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-   -   Problem with reference area (http://www.cfd-online.com/Forums/star-ccm/92427-problem-reference-area.html)

alfaben September 14, 2011 04:54

Problem with reference area
 
Hi,

i have a problem with the definition of the reference area (force coefficient)
Normaly the reference area is the projected area from the part.
Normaly!
In the bluntbody and airfoil tutorial the reference areas have nothing to do with the projected area from this parts.
It seems in the bluntbody tutorial that the projected area is 2 x value from the inlet area.
In the airfoil tutorial the value is ????.
In the automation tutorial the value for train is ok.
My problem is that i calculate a ship resistance.
When i calculate with the projected area (89.8 mm^2)from the ship my values are to high 0.3 - 0.4 instead 0.03 - 0.04 for the half model.
When i calculate with the value from the inlet area (8190 mm^2) the values are more then ok.
I donīt understand the values from the tutorials.
Can anybody help me.

alfaben

abdul099 September 16, 2011 19:11

Every force coefficient is calculated by its own characteristic reference area. All you have to know is how this area is usually defined.

For example, for an airfoil, the reference area is ALWAYS the area of the wing, calculated by span x chord. No matter whether you are interested in lift coefficient or drag coefficient, the reference area is the same while everybody agrees, the projected area changes dramatically.

I don't know about the reference area for a ship hull, but for a blunt body I suspect it to be the frontal area. But for sure, it does have to do NOTHING with the inlet area, as an inlet boundary and it's shape is just an aproximation made by the user. With other words: THERE IS NO INLET IN NATURE! Or which inlet area would you use for an external areo simulation of a car? Width of a highway x thickness of the whole atmosphere? And your neighbour would use the width of the city from north to south x 5m? How would you ever get the same force coefficient? Just think about it: The force coefficient should depend from the geometry of your problem, not from the domain around your body you decided to simulate.

Additionally, the reference area of a force coefficient is just an arbitrary chose of the guy who decided to use this force coefficient. You can create your own coefficient with your own reference area, but can't compare it with an experiment or another simulation with a different reference area.

By the way, that's the same with other characteristic dimensions, for example for the reynolds number. The decision for a specific characteristic length is just arbitrary and as good as another one - as long as all are using the same characteristic length.

alfaben September 17, 2011 03:46

Hi abdul099,

thank you very much for the answer.
I agree with your argumentation. But the reference area from bluntbody has nothing to do with his projected area (frontal area). The projeced area from the bluntbody is 0.0007m^2. The value in the calculation is 0.016m^2 but values for the foerce coefficent seems to be ok. This is what i donīt understand. The same is with the airfoil.

UUPS SORRY they convert the airfoil to a two-dimensional mesh. Now the value is ok.

alfaben


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