# Vortex Shedding Behind a Cylinder

Given how simple the geometry defining a cylinder is, you'd intuitively think that the air flow around it would also be simple. And as with many things fluid you'd be wrong.

Vortex Shedding Caedium CFD SimulationVelocity vectors (high definition video)

If you set up an experiment either in the real or virtual realms to study the flow (air, water, any Newtonian fluid) around a cylinder and configure the flow conditions such that the Reynolds number lies somewhere between 47-100,000 you'll see a complex vortex-shedding phenomenon known as a Karman vortex street.

Vortex shedding can be a dangerous phenomenon if it coincides with the natural frequency of a structure. Ever wondered why you see helical flutes on chimneys? It's to suppress the formation of a Karman vortex street leading to vortex-induced vibrations that can cause structural damage.

Vortex Shedding Caedium CFD SimulationVelocity contours (high definition video)

It's an interesting exercise for CFD to simulate a Karman vortex street. It's a trivial exercise in geometry creation to create the cylinder, but it's more efficient to perform a 2D simulation. In Caedium Professional that means you will need to create a multiblock mesh topology 1 cell thick as in the tutorial "Transonic Flow Over the NACA 0012 Airfoil".

You'll also need to condifure a transient flow solver with a 2nd-order accurate divergence scheme. In Caedium that means:

• Substance->State->Transient = yes
• Substance->Solver->Schemes->Divergence = Linear Upwind

Then adjust your time step (Substance->Solver->Control->Time Step) so that the simulation runs correctly.

In the CFD simulation shown in the animations I used the following flow parameters:

• Velocity = 1 m/s
• Cylinder Diameter = 1 m
• Density = 1.2 kg/m3
• Dynamic Viscosity = 1.82e-5 N.s/m2
• Flow Type = Turbulent

Which gave the Reynolds number = 65,898