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

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

James

Thank you for your reply, most appreciated!

James

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,

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 x-direction

uu, vv & ww = Flow Boundary Condition Velocities

1027.6 is the density of salt water at 5 deg C.

James

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.

The Statement:

force_x()@Section

Gives the total force in the x-direction acting on the wall defined with the name "Section".

James

Hi,

Take a look at Tutorial 20 page 414 (CFX-5.7 Documentation), where the force on the ball due to fluid flow, FFlow, is defined as force_y()Ball

Cheers Janes and Rui