Example: MHKiT-MATLAB WEC-Sim

This example loads simulated data from a WEC-Sim run for a two-body point absorber (Reference Model 3) and demonstrates the application of the MHKiT Wave Module to interact with the simulated data. The analysis is broken down into three parts: load WEC-Sim data, interact with the WEC-Sim data, and apply the MHKiT wave module.

Load WEC-Sim Simulated Data
Analyze WEC-Sim Simulated Data (e.g. Wave Class, Body Class, PTO Class, Constraint Class, and Mooring Class Data)
Apply MHKiT Wave Module

1. Load WEC-Sim Simulated Data

WEC-Sim saves output data as a MATLAB structure, generated by WEC-Sim’s Response Class. The WEC-Sim structure contains data for: wave, bodies, ptos, constraints, mooring, moordyn, and ptosim

Here we will load the WEC-Sim RM3 data run with a mooring matrix.

filename = './data/RM3MooringMatrix_matlabWorkspace.mat'
filename = './data/RM3MooringMatrix_matlabWorkspace.mat'
load(filename,'output')
output
output = struct with fields:
           wave: [1×1 struct]
         bodies: [1×2 struct]
           ptos: [1×1 struct]
    constraints: [1×1 struct]
        mooring: [1×1 struct]
        moorDyn: [1×1 struct]
         ptosim: [1×1 struct]

2. Analyze WEC-Sim Simulated Data

This section will analyze the WEC-Sim RM3 data using the WEC-Sim structure.

Wave Class Data

Data from WEC-Sim’s Wave Class includes information about the wave input, including the the wave type, and wave elevation as a function of time.

% Store WEC-Sim output from the Wave Class to a new variable, called `wave_data`

wave_data = output.wave

wave_data = struct with fields:

type: 'etaImport' time: [40001×1 double] elevation: [40001×1 double]

figure;plot(wave_data.time,wave_data.elevation)

xlabel("Time [s]")

ylabel("Wave Surface Elevation [m]")

legend('elevation')

Body Class Data

Data from WEC-Sim’s Body Class includes information about each body, including the body’s position, velocity, acceleration, forces acting on the body, and the body’s name. For the RM3 example there will be 2 boides a float and a spar.

% Store WEC-Sim output from the Body Class as 'bodies'
bodies = output.bodies
bodies = 1×2 struct 
FieldsnametimepositionvelocityaccelerationforceTotalforceExcitationforceRadiationDampingforceAddedMassforceRestoringforceMorrisonAndViscousforceLinearDamping
1'float'40001×1 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double
2'spar'40001×1 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double40001×6 double
% Store Body Class data for Body 1 as `body1`. 
body1 = output.bodies(1)
body1 = struct with fields:
                       name: 'float'
                       time: [40001×1 double]
                   position: [40001×6 double]
                   velocity: [40001×6 double]
               acceleration: [40001×6 double]
                 forceTotal: [40001×6 double]
            forceExcitation: [40001×6 double]
      forceRadiationDamping: [40001×6 double]
             forceAddedMass: [40001×6 double]
             forceRestoring: [40001×6 double]
    forceMorrisonAndViscous: [40001×6 double]
         forceLinearDamping: [40001×6 double]

Fields	name	time	position	velocity	acceleration	forceTotal	forceExcitation	forceRadiationDamping	forceAddedMass	forceRestoring	forceMorrisonAndViscous	forceLinearDamping
1	'float'	40001×1 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double
2	'spar'	40001×1 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double	40001×6 double

Plot heave position data for Body 1

The RM3 device creates power in the heave direction (DOF 3). Let us consider the float's (body 1) position as a function of time by plotting body1.position(:,3).

figure;plot(body1.time, body1.position(:,3))

xlabel("Time [s]")

ylabel("Heave Position [m]")

title('Body 1')

% Calculate the maximum and minimum heave position of Body 1

fprintf("Body 1 max heave position = %4.5f [m]", max(body1.position(:,3)))

Body 1 max heave position = -0.09745 [m]

fprintf("Body 1 max heave position = %4.5f [m]", min(body1.position(:,3)))

Body 1 max heave position = -1.32645 [m]

Plot Body 1 position data for all DOFs

As an example we could plot multiple postion DOFs by calling a plot on only the position data.

figure;plot(body1.time, body1.position(:,:))

xlabel("Time [s]")

ylabel("Position [m or rad]")

title('Body 1')

3. Apply MHKiT Wave Module

In the example below we will calculate a spectrum from the wecsim wave elevation timeseries data using the MHKiT elevation_spectrum function.

% Use the MHKiT Wave Module to calculate the wave spectrum from the WEC-Sim Wave Class Data

sample_rate=60

sample_rate =

nnft=1000 % Number of bins in the Fast Fourier Transform

nnft =

1000

ws_spectrum = elevation_spectrum(wave_data.elevation, sample_rate, nnft,wave_data.time)

ws_spectrum = struct with fields:

spectrum: [501×1 double] type: 'Spectra from Timeseries' frequency: [501×1 double] sample_rate: 60 nnft: 1000

% Plot calculated wave spectrum

spect_plot = plot_spectrum(ws_spectrum);

xlim([0, 4]);

% Calculate Peak Wave Period (Tp) and Significant Wave Height (Hm0)
Tp = peak_period(ws_spectrum)
   8.333333333333334
Tp = 
   8.333333333333334
Hm0 = significant_wave_height(ws_spectrum)
Hm0 = 
   1.785739037962701