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VAWT Wake Instability at Higher Number of Blade Elements

Greetings QBlade Community,

Recently I started running simulations for a simple H-Darrieus VAWT  and assessing the impact of the amount of blade elements used. At 15 elements per blade, the simulations ran smoothly but at 20 elements per blade, the wake in of the simulation started showing large and sudden fluctuations that seem very much unrealistic, such as seen in the picture below. I am using the default wake and vortex modeling parameters.

I have attached the project file for these simulations here for your convenience. If anyone has any insights or recommendations on how to address this problem it would be greatly appreciated.

Thank you and best regards,

Daniel Roman

 

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Hello Daniel,

the issue is that the wake “explodes”. This is caused by a failed convergence of the fixed point iteration while evaluating the blade bound circulation.

The main reason for this problem is the aspect ratio of the blade panels near the outer sections of each blade. This aspect ratio is significantly influenced by the blade discretization and its geometry. The problem is particularly pronounced when a blade ends abruptly without a tapered tip, leading to a very strong circulation gradient at the blade tip.

There are two things you can do to “fix” the convergence issue.

  • Increase the fixed bound core radius of the bound vortices (try a value of 0.3 or higher)
  • Reduce the relaxation factor (0.05 worked in my quick test)

For the next release I might change a few default values to make the LLFVW more robust for such cases (straight blade without taper).

BR,

David