Why does a floating wind turbine tilt forward when shutting down? with Ashes aeroelastic simulations

This video explains why a spar buoy floating wind turbine tilts towards the wind when the blades are pitched.
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[@SirWiIlbur]

Great explanation, thank you. How much of the oscillation is due to reversing the thrust force, and how much is due to normal overshooting oscillatory motion, which you would get anyway if you immediately turned off the thrust (i.e. a very quick change in blade pitch)?

[@AshesWindTurbineSimulation]

That's an excellent question. To answer it, we reran the same simulation, but turned off aerodynamic loads exactly when the blades start pitching. Result: with aerodynamic loads (i.e. the simulation in the video), the tilt angle goes from +4 to -4.3 degrees. Without aerodynamic loads (i.e. only with restoring forces), the tilt angle goes from +4 to -3.6 degrees. So that would mean that most of the tilting is due to restoring hydrodynamic forces (as expected), but the thrust force does increase the angle by about 10%.

[@rogerbergua6268]

@@AshesWindTurbineSimulationExcellent video and explanation, as always. Just one small comment. In reality, the restoring motion observed for this spar design is driven by the gravitational restoring (e.g., spar center of mass and probably a contribution from the rotor-nacelle-assembly overhang). In a semisubmersible design, the hydrostatic stiffness in pitch would be very large and the restoring would be driven by it. Spot on with the insights about the thrust force reversing due to the angle of attack 😉

[@AshesWindTurbineSimulation]

@@rogerbergua6268 right, as opposed to what we wrote in the comment saying that it was mostly restoring hydrostatic forces? That makes total sense

[@rogerbergua6268]

Right. The main motion in this design comes from gravitational restoring rather than hydrostatic restoring 😉

[@malithmadhuwantha6961]

great.....why the lift force decreases when the angle of attack decreases ?

[@AshesWindTurbineSimulation]

Because the pressure distribution around the airfoil changes to a configuration that gives a smaller lift force. You can actually try that with Ashes, if you use the Investigate option in the airfoil database (see www.simis.io/docs/databases-airfoil-database)