## Some more realistic OpenFOAM runs

As a first test, I am trying to reproduce the graphs of coefficient of lift $$C_l$$ and coefficient of drag $$C_d$$ as functions of angle of attack $$\alpha$$ from http://airfoiltools.com/airfoil/details?airfoil=naca2408-il. The airfoil is 1m long, the airspeed is 10m/s, and the kinematic viscosity $$\nu$$ is 1E-5 for a Reynolds number of 1E6. $$C_l$$ and $$C_d$$ are measured for $$\alpha = \{-5,0,5,10,15,20\}$$. I am unclear on the reference length and area in $$C_l$$ and $$C_d$$, but that will be obvious as a constant factor between the graphs.

On the recommendation of a friend (I do not want to get stuck down any rabbit holes) these use the SST k-omega turbulence model. The constants for incoming turbulence intensity are taken from: k-Omega-SST-DES-Report. At high angles of attack the airfoil sheds vortices:

And the plot of $$C_l$$ and $$C_d$$ vs $$\alpha$$:

Error bars are given from the standard deviation of the forces. The vortex shedding causes periodic changes in each coefficient. I am bothered that $$C_l$$ doesn’t turn over at $$\alpha\approx 15^{\circ}$$ as for the airfoil tools database, and am now trying to figure out what causes the difference.

## OpenFOAM Airfoils

I’ve been playing with OpenFOAM recently and ended up writing my own mesher. It’s been educational to work with a code which uses unstructured meshes for once. See https://github.com/garthwhelan/openfoam-mesher-stuff for details. So far all it does is generate hexahedral meshes with Cartesian coordinates, and merge boundary faces/points if two boundary patches share the same coordinate. It’s pretty straightforward to build more complicated meshes by deforming and combining Cartesian ones. For example, C meshes for airfoils can be made by deforming a rectangular Cartesian mesh so that the right edge surrounds an airfoil and then combining the interface behind the airfoil (automatically removing the duplicate faces and faces that are now internal). O meshes would involve doing the same thing, but using the entire right edge around the airfoil and merging what were the top and bottom edges.

Ultimately, I would like to build an RC plane and be able to say some of the design was inspired by CFD simulations, but this is a tall order. I think I will make sure that I can match the information on www.airfoiltools.com for $$C_l$$ and $$C_d$$ for a named airfoil and then use OpenFOAM to motivate the design of the control structures.

After going through some tutorials, I tried a (very nonphysical) run with an airfoil. The graph below was generated with icoFoam using a 1m airfoil at 1m/s, and with a realistic air viscosity. The boundary conditions are probably something nonphysical. Since then I’ve moved to using the freestream boundary conditions, pisoFoam, and RANS.