Blade-Element/Momentum Theory and Implementation

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Gabriel.Scarlett
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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, (https://www.nrel.gov/docs/fy15osti/63217.pdf). 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.
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CP_CT_Curves.jpg
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Jason.Jonkman
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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)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Ricardo.Amaral
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Organization: TU Delft
Location: Netherlands

Re: Blade-Element/Momentum Theory and Implementation

Postby Ricardo.Amaral » Tue Jan 19, 2021 12:04 pm

Good evening,

I'm sending this message because some of the answers to the previous questions and other posts (Re: Rotor aerodynamics while OFWT is moving
and more recently viewtopic.php?f=19&t=184&p=6316&hilit=momentum+equation#p6316) may have changed meanwhile.

Are these relations still true for the latest version of AeroDyn v15:
- Vrel_n = (Vwind_n + Vstruct_n)*(1-a)?
- Vrel_t = (Vwind_t + Vstruct_t)*(1+ap)?

Where Vwind_n/t ia the local undisturbed wind speed normal/tangential to the rotor plane and Vstruct_n/t is the structural velocity with the same nomenclature.

Also, in the momentum part of the BEM equation for the thrust and torque (For thrust: dT = 4pi*r*rho*Uinf^2*(1-a)*a*dr), is Uinf still the undisturbed wind velocity at a given blade section or does it take into account the structural velocity of the section (under the assumption that the platform motion adds energy to the system)?

Best regards,
Ricardo Amaral

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: Blade-Element/Momentum Theory and Implementation

Postby Jason.Jonkman » Tue Jan 19, 2021 1:21 pm

Dear Ricardo,

In AeroDyn v15, the BEM formulation does not distinguish between the wind velocity and structural velocity. The relative velocity (wind minus structure) is used in all blade-element and momentum equations.

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

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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