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-   -   Forces on wall in unsteady case (http://www.cfd-online.com/Forums/fluent/109866-forces-wall-unsteady-case.html)

sondinho November 28, 2012 11:33

Forces on wall in unsteady case
 
Hi

I am wondering about how to plot forces that acting on valve wall in unsteady simulation. I now that there is something in ,, monitors'' but only
a drag force. But this is not that I want. I would like that y - axis represent force and x- axis represent flow time. Does anybody know how to do it?

And my second question is about the metioned drag force. This value is based on referenced values like area and velocity. How to get information what kind of values they should be I dont understand this meaning of reference values. Maybe somebody can explain me this in easy way.

Crank-Shaft November 30, 2012 22:29

Quote:

Originally Posted by sondinho (Post 394679)
Hi

I am wondering about how to plot forces that acting on valve wall in unsteady simulation. I now that there is something in ,, monitors'' but only
a drag force. But this is not that I want. I would like that y - axis represent force and x- axis represent flow time. Does anybody know how to do it?

And my second question is about the metioned drag force. This value is based on referenced values like area and velocity. How to get information what kind of values they should be I dont understand this meaning of reference values. Maybe somebody can explain me this in easy way.

To plot the forces acting on the wall, you need to either create a representative surface or use any of the existing wall boundaries on your geometry. Under the solution monitors menu item, you should be able to choose drag force as you already mentioned and then check the box to write the history file. Ensure that you have the x-axis defined using the appropriate time-step values, flow time or iterations, based on your preference and these are available when running a transient simulation.

For the reference values, I would guess that a velocity which represents free-stream values or bulk velocity should be appropriate, since that is what we would manually calculate. I also noticed that the default reference area is 1 m^2 and so far I have always left it at this value. However, this should ultimately be changed to represent the surface area of interest. From my limited understanding it would seem using an area of 1 means that the drag force being calculated is essentially a specific 'per-unit area' value and should be multiplied with the surface area, perhaps in a post-processing application.

I am sure someone here will be able to shed some more light on this however, I hope the discussion above helps you.

Good luck.

sondinho December 7, 2012 17:09

I am wondering about other method. If I have defined variable in UDF that stores the value of Force. Is there a UDF Function that I can use to plot this variable values?

sondinho December 8, 2012 08:30

Ok I find solution. As you mentioned I specyfy reference values:
velocity 1 m/s
density 2 m/s
area 1 m^2

And then the Drag Coefficent is exactly as the value of force ( I checked it from force reports). Thank you

Crank-Shaft December 8, 2012 08:55

Quote:

Originally Posted by sondinho (Post 396412)
Ok I find solution. As you mentioned I specyfy reference values:
velocity 1 m/s
density 2 m/s
area 1 m^2

And then the Drag Coefficent is exactly as the value of force ( I checked it from force reports). Thank you

Thank you for verifying this. I will try this out myself and hopefully the results are physically valid and equivalent.

Best of luck with your work.


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