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Old   May 21, 2007, 09:52
Default Drag Coefficient
  #1
Nikos
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Hi guys,

im doing a 2d simulation and I want to get the drag coefficient for the structure, how do I do that through fluent?

Thanks!
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Old   May 21, 2007, 10:00
Default Re: Drag Coefficient
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Hussein
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Solve>monitors>force>drag
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Old   May 21, 2007, 11:35
Default Re: Drag Coefficient
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Nikos
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Hmm, thanks! I did that and it worked.

However I get a drag coefficient of about (whats you said calculates drag coefficient, not just drag, right?) -19. It sounds like a strange value considering that drag for aerodynamic bodies is around 0.3. And this is quite aerodynamic... Any ideas?
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Old   May 21, 2007, 13:40
Default Re: Drag Coefficient
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Matt
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check your reference values, you will require to input a reference depth (because your modelling in 2D) and a reference area.

In the menu...

report/reference values
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Old   May 21, 2007, 17:24
Default Re: Drag Coefficient
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Nikos
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Thanks! Which values of those are relevant to to drag coefficient? The values are Area/Density/Depth/Enthalpy/Length/Pressure/Temperature/Velocity/Viscocity/Ration of specific Heats.

Reference depth is correct, reference area im not sure about. Which area do I use? The are of the whole structure?
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Old   May 21, 2007, 17:43
Default Re: Drag Coefficient
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Nikos
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Or maybe the frontal area? I used the are of the whole structure for trial and error and the Cd came out too small, around 0.0019, which doesnt seem right either...
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Old   May 21, 2007, 18:29
Default Re: Drag Coefficient
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Matt
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Its up to the engineer to decide what is most suitable.

Frontal area is frequently used in the automotive industry for example.

Plan area, however, is used for an airfoil.
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Old   May 22, 2007, 04:12
Default Re: Drag Coefficient
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Nikos
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I cant really use frontal area in a 2d analysis though can I? I am bound to use plan area...
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Old   May 22, 2007, 04:37
Default Re: Drag Coefficient
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Nikos
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Also, when it asks for the length, which length does it mean? the length of structure or the length of the domain?
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Old   May 22, 2007, 04:41
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Nikos
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Also, whats the ratio of specific heats for water and seawater??
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Old   May 22, 2007, 05:43
Default Re: Drag Coefficient
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Matt
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its just a reference area, its no referring to an actual dimension on the 2D CAD model.

For example imagine i was modelling flow over an elliptical cylinder, but in 2D.

I would basically model a cross sectional slice through the geometry, in other words a 2D ellipse.

Generally the engineer would choose to use the frontal area to obtain a drag coefficient. Obviously there is no third dimension in the model to be able to obtain the frontal area. That is why you specify it in the reference values toolbox. If the cylinder is 2m long (ie 2m into the page) and 0.5m in diameter the frontal area is simply 2*0.5= 1 metres squared. Put this value into the Area box.

You will need to put in a reference velocity and density also which are usually the values used in the inlet boundary condition. Dont concern yourself with any other other reference value that isnt in the drag coefficient equation.

Cd = Drag/(0.5*denisty*velocity^2*reference_area)

Thats me over and out on this one, look up a fundemental fluids textbook if your still struggling. Its just basic 2D fluids.

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