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-   -   Force calculation help (http://www.cfd-online.com/Forums/cfx/96538-force-calculation-help.html)

tomwalker January 25, 2012 09:38

Force calculation help
 
I have set up an SST flow to simulate the air flow around an underground train. With these settings I am trying to calculate the drag coefficient of the train but when I do an expression to calculate the force on the train then it keeps giving me a value of zero. The train is only moving at 15 m/s so i know the force will be low but zero seems a little extreme.

Is there a really obvious reason why it is doing this?

All help is highly appreciated

Thanks

Tom

Lance January 25, 2012 10:27

Could you post your expression for the drag coeff?

tomwalker January 25, 2012 10:30

i was going to work out the drag coefficient in excel later i just needed ansys to measure the drag force so that I can incorporate it into my equation

Thanks

Lance January 25, 2012 10:34

Quote:

Originally Posted by tomwalker (Post 341078)
... but when I do an expression to calculate the force...

so how does this expression look like? What do you want to export to Excel (and why?).

tomwalker January 25, 2012 10:37

The only value i need out of ansys is the value for the force acting on the body in the same direction as the air flow (i.e. the drag force) using this value I can then use:

Cd = (2Fd)/(density*velocity*velocity*area)

Lance January 25, 2012 10:40

Yes, that's the expression for the drag coeff, but I asked about your expression for the force (that apparently becomes zero...)

Is is something like:
force_x()@some_area?
Or its equivalent
areaInt_X(Pressure)@some_area+areaInt(Wall Shear X)@some_area

tomwalker January 25, 2012 10:42

oh right sorry, miscommunication there

Yes that is what my equation looks like using x because that is the direction of flow and the area is front (because that is the domain that is on the front of my train)

Thanks

Lance January 25, 2012 10:56

Quote:

Originally Posted by tomwalker (Post 341106)
oh right sorry, miscommunication there

Yes that is what my equation looks like using x because that is the direction of flow and the area is front (because that is the domain that is on the front of my train)

Thanks

Well, it shouldnt be zero (as you pointed out :)). Is it the same for other surfaces? Is the pressure integral also zero?

tomwalker January 25, 2012 10:57

I haven't checked the pressure integrals but looking at the force on the surface of the whole body it has given me a reasonable looking results :D

Thanks for your help

Tom Walker

sheikh nasir February 14, 2012 10:07

train moving in tunnel help
 
hello
i am working on train moving in tunnel. i am getting floating error:invalid number. can any one help me. my email is sheikhnasir39@gmail.com
plz help me
thanks

:confused:


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