Sunday, March 2, 2014

Cyclorotor CFD Simulations in OpenFOAM

A few months ago, I became interested in the cyclorotor concept. This concept goes by many names, including cyclogyro and cyclocopter. The cyclorotor concept has been around since the early 1900's, and has recently had revived interest. I think this is due to modern materials and controls making the concept more feasible. Some universities I have seen in research papers include the University of Maryland and Seoul National University. To see one in action, check out this video:

I ran a few simulations in OpenFOAM to explore the capabilities of the cyclorotor. Here, I will describe validation runs that show that OpenFOAM can accurately predict cyclorotor performance. I used the pimpleDyMFoam solver in PISO mode (no SIMPLE iterations) and a Spalart-Allmaras RANS turbulence model with curvature correction.

Some more solver details:
-Max Courant number of 2
-First order in space and time
-Upwind spatial discretization scheme

OpenFOAM cannot run cyclorotor simulations 'out of the box'. It requires some custom solidBodyMotionFunctions, because the blades rotate in a certain way about its own pitch center, while being displaced by rotating around the rotor center. I have uploaded the code I used to Github here:

The mesh I used consisted of a square far-field boundary with inflation layers near the shaft and blades. I meshed it in Gmsh.

The following videos show the velocity magnitude and pressure fields.

The following shows experimental results from a Seoul National University research paper, and my OpenFOAM simulations. OpenFOAM results match quite closely. The OpenFOAM simulations were performed in 2D.

Ultimately, I found the cyclorotor concept not very practical (at the time of writing this). These are the reasons:
-Complicated mechanisms
-Forward flight performance not favorable
-Structures. Centrifugal loads are very high, and the blades have to resist this in a weak orientation (i.e. tangential vs radial chord line alignment).

Although there is an interesting track mechanism that I have seen in research reports that can alleviate forward flight problems and greater blade pitch control in general, but in my opinion it is still complicated and sensitive to geometry. The mechanism involves wheels on the tips of the blades that ride on the inside of a specially-shaped track. The wheel stays on the track by the inherent centrifugal forces on the blades. Control comes from translating the track in the plane of rotation, as well as the in the direction of the rotation vector. The track changes shape smoothly along the direction of the rotation vector so as to get the desired pitches.

In my opinion the most significant advantage of the cyclorotor is thrust direction freedom.

At the request of Francois, you can find the mesh files here:

At the request of Thomas, I have posted the case and mesh files here: