An operational dispersion model for wind over swell seas

FFI-Report 2024

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Espen Åkervik Andreas Nygård Osnes
In this report, we develop an operational contaminant transport model suitable for offshore conditions. Although the model is, in principle, independent of sea state, we expect it to be most useful for swell sea calculations, that is, for long and fast waves. The model is based on tracking of Lagrangian particles with turbulent motion stemming from a random walk procedure. As in standard operational models, the mean wind is given as a simple logarithmic vertical profile. In this model, the novel element is the addition of quasi-laminar wave-correlated forcing. In order to properly model the influence of waves on the flow of the atmospheric boundary layer, it is generally necessary to utilize high fidelity computational fluid dynamics models (CFD models). These models lead to computationally demanding simulations, which makes them infeasible for operational dispersion modeling. For a less demanding alternative, we extend the laboratory-scale framework proposed by Åkervik and Vartdal in “The role of wave kinematics in turbulent flow over waves” (2019) to create a simple flow model on an operational scale, in other words on spatial scales ranging from meters to kilometers. This model does not fully model the influence of waves on the flow, but we believe that it covers the most important features from a dispersion point of view. The resulting dispersion problem can be solved on a regular computer in minutes. We develop and test our transport model on a laboratory scale, for which we have high-fidelity reference data. Thereafter, we extend the model to full scale and compare it with a Gaussian puff model as well as published high-fidelity data. We demonstrate that the model yields reasonable results and that it can describe the effect of swell waves on the dispersion of aerosols.

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