| One of the most common methods of reproduction in the sea is broadcast spawning where marine invertebrates release eggs and sperm into the ambient flow and fertilization occurs externally. Gamete coalescence at large scales is dominated by fluid stirring, and may be influenced by the presence of flow obstructions (e.g., coral heads, bed topography). The effect of turbulent wake behind a round obstacle on the second-order reaction between two initially distant scalars has been investigated by series of planar laser-induced fluorescence experiments. The scalars are released continuously, and are separated from each other by a lateral distance that initially impedes the reaction. The direct effect of the wake on mixing enhancement is determined by comparing segregation coefficient for cases with and without the cylinder wake. We measured mixing statistics for variety of flow regimes, streamwise locations, and scalar release geometries. This study suggests that the presence of turbulent obstacle wakes in spawning regions may substantially raise the efficacy of external fertilization. In addition, the effect of viscosity and non-Newtonian (shear-thinning) rheology on mixing and reaction between two initially distant scalars has been investigated. In this case, the ambient flow is pure water, but the scalar solutions include Xanthan gum to alter their rheology. Results indicate that mixing and reaction rates in the low-Damkohler limit between the two scalars plumes increase with the increase in viscosity of the scalars. The results of this study have broad implications for biological and ecological mixing processes involving now-Newtonian fluids. |
