest and lowest pressure expected in the system. If the system includes a section which is atmospheric it makes sense to use atmospheric pressure as the reference.

[Ahmed Saeed Mansour]

Thank you!

[AS_Aero]

Hi Glenn

Then does it mean that if there is an outlet to atmosphere (which is common in most cases) both Reference pressure and the Outlet boundary condition (Pressure ) to be given as Zero or 1 atm ?

[ghorrocks]

If a system outlets to normal atmosphere, then use a reference pressure of 1 atmosphere with an outlet pressure of 0.

[AS_Aero]

And is it possible that we can give static pressure at the inlet (as in experimental setup the barometer reads static pressure) and the outlet as 0 pa with ref pressure as 1 bar ?
So here basically we are giving both pressure bc'n is it ok to give like that or is it ill posed problem ?

[Ahmed Saeed Mansour]

Okay, I have inlet mass flow rate and outlet static pressure 290 KPa, So what is the reference pressure and the outlet static pressure?
Thanks

[evcelica]

The point of the reference pressure is to limit round off errors in floating point operations.
Simple example: it is easier to say 2-1=1 than 100000002-100000001=1. So In that calculation I would use 100000000 for my "reference number", to make all the math easier.

For a computer using floating point numbers, it is more accurate as well.
2E0-1E0=1E0
or
1.00000002E8 - 1.00000001E8 = 1E0 (you can see you can lose accuracy this way as we are using up more of our floating point number with meaningless numbers, therefore wasting them.)

All other pressure boundary conditions and variables are now relative to the reference pressure. Even in the solution, the "Pressure" variable is relative to this reference. If you want absolute pressure, you would use the "Absolute Pressure" variable.

You could just as easy run a simulation with 0 reference pressure, and your outlet at 1 atm, this is mathematically correct. Heck, even 10 atm reference pressure, and an outlet relative pressure of -9 atm would work fine. You will just not be as accurate numerically as using 1 atm reference pressure, and 0 at outlet.

[turbo]

Just ignore the reference pressure in CFX, always putting 0 there.
Then use whatever real pressures you have. Free of worries.

[ghorrocks]

Free of worries.

Providing it converges, then yes, free of worries. If it does not converge then what? I presume then you can worry

The point of reference pressure is to reduce round off error and make convergence easier. If your simulation is easy to converge then you can do anything you like as turbo says. But if your simulation is tricky to converge you need to be careful about it or it will cause a failure to converge. Personally I always set the reference pressure to a sensible value as it is one of the easiest parameters to get right as about 10 seconds thought is all which is required to correctly set it.

[turbo]

Originally Posted by ghorrocks

Providing it converges, then yes, free of worries. If it does not converge then what? I presume then you can worry

The point of reference pressure is to reduce round off error and make convergence easier. If your simulation is easy to converge then you can do anything you like as turbo says. But if your simulation is tricky to converge you need to be careful about it or it will cause a failure to converge. Personally I always set the reference pressure to a sensible value as it is one of the easiest parameters to get right as about 10 seconds thought is all which is required to correctly set it.

Both you and "evcelica" stated the issue of round-off errors which will affect solution stability, as CFX manual says. However, my thought is different : As for the special case where the inlet and exit boundary pressure undergoes very tiny changes not enough to induce dynamic flows, the use of pressure boundary conditions should be avoided because of non-physical. Instead, the velocity inlet and the exit Neuman pressure gradient, or the total pressure inlet and massflow exit will be better in such a highly incompressibe flow case. The original post was asking on centrifugal pump simulation, and of course due to a significant pressure jump across the pump the zero reference pressure should be absolutely fine. In most of turbomachinery flows it will be OK. The non-zero reference pressure makes me crazy in the Post process bringing confusion.

[ghorrocks]

Agreed - there are configurations where it is not important. But there are others where it is. Rather than having to figure out whether it is important or not I just set it as recommended and then I don't have to worry about it.

[AS_Aero]

So what if my real Total pressure is 108000 Pa (10.08 bar) and outlet is atmospheric pressure (1 bar) and my Reference Pressure ?

If I give Reference pressure = 1 bar my outlet pressure (static ) = 0 bar and what about inlet ? Should I give 10.08bar or 9.08bar ?? Kindly let me know.

[Opaque]

Relative Pressure = Absolute Pressure - Reference Pressure

and

Total "Relative" Pressure = "Total" Absolute Pressure - Reference Pressure

Keep in mind that

Total Pressure = Static Pressure + 1/2 * Density * V^2 (for incompressible flows)

If you subtract Reference Pressure from both sides, you will see how they are related.

Hope the above helps,

[AS_Aero]

So In my case I have to give 9.08 as my Total Pressure at Inlet and 0 at Outlet and Reference = 1 bar
So my real pressure (total Pressure ) (as in experiment) = 10.08 bar
So how does it works in CFX and in Post Processing of CFX Pressure is Static Pressure right ??

[guillaume74]

You can either choose to use 10.08 bar at inlet and 1 bar at outlet (with 0 bar ref pressure), or use 9.08 bar at inlet and 0 bar at outlet (with 1 bar ref pressure). The results will be the same as the flow is incompressible.

However, in the post processing, be aware that you might have to add the ref pressure to your static pressure depending which option you chose.

[mahesh1819]

Hi, I have solved a case for hydrostatic force in a tank. The profile (Fig attached) I am getting correct nut the values are not matching with manual calculation. Height of water is 4.2m. Please help where I am wrong.

[Gert-Jan]

What about the variable "Absolute Pressure"?

[ghorrocks]

As Gert-Jan says, you probably want Absolute Pressure.

Read the CFX manual about the pressure variable. In flows with gravity it has the hydrostatic component removed by the reference point and a reference density. The fact that you are seeing a variation in pressure at all in a hydrostatic simulation means you have not set these reference values correctly. The pressure variable should be constant in a hydrostatic simulation in CFX.

[mahesh1819]

Dear Gert-Jan,

Thanks for your time and reply. Please suggest what shall be the reference pressure and density values to be given to get the hydrostatic profile as per hand calculation?

I have currently mentioned atmospheric pressure (1 atm) and relative density as 1.185 kg/m3) as per attached fig. and could not plot the required hydrostatic profile.

Thanks in advance.