determine the right settings
 

how i decide the time step?

my 2d problem const of a beam section (0.3m x 0.3m ) invested by a flow at 15 m/s. the domain along x axle is from -5 to 10 m and and from -4 to 4 along y axle.
i would like to investigate the oscillation of kolmogorov scale of vortex, so i made a mesh from 0.001 at the section to expand up to 0.150 at the boundary. i use a k-om sst unsteady model with a time step of 0.005, but the lift and drag plot are so bad..