# Force calculation help

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 January 25, 2012, 09:38 Force calculation help #1 New Member   Tom Walker Join Date: Jan 2012 Posts: 10 Rep Power: 6 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 Senior Member   Lance Join Date: Mar 2009 Posts: 601 Rep Power: 13 Could you post your expression for the drag coeff?

 January 25, 2012, 10:30 #3 New Member   Tom Walker Join Date: Jan 2012 Posts: 10 Rep Power: 6 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
Join Date: Mar 2009
Posts: 601
Rep Power: 13
Quote:
 Originally Posted by tomwalker ... 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?).

 January 25, 2012, 10:37 #5 New Member   Tom Walker Join Date: Jan 2012 Posts: 10 Rep Power: 6 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 Senior Member   Lance Join Date: Mar 2009 Posts: 601 Rep Power: 13 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 New Member   Tom Walker Join Date: Jan 2012 Posts: 10 Rep Power: 6 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
Join Date: Mar 2009
Posts: 601
Rep Power: 13
Quote:
 Originally Posted by tomwalker 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?

 January 25, 2012, 10:57 #9 New Member   Tom Walker Join Date: Jan 2012 Posts: 10 Rep Power: 6 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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