Efficient non-hydrostatic modelling of 3D wave-induced currents using a subgrid approach

Efficient non-hydrostatic modelling of 3D wave-induced currents using a subgrid approach

Rijnsdorp, D.P. (TU Delft Environmental Fluid Mechanics; University of Western Australia; Centre for Offshore Foundation Systems)
Smit, Pieter B. (Spoondrift Technologies, Inc.)
Zijlema, Marcel (TU Delft Environmental Fluid Mechanics)
Reniers, A.J.H.M. (TU Delft Environmental Fluid Mechanics)

2017-08-01

Wave-induced currents are an ubiquitous feature in coastal waters that can spread material over the surf zone and the inner shelf. These currents are typically under resolved in non-hydrostatic wave-flow models due to computational constraints. Specifically, the low vertical resolutions adequate to describe the wave dynamics – and required to feasibly compute at the scales of a field site – are too coarse to account for the relevant details of the three-dimensional (3D) flow field. To describe the relevant dynamics of both wave and currents, while retaining a model framework that can be applied at field scales, we propose a two grid approach to solve the governing equations. With this approach, the vertical accelerations and non-hydrostatic pressures are resolved on a relatively coarse vertical grid (which is sufficient to accurately resolve the wave dynamics), whereas the horizontal velocities and turbulent stresses are resolved on a much finer subgrid (of which the resolution is dictated by the vertical scale of the mean flows). This approach ensures that the discrete pressure Poisson equation – the solution of which dominates the computational effort – is evaluated on the coarse grid scale, thereby greatly improving efficiency, while providing a fine vertical resolution to resolve the vertical variation of the mean flow. This work presents the general methodology, and discusses the numerical implementation in the SWASH wave-flow model. Model predictions are compared with observations of three flume experiments to demonstrate that the subgrid approach captures both the nearshore evolution of the waves, and the wave-induced flows like the undertow profile and longshore current. The accuracy of the subgrid predictions is comparable to fully resolved 3D simulations – but at much reduced computational costs. The findings of this work thereby demonstrate that the subgrid approach has the potential to make 3D non-hydrostatic simulations feasible at the scale of a realistic coastal region.

Nearshore circulation
Non-hydrostatic modelling
Subgrid
SWASH
Wave breaking
Wave-induced currents

http://resolver.tudelft.nl/uuid:2fdef90e-96f1-42da-93fc-2f65245f8753

https://doi.org/10.1016/j.ocemod.2017.06.012

2019-07-04

1463-5003

Ocean Modelling, 116, 118-133

Institutional Repository

journal article

© 2017 D.P. Rijnsdorp, Pieter B. Smit, Marcel Zijlema, A.J.H.M. Reniers