Blade Spar / Spar Cap Optimisation

Connor.Evans · September 1, 2022, 10:18am

I have a question about the aerostructure optimsation caclulations.

I have created 2 turbines, a reference turbine and the same turbine but with an increased stiffness carbon used throughout. I ran the aerostructure optimisation and compared the results - see image.

NREL

I would have thought the tip deflection would be maintained and then the spar cap thickness reduced to capitalise on the better material properties to reduce the weight and cost of the blade further. But this is not the case - tip deflection reduces with the better performing material. Is there another criteria that is being used to size the blades instead of deflection/stiffness? Is there a strength criteria that is used? I think this boils down to my main question : what is driving the spar cap thickness?

Pietro.Bortolotti · September 2, 2022, 2:16pm

Hello Connor,
thanks for using WISDEM and contributing to the wind energy research.
You can set the design variables and constraints in the analysis options yaml.
If I understand your problem correctly, I would start by running a blade structural optimization problem. You can do it by calling wisdem with this yaml file

github.com

WISDEM/WISDEM/blob/master/examples/03_blade/analysis_options_struct.yaml

general:
    folder_output: outputs_struct
    fname_output: blade_out

design_variables:
    blade:
        structure:
            spar_cap_ss:
                flag: True             # Flag to optimize the spar cap thickness on the suction side
                n_opt: 4               # Number of control points along blade span
                max_decrease: 0.7      # Maximum nondimensional decrease at the n_opt locations
                max_increase: 1.3      # Maximum nondimensional increase at the n_opt locations
                index_start: 1         # Lock the first DV from blade root
                index_end: 3           # The last DV at blade tip is locked
            spar_cap_ps:
                flag: True             # Flag to optimize the spar cap thickness on the pressure side
                equal_to_suction: True # Flag to impose the spar cap thickness on pressure and suction sides equal
                n_opt: 4               # Number of control points along blade span
                max_decrease: 0.7      # Maximum nondimensional decrease at the n_opt locations
                max_increase: 1.3      # Maximum nondimensional increase at the n_opt locations

This file has been truncated. show original

To keep things as simple as possible, you could turn off the design variables controlling the thickness of the TE reinforcement (lines 24 and 31), and the corresponding max strain constraints (lines 54 and 59). At this point, the only two constraints driving your spar cap thickness will be max strains and ultimate tip deflection.
Let us know how this design run goes.
Best regards,
Pietro

Connor.Evans · September 5, 2022, 9:25am

Hello,

Thanks for your help. That did indeed work. Do you have an explanation for why this happened for my understanding?

I also did a test to increase the power to 6.6MW but the rotor diameter has gone to the minimum - I thought it would have increased. Do you know what might be happening here?

NREL_2

Many thanks,
Connor.

Pietro.Bortolotti · September 5, 2022, 8:30pm

Hi Connor,
I’m not sure I understand your questions. You’ve increased nameplate power. Are you trying to optimize rotor D? What figure of merit are you using? If it’s blade mass, I’m not surprised that the optimizer will try to minimize it. My general recommendation is to keep a close eye on your openmdao recorder log and check design variables, figure of merit, and constraints during the optimization. You can call this script WISDEM/create_conv_plots.py at develop · WISDEM/WISDEM · GitHub
to visualize the outputs in the log.
Good luck!
Pietro