Dear all,
I have tried running the MHK_RM1_Fixed turbine through a sweep of TSR at constant inflow of 2 m/s. Graph are shown below, and please ignore the different cases, these are just testing the impact of enabling ‘TwrPotent’ and ‘TwrAero’:
the simulation fails to properly simulate at lower TSRs and looks sensible for simulations after TSRs of around 4.5. When conducting a single simulation at the lower TSRs the logs noted that cavitation is occuring.
As a beginner user, may I ask what is happening at these lower TSRs?
Regards,
Ariff
Hello Ariff,
At low TSR’s the angle of attack increases, moving towards or past stall. In this region, the angle of attack is also more sensitive to small changes in the rotational speed. There is some instability in the drivetrain flexibility in this region for this reference design. Disabling the drivetrain flexibility (DrTrDOF in MHK_RM1_Fixed_ElastoDyn.dat) should give you more physically expected results for the full range of TSR’s. I suspect that the cavitation warning you are seeing is due to this unstable motion, and will go away without the drivetrain DOF.
Thanks,
Will
Hello Will,
Thank you very much for your advice. I have made the necessary changes and the results have shown to be close to the expected results as shown below, apart from the really low TSR region which I suspect the stall to be most prominent at those regions.
I have conducted simulations for a self-defined rigid and fixed pitch blade, with same conditions and simulations parameters of AeroDyn as MHK_RM1, but what I found was that the Ct against TSR graph continues increases linearly towards higher TSRs, getting results of Ct > 1. Results shown below:
I had thought that this was due to the elastic properties that were being calculated in MHK_RM1 from the blade and tower influence, however when conducting a simulation of MHK_RM1 with rigid blades and no influence of towers, the results were close to the expected results:
May I ask on what factors I should look into which may cause this discrepancy, an initial hypothesis I thought would be that definitions of the self-defined blade itself has problems or perhaps the airfoil polar information being used? Or is it entirely outside of the hydrodynamics considered?
Thank you for your time.
Best regards,
Ariff
Hello Ariff,
I’m glad that the results look better without the drivetrain flexibility.
For the thrust curves that you have shown, I’m not sure if I see anything that is inherently wrong. Depending on the blade definition you have set up, it can make sense that the Ct exceeds 1 at high TSR’s, and makes sense that it looks different than the thrust curve from the RM1 blade.
Is there something that you think is being modeled incorrectly, or just wondering about improvements to the blade design?
Thanks,
Will
Hello Will,
Thank you for your fast reply.
I suppose that I am trying to understand it on a more fundamental level, if the Ct is more than 1 does that mean BEM is calculating as if a force is added to the blades in addition to the flow at higher TSRs, which would then lead me into looking at improvements to the blade design?
In the context of modelling, it might not be incorrect but I have assumed that the root sections follows the polar data from a traditional NACA airfoil as suppose to a cylindrical foil. Would this pose much error in the simulation and is this what causes the prompt during simulation of:
Thank you.
Kind regards,
Ariff
Hello Ariff,
Discussion of the Glauert correction in the AeroDyn theory manual (bottom of page 6 https://www.nrel.gov/docs/fy05osti/36881.pdf) might be helpful.
A non-cylindrical root isn’t necessarily a problem, but might be better with a large twist angle.
Thanks,
Will
Hello Will,
Thank you for directing me to the related theory and my inquiry on the root polar data. Will look into it.
Thanks,
Ariff
Hello Will,
Thank you very much for your guidance regarding the previous inquiries.
After testing the MHK_RM1_Fixed, I have tried conducting my own simulation with a defined blade using PreComp and BModes to get the ElastoDyn properties. The simulation is encountering issues, and I would like to refer to the reference turbine in order to understand the inputs that ElastoDyn is required to define the blade structural properties.
Regarding the ElastoDyn blade properties in MHK_RM1_Fixed, I have a couple of questions:
- Could you explain the reasoning behind the adjustment factors applied to the blade mass density and stiffness values?
- How significantly does the number of blade input stations impact the stability of the aero-elastic simulation?
I greatly appreciate your time and guidance.
Regards,
Ariff
