Friday, December 24, 2010

Fanwing simulation in OpenFOAM (a free CFD program)

This is one of my many Fanwing simulation (and other) cases run in OpenFoam, which is an open-source CFD code. The flow field patterns match well with the few research papers that have been done on the Fanwing (which used CFX/Fluent, and compared to physical experimental observations). The colors represent pressure values and the white lines are streamlines.

As you can see, the primary lift mechanism is the low-pressure eccentric vortex formed by the fan. Also, if you look at videos of the fanwing, you will notice the sound created by it can be explained by the pressure oscillation at the front of the wing (~400 Hz). The pressures within the rotor cannot be assessed by Bernoulli's equation for incompressible flow, because of vorticity. Vorticity is causing the stagnation pressure of the rotor interior to change, specifically to lower greatly. Inside the rotor the static pressure takes on the value of this lowered stagnation pressure, as the velocity inside the rotor at the center of the eccentric vortex is near zero. This simulation uses a transient solver. The turbulence model used is k-Omega SST. The blades are turning at 1480 RPM. The freestream is 6.8 m/s. The whole simulation takes place over a quarter of a second.

This case uses GGI (generalized grid interface), where the rotor mesh (blades) rotates with respect to the casing. The interface is resolved by using weighting factors on the overlapping and interfacing cells, without the mesh actually deforming, unlike sliding interface meshes (Google it for more info).
You may have noticed the weird streamline behavior at the interface of the rotor and outside air (that they disappear and appear). I do not think it is an indicator of something wrong with the simulation, but probably merely that the streamline integrator cannot handle the tiny gaps at the rotor-stator interface.

The mesh was created using gmsh. A great tutorial for setting up 2D cases for OpenFOAM can be found here. The whole simulation was run in Ubuntu (necessarily, OpenFoam doesnt work on Windows.....yet). The post-processing was done in ParaView. is a great resource for beginning with OpenFOAM. I have had important questions of mine answered there; it is a pretty active forum. I could not have done this simulation without it.

So everything needed for this was FREE! And a talented high-schooler could set up and use this.

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