WECCCOMP – Forums › WECCCOMP forum › Restoring force is shifted up?
 This topic has 7 replies, 3 voices, and was last updated 6 years, 5 months ago by IFP Energies nouvelles.

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3rd April 2018 at 12:33 am #1368
Hi WECCC team
Please, I have noticed that the restoring force obtained from the simulation of the WECCC Simulink model is shifted up by about 2 units. All other forces have almost zero mean (which is expected).
When I use a state space model with WEC parameters taken from the EWTEC conference paper (Restoring force constant 92.33, moment of inertia 1 and the provided state space model for radiation force), we expect it would give the same results as the WECCC simulink model. But I found that the restoring force is not shift.Please, could you help us clarify this.
Best Regards
Mustafa Abdelrahman
Hull University Team4th April 2018 at 5:42 pm #1369Mustafa Abdelrahman,
Thank you for your comment. I have a few questions to try in order to better understand your question.1) Can your clarify what “2 units” refers to as vertical or rotational displacement. There is a
slight offset when running the WECCCOMP WECSIM model with no waves (~1/2 a deg). This is
because the hydrostatic restoring forces are taken from WAMIT. There will be inevitably some
uncertainty between the numerical and experimental models that competitors should try to
account for in their control algorithm.
2) The restoring force constant 92.33, is this rotation about hinge A?
3) Are the values of Moment of Inertia of 1 kg.m2 and the state space values being obtained from
Ringwood 2017?The values presented in Ringwood 2017 assume the equation of motion was derived at hinge A as a onedegree of freedom equation of motion. The WECSim model is a multidegree of freedom system as each body and link is modeled as a separate body and the hydrodynamic and mass properties are defined at each center of gravity.
Please let me know if this has helped to resolve your questions. If not, please add further comments and questions so we can find the clarify the issues you are having.
Cheers,
Nathan Tom
National Renewable Energy Laboratory5th April 2018 at 1:27 am #1370Hi Nathan
Thanks a lot for your helpful reply.
1. After running the wecSim to simulate the WECCC model the function userDefinedFunctions.m plots the response (states, forces and power). The plot for forces, figure 3, shows all forces are almost symmetric around the zero line except the restoring which is shifted up by 1.8 N (or Nm). So the 2 units refers to 2 N (or Nm). (Here I have another question please, should we consider the forces generated by the wecSim here as moments so the unit is Nm because the motion is restricted in the pitch direction only, or they literally forces so the unit is N?. The figure ylabel indicate force in (N) or (Nm) without specifying which is the valid unit for this case)
I agree with you that this mismatches between the simplified onedegreeoffreedom case (Ringwood 2017) and the multibody case (wecSim) is expected and it should be handled by the controller.
2. Yes, I take the restoring constant of 92.33 Nm/rad from Ringwood 2017 (EWTEC conference paper). It is around point A as I understand. I am trying to double check this
3. Yes, the Moment of Inertia of 1 kg.m2 and the statespace approximation of radiation force dynamic are taken from Ringwood 2017.
My point of view is that the wecSim for the WECCC model should be close to the simple ODF case (Ringwood 2017) as the motion is considered in pitch degree only. Some mismatch should exist indeed.
Thanks a lot
Regards
Mustafa9th April 2018 at 10:57 pm #1387Mustafa,
Thanks for clarifying some of my questions:1. Yes, the restoring force in Figure 3 hovers at 1.8 N. This is because this force is only
calculated from the hydrostatic stiffness and the difference between the mass and buoyancy of
the float (The center of gravity sits farther forward than the center of buoyancy).
Therefore, this is the rotational restoring force on the float itself and should not be
compared against the restoring torque about Point A. In this plot, all lines correspond to
torque so N.m, the labeling is not modified in the function plotForces that is used in the
userdefined function to plot results.
2. Thank you, this value was calculated about Point A. This value is close to the heave
hydrostatic stiffness of the float multiplied by the lever arm between the float and Point A.
Again, there is a slight difference in the displaced volume, so the float/arm pair does
rotate slightly in the WECSim model.
3. Thank you, the 1 kg.m2 was converted about the center of gravity of the arm EC and the float.The organizers agree that the WECCCOMP model should be as accurate as possible in modeling the
real system. We have completed several validation cases using the WECSim model and experimental
tests to tune the WECSim model. We will provide access to this report once it is ready.Please let me know if this doesn’t answer your questions or want clarification on any other parts of the model.
Cheers,
Nathan Tom
National Renewable Energy Laboratory10th April 2018 at 12:53 pm #1388Nathan,
The WECSim online documentation states that “WECSim requires that all hydrodynamic coefficients must be given at the center of gravity for each body”. Could you give me the position of the point used to compute the hydro coefficients in ‘wavestar.out’ and ‘wavestar.h5’, with respect to point A?
Thanks
Cheers,
Paolo
11th April 2018 at 5:13 pm #1389Hi Nathan
Thanks a lot that is a great help.
Regards
Mustafa11th April 2018 at 6:07 pm #1390Paolo,
That is correct, in WECSim the equations of motion for each body (either hydrodynamic or nonhydrodynamic) are defined about the center of gravity of each body. The origin of the model is at the still water line above the float center of buoyancy and all bodies, constraints, and ptos are defined from that origin. the The center of gravity of the float can be obtained from “body(1).cg” after loading the .h5 file which is [0.0511 m, 0 m, 0.0533 m]. The location of Point A can be found from “constraint(2).loc” which is [0.438 m, 0 m, 0.302 m]. Therefore the vector from Point A to the center of gravity of the float would be [0.4891 m, 0 m, 0.2487 m]. Please let us know if you have any other questions or need more clarification on model parameters.Cheers,
Nathan Tom
National Renewable Energy Laboratory12th April 2018 at 8:44 am #1391Thanks, Nathan.
Cheers,
Paolo

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