Wind turbine blade characteristics

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Discussion Overview

The discussion revolves around the characteristics and testing of wind turbine blades, specifically focusing on the S809 airfoil. Participants explore methods for determining lift and drag coefficients using a wind tunnel and discuss various modeling and measurement techniques relevant to wind turbine blade design and analysis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Experimental/applied

Main Points Raised

  • One participant is seeking guidance on how to determine lift and drag coefficients for a wind turbine blade model in a wind tunnel, having access to modeling software and a wind tunnel.
  • Another participant inquires about the mounting apparatus for the turbine blade and whether it can be equipped with force-measuring devices.
  • It is suggested that measuring the forces on the blade in the wind tunnel is necessary.
  • A participant proposes that if the blade's rotation speed is significantly slower than the wind speed, it may be sufficient to fix the blade in the airflow and measure the torque at the root.
  • Discussion includes the consideration of whether to test just the S809 airfoil or a complete rotor assembly, with a suggestion that a complete blade set would be preferable.
  • One participant advises measuring thrust and bending moment to estimate lift and drag, noting that the lift/drag center typically occurs around 75% of the blade radius.
  • There is a recommendation to optimize the blade design using an Excel-based blade element model before conducting computational fluid dynamics (CFD) or physical testing.
  • A participant mentions the potential for estimating lift for the S809 section using an external resource.

Areas of Agreement / Disagreement

Participants generally agree on the need to measure forces and consider the complete rotor assembly for accurate testing. However, there are differing opinions on the best approach to testing, including whether to fix the blade or allow it to rotate, and the optimal design considerations for the blade.

Contextual Notes

Participants express uncertainty regarding the appropriate offset for counterbalancing forces and the aerodynamic forces involved, indicating that these factors remain unresolved. Additionally, there are assumptions about the relationship between rotation speed and wind speed that are not fully explored.

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I'm about to start a project in which I want to study the effects of making modifications to a wind turbine blade. I am going to be able to use various external flow modeling software our aerospace labs. I am a mechanical engineer and am used to using solidworks and I understand that that can be used for some external flow. My real question is here:

-I have access to a wind tunnel and rapid prototype machine. I wish to model and place in the wind tunnel a wind turbine blade using the S809 airfoil. There is just a major point I am missing: how do I go about determining the lift/drag coefficients within the wind tunnel? I am far removed from the process of determining these and was hoping someone could point me in a general direction.

thanks.
 
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Upon what will your turbine blade model be mounted? Can the mount apparatus be fited with force-measuring devices?
 
Yup. You need to measure the forces on the blade in the tunnel.
 
I'll get a chance to see the tunnel some time soon and then I can better answer those questions.

Regards,
Kane
 
If the blades rotation speed is slow compared to the wind speed would it be enough to mount the blade fixed in the airflow and just measure the torque at the root as it tries to turn?
It would be much convenient to build and instrument than a rotating blade set.
 
So you want to measure the characteristics of just the S809 or a partial rotor assy? I'll assume you mean assy for now. You'd be better off with the complete blade set, but could use an offset counterbalance for the centrifugal and aerodynamic forces. Trouble is what offset to use because you don't know the aerodynamic forces...

You will need to measure thrust and bending moment to get an idea of lift and lift centre. Same for drag. Lift/drag centre usually works out about 75% of radius for a straight blade, because the aerodynamic and centrifugal forces increase with radius squared (since proportional to v^2, and v increases with radius).

My advice would be optimise your design with an Excel based blade element model first, before you do your CFD or testing. This will get the design as close to right as practical. The general rules are that an "ideal" aerofoil is tapered with chord proportional to 1/Radius, and that this should be superimposed with an elliptical distribution for tip vortex losses. Most practical designs compromise the root choord, since the wind is free...

You can estimate the lift for the S809 section from:
http://www.engapplets.vt.edu/fluids/vpm/index.html

Actually, i'd be curious to develop my own blade element program to handle the windmill brake state of a helicopter rotor. ;)

Mart
 

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