[Mike97]

Hi guys,
i'm working on my thesis which consist in study the flow past a pitching airfoil NACA 0015 at Re 1.5e6 focusing on the transition and vortex shedding. Before i do that i run some steady and then transient simulations for the AoA between 1 and 23 to validate my mesh. The cord length is of 0.5459m, i'm using a duoble O-mesh with the external circumference of 40m radius while is the internal of 2m. I'm interested in study the transition so i use 1600 point on the airfoil which are thickened on the leading edge and the trailing edge and an inflation of 32 level with first cells height of 8*10e-6 and a growth rate of 1.1. The external mesh is structured while the internal is unstructured and the circumference are divided with 280 points. I check my quality and seems to be good, my teacher thinks is good too.


Steady:

In fluent i set pressure-far-field for the external vicumference as b.c., SST Transition (4 eq) as viscous model and SIMPLE + second order discretization. I run over 30000 iteration for every AoA.



The problem is:

my Cl and Cd are good, in agreement with other simulation and the experimental data, contours are good too, but if i look at pathlines there are some vortices in the pressure side for the AoA 5:11 and we don't know the reason!

I try to modify the mesh but it doesn't work, so i change the scheme and with SIMPLEC is the same while with PISO they disappear. The problem with PISO is that the lift coefficient is too small. So i use again SIMPLEC and i set adaptive mesh based on iso-values of vorticity, FLUENT create a mesh with over 1 milions cells and of course it take too much time.
At that point i switch to the transient one thinking that it was a problem of the stady state simulation.



Transient:
time step size of 0.001s and over 7000 time step. I use PISO and second order implicit for time discretization. I run at 11 AoA and the problem is now in on the suction side because there are zones in which velocity became of 0.5 Mach. I don't know if it is right so i try to switch to SIMPLEC and set the time step to 1e-5 for try. The vortices are disappeared, the pathlines and the contours are good so as the lift coef. but the time step is too small and i have to simulate about 8s.

Reading on the forum i discovered that Coupled scheme is better when the time step is large and so i do. He provide the same results of SIMPLEC.
When i switch to 9 AoA the Coupled scheme doesn't work anymore and now i run one with SIMPLEC and time step of 0.0001. Tomorrow i'm going to check what happen.


My questions are:
1. Why there are vortices on the pressure side?
2. Why the flow field is deeply influenced by the scheme and give me different solution?
3. Do you have any suggest to improve my simulations?



I made geometry and mesh in workbench.
Sorry for my english guys, i hope you will undestand the problem. Any suggest and tip will be appreciated.


Thanks for your attention.