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/m3
- Dynamic Viscosity = 1.82e-5 N.s/m2
- Flow Type = Turbulent
Which gave the Reynolds number = 65,898