1. Introduction

1.1. The ComFLOW projects

The ComFLOW program is one of the deliverables of the ComFLOW-3 project, as was the case for the SafeFLOW and ComFLOW-2 projects. The main objective of these ComFLOW projects can be summarized as:

To develop a user-friendly and validated numerical tool for marine and offshore industries to study complex free-surface problems, which is flexible in its application and has a coupling possibility to the other tools of participants.

The emphasis in the ComFLOW projects is put on the research aspect.

The following deliverables are presented to the participants during and at the end of a ComFLOW project:

  • executable versions of the numerical tools,
  • pre- and post-processing tools for data visualization,
  • user manual (the current document).

ComFLOW is a program for the numerical simulation of fluid flow, based on the Navier-Stokes equations. The program has been developed initially by the University of Groningen (RuG) to study the sloshing of liquid fuel in spacecraft. This micro-gravity environment requires a very accurate and robust description of the free surface. In close co-operation with MARIN, the methodology was extended to the simulation of green water loading on a fixed bow deck and the analysis of anti-roll tanks.

In the first ComFLOW project, called SafeFLOW, the development of the program has been extended further, with physical focus on impact loading due to green water and steep waves. During the SafeFLOW project, the emphasis was on the following items, which have been included in the final deliverable of SafeFLOW:

  • implementation of regular waves
  • implementation of an outflow boundary condition / damping zone
  • implementation of moving objects

In the ComFLOW-2 project the physical focus was on wave impact loading, steep waves, overturning waves and internal sloshing. From a computational point of view, progress has been achieved in:

  • improved accuracy in simulations
  • improved simulation of impact physics by two-phase modeling
  • improved simulation of moving bodies
  • reduced computational effort by improved boundary conditions

Large steps forward have been made in SafeFLOW and ComFLOW-2, but several challenges remained, such as the large computational times and high artificial (numerical) dissipation of waves. Therefore, the ComFLOW-3 project aimed at further development of ComFLOW, with focus on new functionality and its validation, as well as on further improvement of existing functionality. The main objective of the ComFLOW-3 project is:

To further improve, develop and validate the ComFLOW program for complex free-surface flows in the offshore industry and make it usable for advanced engineering applications by improved functionality and speed-up of the algorithms.

The following issues are addressed in ComFLOW-3:

  • possibility to model viscous effects in the flow near sharp corners
  • the ability to study local flow problems by using a local dense grid
  • possibility to model the dynamics of mooring lines and their influence on motions of offshore structures
  • computational efficiency by applying parallelization using multi-core PC’s
  • computational efficiency by using better absorbing boundary conditions, which makes it possible to use a small computational domain
  • better algorithms for wave propagation

The ComFLOW package is available for Windows (Vista, 7 or 8) and Linux systems. All versions of the deliverable are designed for 64 bits operating systems. As of April 2014, Microsoft has ended mainstream support for the XP operating system. Since this date also ComFLOW does no longer provide support for Windows XP.

System requirements and recommendations:

  • a 64 bits PC system with one of:

    • 64-bits Windows Vista/7/8/10
    • 64-bits Linux operating system 1
    • 32-bits Windows Vista/7/8/10 2

  • a recent CPU model (Intel i5/i7/Xeon series is recommended)

  • 8 Gb RAM

  • data visualization: MATLAB version 2012b or higher

  • simulation setup: a text editor; XML editor (optional)

1.2. Contents of this manual

In this manual, first a short introduction into the mathematics and numerics of the ComFLOW program is given in Chapter 2. A detailed description can be found in the theses of :cite:`gerrits2001dynamics`, :cite:`fekken2004numerical`, :cite:`kleefsman2005water` and :cite:`wemmenhove2008numerical`. The history of the delivered versions is described in Chapter %s.

For a first run with ComFLOW, read the instructions in Chapter 3, where all steps are described for installing ComFLOW and running a test simulation. A run of ComFLOW consists of the definition of the geometry (using GEODEF) and the run of the fluid solver (ComFLOW). The input files needed for the geometry definition and the initial liquid distribution are described in Chapter %s.

The input files for simulation setup, used by the preprocessor GEODEF as well as by ComFLOW, are described in chapters 28 and xml_input. The local grid refinement functionality (available since version 3.4.1) is described in more detail in Chapter 7.

Guidelines for grid and time step parameters are given in Chapter 24. The main input file is described in Chapter 28. When moving objects are used, another input file is needed to either prescribe the motion of the object or to compute the body motion interactively. The input files for moving objects are described in Chapter 12. The possibility to couple ComFLOW to other codes by means of the ComFLOW library is described in Chapter 25.

All post-processing data files that have been created during a ComFLOW run can be visualized using the MATLAB graphical user interface (GUI). This GUI is described in a separate manual which can be found in the subdirectory documentation/. Each ComFLOW distribution comes packed with a set of simulation examples. These can be found in the subdirectory examples/. Together with the example input files you can find documents that give background information and highlight the relevant parts of the input and output files.

Note

Important remarks for using ComFLOW version 4.3.1

  • The GABC boundary condition is only functional for one-phase simulations (see fixed_form_input/gabc).
  • The restart facility has been improved significantly with respect to older versions and can be considered to be stable. However, ASCII output files and monitor files are not correctly restored yet, which may result in overlapping output.

Footnotes

[1]To use all functionality of the ComFLOW package, (E)GLIBC version 2.12 or higher is required; to see which version is installed on your system, type:ldd --version. On older operating systems use the static versions of the executables.
[2]Limited support: parallelization and coupling to external libraries may not work in this version.

References