Learning Forum

[Conan Butler]

I am currently working on a 2D cone analysis, where I have set up the drag equation and used the 3D circumference area as a reference value for "Area" in FLUENT. However, I am unsure about the validity of the Area I used for the reference value in the drag calculations. I would like to know what area should be used instead. I chose a 2D analysis because it is easier to solve for Ansys and is also symmetrical. My aim is to represent somewhat the drag value for a 3D version of the cone.

[Rob]

Check the report definitions. Some of the coefficients are based on PI radians rather than 2PI. 

[Conan Butler]

I took the area from the cad software where I made the cone, so nothing is going wrong there. First, let's get this clear: I am getting results so there are no problems with the formula or anything. I am just curious if the method I use now is valid for a cone in 3d instead of now doing 2D. To conclude, how do I determine which Area I have to choose for the drag Area since it's a 2d analysis, and are the results valid for the 3d cone?

[Rob]

2d or 2d axisymmetric? Former is a wedge, latter is a cone as far as Fluent is concerned. 

The 2d options are generally a good option if both geometry and flow obey the symmetry constraint. Using a wing as an example, you'll get much the same result as for 3d for a long wing but with much lower cell count. It'll start to differ near the wing root or tip where 3d effects come into play. 

[Conan Butler]

This is the "cone" I used. My solving mode is set on planar, so it isn't axisymmetric. I could have made it axis-symmetric, but I didn't. What Area must I use for the "reference values" in fluent, which are used in the drag calculations? And I used the shell plane of the 3d version of this cone for its area, but I have to make sure first of all that is correct and second if the Drag CD is valid for the 3d cone or just for the 2d plane, which is visible in the picture above? This is the reference value tab:

[Rob]

If it's 2d planar it's not a cone, so you'll struggle to get a surface area for the coefficients to match a cone. The reference values are explained in the various air/aero foil tutorials (NACA for example). 

[Conan Butler]

I understand what the reference values are. That's also not the problem.

Maybe let's formulate my question differently: what will you choose for the Area I have to fill in for the reference value for this shape (is it the area of the 2d cone sketch? etc)? And let's say, assuming looking at the "cone picture" the height is in the y direction and width in the x direction. The velocity is coming from the negative y direction towards the cone. And I want to know what the drag will be on the cone with these assumptions.

Again mentioning that the problem is not the not understanding of the drag etc. it's just what I assume the Area to be. And since this is not an Airfoil just using the chord length times the height isn't valid.

[Rob]

Coefficients are always an odd one: the forces are just read from the pressure/shear results. As it's a coefficient the area you need is based on whatever you're comparing with: ie how did you compute the coefficient from the experiment. However, they are convenient and often used inside Fluent for the particle drag laws. 

More usefully, Google of "drag coefficient formula cones" (lose the quotes) has a decent article from Aerospaceweb.org which seems fairly comphrensive.