[aakash8055]

I am trying to use CFD Fluent to find the drag coefficient on a 2D generic SUV model build using SolidWorks and while doing the interation process if in the reference values option if i keep the reference value of area as 1 sq.meter then the results are something abrupt but if put the value as the area of the car from the solidworks sketch(side area) then the result shown are something a bit realistic.So can plase any one give me an idea about what exactly is the significance of the reference area variable?

Thanks in advance.

[LuckyTran]

I answer using the Socratic method. What is your definition of drag coefficient?

The reference values are super important! You must specify the correct reference values for area, density, velocity, and so on. This is not a joke. If you understand what drag coefficient is, you will appreciate that there are tons of reference parameters in it and in order to get a meaningful coefficient, you must specify the correct ones.

[aakash8055]

Thanks for the reply sir.
After using various values for the reference variable I am able to understand that the reference value plays a great role in getting the correct value of drag coefficient.But I am still not able to understand that which area, volume, depth, length should I consider will it be the values corresponding to the car model or the 2-D enclosure around the car model.

[LuckyTran]

Two things. First for 3D simulations:

Although pretty much everyone uses the reference values, I believe it is best for you to learn for yourself! It is very simple and you simply need to answer my earlier question.

Simply write down your definition of drag coefficient. The reference values that you need to give to Fluent are embedded in that definition. Drag coefficient is meaningless unless two people agree on the same reference parameters. Otherwise, the reference values are completely arbitrary. Some people use actual exposed surface areas, some people use project areas. However, some people use the projected frontal area for the drag coefficient but the planform area for the lift coefficient, whereas some people use the planform area for both the drag and lift coefficients for consistency.

If you have no idea what these reference values should be, then I offer you a second question. Why the heck do you even care what the drag coefficient is? Drag/force/lift coefficient only makes sense after you have defined the reference parameters.

Now for 2D simulations there is another reference parameter that is not really a reference parameter, the reference depth. Before Fluent can compute the force it needs to know the length of the object/domain in the 3rd dimension. Hence you must explicitly give the reference depth. Be careful, because the value you choose for reference depth needs to be consistent with the reference area, otherwise you can calculate a different coefficient than what you intended.