[Robin]

im modelling a low speed (50m/s) airfoil (NACA 4415) in fluent and at zero angle of attack i have a higher than expected drag coefficient at about 0.02. when the angle of attack is increased to 5 degrees the value for the drag coefficient then becomes negative for some reason?? the lift coefficient in first case is 10% out and 45% out in the latter.

to mesh the airfoil i followed the tutorial http://courses.cit.cornell.edu/fluent/airfoil/step1.htm as suggested in many other posts. i also tried using the tranisent SST solver technique as opposed to SKE as the fluent user manual suggests that that is a better solver for negative pressure gradients. also i read that it was more suitable for low Y+ values that occur with finer meshes although i have no idea why?

for the solution methods i used SIMPLEC green gauss cell based for the gradient standard for the pressure and second order upwind for all the others as recomended in many other topics.

the problem is set the the standard solution controls (under relaxations etc) and converges after 162 iterations and comes up with no messages saying turbulent viscosity ratio limited etc.

ive attached a residuals plot, force report and summary report if that helps.

could anyone suggest what is wrong / how i could get more accurate answers?