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-   -   Monitor pressure on the rotating blade (http://www.cfd-online.com/Forums/cfx/93516-monitor-pressure-rotating-blade.html)

BalanceChen October 18, 2011 04:45

Monitor pressure on the rotating blade
 
Hi my friends, I am doing a transient simulation of turbomachinery. And I want to get out the pressure of a point on the rotating blade, whose location varies with time. If I set up a point in the output control, will it then rotates with the blade? If not, what should I do to track the rotating point?

Thanks very much,
BalanceChen

flilk October 18, 2011 04:59

i think you needn't set up a point in the output control ,becasue all datas will be saved ,and you can read it anyway

BalanceChen October 18, 2011 21:09

I think I didn't put it clear. For this transient simulation, I didn't save all results during the run. But if I put a monitor point, all the resultss of that point during the run can be saved. This is what I want. My question is that, If this point is in the rotating frame, then will it rotates as time proceed?

Far October 19, 2011 05:47

If it is in stationary domain then It will not otherwise it will :p

bhatiadinesh October 19, 2011 12:19

Hi,
As per my knowledge, The path of a point in any frame in transient simulation cannot be traced unless and until you use particle track, that too it has to be put in the def file before the simulation begins.

Actually I am looking for something exactly like this. If you or anyone else knows about any other way or the exact way by using the particle track please do let me know as well.

-Dinesh

BalanceChen October 20, 2011 08:24

Got it, Thank you very much Far~

Far October 20, 2011 08:39

It is same as I though or some thing else ?

BalanceChen October 20, 2011 08:51

I am not sure.
I put a point on the blade surface and monitor the pressure of the point. I found that there is always a value for the pressure, does this mean the point is rotating? Because if the point doesn't rotate, it will move into the blade as the blade rotating, and then no pressure value on the point.

Far October 20, 2011 08:55

As I know that in rotating reference frame every thing is moving unless you specify it is stationary. for example for casing you have to specify explicitly that it is stationary wall (counter rotating wall in CFX and wall with zero rpm in fluent in stationary reference frame )

bhatiadinesh October 21, 2011 06:40

What about the possibility that it is just keeping the point at the same position wrt rotating frame of reference. when you are loading the new time step, the fan should rotate ( IS THIS HAPPENING ? ) Cause I tried to do exactly the same and my point didnt move. The point just shows pressure at the same point in stationary space.

Far October 21, 2011 08:05

Quote:

Hi my friends, I am doing a transient simulation of turbomachinery.
If he is conducting the transient simulation then how come it is possible that the single point in the domain does not move with geometry. If this is the case then blade should also not be moving, as boundary condition is defined for the fluid equivalent to the rotor rpm.

This point is stationary with respect to the blade and hub but moving in the absolute reference frame. Just think as if you are also moving with the rotor.

bhatiadinesh October 24, 2011 00:21

Quote:

Originally Posted by Far (Post 328887)
If he is conducting the transient simulation then how come it is possible that the single point in the domain does not move with geometry. If this is the case then blade should also not be moving, as boundary condition is defined for the fluid equivalent to the rotor rpm.

This point is stationary with respect to the blade and hub but moving in the absolute reference frame. Just think as if you are also moving with the rotor.

Hi,
I understand what rotating frame of reference is. But only in post processing how can the solver know the flow trajectory taken by a point between two time steps and hence the location of one point in the next time step ?
Also one could say that the point shows the value of any one variable at a particular time step at a fixed location. for example say a point to monitor pressure fluctuations at a point in space for eg for noise analysis.

One way to check is define the point. see its position. Load the some other time step. if Point moves the considers it moving with the flow else its stationary in space.

I did the above exercise and found the piont to be stationary in my case.

-Dinesh

bhatiadinesh October 24, 2011 00:26

BalanceChen,
Do you want to check the pressure fluctuations on the blade surface with time ?
This can be done by defining a monitor in your pre file.

Far October 24, 2011 02:25

Quote:

I did the above exercise and found the piont to be stationary in my case
then I would be interested to check wats happening there.


So far I understood, from this discussion, though I am not convinced, that point on blade has transient pressure fluctuations (moving with blade) in solver but in post it is stationary.

Far October 24, 2011 02:27

Quote:

Because if the point doesn't rotate, it will move into the blade as the blade rotating, and then no pressure value on the point.
Yes this can be the another simple explanation

BalanceChen October 26, 2011 10:02

I have just found that there is notice containing following words in the out file:

+--------------------------------------------------------------------+
| User Defined Monitor Information |
+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
| ****** Notice ****** |
| |
| Monitor points are assigned to the nearest vertex and will |
| change position in case of moving mesh or rotating domains. |
+--------------------------------------------------------------------+

So, we can now be certain that the point in the rotating frame is rotating.

bhatiadinesh October 31, 2011 05:38

Hi,
Thanks for the update. This is precisely what the doubt was about. When you define a point in pre, it takes it as the point in the rotating domain and it moves with the mesh. But again it will be stationary in rotating frame of reference. That is it will simply rotate and not flow with the fluid.
But when defined in post it is static wrt stationary frame of reference. That is it is not moving with the mesh.

-Dinesh

Far October 31, 2011 06:05

Quote:

static wrt stationary frame of reference.
So you want to say, the point in the rotating reference frame is not moving w.r.t stationary reference frame. I think it is problem of understanding of pre and post functions.

1. when you define the point in the pre and solve it then all files with different time steps will have same point at the same location on blade but will be having different coordinates. because now blade is changing its position w.r.t stationary reference frame.

2. On the other hand in CFD post you will have to define the point in all files with different time steps so that it will have the same location on blade but with differnet x,y,z coordinates as mentioned above. For this either you have to do it manually or use some script.

3. If your point is valid (that is CFD post can not extract data on some rotating point), then it is bug and must be reported to CFX support. Moreover why people in CFD community are using the Techplot and Fieldview to extract the similar type of data.

bhatiadinesh October 31, 2011 07:12

Quote:

Originally Posted by Far (Post 330123)
So you want to say, the point in the rotating reference frame is not moving w.r.t stationary reference frame. I think it is problem of understanding of pre and post functions.

1. when you define the point in the pre and solve it then all files with different time steps will have same point at the same location on blade but will be having different coordinates. because now blade is changing its position w.r.t stationary reference frame.

2. On the other hand in CFD post you will have to define the point in all files with different time steps so that it will have the same location on blade but with differnet x,y,z coordinates as mentioned above. For this either you have to do it manually or use some script.

3. If your point is valid (that is CFD post can not extract data on some rotating point), then it is bug and must be reported to CFX support. Moreover why people in CFD community are using the Techplot and Fieldview to extract the similar type of data.

I would not say that it is a bug necessarily. It is just that they should introduce a feature which allows you to select the frame of reference of a user defined location ( like point, line etc..)


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