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Negative Power
My team and I are engineering students in the process of simulating a three-bladed H-rotor for our design team. The main issue we are encountering is that all of our simulations are generating negative power. I have attached a project below that demonstrates this. We were wondering if you have any ideas on why this might be happening.
Thanks
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Hello,
similar to the last answer on this topic (with a very similar rotor geometry, if not the same):
The rotor you are modeling is particularly challenging to model aerodynamically:
- The rotor is high-solidity. With 3 blades, chord ≈ 0.4 m and radius ≈ 0.5 m, the solidity is about σ ≈ 0.38, which is well into the high-solidity regime.
- The chord-to-radius ratio is very large (c/R ≈ 0.8), leading to very strong induction and intense blade-wake interaction.
- The blade aspect ratio is low (H/c ≈ 2.5), so 3D effects and deep stall dominate large parts of the rotation.
- Large azimuthal regions operate in dynamic stall, producing strong, rapidly varying vorticity that makes free-wake models especially sensitive to numerical settings.
- Overlapping bound vortex elements from the struts at r=0
Besides, this rotor concept is not expected to produce significant power in practice. High-solidity H-rotors typically operate at very low tip-speed ratios, where large portions of the blades are in deep stall for most of the revolution. Combined with the very large struts (chord ≈ 0.2 m, i.e. ~50% of the blade chord), the parasitic drag losses are substantial and can easily dominate any useful aerodynamic torque. As a result, even if the wake is numerically stabilized, the achievable power coefficient will be low and the solution will remain highly sensitive to numerical and modeling parameters.
I would suggest significantly reducing the blade chord, and ensuring that the struts do not end at r = 0 to avoid panel overlap.
BR,
David
