Setting up a problem¶
Defining a test matrix with xarray¶
The shortest way to set the parameters of the potential flow problems to be solved is to build a xarray dataset.
Then the function fill_dataset will automatically make all the simulations defined in the test matrix.
import numpy as np
import xarray as xr
import capytaine as cpt
body = ... # Set up the body and its dofs here.
test_matrix = xr.Dataset(coords={
'omega': np.linspace(0.1, 4, 40),
'wave_direction': [0, np.pi/2],
'radiating_dof': list(body.dofs),
'water_depth': [np.inf],
})
dataset = cpt.BEMSolver().fill_dataset(test_matrix, body)
The first argument of fill_dataset is an xarray.Dataset
specifying the cases to be run. When a parameter is not specified in the
dataset, the default value is used (see next section).
The second argument is either a single FloatingBody or a list of
FloatingBody. In the latter case, each body in the list is studied
independently and the output dataset contains one dimension more.
fill_dataset returns an xarray.Dataset with the same
coordinates as its input but filled with additional output data.
If the coordinate theta is added to the test matrix, the code will
compute the Kochin function for these values of \(\theta\).
The LinearPotentialFlowProblem class¶
For a finer grain control of the problems to be solved, a list of LinearPotentialFlowProblem should be defined.
It is defined as, e.g.:
problem = cpt.RadiationProblem(body=my_body, omega=1.0, radiating_dof="Heave")
other_problem = cpt.DiffractionProblem(body=my_body, omega=1.0, wave_direction=pi/2)
Besides the body, all the parameters are optional. The table below gives their definitions and their default values.
Parameter |
Description (unit) |
Default value |
|---|---|---|
|
Position of the free surface [1] (m) |
\(0.0\) m |
|
Constant depth of water (m) |
\(\infty\) m |
|
Acceleration of gravity \(g\) (m/s²) |
\(9.81\) m/s² |
|
Water density (kg/m³) |
\(1000.0\) kg/m³ |
|
Direction of the incoming waves (rad) (for diffraction or forward speed) |
\(0.0\) rad [2] |
|
Name of radiating dof (only for radiation) |
first one found |
|
Speed of the body in the \(x\) direction (m/s) |
\(0.0\) m/s |
Warning
Unlike other software such as Nemoh, the wave direction in Capytaine is expressed in radians.
The wave height is implicitly assumed to be \(1\) m. Since all computations are linear, any wave height or motion amplitude can be retrieved by multiplying the result by the desired value.
Setting the frequency is done by passing one and only one of the following magnitude.
Parameter |
Description (unit) |
|---|---|
|
Angular frequency \(\omega\) (rad/s) |
|
Period \(T = \frac{2\pi}{\omega}\) (s) |
|
Wavelength \(\lambda\) (m) |
|
Angular wavenumber \(k = \frac{2\pi}{\lambda}\) (rad/m) |
If no frequency is provided, a frequency omega = 1.0 rad/s is used by default.
Once the problem has been initialized, the other parameters can be retrieved as:
problem.omega
problem.period
problem.wavelength
problem.wavenumber
When forward speed is non zero, the encounter frequency is computed and can be retrieved as:
problem.encounter_omega
problem.encounter_period
problem.encounter_wavelength
problem.encounter_wavenumber
In some cases (radiation problems in infinite depth), setting the frequency to zero or infinity is possible. Simply pass the value 0.0 or float(‘inf’) to one of the above magnitude.
Legacy Nemoh.cal parameters files¶
The legacy parameters files from Nemoh can be read by a dedicated function:
from capytaine.io.legacy import import_cal_file
list_of_problems = import_cal_file("path/to/Nemoh.cal")
The function returns a list of LinearPotentialFlowProblems.
Warning
This feature is experimental.
Some of the settings in the files (such as the free surface computation or the Kochin function) are ignored for the moment.
See the example Nemoh.cal below.
--- Environment ---
1000 ! RHO ! KG/M**3 ! Fluid specific volume
9.81 ! G ! M/S**2 ! Gravity
0 ! DEPTH ! M ! Water depth
0 0 ! Wave measurement point !! This line is ignored !!
--- Description of floating bodies ---
1 ! Number of bodies
--- Body 1 ---
Cylinder.dat ! Name of mesh file
540 300 ! Number of vertices and panels in mesh !! This line is ignored !!
6 ! Number of degrees of freedom
1 1 0 0 0 0 0 ! Surge
1 0 1 0 0 0 0 ! Sway
1 0 0 1 0 0 0 ! Heave
2 1 0 0 0 0 -7.5 ! Roll about CdG
2 0 1 0 0 0 -7.5 ! Pitch about CdG
2 0 0 1 0 0 -7.5 ! Yaw about CdG
6 ! Number of resulting generalised forces
1 1 0 0 0 0 0 ! Force in x direction !! This line is ignored !!
1 0 1 0 0 0 0 ! Force in y direction !! This line is ignored !!
1 0 0 1 0 0 0 ! Force in z direction !! This line is ignored !!
2 1 0 0 0 0 -7.5 ! Moment force in x direction about CdG !! This line is ignored !!
2 0 1 0 0 0 -7.5 ! Moment force in y direction about CdG !! This line is ignored !!
2 0 0 1 0 0 -7.5 ! Moment force in z direction about CdG !! This line is ignored !!
0 ! Number of lines of additional information !! All the additional information is ignored !!
--- Load cases to be solved ---
1 2 0.1 2.0 ! Freq type 1,2,3=[rad/s,Hz,s], Number of wave frequencies/periods, Min, and Max
1 0.0 0.0 ! Number of wave directions, Min and Max (degrees)
--- Post processing ---
!! All lines below are ignored by Capytaine !!
Note
The line setting up the frequencies is slightly different in Nemoh v2 and Nemoh v3. Both format are supported by Capytaine.
Command-line interface¶
Warning
This feature is experimental.
Capytaine comes with a command-line command capytaine which can be used as:
$ capytaine path/to/directory/parameter.cal
The parameter file (in Nemoh.cal format) passed as argument is read and legacy tecplot output file are written in the directory path/to/directory/results/.
Warning
If results files already exist, they will be overwritten!
If no argument is provided to the command, the code looks for a file Nemoh.cal in the current directory.