Post processing

Post-processing

The post-processing of outputs of HOS-ocean is available in sources/PostProcessing directory. This is written as a program to compile and execute with a given input file named input_post_process.dat containing necessary informations and described hereafter.

## Installation

Installation steps to create the executable program are similar to the ones of the main program. Particularly, note that same library is used for Fast Fourier Transforms (FFTW) with identical linking.

Linux/Unix environment

A Makefile is provided with the source code for compiling PostProcessing in Linux/Unix environment. The GNU Fortran compiler gfortran shall be used. Other Fortran compilers may be used with small adjustments of compiler options, libraries...

Makefile makes use of executable makedepf90 to extract automatic dependencies available here

Windows environment

Description of compilation, linking... with Windows environment will be completed later.

Input file

The input file has the following form and is assumed to be named input_post_process.dat

input file

Choice of post-processing options

i_ana defines the wavefield analysis to perform (i_ana /= 0)
- i_ana = 1 : computation of the moments of free surface elevation η
- i_ana = 2 : computation of moments and wave-by-wave analysis along x-direction (for 2D or 3D wavefields)
- i_ana = 3 : computation of moments and wave-by-wave analysis along x and y direction (for 3D wavefields with sufficient directional spreading and domain size)
i_card defines the output of velocity and pressure cards (i_card /= 0)
- i_card = 1 : Mesh is defined with constant z and (x,y) from HOS-ocean simulation. This is the most efficient way to generate cards or for coupling since it allows use of FFTs
- i_card = 2 : Mesh is defined with boundary (free-surface) fitted z and (x,y) from HOS-ocean simulation.
T_start gives the time of the beginning of post-processing (wavefield wanalysis and/or cards)
T_stop gives the time of the end of post-processing (wavefield wanalysis and/or cards)

Velocities/pressure cards

This section of input file describes the mesh of the output of velocity and pressure card:

x_min is the minimum of the x-mesh
x_max is the maximum of the x-mesh
y_min is the minimum of the y-mesh (for 3D simulation)
y_max is the maximum of the y-mesh (for 3D simulation)
z_min is the minimum of the z-mesh
z_max is the maximum of the z-mesh (for i_card = 1)
i_zvect is the number of points to describe z vector

Input files

tecplot gives the tecplot version of output files
file_3d gives the name of file describing free surface elevation (with possible relative path)
file_mod gives the name of file of modal amplitudes of volumic informations (with possible relative path)

Analysis of wavefield

The main idea of the different wavefield analysis provided in postprocessing of HOS-ocean is summarized hereafter

Moments of free surface elevation η are always computed: mean, standard deviation, skewness, kurtosis. These are basic informations useful for analysis
- Analysis.dat file is created with these informations
Wave-by-wave analysis along x-direction (i_ana = 2)
- 2D wave fields: compute H_1/3, H_max, A_max and detect freak waves (i.e. waves with H>2H_s)
- 3D wave fields: same as 2D wave fields with analysis done on eta(1:n1,1) (i.e. y=0)
- Analysis.dat contains those informations
- freak_waves.dat gives the free surface profile of each detected freak wave
- Caract_freak_waves.dat gives the characteristics of detected freak waves (size, amplitude...)
Wave-by-wave analysis along x and y direction (i_ana = 3)
- Available for 3D wavefields with sufficient directional spreading and domain size
- 3D waves are detected assuming they are propagating along x direction and that their transverse direction is along y
- Compute H_1/3, H_max, A_max and detect freak waves (i.e. waves with H>2H_s)
- Analysis.dat contains those informations
- freak_waves.dat gives the 3D free surface profile of each detected freak wave
- Caract_freak_waves.dat gives the characteristics of detected freak waves (size, amplitude...)

Description of kinematics inside fluid domain

HOS-ocean is able to create a modal description of some volumic informations in the fluids. All necessary informations to compute velocities, pressure, acceleration (or whatever derivatives of these quantities) are included in this file usually named modes_HOS_SWENSE.dat

We give as an example of treatment of this file the possibility to create representation of velocities and pressure field (i_card /= 0). Depending on the choice of user following files are created:

data_VP_card.dat gives useful informations about the card created in VP_card.dat. This is used at ECN as coupling informations file. (i_card=1)
VP_card.dat contains the velocities and pressure card in the case i_card=1, i.e. when the mesh is at constant z. This file is adapted to Tecplot visualization.
VP_card_fitted.dat contains the velocities and pressure card in the case i_card=2, i.e. when a boundary fitted (free surface) mesh is used. This file is adapted to Tecplot visualization.

Note that the process used to create those cards is similar to the one used at ECN for coupling between HOS and RANS solver with the SWENS method (Spectral Wave Explicit Navier-Stokes). The quantities to reconstruct have to be adapted for the problem solved but the core is the one proposed in this post-processing routines.

We refer to the following publications for details about the SWENS coupling strategy

Monroy, C., Ducrozet, G., Bonnefoy, F., Babarit, A., Gentaz, L., Ferrant, P., RANS Simulations of a calm buoy in regular and irregular seas using the SWENSE method, International Journal of Offshore and Polar Engineering, Vol. 21, No 4, 264-271, 2011.
Luquet, R., Gentaz, L., Ferrant, P., Alessandrini, B., Viscous flow simulation past a ship in waves using the SWENSE approach, 25th Symposium on Naval Hydrodynamics, St. John’s, Canada, 2004
Ferrant, P., Gentaz, L., Alessandrini, B., Le Touzé, D., A potential / RANSE approach for regular water wave diffraction about 2D structures, Ship Technology Research, Vol. 50, No 4, 2003.

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