New custom-made software for aerodynamic and acoustic optimization of propellers
This blog post is about the new simulation software that was developed to suit Mejzlik’s growing needs for simulating propellers in both single and complex configurations.
The performance of propellers can be evaluated either by measurement or through calculation. Both ways play an important role in the process of propeller design. Today we will focus on the more theoretical part - calculation.
On one side, there are many applications, scripts and excel sheets that allow quick estimation of propeller performance based on its geometry. These belong to the group of specialized propeller software, since we can use it to simulate a propeller, sometimes a ducted fan or even a wind turbine - but not much more. On the other side, there are CFD (Computational fluid dynamics) packages which can be used generally on any fluid dynamics problem. This second group we can call general CFD software.
Members of each of the groups come in many flavors, freeware, open-source, or payware with yearly subscription fees becoming the new standard. But let’s not focus on the cost for a moment and let’s simply select the best software for our needs.
The main drawback of CFD packages for propeller simulation is also its biggest strength - versatility. A general CFD software does not know it simulates a propeller. We could swap the geometry for a rubber duck and it would happily simulate the performance of a rotating rubber duck. The Navier Stokes equations solver contained in most CFD packages needs a good volume mesh with millions of cells to start with. The quality of the mesh is very important as well as having the right size of the mesh at the right places. There is nothing wrong with choosing a major brand CFD software as the main development tool for a propeller, but users must be aware of long preparation, computation and postprocessing times. A great danger also lies in the use of CFD without proper validation or expertise. It is easy to calculate very inaccurate lift and (especially) drag values of wing shaped bodies, because the inaccurately chosen settings and turbulence model may work better for road vehicle applications or simulating a toilet flush.
As a result, the general CFD packages are used typically by big turboprop propeller manufacturers where the design and certification of a new propeller takes several years.
For small propeller design and optimization, specialized propeller analysis software was chosen as the best route to follow.
The software was written in python and takes full advantage of interactive graphical user interface with workflow-oriented menus and controls. The user can quickly modify propeller geometry shape parameters, view the progress of aerodynamic simulation and browse through results thanks to 2D and 3D OpenGL accelerated plotting capabilities. The software was built around the extensive list of Mejzlik’s requirements which included optimization capabilities, contra-rotating propellers support and noise simulation.
The software uses a sophisticated method of replacing the blade and its wake by a system of vortices. The strength of the vortices is determined by the local lift and drag coefficients. Precise airfoil polars are thus required. The software features an automated process of extracting airfoils from the blade geometry and calculating polars for a wide range of Reynolds numbers. If needed the software can also load external files with measured polars.
The key feature of the new software is a force-free vortex wake model attached to each blade. The computational model of the wake allows mutual interaction and overlapping of as many wakes as needed. This makes it suitable for calculating contra-rotating propellers, or any set of propellers in close proximity.
Another very convenient feature of the software is the ability to estimate the noise of the simulated propeller. Both the noise spectrum and overall sound pressure level can be calculated for any location around the propeller. The acoustic computation module contains a model of acoustic sources that are placed on the surface of blades and standard Ffowcs Williams - Hawking equation for wave propagation.
The software can be used either in analysis mode, with fixed propeller geometry, or in optimization mode, where the propeller shape is optimized with respect to user-defined goal function. Aerodynamic shape optimization takes advantage of genetic algorithms which are perfectly suitable for such multiparametric highly non-linear problems.The goal of the optimization can be either aerodynamic, such as increasing propulsion efficiency or acoustic, such as reducing overall noise, or combination of both.
The versatility of the software can be demonstrated on the ability to simulate the effect of a close wall or a ground effect, just by mirroring the propeller into its image twin using wall or ground as the mirror plane.