Cylinder in Cross Flow Problem
 

I am trying to run a verification case for a cylinder in cross flow (trying to get Strouhal number = 0.2) and am encountering a consistent error across simulations. There is a subharmonic which shows up in the velocity/pressure and which has a very large contribution. I've attached an image of an FFT and a time history of pressure. The data in the plots is from a point a few diameters downstream of the cylinder.



What could cause this kind of error? Mesh too coarse? Time steps too great? The software I am using is SU2 but I figured that this kind of error may not be software specific so I posted here.



Thanks