Learning Forum

[Lewis Bell]

Hi,

I am continually getting incorrect Cl and Cd values for my simulation. With some simulations, the value I get at 0 Degrees AOA my answers are within 1.5%, however sometimes are out by an order of magnitude. 

My simulation is of an Eppler 908 foil, at this point, I am just trying to validate my model against data found on Airfoiltools database. Eventually, I would like to perform simulations of this foil against a multitude of AOA and velocities within water.

However, currently my simulation is in air, velocity 7.1m/s chord length 0.1m, this gives Reynolds number of 50000. I am using a C shaped mesh. For y+ = 1, my wall spacing was calculated to be 3.9x10^-5. I have attached images to show that my wall spacing is significantly below this threshold. 

My settings for the fluent app are attached to an imgur link, as well as the expected output vs. actual output. Any help would be greatly apprecitaed.

Imgur Link to relevant images

[Rob]

Did you adjust the reference values? Check the lift and drag force too. It's very common to not get the area correct so the force is OK but the coefficient isn't.  There are a few NACA airfoil examples/tutorials in the Fluent Help, they should explain what's needed. 

As an aside, staff are not permitted to open/download files and attachments. It's to keep us out of trouble with Legal (export compliance laws) and IT. 

[SRP]

Hi,

To add, you can also refer to Ansys Innovation Course on flow over an airfoil: Flow over an Airfoil - ANSYS Innovation Courses

Hope you will find this useful.

Thank you

[Lewis Bell]

These are my reference values, I will check the Lift and Drag values, further to this I want to find the Cp vs x/c plot. I know how to do this, however, will this be affected by the same issue as the Lift and Drag Coefficients?

 

[Rob]

All coefficients are defined in the User's Guide, and most use one of the values you posted in that calculation. By Cp do you mean pressure coefficient, if so, yes it'll be affected. 

[Lewis Bell]

Yes, I do mean the Pressure Co-efficient. If affected similarly to Cl and Cd, how will I calculate Cp?

[Rob]

The Cp value come from Fluent - read https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_fvdefs.html   and will tell you which reference value(s) are used. How to change those is covered in the materials SRP and I have already mentioned.