# Calculate Drag & Lift

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How to measure coefficient of drag & lift in Acusolve using AcuProbe or whatever ?

What is the user defined function for calculating cd value?

How to calculate the frontal area of a model ?

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AcuSolve does not compute the lift and drag coefficients directly.  It does, however, compute the forces on the body which you can then use to compute the lift and drag coefficients.  The forces on the body are the x-traction, y-traction, and z-traction that are displayed as part of the SURFACE_OUTPUT (either in AcuTrans or AcuProbe).

To get the lift and drag coefficients (assuming x is drag direction, and y is lift direction), you need to compute:

Cd = 2*xTraction/(area*rho*referenceVelocity^2)

Cl = 2*yTraction/(area*rho*referenceVelocity^2)

The area that is used generally depends on the type of analysis that you are using.  For airfoils, it is typically the planform area.  For vehicles, it is typically the projected area.  You can compute the projected area in a CAD package, using HyperMesh, or manually for simple shapes.

jegan260 likes this

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I'm doing a analysis of Ahmed body of slant angle = 25 degree.
I got the plot of cd with the above formula. But how to get a exact value, like overall drag of the body. For example, When I used Virtual Wind Tunnel. I got a exact drag value of 0.2981 which is the exact experimental value in the generated pdf report. How it calculated that value ?

When I exported the drag plot & viewed in excel, that 0.2981 value was in the time step of 684. My total time step is 1389. Previously I was using Star-CCM+, In that, If I give the drag equation, It will show me the exact drag value & with that also I can plot a graph, But in Acuprobe, its just generating a plot, its not showing the exact drag value.

I've attached the screenshot of the cd plot from Acuprobe & the drag value using Star-CCM+ for your reference.

Thank You

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in VWT 12.1 the drag is averaged over the last 50 iterations. This is good for transient runs, but might not be the best solution for a steady state run. As far as I know in VWT 13.0 there will be user defined number of iterations used for averaging.

If you go to the run directory, there you will find a file called Drag_PLots.Drag_Coeff.dat (on Windows), this contains the history of the drag.

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Thank you for your reply. I counld'nt find the Drag_PLots.Drag_Coeff.dat in windows. But anyway using UDF with the above formula, I generated a text file of the plot. So how to calculate the exact/overall drag of a vehicle in steady & Transient analysis. I'm least internested in the drag @ each time step, I just want the overall drag of the vehicle/any body ?

Thank You

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for transient runs you can average the drag over several time steps to get the overall drag of the body. How many time steps you need to include depends on the time increment and the fluctuation frequency of your flow. For steady you can use the last time step or average of the last 5 time steps.

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Thank you. Can you please explain bit more detail on how many time steps is needed to calculate drag for transient analysis. My time increment is 0.00056333s, Its a Ahmed body with 25 degree slant angle. inflow = 40m/s, nTimeSteps   =   1389.

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You could right click and choose to export the traction, that will produce a text file of the traction history at each time step.

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How to calculate the Frontal area using hyperworks tool ?

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Virtual Wind Tunnel is the easiest way to get the frontal area. Load the mesh (scaled in meters) als .NAS (to be precise Nastran Fluent Format from HyperMesh) into  VWT.

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