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February 15, 2005, 06:18 
Lift, Drag Vs time chart,calculations

#1 
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Hi
I am interested in calculating the lift and drag coefficients over time for a bluff body. Is it possible to have such a graph made in post processor or solver, or do I need to calculate the coefficients manually for each timestep? Regards James 

February 15, 2005, 07:00 
Re: Lift, Drag Vs time chart,calculations

#2 
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You need to create expressions for Cl, Cd and then monitor them in the output menu.
This should give you an idea of what to do. The expressions go something like this: LIBRARY: CEL: EXPRESSIONS: Angle = 0 [deg] Span = 0.1 [m] Chord = 0.18574 [m] SectionArea = Chord*Span Drag = ((force_x()@Section)*cos(Angle))+((force_y()@Secti on)*sin(Angle)) Free Stream = 4.1 [m s^1] CD = Drag/(0.5*1027.6*SectionArea*Free Stream^2) Lift = (1.0*(force_x()@Section)*sin(Angle))+((force_y()@Se ction)*cos(Angle)) CL = Lift/(0.5*1027.6*SectionArea*Free Stream^2) CLCD = Lift/Drag CM = torque_z()@Section/(0.5*1027.6*SectionArea*Free Stream^2) NormalF = (Lift*cos(Angle))+(Drag*sin(Angle)) CNN = NormalF/(0.5*1027.6*SectionArea*Free Stream^2) CPX = torque_z()@Section/NormalF FX = force_x()@Section FY = force_y()@Section XTorque = torque_x()@Section YTorque = torque_y()@Section ZTorque = torque_z()@Section uu = Free Stream*cos(Angle) vv = Free Stream*sin(Angle) ww = 0 [m s^1] END END The monitor points go something like this: OUTPUT CONTROL: MONITOR OBJECTS: MONITOR BALANCES: Option = Full END MONITOR FORCES: Option = Full END MONITOR PARTICLES: Option = Full END MONITOR POINT: CLCD Ratio Expression Value = CLCD Option = Expression END MONITOR POINT: Centre of Pressure Expression Value = CPX Option = Expression END MONITOR POINT: Drag Coefficient Expression Value = CD Option = Expression END MONITOR POINT: Drag Force Expression Value = Drag Option = Expression END MONITOR POINT: Force X Expression Value = FX Option = Expression END MONITOR POINT: Force Y Expression Value = FY Option = Expression END MONITOR POINT: Lift Coefficient Expression Value = CL Option = Expression END MONITOR POINT: Lift Force Expression Value = Lift Option = Expression END MONITOR POINT: Moment Coefficient Expression Value = CM Option = Expression END MONITOR POINT: Normal Coefficient Expression Value = CNN Option = Expression END MONITOR POINT: Normal Force Expression Value = NormalF Option = Expression END MONITOR POINT: TorqueX Expression Value = XTorque Option = Expression END MONITOR POINT: TorqueY Expression Value = YTorque Option = Expression END MONITOR POINT: TorqueZ Expression Value = ZTorque Option = Expression END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: File Compression Level = Default Option = Full Output Boundary Flows = All Output Equation Residuals = All Output Variable Operators = All END END James 

February 15, 2005, 07:19 
Re: Lift, Drag Vs time chart,calculations

#3 
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James
Thank you for your reply, most appreciated! James 

February 15, 2005, 08:43 
Re: Lift, Drag Vs time chart,calculations

#4 
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James, Excellent !, any chance you can define some of the variables ? and some explantion of the commands , for instance what's 1027.6 related to or I am asking too much,


February 15, 2005, 14:25 
Re: Lift, Drag Vs time chart,calculations

#5 
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You're best bet is to have a look at the CCL guide in the CFX user manual to work out any of the commands which I've used which aren't familiar.
Angle = Angle of Incidence Span = Span of Section Chord = Chord of Section Free Stream = Free Stream Flow Velocity CD = Drag Coefficient CL = Lift Coefficient CNN = Normal Force Coefficient CLCD = Lift/Drag Ratio CM = Moment Coefficient CPX = Centre of Pressure in the xdirection uu, vv & ww = Flow Boundary Condition Velocities 1027.6 is the density of salt water at 5 deg C. James 

February 16, 2005, 14:29 
Re: Lift, Drag Vs time chart,calculations

#6 
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James, I am not sure CFX produce a force_x,force_y or z for that matter. Also the statement @section, refers to the wall in question, please clarify.


February 16, 2005, 17:58 
Re: Lift, Drag Vs time chart,calculations

#7 
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The Statement:
force_x()@Section Gives the total force in the xdirection acting on the wall defined with the name "Section". James 

February 17, 2005, 09:43 
Re: Lift, Drag Vs time chart,calculations

#8 
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Hi,
Take a look at Tutorial 20 page 414 (CFX5.7 Documentation), where the force on the ball due to fluid flow, FFlow, is defined as force_y()Ball 

February 17, 2005, 17:23 
Re: Lift, Drag Vs time chart,calculations

#9 
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Cheers Janes and Rui


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