no karman vortex street occur in the simulation of the flow over a cylinder
 

Hello,
I have trouble making a simulation of the flow over a circle cylinder.
Fig.1 is the 2D case I did before, and Karman vortex street appeared obviously.

But when I try to do a 3D case, the wake becomes straight and no periodic vortex shedding (Fig.2-Fig.5)

Parameters of the case:
·Diameter=0.01m
·Height=0.03m
·Velocity of the incoming flow=0.01m/s
·Reynolds number=100 (laminar flow)

The fluid region:
·X Direction(incoming flow): -7.5D/+25D from the axis of the cylinder
·Y Direction: ±10D from the axis
·Z Direction: 1D from the top/bottom face of the cylinder

Boundaries:
·Faces of the cylinder: walls
·Inlet/ outlet: velocity inlet/ pressure outlet
·Top & bottom faces(along the Z axis): symmetry
·Side faces(along the Y axis): walls

Physics:
·3D
·Segregated Flow
·Laminar
·Constant Density
·Gradients
·liquid(water, μ=1.002e-3Pa·s, ρ=998kg/m³
·Implicit Unsteady

I have refined the mesh from 800 thousand to 5 million, so it is unlikely to be a problem of grid.
In fact, I had also try to simulate the turbulance case using k-ε and k-omega SST(u=0.394m/s, Re=3900), but the flow field looks similar to the laminar one (Fig.2-Fig.5)
Does anyone have an idea? Please help me.