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New Open-Source Modeling Tool Enables Design of Large, Flexible Wind Turbines

OLAF Updates 1930s-Era Blade Design Models for the 21st Century

Sept. 1, 2020

The past few decades have seen significant reductions in the cost of wind energy, thanks in part to increases in the size of turbine rotors. Larger rotors create a greater swept area, which allows wind power plants to more consistently capture wind energy and access higher wind speeds at elevated heights—helping reduce the cost of wind energy.

However, as turbine blades get bigger, they also get more flexible, which creates several challenges for turbine designers.

Since the 1930s, blade designers have used traditional models like blade element momentum (BEM) theory to calculate blade aerodynamics. While these models offer efficient methods of calculation, they can be too simplistic for the scale and complexity of modern turbines. For example, the foundation on which traditional aerodynamic models are built assumes that the turbine rotor stays in one plane, but large, flexible turbine blades tend to deflect from the rotor plane.

As wind turbines continue to increase in size, turbine designers need modeling tools that can provide accurate representations of the forces acting on larger turbine blades that cause them to flex and bend as they rotate.

The National Renewable Energy Laboratory (NREL) recently released cOnvecting LAgrangian Filaments (OLAF), a new wake module included in NREL’s OpenFAST wind turbine simulation tool. OLAF models the turbine wake using particles connected via filaments and is programmed to generate realistic representations of the wakes of large, flexible turbine blades, providing users an alternative to traditional aerodynamic models.

Video of computer-generated video of spinning turbine blades, with the turbine’s wake represented by filaments.

This video shows how OLAF uses filaments to represent a turbine's wake. OLAF is programmed to generate realistic representations of the wakes of large, flexible turbine blades, providing users an alternative to traditional aerodynamic models. Video by Kelsey Shaler, NREL

"OLAF is a more generalized code than traditional models, which require users to adjust a lot of input parameters to increase the code's accuracy," said NREL researcher Kelsey Shaler, who led OLAF's development along with researchers Emmanuel Branlard and Andy Platt. "OLAF's method provides a better representation of the physics of turbine wakes and makes the tool more accessible to users."

Adding to that enhanced accessibility, OLAF is a mid-fidelity modeling tool, meaning it can provide a more complex and accurate level of representation without the computational expense of a higher-fidelity model.

“At this level of fidelity, OLAF can be run on a personal computer, making it accessible to a wide range of users including industry, academia, and laboratories,” said NREL researcher Nick Johnson, who acts as principal investigator for the Department of Energy's (DOE’s) Big Adaptive Rotor project, which funded OLAF’s initial development. “It’s great for students, because they can use OLAF during their studies, which will give them a competitive edge in the job market.”

OLAF is an open-source tool, which enables designers throughout the wind industry to model their own designs more accurately and predictably, thereby reducing development costs while further refining the software collaboratively.

OLAF can also be used to design both land-based and floating offshore wind turbines. Not only can OLAF account for the massive size of floating offshore turbines, but OLAF also accounts for the motion of the turbine and the interaction with its wake, allowing offshore wind turbine designers to create turbine models that more closely reflect reality.

NREL will continue to enhance OLAF through DOE’s Advanced Research Projects Agency-Energy (ARPA-E) Aerodynamic Turbines, Lighter and Afloat, with Nautical Technologies and Integrated Servo-control (ATLANTIS) program.

For more information, read the OLAF User’s Guide and Theory Manual. OLAF is available now on GitHub.