# 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.

high definition video) Velocity vectors (

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.

high definition video) Velocity contours (

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/m
^{3} - Dynamic Viscosity = 1.82e-5 N.s/m
^{2} - Flow Type = Turbulent

Which gave the Reynolds number = 65,898

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## Comments

## Caedium project file available

The Caedium project file for this simulation is available at "Transient Airflow Around a Cylinder".