Learning Forum

[janetj]

Hello everyone,

I am working on two element airfoil with a trailing edge flap at 25 degree. I have done quad structured meshing. I have some questions regarding that.

My Y-plus value is 7 when I am using Spalart Allmaras model with Reynolds number of 200,000. (Density 1.225 kg/m3, Chord Length 1m, Viscosity 1.7894e-5, Velocity 2.92 m/s). Looking at the mesh and model, is my Y-plus value of 7 okay for S-A turbulence model? What should be the the minimum or maximum Y-plus value?

Secondly, when I use K-epsilon (2 equation) model with Realizable K-Epsilon Model and Scalable Wall Function, my Y-plus reaches nearly 16. I am getting lower Cd and higher Cl using this model as compare to Spalart Allmaras. I know that Scalable Wall Function increases the Y-plus above 15. My question is, is it okay to use Realizable K-Epsilon Model with Scalable Wall Function for Airfoil?

[Kalyan Goparaju]

Hello,
In Ansys Fluent, the Spalart-Allmaras model has a y+ insensitive wall treatment. On intermediate grids, 1 I would recommend using the Realizable K-epsilon model with Enhanced Wall Treatment. This is a y+ insensitive wall treatment and will automatically adjust based on the local y+.
Thanks,
Kalyan

[janetj]

Dear Kalyan I am still not getting a lower Cd after using the Realizable K-epsilon model with Enhanced Wall Treatment.What could possibly be wrong? My airfoil is a high-lift one (Selig1223) and with the flap, the idea is the get an increased Cl with nominal increase in Cd. But I am getting a Cl of 1.8, whereas Cd is 0.12 and the Cl/Cd is 14.66 at 0 degree alpha which is quite low.

[Rob]

Can you check the forces are correct? Cd and Cl are coefficients so if the forces are as expected check you're using the correct reference areas etc

[Kalyan Goparaju]

Hello,
A couple of things
do you have any validation for the numbers that you are getting? experimental or theoretical?
are both Cl and Cd wrong or is it only the Cd? Drag is notorious for being strongly dependent on the mesh. is your mesh fine enough to capture all the relevant physics? Boundary layers, separation (if any), gap between the elements etc.
As my colleague suggested, please also check if you are using the correct reference quantities.
Thanks Kalyan