Morison model - Benchmark FRyDoM / DeeplinesWind

This benchmark is dedicated to the validation of the full morison model in FRyDoM. Comparisons with the results from DeeplinesWind [DLW2013] have been made for one morison element both completly immersed and piercing the free surface.

The properties of the morison element are listed into the following table

Propertie Value
Length 4 m
Diameter 1 m
Normal drag coeff (Cd) 0.6
Normal added mass coeff (Cm) 1.8
Axial drga coeff (Cd_a) 0.0
Axial added mass coeff (Cm_a) 0.0

Several direction are considered for the morison element:

  • aligned the main axis (x-axis, y-axis and z-axis) as represented in the next figure.
direction morison
  • aligned with diagnonal axis (xy-axis, yz-axis and xyz-axis).

Immersed elements

The following results concern elements with center placed at 10 m under the free surface. A regular wave with 0.1 m of amplitude and 8s period is considered. Elements are completely immersed during the simulation. The buoyancy contribution have been removed from this results since they are constant in time.

Immersed Morison Main Axis

Fig. 51 Time series of the morison force for a morison element aligned with the x-axis (left plots), y-axis (center plots) and the z-axis (right plots). Comparison of the FRyDoM results (green) to DeeplinesWind results (blue).

Immersed Morison Diagonal Axis

Fig. 52 Time series of the morison force for a morison element aligned with the xy-axis (left plots), yz-axis (center plots) and the xyz-axis (right plots). Comparison of the FRyDoM results (green) to DeeplinesWind results (blue).

Very good agreement between the FRyDoM and DeeplinesWind results are found in all the direction of the morison element.

Surface piercing elements

The following results concern elements with center placed at the free surface (z=0). A regular wave with amplitude equal to 0.1m and 8s period is considered. Elements are crossing the free surface in this case.

Piercing Morison Main Axis

Fig. 53 Time series of the morison force for a morison element aligned with x-axis (left plots), y-axis (center plots), and the z-axis (right plots). Comparison of the FRyDoM results (green) to DeeplinesWind results (blue).

Piercing Morison Diag Axis

Fig. 54 Time series of the morison force for a morison element aligned with xy-axis (left plots), yz-axis (center plots) and the xyz-axis (right plots). Comparison of the FRyDoM results (green), to DeeplinesWind (blue).

Good agreement can be observed for the elements aligned with the z-axis, yz-axis and xyz-axis. Small depreciancies and discontinuities on the FRyDoM results can be observed since the no progressive variation of the buoyancy is used when the element are crossing the surface. A discontinuity appears when elements switch from immersed to emmerged element. Those discontinuities are more pronouced for horizontal bar elements (aligned with x-axis, y-axis and xy-axis) as it can be observed on the previous figures. Improvment on the buoyancy force would reduce this differences.

References

[DLW2013]
  1. Perdrizet, J-C Gilloteaux, D. Teixeira, G. Ferrer, L. Piriou, D. Cadiou, J-H Heurtier, C. Le Cunff, “Fully coupled floating wind turbine simulator based on nonlinear finite element method - Part II : Validation results”, OMAE, 2013, Nantes