ComFLOW

Wave fields in the close vicinity (say, a few wave lengths) of a coastal structure (dike, breakwater, offshore platform, groynes etc.) can be very complex. Wave breaking, the generation of super- and sub-harmonics, wave set-down and set-up, shoaling and refraction all significantly transform the wave field nearby the structure. Obviously, these wave transformations affect the subsequent wave impacts on the structure. These impacts, on their turn, may cause damage to the structure or even complete failure. Also, processes related to wave impacts, like wave run-up on and wave overtopping, are strongly influenced by these wave field transformations. A computational model that is capable of accurate and efficient simulation of the complex wave field near, and the wave impacts on, coastal structures can be of great help to assess the quality of the structure. ComFLOW is such a model. ComFLOW is developed in cooperation with Rijksuniversiteit Groningen (owner of the code) and Marin.

Characteristics

ComFLOW is a Volume-of-Fluid (VOF) model. It solves the incompressible Navier-Stokes equations and the free-surface motion. ComFLOW has the following characteristics:

• For each computational cell, the ‘VOF’-function indicates which fraction of it is filled with liquid. The occurrence of unwanted ‘flotsam and jetsam’ (loose droplets that separate from the liquid, being a numerical artefact of the liquid displacement) is reduced by introduction of a local height function in the VOF-function. This allows for accurate simulation of breaking waves.
• ComFLOW can be applied in 2DV (‘flume’) and in 3D mode (‘basin’).
• Both a one-phase (water) and two-phase (water and air) option are present, with the latter option being particularly suited to account for the cushioning effect of air entrapment during wave impact.
• ComFLOW uses simple rectangular (Cartesian) grids, which has several advantages compared to other grid types (e.g. unstructured grids): a more accurate and crisp treatment of the free surface; easier grid generation; simpler and more efficient data structures. The employed cut-cell technique allows for the incorporation of arbitrarily shaped bodies.
• A staggered positioning of the primary variables (pressure and velocity) on the grid is used, which avoids the need to apply artificial remedies against unwanted pressure-velocity oscillations (so- called 2Dx waves).
• (under development) The interaction of the flow with a permeable structure is modeled in ComFLOW. It is parameterised by a Darcy coefficient times the flow velocity and a Forchheimer coefficient multiplied by the velocity squared. The parameterisation is added to the Navier-Stokes equations.

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