Cl and Cd oscillate in steady simulation for higher angles of attack

    • Sahil1028

      Hi, I am new to ANSYS FLUENT and I am using it to simulate flow over a s809 airfoil. The simulation is 2D and i created a c-type mesh using ICEM. The Reynolds number of the flow is 2 million and my chord length is 1m. So i calculated the appropriate wall spacing value online with a y+ value of less than 1. I am using the transition SST model as I am trying to validate the results i get with another research paper. So far I have kept all parameters the same to ensure accurate results. I run the steady state simulations while varying the angle of attack (0-15 degrees). The reynolds number is always kept the same and I know for a fact that the airfoil should stall at around 20 degrees. I receive accurate Cl and Cd values up to 10 degrees angle of attack and from 11-15 degrees the values of Cl and Cd start to oscillate. I have attached the images here. 

      I originally thought it was the mesh so i refined it. But refining the mesh did not change the results I obtained as the mesh is already fine. I dont know why the results oscillate and I think it has something to do with the unsteadiness of the flow. But the airfoil is supposed to stall at 20 degrees so why cant i get converged values of Cl and Cd for angle of attack lower than 20?

      Any help would be appreciated!!!

      Cl graph for AOA - 15°Cd graph for AOA - 15°

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