O’CONNOR, M.P.; Drexel Univ.: Importance of upstream flow obstructions to convective heat exchange in terrestrial ectotherms

Convective heat transfers can be dominant exchange pathways in terrestrial ectotherms. For animals in the field, estimated convective exchanges, whether based on empirical measurements or on biophysical models, depend on assumptions about air flows. Further, such estimates depend on the translation of results obtained with simple air flows into complex conditions in the field. Empirical and theoretical studies have shown that nearby objects (e.g., tree limbs for arboreal species and the substrate for ground dwellers) can alter air flows and convective exchanges from those seen in laboratory experiments. Another complication in the field is the presence of flow obstructions that disrupt air flows and temperature patterns. To delimit the import of such obstructions, I simulated air flows and heat exchanges near bushes or rocks on the ground. I used a finite element model of both laminar air flows and heat exchanges near a hypothetical lizard to examine the effects of wind speed, body and environmental temperatures, and proximity of the lizard to small or large flow obstruction on convective heat exchanges. The size of the lizard, the size of the flow obstruction (e.g., a relatively small rock vs a large bush), and the distance downwind from the obstruction all had noticeable affects on estimated convective exchanges. To some extent the sizes and distances could all be scaled to an independent length, i.e., doubling the size of the rock, the lizard and the downwind distance gave similar results. But because of wind speed and temperature gradients near the ground upstream of the rock or bush, some effects depended on absolute rather than relative distances. Convection disturbances extended for several diameters downstream of the rock or bush, suggesting that animals in complex thermal habitats may nearly always be in the wind-shadow of environmental objects.