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-   -   Assigning pressure to closed fluid domains (http://www.cfd-online.com/Forums/cfx/100379-assigning-pressure-closed-fluid-domains.html)

Torque_Converter April 24, 2012 13:31

Assigning pressure to closed fluid domains
 
Hi All,

I have what is basically a torque converter, and would like to explore the effect of charge pressure on performance. Basically the fluid is filled into the cavity of the geometry at 500kPa before it starts running. I have tried 2 things mainly to simulate this. One is to assign 500kPa to the reference pressure in the domain settings, and the other is to assign it to the initialization. Neither seem to have the effect my fluid dynamics judgement would suggest they have.
When I do this i get the warning at the start of a run:
Domain Group:rotor

Pressure has not been set at any boundary conditions.
The pressure will be set to 0.00000E+00 at the following location:
Domain: Rotor
Node: 1 (equation 1)
Coordinates:.......

This leads me to strongly believe I am certainly not simulating the 500 kPa charge pressure.

ghorrocks April 25, 2012 07:45

Is the simulation incompressible or compressible?

Torque_Converter April 25, 2012 07:47

Incompressible

ghorrocks April 25, 2012 08:36

So that means the absolute pressure level makes no difference, and the flow field is just offset from the reference pressure.

To model the effect of pressure on the model, you need to include some physics which depend on pressure. Maybe the flow cavitates? Or is slightly compressible? Or some other physics dependant on pressure?

Until you find some physics which depends on pressure nothing will change as you change the reference pressure level.

Torque_Converter April 25, 2012 09:06

I see what you're saying. That is definitely what it does, just offsets from the pressure of reference. I suppose the only thing I can do is model in the inlet to that initially charges the domains, but since it is not periodic all the way around the device I would likely have to model the entire device instead of just one period.

ghorrocks April 25, 2012 19:27

I do not understand your comment. If the flow is perioidic then you should be able to use a periodic boundary, regardless of pressure level.

But again, what physics is dependant on pressure? If you have no physics dependant on pressure then nothing will happen in this model either.

Torque_Converter April 25, 2012 20:00

Yea, I see how it was confusing.
The periodic issue was having to do with modelling just one blade passage of the rotor and stator. The fluid is pumped in from ports that are not fully around the item so you could not employ any sort of geometric symmetry or periodicity if you would want to model in this the ports with their boundary condition pressure condition.

ghorrocks April 26, 2012 05:51

OK, so the flow is not periodic. But as it now seems you have inlets and outlets then is it not a trivial matter to assign it any pressure you like?

Why are you getting an unassigned pressure warning? Surely you have a pressure boundary in your model.

Aladdin July 31, 2012 17:06

Did you find a solution
 
Hi, I am also modeling torque converter and I received the same pressure warning message. Did you find a solution to assign charge pressure?

Thanks in advance for help.

Torque_Converter July 31, 2012 17:27

Aladdin,

I hope what I have learned helps as the problem was quite easy to resolve once I understood the torque converter, not just the fluid physics....
In the torque converter the fluid is incompressible, unlike a retarder for example, since there is no cavitation. What this means is that the determinant of torque is not the pressure everywhere in the domain, but is simply the ratio of pressure of on the blade pressure vs. suction side. So having flow thru the torque converter, or trying to get pressure up inside does not change the torque number. This can be demonstrated by looking at performance curves for the converter and noticing that once enough pressure is applied to fill the domain, additional pressure does not increase performance.
This is also verified by several published papers I found from Chrysler, Komatsu, GM, and Mercedes. It is working well for my applications as well which has built confidence.
Basically you can ignore that warning, and only worry about initial or input pressure to the domain if things like cavitation are going to be significant.

ghorrocks July 31, 2012 19:01

Quote:

is simply the ratio of pressure of on the blade pressure vs. suction side.
A small but important point - No, not the ratio but the pressure difference over the blade.

Aladdin July 31, 2012 19:10

Thanks
 
Thank you for your reply.

I have a study paper titled "COMPUTATIONAL FLUID DYNAMICS ON TORQUE CONVERTERS – VALIDATION AND APPLICATION" that shows a figure of pressure distribution along torque converter parts at charge pressure 40 psi and different pump speeds . How can I simulate this pressure? and how can I check the charge pressure value for my simulation?

ghorrocks July 31, 2012 19:23

If the flow is incompressible in a closed domain the reference pressure is irrelevant. You can add any amount you like to it to get the reference pressure you want.

Aladdin July 31, 2012 20:09

Thanks Glenn Horrocks . Does this mean in CFX simulation of torque converter I define the value of reference pressure equal to the value of charge pressure?

ghorrocks August 1, 2012 07:54

I am not an expert in torque converter design so do not know what charge pressure is - but if that is the base pressure of the device then yes, that would be a good reference pressure.

Torque_Converter August 1, 2012 08:19

Charge pressure is a tricky thing in torque converters. It is going to be dependent on how and where it is measured, and its only real use is to ensure the cavity is filled. When at 40 kPa the entire domain may not be filled due to cooler flow path, but at 180 kPa it is, and as you increase pressure you do not change performance.
Charge pressure may be measured at a wall on the pump hub, or sometimes even at the stator shroud. So one company's charge pressure is not relatable to another necessarily. The charge pressure is really only modulated by one control, and that is mass flow the cooler pump sends in. Beyond that, the factors of cooler pressure drop, RPM, none of those are variables used to control this charge pressure.

Aladdin August 1, 2012 12:11

Thank you all for your reply.

Aladdin August 26, 2012 14:50

Torque Converter charge pressure again
 
Hi all. I changed reference pressure in modelling a torque converter in CFX, the output torque on all elements did not change. Do you have any suggestions to order to model charge pressure?

Torque_Converter August 26, 2012 14:57

It is a good thing the charge pressure did not change the performance, that is physically real. The only scenario where your performance should change with changing pressure is if you have cavitation or significant air entrainment. Neither of these is significant in a fully functioning torque converter.

If you do have an odd situation that has compressible flow issues or works with a partially filled TC you will need to simulate the inlet/outlet conditions. Most TC's are periodic, so you can keep one blade passage, and you will need mass flow rate in and pressure out. This will likely be very difficult to obtain.

Aladdin August 26, 2012 16:21

Thank you for your quick reply.
I think I'll go with modeling partially filled torque converter. I am doing a PhD research and my supervisor thinks that one of the unknown problems in torque converters is partially filled torque converter performance.


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