Learning Forum

[Abubakar]

Hi @Kremella sir,

Hope you are doing great!!

I have a question related to the Solver setup of Aerodynamics of an FSAE Car course. I just wanted to know did the instructor use the same area (shown below) to find the Coefficient of lift?

[Karthik Remella]

Hello The lift force is computed on the entire surface area of the car. But, when estimating the lift (or in this case, downforce), you need to pick a reference area. For this, we chose to use the projected area of the car (for both lift and drag coefficients). This is just a reference area to estimate the dimensionless parameters that quantify lift and drag. I hope this clarifies your question.
Karthik

[Abubakar]

But sir, how can the reference area be the same for calculating lift and drag coefficient?
And also I tried to run the Ahmed body simulation with having a frontal projected area as a reference area and I computed the drag and lift coefficient and when I compared this coefficient with the experimental one I am getting the wrong lift coefficient but the drag coefficient is correct.
Also, when I keep the perpendicular area as a reference area I got the lift coefficient correct but the drag coefficient is wrong.
Even the below discussion suggesting me to use a perpendicular area to calculate the lift coefficient and frontal projected area for calculating the drag coefficient.
I hope you understand my concern.

[Abubakar]

Hi Sir Can you please help me with this?
Thank you Abubakar khan

[Karthik Remella]

Hello To be honest, the drag and lift (or downforce) coefficients are non-dimensional forms used generally to compare the data with experiments. Usually, a reference area is chosen to non-dimensionalize the obtained forces. When comparing with experiments, it is very important to use the same consistent reference area as used in the experiments. Therefore, in your Ahmed body case, if you are comparing the simulation data with experiments, you should double-check and use the same reference area. It is possible that the drag and lift coefficients are non-dimensionalized using different areas in the experiments. But, this practice is not universal and is certainly not driven by physics. It just scales the forces consistently to facilitate good comparison between different configurations. Having said that, care should be administered in picking a consistent reference value when comparing these data with other results.
Since the scope of our FSAE course did not include any experimental comparison, as it was intended for teaching the problem set-up and how to post-process the obtained results, we decided to use the 'Frontal Area' of the car as our reference value. If you choose to use a different value, please feel free to change the reference value and compute the forces. Note that you do not have to re-run the simulation if you make changes to the reference values as they do not affect the absolute values of the forces (they only affect the corresponding coefficients).
I hope this answers your question. Also, my apologies for not replying to your post earlier.
Karthik

[Abubakar]

Thank you so much sir, for your generous answer and yes now I understood the concept of the Reference area. No need to apologize sir.