Nonlinear Buoyance and Froude-Krylov Wave Excitation Tutorial results seem excessive #1485
Description
Is your question request related to a problem? Please describe.
I need to consider nonlinear buoyancy and Froude-Krylov excitation for my
simulation. However, changing body(ii).nonlinearHydro=2 results in
unrealistically large responses and power generation compared to simulations
where body(ii).nonlinearHydro=0 or 1.
Describe the theory or implementation approach that you have a question
To investigate this issue, I used the example provided in
Wec-Sim_applications/Nonlinear_Hydro/, specifically a heaving ellipsoid.
The table below compares the results when changing the body.nonlinearHydro
parameter for all cases provided in the example.
Wave profile: 6s 4m; PTO damping: 1.2e6
| ODE | Wave | NL setting | Ave Power (kW) | Peak pos (m) | Peak vel (m/s) |
|---|---|---|---|---|---|
| ode4 | Regular | 2 | 1127 | 1.15 | 1.25 |
| 1 | 249 | 0.60 | 0.64 | ||
| 0 | 244 | 0.60 | 0.64 | ||
| ode4 | RegularCIC | 2 | 1127 | 1.15 | 1.25 |
| 1 | 249 | 0.60 | 0.64 | ||
| 0 | 244 | 0.60 | 0.64 | ||
| ode45 | Regular | 2 | 1189 | 1.20 | 1.25 |
| 1 | 263 | 0.64 | 0.66 | ||
| 0 | 244 | 0.60 | 0.64 | ||
| ode45 | RegularCIC | 2 | 1184 | 1.20 | 1.25 |
| 1 | 261 | 0.64 | 0.66 | ||
| 0 | 243 | 0.60 | 0.64 |
All four cases show similar behaviors. There is a significant increase in power
generation for NL=2, while for NL=1, the results differ slightly from the
linear case (NL=0).
This behavior seems unrealistic. Further investigations, such as reducing the
wave conditions to 8s and 0.25m, or removing PTO damping, still show
approximately a doubling of position and velocity for the nonlinear case.
When removing the wave load (waveClass('noWave')) and prescribing an initial
displacement, the response for the nonlinear case becomes much closer to the
linear case, even with a very large initial amplitude.
These results suggest that the Wec-Sim nonlinear case may have inadvertently
doubled the wave force when nonlinearHydro=2.
Personal project context
I need to perform nonlinear hydrodynamics for our simulation, but the observed
doubling of response amplitude and quadrupling of power generation, compared to
the linear case, seems incorrect.
Describe the type of conclusion or resolution you are looking for
Please check for a bug in Wec-Sim's nonlinear hydrodynamics, or provide
validated cases to support the results.
Additional context
This issue may be related to issues #878 and #886.