GDW for wind speeds below 8 m/s

Wind & Water Computer-Aided Engineering Software Tools
1

Hi,

I’m using AeroDyn coupled to Riflex for a floating turbine, and I have been doing some comparisons of BEM and GDW in AeroDyn (EQUIL/DYNIN) for different mean wind speeds(turbulent). I can see that there’s a block that switches off dynamic inflow at 8 m/s, but I have trouble to figure out which method I’m using if I use the DYNIN option and the mean wind speed is below 8 m/s. I ran a case with 7 m/s turbulent wind with EQUIL and with DYNIN, and one with DYNIN where I set the limit for dynamic inflow lower than 7 m/s, but got different results for all three options.

So my question is which method do I end up using with if I choose GDW with a mean wind speed below 8 m/s? Why doesn’t the EQUIL and the DYNIN options give me the same result when the wind speed is below 8 m/s?

Thanks for your help,
Marit

2

Dear Marit,

Good question.

It is well known that the generalized dynamic wake model currently implemented in AeroDyn is unstable for highly loaded rotors (i.e., for high axial induction factors, say, higher than a = 1/3). The 8-m/s limit was built into the code based on the analysis of several wind turbine models, even though the exact limit is likely turbine dependent.

The result you are seeing is caused by a flaw in the logic of AeroDyn’s initialization routines. The basic problem is that when “DYNIN” is selected, AeroDyn never reads in the hub- or tip-loss model settings because hub and tip losses are intrinsically included the generalized dynamic wake solution. When “DYNIN” is automatically swithed to “EQUIL” below 8m/s, AeroDyn essentially assumes that the hub- and tip-loss settings are “NONE”, meaning that the equilibrium wake solution has no hub or tip losses. Thus, the “DYNIN” solution in this case will not match the “EQUIL” solution unless the “EQUIL” solution used hub- and tip-loss settings of “NONE”. To get around this problem, I suggest you manually choose the “EQUIL” option in place of the “DYNIN” option when modeling your turbine with wind speeds below 8 m/s.

We plan to resolve these problems in a future release of AeroDyn.

Best regards,