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Vortex core parameters
Dear,
In the estimation of the vortex core size two empirical parameters appear. The first one is turbulent viscosity coefficient $\delta_{\nu}$ and the second one is the time offset parameter. Sant et. al. ( “estimating the angle of attack from blade pressure measurements on the nrel phase VI rotor using a free-wake vortex model: Axial conditions, 2016″,
) suggest for small rotors that the viscosity coefficient is 10 for small rotors and 100-1000 for large rotors. However, on the second parameter, I can not find any pointers to what value the time offset parameter should be. Only in Balduzzi et. al. (“Three dimensional aerodynamic analysis of a darrieus wind turbine blade using computational fluid dynamics and lifting line theory, 2017″) a paramater is documented and set to 0.0001s.
Is this the typical value, or is it guessed almost randomly?
Hello LTW,
It appears that you might be referring to an older version of QBlade. In the latest release, the time offset parameter is no longer utilized. Instead, there is now an introduction of an initial vortex core radius, which serves the same purpose as the previous time offset parameter, governing the initial vortex size.
With this improvement, the simulation process becomes more straightforward, as you can now directly set the initial vortex core radius to achieve the desired results, without the need for a separate time offset parameter.
BR,
David
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Dear David,
Thanks you for your reply. Although the time-offset parameter is not longer relevant, the underlying question of estimating the initial vortex core size remains, right? Are there some guidelines for that? Additionally, there must be a distinction in bound vortex core size and vortex core size in the wake. How does one deal with this in QBlade, and how does one determine both? I presume something along the lines of estimating the thickness of the boundary layer.
Best,
LTW
Whoops, I missed your attached image. Nevertheless, how does one decide to set the initial radius to 0.05 panel width? From a physical point of view, this would mean the radius decreases when the number of panels is increasing. I do not see why that would be the case as the paneling is solely numerical while the boundary layer does not care about panels, right? Maybe, a percentage based on chord would make more sense, but this is just a suggestion.
Hi LTW,
you are correct that from a physical point of view the relation between bound vortex core size and panel width doesnt make much sense.
The main point of relating the bound vortex core size to the panel width is to facilitate a convergence for the bound circulation iteration. Since in many scenarios the panel spacing is finer in the tip region (with cosine spacing) to better resolve the large circulation gradient in this region, very small panel widths can cause issues when the core size is too large (such as a divergence of the bound circulation iteration).
Unlike the “free vortex core size” that relates to the panel length and has an influence on the physical development of the free wake, the bound vortex core size serves more as a numerical parameter. It’s not directly linked to boundary layer thickness and has minimal impact on the development of the free wake, but rather on the bound circulation convergence behavior.
BR,
David
