Development and Validation of Computational Fluid Dynamics Models for Prediction of Heat Transfer and Thermal Microenvironments of Corals Robert H. Ong 1 *, Andrew J. C. King 1 , Benjamin J. Mullins 1,2,3 , Timothy F. Cooper 4 , M. Julian Caley 5 1 Fluid Dynamics Research Group, Curtin University, Perth, Western Australia, Australia, 2 Atmospheric Environment Research Centre, Griffith University, Brisbane, Queensland, Australia, 3 Curtin Health Innovation Research Institute, Curtin University, Perth, Western Australia, Australia, 4 Australian Institute of Marine Science, UWA Oceans Institute, Perth, Western Australia, Australia, 5 Australian Institute of Marine Science, Townsville, Queensland, Australia Abstract We present Computational Fluid Dynamics (CFD) models of the coupled dynamics of water flow, heat transfer and irradiance in and around corals to predict temperatures experienced by corals. These models were validated against controlled laboratory experiments, under constant and transient irradiance, for hemispherical and branching corals. Our CFD models agree very well with experimental studies. A linear relationship between irradiance and coral surface warming was evident in both the simulation and experimental result agreeing with heat transfer theory. However, CFD models for the steady state simulation produced a better fit to the linear relationship than the experimental data, likely due to experimental error in the empirical measurements. The consistency of our modelling results with experimental observations demonstrates the applicability of CFD simulations, such as the models developed here, to coral bleaching studies. A study of the influence of coral skeletal porosity and skeletal bulk density on surface warming was also undertaken, demonstrating boundary layer behaviour, and interstitial flow magnitude and temperature profiles in coral cross sections. Our models compliment recent studies showing systematic changes in these parameters in some coral colonies and have utility in the prediction of coral bleaching. Citation: Ong RH, King AJC, Mullins BJ, Cooper TF, Caley MJ (2012) Development and Validation of Computational Fluid Dynamics Models for Prediction of Heat Transfer and Thermal Microenvironments of Corals. PLoS ONE 7(6): e37842. doi:10.1371/journal.pone.0037842 Editor: Sebastian C. A. Ferse, Leibniz Center for Tropical Marine Ecology, Germany Received September 6, 2011; Accepted April 29, 2012; Published June 11, 2012 Copyright: ß 2012 Ong et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors acknowledge both the Western Australia Supercomputing Hub (iVEC) and the Society for Underwater Technology (SUT) for external funding sources. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: robert.ong@postgrad.curtin.edu.au Introduction An increase in the magnitude and frequency of stress-induced coral bleaching in the past two decades is likely due to a variety of stressors [1]. The most common cause of coral bleaching is an elevation of sea surface temperature (SST) combined with elevated solar irradiance [2–4]. Because corals thrive close to their upper thermal tolerance threshold [5], bleaching is expected in response to a 1–2uC temperature increase over a prolonged period. Some coral species, however, bleach more readily than others [3,6]. While bleaching can be strongly correlated with SST, several experimental studies have also shown a clear difference between coral surface (tissue) temperature and SST [7,8]. This temperature divergence is likely due to the physics of heat transfer and fluid flow, coupled with other interacting phenomena, such as the influence of coral porosity and permeability, as well as differences in the structure and growth forms of different coral species. Here, we begin to explore the effects of these coupled processes using a computational fluid dynamics framework with a view to providing a better understanding of the role these parameters play in coral warming and resultant bleaching. The calcium carbonate skeleton of corals is predominantly composed of the mineral’s aragonite polymorph, which has a density of 2.94 g cm 23 [9,10]. The highly porous structure and permeability of coral skeletons, and the morphologies of their colonies, may play a significant role in determining coral surface temperatures. In spite of their potential importance, the influence of coral porosity and permeability and colony shape on coral thermal microenvironments and their roles in determining the susceptibility of corals to bleaching is yet to be properly addressed. Recent suggestions of changes in growth rates of massive and branching corals on the Great Barrier Reef [10,11] and West Australian Reefs [12] would indicate potential changes in bleaching susceptibility should these mechanisms prove to be important. Furthermore, the growth of coral reefs is highly dependent on the framework provided by corals and its degradation by physical, chemical and biological processes [13]. While bioerosion, predation, sedimentation and hurricanes can all reduce coral growth by damaging coral tissues, they may also affect any relationship between fluid dynamics and heat transfer, and consequently, the susceptibility of corals to bleaching. For example, the bioerosion of corals through boring, etching and grazing organisms, will lead to increased (local) skeletal porosity [13,14]. The mechanisms that underpin coral bleaching remain unclear, due in part, to the difficulty of obtaining accurate measurements and predictions from in-situ monitoring of the complex environ- ments experienced by corals in both time and space [1]. Meanwhile, laboratory studies can be confounded by the PLoS ONE | www.plosone.org 1 June 2012 | Volume 7 | Issue 6 | e37842