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Waves

Waves in OASIS drive hydrodynamic excitation forces on floating bodies. The wave system supports regular, irregular (spectral), and user-defined wave inputs.

Wave Types

graph TD
    W[Wave] --> R[Regular Wave]
    W --> I[Irregular Wave]
    I --> J[JONSWAP Spectrum]
    I --> T[Time-Series Input]
    I --> F[Frequency-Domain Input]

Regular Waves (REG)

Single monochromatic sinusoidal wave.

Parameter Symbol Units Description
Height \(H\) m Wave height (crest to trough)
Period \(T\) s Wave period
Heading \(D\) deg Propagation direction
Ramp time \(t_r\) s Linear ramp-up duration

The surface elevation is:

\[\eta(x,t) = \frac{H}{2} \cos(kx - \omega t) \cdot R(t)\]

where \(R(t)\) is a linear ramp from 0 to 1 over \(t_r\) seconds.

Irregular Waves (IRR)

Spectral sea states composed of multiple wave components. Three input modes are available, selected by specType_flag:

1. JONSWAP Spectrum (specType_flag = 1)

Generates a JONSWAP spectrum analytically following the IEC 61400-3 standard:

\[S(\omega) = \alpha_{\text{PM}} \cdot \gamma^{\exp\left[-\frac{(\omega - \omega_p)^2}{2\sigma^2 \omega_p^2}\right]}\]
Parameter Description Typical value
\(H_s\) Significant wave height —
\(T_p\) Peak period —
\(\gamma\) Peak enhancement factor 3.3
\(s\) Directional spreading exponent < 1 = no spreading
\(d\theta\) Heading discretisation 10°

Supports multi-directional spreading via the cosine-power model:

\[G(\theta) = \cos^s(\theta - \theta_0)\]

Supports piecewise decomposition: the simulation time is split into windows for faster wave elevation computation.

Random Phases

Set readPhases_flag = 1 to read phase angles from a file for reproducible results. Otherwise, random phases are generated each run.

2. Time-Series Input (specType_flag = 2)

Reads a pre-computed wave elevation time series and converts to frequency domain via FFT.

File format (wave.dat):

3001 0
0.0   0.523
0.1   0.612
0.2   0.487
...
  • Line 1: number of points, followed by 0
  • Data lines: time [s] elevation [m]

Supports piecewise decomposition.

3. Frequency-Domain Input (specType_flag = 3)

Reads wave components directly in the frequency domain.

File format (wavefreq.dat):

150
0.0500  0.0  0.0312  1.2345
0.0600  0.0  0.0456  2.3456
0.0500  15.0 0.0112  0.5678
...
  • Line 1: total number of components
  • Data lines: frequency [Hz] heading [deg] amplitude [m] phase [rad]

Each frequency-heading combination is listed separately. This format supports multi-directional, non-parametric wave fields from external models (e.g., numerical wave propagation tools).

Note

Piecewise decomposition is not available for frequency-domain input.

Input Configuration

In dataWaves.dat:

//////////////////
////////////////// Basic wave data
//////////////////
IRR             // Wave type
2.0             // Hs [m]
10.0            // Tp [s]
0.0             // Heading [deg]
5.0             // Ramp time [s]
//////////////////
////////////////// Irregular wave data
//////////////////
1               // specType_flag (JONSWAP)
0               // piecewise_flag
//// JONSWAP
3.3             // gamma
0.0             // s (spreading, <1 = none)
10.0            // dtheta [deg]
0.001           // factor
0               // readPhases_flag
0.05            // relTol
0.1             // dt [s]
none            // phase filename
//// CUSTOM
none            // data filename

In dataProblem.yaml:

waves:
  type: IRR
  Hs: 2.0
  Tp: 10.0
  heading: 0.0
  ramp_time: 5.0
  spectrum:
    type: 1
    piecewise: 0
    gamma: 3.3
    s: 0.0
    dtheta: 10.0
    factor: 0.001
    read_phases: 0
    rel_tol: 0.05
    dt: 0.1
    phase_file: none
    data_file: none

Output Files

File Content Header
WaveSpectrum.txt Frequency [Hz], Spectral density [m²/Hz], Amplitude [m] # Wave Spectrum
WaveTimeSeries.txt Time [s], Elevation [m] # Wave Time Series
WavePhases.txt Phase matrix (Armadillo ASCII format) —

Constraints

  • Wave period must be non-zero
  • Wave height must satisfy the Miche breaking criterion
  • Heading discretisation \(d\theta\) must divide 360° evenly (e.g., 10°, 15°, 30°, 45°)
  • For piecewise: spectrum frequency step \(df = 1/(N \cdot dt)\)
Example Feature
waves/regular Regular wave
waves/jonswap JONSWAP spectrum
waves/jonswap_piecewise Piecewise-decomposed JONSWAP
waves/multidirectional Multi-directional spreading
waves/timeseries Time-series input
waves/frequency_domain Frequency-domain input