Prediction of surface warming in corals using coupled irradiance and computational fluid dynamics modelling

Abstract

The WWF estimates that coral reefs are responsible for almost $30 billion USD net benefit to world economies. However, these reefs are under threat from coral bleaching, events which are have been a regular occurrence worldwide over the past two decades. While the reported causes of coral bleaching are varied, the most commonly identified is thermal stress caused chiefly by a combination of increased sea surface temperature and elevated levels of solar irradiance. Detailed analysis of the mechanisms responsible for thermally induced bleaching is difficult – in-situ studies are expensive and often prompt more questions than they answer, and laboratory studies generally fail to accurately replicate the conditions under which bleaching is observed. Conducting these studies using Computational fluid dynamics (CFD) can overcome a number of these difficulties. In this paper we present a method for coupling accurate ray-tracing techniques with CFD to replicate the conditions for coral bleaching. We use the physically derived software RADIANCE to determine the incident solar radiation on the coral, which is then used to calculate a volumetric heat source field for use in the CFD simulation. The CFD component is implemented using the OPENFOAM CFD libraries, and is developed to account for the flow through the porous medium, and heat transfer in the system. Finally, an illustrative example is presented to demonstrate the feasibility of the approach. A branching coral in cross-flow is simulated in the presence of solar radiation, and the temperature rise of the coral surface predicted. The irradiance at the surface and the resulting surface temperature rise are shown in Figure 1.