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January 25, 2012, 09:38 
Force calculation help

#1 
New Member
Tom Walker
Join Date: Jan 2012
Posts: 10
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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 

January 25, 2012, 10:27 

#2 
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Lance
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Could you post your expression for the drag coeff?


January 25, 2012, 10:30 

#3 
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Tom Walker
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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 

January 25, 2012, 10:34 

#4 
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Lance
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January 25, 2012, 10:37 

#5 
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Tom Walker
Join Date: Jan 2012
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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) 

January 25, 2012, 10:40 

#6 
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Lance
Join Date: Mar 2009
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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 Last edited by Lance; January 25, 2012 at 10:54. Reason: typo 

January 25, 2012, 10:42 

#7 
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Tom Walker
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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 

January 25, 2012, 10:56 

#8 
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Lance
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Well, it shouldnt be zero (as you pointed out ). Is it the same for other surfaces? Is the pressure integral also zero?


January 25, 2012, 10:57 

#9 
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Tom Walker
Join Date: Jan 2012
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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
Thanks for your help Tom Walker 

February 14, 2012, 10:07 
train moving in tunnel help

#10 
Member
Join Date: Jan 2012
Posts: 58
Rep Power: 6 
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 

Tags 
cfx, drag coefficient, force 
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