Experimental Analysis of Gravity Currents on Smooth and Rough Bottom and in an Unstratified and Stratified Ambient

Gravity currents are a common phenomenon that occurs in ocean, lakes, reservoirs, atmosphere, and in many industrial processes. 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, covering a range of Δρ = 1 to 30 kg/m³ in three different ambient configurations: an unstratified ambient with smooth bottom, a stratified ambient with smooth bottom, and an unstratified ambient with rough bottom. With respect to the two-layer ambient, we observed clearly the formation of internal waves in case of S=(ρ₂-ρ₁)/(ρ_c-ρ₂) > 0.75 and, using spectral analysis, also when S > 0.68, which indicates that internal waves formed in front of the gravity current, which also influences periodically the current speed. Although we did not observe a real contribution of internal waves to vertical mixing, the energy is dissipated faster in this cases, and consequently a gently deceleration of the gravity current occurs. 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. This suggests that in many natural environments, given the small density difference, the gravity currents are umperturbed by the small irregularities of the topography.