About propeller representation question

[Eric Cheng]

Hi everyone,  

In VSPAERO, it is well known that "Actuator Disk" is based on momentum theory.

I want to know whether the "Rotating Blades" method in OpenVSP is based on blade element theory (BET) or blade element momentum theory (BEMT)?

Looking forward to your answers, thanks in advance!

Eric

[Brandon Litherland]

it's neither.  The rotating blades mode is an unsteady, time accurate model that treats the blades as their own panel or VLM surfaces with wakes.  Think of it just like a wing in VLM mode that's rotating through the flow field.  Locally, the blade experiences its own velocity and produces lift in the thrust direction and drags in the torque direction.

BEMT is more about discretizing the local blade sections (twist, chord, etc) into slices that each impart some additional momentum to the flow.  Corrections to this model relating rotor inflow, tip losses (Prandtl tip loss factor), viscous approximation,  rotor solidity, etc. are all accounted for at varying fidelities depending on the implementation.  For more on this subject, I highly recommend Leishmann's text, "Principles of Helicopter Aerodynamics".

[Rob McDonald]

Also, the actuator disk model is not your simple textbook momentum theory approach.  While textbook momentum theory gives some important results, it is too simple to be useful in this context.

Instead, we use an analytical model that gives the complete 3D flowfield induced by an isolated actuator disk.  This solution is by Conway, you should be able to find his papers online.

Rob

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[Eric Cheng]

I really learned a lot from it, thanks!

[Eric Cheng]

Hi all,

If the "Rotating Blades" method in OpenVSP does not belong to blade element theory (BET) nor blade element momentum theory (BEMT).

"Rotating Blades" method in OpenVSP, as Brandon said, it is an unsteady, time accurate model that treats the blades as their own panel or VLM surfaces with wakes. 

Then I have a new question, then "Rotating Blades" Methodan  based on which computational fluid dynamics method is used to calculate?? 

For example, is it a high-order panel method or a high-fidelity CFD method like RANS? (I really want to know it)

Looking forward to your answer,Thanks.

Eric

[cibinj...@gmail.com]

Eric, I can't help but point out that Brandon already answered this question in the same thread a few hours back. It's based on the Vortex Lattice Method (VLM) just like the fixed wings are.

To quote him:

"It's neither [BEMT or BET].  The rotating blades mode is an unsteady, time accurate model that treats the blades as their own panel or VLM surfaces with wakes.  Think of it just like a wing in VLM mode that's rotating through the flow field.  Locally, the blade experiences its own velocity and produces lift in the thrust direction and drags in the torque direction"

[cibinj...@gmail.com]

VLM is a (mid-fidelity) potential flow-based method where the  lifting surfaces are discretised using panels. Each panel will have a vortex or doublet element to model the circulation at that location. It is not a high-fidelity method like CFD. The wikipedia article on Vortex Lattice method is a good starting point if you're new to the approach. Most textbooks on Low-speed aerodynamics also have details on the implementation.