Wind turbine blades and bearing calculation help

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Discussion Overview

The discussion revolves around the design and calculation of wind turbine blades, specifically Savonius blades, and the associated bearing calculations. Participants are exploring how to ensure the blades will rotate effectively at a specified wind speed, considering factors such as wind force, blade dimensions, and bearing friction.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant is seeking formulas or directions to calculate the performance of Savonius blades at various wind speeds, starting with 10 mph, while considering the weight and dimensions of the blades and the friction coefficient of the bearings.
  • Another participant suggests using Newton's method to calculate the lift generated by the blades and comparing it to CFD simulations, noting that working backwards from a required speed may be challenging.
  • It is mentioned that Savonius blades have an estimated tip speed ratio (TSR) of one, as they function as drag devices rather than lift devices, and that the area of the blades exposed to the wind can be calculated.
  • One participant expresses that bearing friction is likely negligible compared to the torque required to accelerate the turbine and generator, citing an example of easily turning a heavy rotor by hand.
  • A later reply reiterates the low impact of bearing friction and emphasizes the focus on the wind speed's effect on blade rotation.
  • Another participant requests information on how to calculate bearing load.

Areas of Agreement / Disagreement

Participants generally agree that bearing friction is not a significant concern compared to other factors affecting blade rotation. However, there are multiple competing views on the best methods to calculate the performance of the blades and the influence of wind speed.

Contextual Notes

Participants have not resolved the specific calculations for bearing load or the exact methods for determining blade performance at varying wind speeds. There are also assumptions regarding the applicability of the TSR and the integration of pressure coefficients that remain unexamined.

Isimanica
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Ok I have a design project that I am working on for the University this semester. Having a bit of problems trying to figure out and issue revolving around the wind force on our savonius blades. We are trying to make sure our blades will rotate with a wind speed of 10 mph. We have the blade total weight of approximately 75 lbs and are 6 feet tall by 4 feet wide. The bearings that the blades will be sitting and rotating on are tapered roller bearings, a set of two. I have read some where that my coefficient of friction for these bearings is around 0.0018 but to cover our selves I would like to use 0.05

Any formulas or directions on how to calculate this not just at 10 but other wind speeds would be wonderful.
 
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You could just use Newton's method to calculate the lift generated by the profile of your blades. This method just integrates the pressure coefficient over the surface of the profile. That will give you the driving force and therefore the speed that they will rotate at. I would then compare this to CFD simulations. Come to think of it, this would make quite a nice Uni project. Working backwards from a required speed might be tricky though.

As a way of cheating, you could look up to see if a TSR (tip speed ratio) exists for your blade profile and that will give you the speed at which it will rotate at a particular wind speed. The other method would be impressive from a uni perspective though.
 
Last edited:
savonius blades have a estimated TSR of one since it is a drag device and not a lift device. We can calculate the area of the blade that will have the wind pushing against. What I need to determine is if the blades will rotate on our bearings based upon the wind speed that is hitting it.
 
I wouldn't worry too much about friction at the bearings. That will be negligible compared with the torque needed to accelerate the turbne and generator reasonably quickly from rest up to a practical RPM.

To give you an idea how low bearing friction is, you can easily turn the low speed rotor of a big jet engine (mass of rotor = about 2000lb) by hand with one finger. In fact for some test procedures you need to keep the rotor turning slowly (at 1 or 2 RPM) and one way to do that is to use an ordinary office fan to blow air onto the engine fan blades.
 
Isimanica said:
savonius blades have a estimated TSR of one since it is a drag device and not a lift device. We can calculate the area of the blade that will have the wind pushing against. What I need to determine is if the blades will rotate on our bearings based upon the wind speed that is hitting it.

My apologies, I just asumed that's what you were after since the friction on the bearing would usuually not even be considered. Pretty much what Zero said.
 
I would like to know how to calculate bearing load
 

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