I am performing CFD simulation upon NACA 4418 airfoil at 45 m/s free steam velocity and Reynolds number of 3 million. I am following a particular paper where the experimental results of NACA airfoils are shown. The turbulence model I am using is k-omega SST turbulence model and the transition is ticked on. The mesh is also properly refined and since my computational power is limited, therefore I am choosing my y+ value to be greater than 30. But the following problems are observant from my simulation
01. Lift coefficient seems to be around the mark but the error percentage is still a bit above 10%
02. Drag coefficient is the real issue as it is exceeds the supposed value and produces.much higher value. Almost double the value that I want
What is the issue here and what should I do? I have been going through the theory behind drag and lift coefficient and the Computational process used behind but it's not helping me so far and I really need help regarding this right now.
Thank you very much
You can fix it by reducing y+. The y+ should be around 1 for that turbulence model.
Hi Nehal Mahmud Khan,
For this model, y+ should be near 1 to get the best results. You can refine the mesh in meshing software or use mesh adaption in fluent.
The issue is that the lift depends on the pressure distribution, that can be accurately quite well even using Euler model. The drag requires the correct evaluation of the tau at the wall and using the first cell at y+=30 you simply have a wrong derivative. For high y+ values you should think to use wall model BCs, but that means you prescribe somehow the wall stress, not that you predict it.
Without a ufficiently refined grid, your model will not work for the drag.
@Filippo Maria Denaro @Ashkan Ghafari @Mohd Shahrul Nizam Ibrahim, thank you for the suggestion.
I tried once again using the said suggestion of applying y+ = 1 and through y+ calculator the value of first layer thickness I acquired was 8.5 e -6 (for free steam velocity of 45 m/s and viscosity of 1.95 e -5) and I applied that during Inflation.
After my simulation, I even checked my y+ value through Surface integrals on Reports and for airfoil the minimum face value for Wall Y plus is showing 0.0138 and the average value is 0.422
But in spite of all these my drag and lift coefficient is still higher than it should be. Both are way higher. Lift is 1.48 (15.625% higher) and Drag Coefficient 0.0198 (43.48% higher).
Can anyone explain me what went wrong this time? And advice me on how to rectify this particular situation
Hi Nehal,
If you have refined the mesh and used the appropriate turbulence model, then I suggest you to please check the convergence of the result.
After refined meshing choose appropriate turbulent model. Select proper boundary conditions especially for left side and Right side. And let up and down side to be velocity inlet. Pressure as 0 pascal. At the begining simple controllers. And choose Reference value precisely and area as chord * span. And no of iterations should be few thousands and depends.
You can try with transition off, I think your flow is not in transition.
I suggest you visit https://aeroptimal.com/mesh (you must create an account to use this module), where you can create a full structured airfoil mesh - https://youtu.be/4Opu0zk7gFk . You can export .su2 .msh .foam .vtk