Author(s): Rafael Bueno, Lucas Hoeltgebaum, Geovana Colombo, Michael Mannich, Tobias Bleninger
Keywords: Internal waves; Density mixing; Density currents; Bed conditions; Stratification conditions;
Abstract: Gravity currents are a common phenomenon that occurs in ocean, lakes, and atmosphere. Many studies have explored the propagation of gravity currents in ideal situations, thus avoiding the comparison between the well-known theory and specific situations, such as when the gravity current flows in a roughness bottom or in a stratified ambient. This paper focuses on the experimental comparison between the gravity currents in different configurations and the density difference effect for each configuration. We ran out 13 laboratory experiments in different ambient configurations, unstratified and stratified ambient, and with smooth and rough bottoms. With respect to the two-layer ambient, we observed clearly the formation of internal waves through spectral analysis of velocity measurements in case of low speed gravity currents, which indicates that internal waves formed in front of the gravity current are influenced periodically by the current speed. Although we did not observe a real contribution of internal waves to vertical mixing, the energy is dissipated faster in these cases. In addition, we observed that the pycnocline prevents vertical mixing, and the disturbance occurs mainly within the second layer. With respect to the rough bottom, we observed lower mixing than other experiments. Furthermore, we concluded that the roughness elements have higher impact on the average velocity and mixing when the density difference gets higher. For small density difference (Δρ ≅ 3.0 kg/m³), the system is practically unperturbed by the elements, following exactly the same behavior as observed in smooth bed experiments, which tend to conserve energy.