Which method is used in OpenFAST? BEM or Actuator Line method?

Wind & Water Computer-Aided Engineering Software Tools
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Hello,

I started learning OpenFAST recently. I have a basic question regarding the aerodynamic method:

  • Which method is used to compute the load on the blades? is it Blade Element Momentum (BEM) method or Actuator line method?

I appreciate your help.

Regards,
Y. Thiziri

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Dear @Yoor.Thiziri,

Well, BEM–as implemented in the AeroDyn module of OpenFAST–is a combination of blade-element and momentum theories and the blade elements can be considered actuator lines. So, I would call BEM an “actuator line method”. AeroDyn also has a free-vortex wake wake model (OLAF), which replaces the momentum theory while keeping the blade-element theory, so, OLAF is also an “actuator line method”.

That said, the term “actuator line method” is often used in CFD to represent the blade as an actuator line rather than resolving the surface geometry of the blade. OpenFAST is coupled to CFD through an actuator line method in both the SOWFA (OpenFOAM) and ExaWind (AMR-Wind) CFD solvers.

Best regards,

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Dear @Jason.Jonkman,

Thank you for your detailed reply.

I realize that my original question was unclear. by BEM I actually meant Actuator Disk. So my question should be: Which method is used by OpenFAST: Actuator Disk or Actuator Line? or both?

Best regards

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Dear @Yoor.Thiziri,

I would call the BEM and free-vortex wake models of AeroDyn “actuator line methods” given that the blade-element part represents the blade as a line.

As of OpenFAST v4.0 released earlier this week, a new AeroDisk module was introduced in OpenFAST (mostly for use in conjunction with FAST.Farm) that does represent the rotor aerodynamics as an actuator disk that works when the rotor is modeled rigidly (mostly for use in conjunction with the new simplified ElastoDyn module of OpenFAST). AeroDisk uses, among other inputs, a user-specified table of rotor-thrust coefficients.

An “actuator disk method” is available in CFD through the couplings to the SOWFA (OpenFOAM) and ExaWind (AMR-Wind) CFD solvers.

Best regards,

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How can BEM method represent the blades as lines? AFAIK, BEM can only represent the rotor as an actuator disk. Could you please point me to some resources to read about the AeroDyn implementation of BEM as actuator lines?

Thank you.

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Dear @Yoor.Thiziri,

BEM is a combination of blade-element theory and momentum theory.

The blade-element theory used by AeroDyn can be considered as an actuator line because the aerodynamic applied loads are represented by lift forces, drag forces, and pitching moments at nodes distributed along a line representation of the blade.

The momentum theory used by AeroDyn is disk-like, but is actually treated as sectors of the full disk, that is, the disk is split into 3 sectors for a 3-bladed rotor.

BEM combines the blade-element and momentum theories such that the line loads are in equilibrium with the induction in the sectors.

Best regards,