Blade-Element/Momentum Theory and Implementation

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Re: Blade-Element/Momentum Theory and Implementation

Postby Gabriel.Scarlett » Mon Oct 09, 2017 10:40 am

Dear All,

I'm Gabriel a PhD student from Edinburgh and I'm investigating the effects of unsteady hydrodynamics on tidal turbine blades.

I have implemented the BEMT model used in Aerodyn v15, exactly as described by Ning et al. 2015, AIAA, ( I have attempted to validate my implementation with CP and CT values generated using AeroDyn v15. It is a simple steady-state case, the blade comprises of constant thickness NREL S814 profiles, zero yaw, zero pitch, default AeroDyn settings.

For CP agreement is good for tip-speed ratios (TSR) up to about 5. However, above this, my implementation is underpredicting as shown in the attached figure. For CT agreement is worse, I am overpredicting from 3.5 - 9, then underpredicting. I encounter negative CL and CD values above TSR = 8 due to high flow angles. I wonder if anyone has any insight into where I am going wrong? I have read the AeroDyn change log and believe my implementation in terms of theory and convergence method is the same. I suspect it is a case of numerical implementation.
CP_CT_Curves.jpg (95.72 KiB) Viewed 749 times

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Re: Blade-Element/Momentum Theory and Implementation

Postby Jason.Jonkman » Mon Oct 09, 2017 12:22 pm

Dear Gabriel,

Ning et al's AIAA SciTech 2015 paper formed a solid basis for the implementation of BEMT within AeroDyn v15; however, there were several modifications (especially in newer versions of AeroDyn v15) that were made to ensure the robustness of the implementation within the aero-elastic solution of FAST. Unfortunately, we have not had the time/funding to publish an update to the BEMT algorithm. It is difficult for me to guess what specifically would lead to differences between your and AeroDyn's solutions.

I would suggest simplifying the model as mush as possible (e.g. fewer analysis nodes, no skewed flow, steady inflow) and looking at the details of the solution (e.g. calculated values of the inflow angle, inductions, and lift and drag coefficients) to identify the sources of the differences. You could also look at the AeroDyn source code to see how the BEMT has actually been implemented within AeroDyn v15.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
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