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Bradley,
Apologies for the delay in our response. The evaluation criteria will be calculated for each of the sea states stated on the WECCOMP info sheet and an unweighted average will be taken. The unweighted average for all the sea states will be used as the final evaluation criteria for each competitor.If you have other questions or need clarification, please do not hesitate to ask.
Cheers,
Nathan Tom
National Renewable Energy LaboratoryPaolo,
That is correct, in WEC-Sim the equations of motion for each body (either hydrodynamic or non-hydrodynamic) 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 LaboratoryMustafa,
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 WEC-Sim 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 WEC-Sim model and experimental
tests to tune the WEC-Sim 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 LaboratoryMustafa 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 WEC-SIM 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 one-degree of freedom equation of motion. The WEC-Sim model is a multi-degree 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 LaboratoryBME,
After some discussion between the WECCCOMP organizers, we have decided to increase the maximum allowable motor linear displacement.1. The value for Z_{max} is now 0.08 m (8 cm), please use this value for calculating the evaluation
criteria. The WECCCOMP-info-sheet_V6.pdf has been updated for clarity.Apologies for any confusion this has caused. If you require additional clarification, please post to the forum and the organizers will respond as quickly as possible.
Cheers,
Nathan Tom
National Renewable Energy LaboratoryHull University,
After some discussion between the WECCCOMP organizers, we have decided to increase the maximum allowable motor linear displacement.
1. The value for Z_{max} is now 0.08 m (8 cm), please use this value for calculating the evaluation
criteria.Apologies for any confusion this has caused. If you require additional clarification, please post to the forum and the organizers will respond as quickly as possible.
Cheers,
Nathan Tom
National Renewable Energy LaboratoryPaolino Tona,
Thank you for notifying us of that version issue. We have just pushed the changes to the GitHub Wec-Sim repository. You should now be able to use the additional wave gauges “waves.wavegauge<i>loc.” Please let us know if you have any further issues.Cheers,
Nathan Tom
National Renewable Energy LaboratoryBME Team,
The WECCCOMP organizers wanted to update you on point 6 from your initial questions.6. WEC-Sim has been updated so there are three wave gauges available for competitors to use. The location and details of the implementation can be found in the WECCCOMP_WECSIM_info_sheet_v7.pdf that has been uploaded to the WECCCOMP GitHub site. Please pull the latest version of WEC-Sim and the WECCCOMP folders.
Again, thank you for your comments! If you have additional questions or want further clarification please leave a comment on the forum and the organizing team will respond as soon as possible.
Cheers,
Nathan Tom
National Renewable Energy LaboratoryBME Team,
Thank you for your questions, the WECCCOMP organizer responses can be found below:1. Each WEC-Sim simulation should have a ramp time of 5*Tp and a simulation end time of 100*Tp.
The first 25 seconds of the WEC-Sim simulation will be discarded for calculation of the
competition metrics (the evaluation criteria will only be calculated for time > 25 s). The
WECCCOMP WEC-Sim info sheet has been updated with this text.2. The organizers apologize for the confusion. The evaluation criteria will be based on the 98th
percentile as written on the WECCCOMP-info-sheet. We will update the website.3. The displacement constraint, Z_{max}, has been set at 0.06 m (6 cm) and the WECCCOMP-info-
sheet has been updated for clarity.4. Yes, the linear motor efficiency has been set at 70%. The efficiency value has been added to
the WECCCOMP-info-sheet and includes the equation on how the piece-wise calculation for
generator/motor power is calculated5. The |P|_{98} percentile does refer to the electrical power. The WECCCOMP-info-sheet has been
updated for clarity.6. WEC-Sim currently has one wave gauge that can be placed anywhere along the x-axis by using the
waves.waveAmpTimex option in WEC-Sim. Two more wave gauges will be added to WEC-Sim once we
have confirmed with AAU on the wave probe location for the experimental portion. This will be
updated in the WEC-Sim code early next week.Again, thank you for your comments! If you have additional questions or want further clarification please leave a comment on the forum and the organizing team will respond as soon as possible.
Cheers,
Nathan Tom
National Renewable Energy LaboratoryHull University,
Thank you for your questions. The WECCCOMP organizers’ responses are below:
1. The value for Z_{max} is 0.06 m (6 cm), please use this value for calculating the evaluation
criteria.
2. A validation study was completed that included a linear rotational damping term to tune the
WEC-Sim model to match experimental measurements of the Wavestar. The
wecSimInputFile.m was modified to include a body(1).linearDamping term that was
determined from experimental tests. This term includes the combination of bearing friction
and fluid viscous damping. Please pull the latest version of WECCCOMP from the Github
repository.Thank you again for your questions, if you need additional clarification, please post to the forum and the organizers will respond as quickly as possible.
Cheers,
Nathan Tom
National Renewable Energy LaboratoryPaolino Tona,
Apologies for the delay in our response. We want to confirm that your statements on 1/12/18 are correct. (1) The ‘motor_pos_rot’ does provide the relative pitch displacement. (2) There was an unnecessary conversion step in the lin2rot conversion block. This conversion block was designed to correct the measured acceleration from the accelerometer attached to the Wavestar arm. This accelerometer will be used in the experimental portion of the competition to provide input to the controller. The WEC-Sim developers are working with Aalborg University to construct a sensor model for the accelerometer. As of now the conversion block has been removed and the ‘float_accel’ is the angular acceleration of the WEC as calculated by WEC-Sim. We will notify participants when the developers have included a sensor model for the accelerometer.Please pull the latest version of the WECCCOMP WEC-Sim model from the Github website.
Thank you,
Nathan Tom
National Renewable Energy Laboratory