Learning Forum

[MCFDHELP]

When simulating a 2D airfoil in fluent, i get a CD of around 0.05 which seems sensible but get a CL of 2.6 which seems too high. When i alter the angle of attack to 3 degrees the CL goes upto 6. I have set the ground to the same velocity as inlet as its in ground effect. Any help of why my CL is so high woud be appreciated

[Rob]

Have you created a higher pressure region under the airfoil because of it's proximity to the ground? Posting pictures will help.

[MCFDHELP]

Hi Rob No i haven't created a higher pressure region under the wing. I have created an inflation layer on the airfoil and done the rest of the free stream as unstructured. I believe in the setup the settings i have used are correct, i have set the inlet velocity, set the outlet as a pressure outlet. I have set the reference value as the inlet and selelcted CD and CL for the airfoil. My max Yplus is also 0.7 on the airfoil

[MCFDHELP]

if i reduce the velocity from 78 m/s to 41 m/s the Cl reduces to 1.78 which is much more sensible but the aircraft will be travelling at 78 m/s

[Rob]

I'm not permitted to open the attachments, please post images in line with the text.

[MCFDHELP]

[MCFDHELP]

[Rob]

What's the pressure & velocity look like around (and under) the wing?

[MCFDHELP]

[MCFDHELP]

[Rob]

That looks a fairly sensible result. Ground effect vehicles work by riding on a sort of air cushion so can carry very high loads. If you get too high you lose the effect and you need to hope the effect re-establishes before you hit the floor.
For a hang glider you'd expect a glide angle of 15-20:1 Near the floor (ie trying to land) if the field slopes slightly away from you it's quite possible for the glide angle to reach 25-30:1 which makes landing much more fun than it needs to be. In the UK, the landing fields are quite short so the dry stone walls tend to slow you up fairly abruptly.....

[MCFDHELP]

yes i thought these contours looked sensible but the CL was 6 which seemed too high hence i doubted the validity of the contour plots. I was hoping to show the CL decreasing as the height increases from the ground but wasn't expecting a CL of 6

[Rob]

Pass, I tend not to have to worry about lift & drag in most of my models. They're generally very well attached to something! If you read up on ground effect vehicles you may get a better feeling for lift coefficients: given the application they may be much higher than you'd expect. I was far too busy worrying about the wall in the practical application....

[MCFDHELP]

Yes I agree however the velocity shouldn't really change the coefficient of lift. I am not sure why when I change the velocity the coefficient of lift varies so much. It could be something in my settings

[Rob]

Transition, flow splits? Did you alter the reference value?

[MCFDHELP]

No I have not altered the reference values. Should they be changed, I just referenced from the inlet

[Rob]

Check the NACA tutorials - I can't remember!

[Timothy Oladapo]

Hey guys,  does anyone know how to stimulate ground effect for 2D airfoils with Fluent?