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PIV Investigation on Lock-Exchange Gravity Currents Propagating over Roughness Elements

Author(s): Maria Rita Maggi; Claudia Adduce; Maria Eletta Negretti

Linked Author(s): Maria Rita Maggi, Claudia Adduce

Keywords: Gravity currents; Effects of bottom roughness; PIV; Laboratory experiments

Abstract: Gravity currents are buoyancy-driven flows generated by a density gradient due to a temperature or a salinity difference or solid particles in suspension. These dense water masses are generally located at high latitudes where the strong atmospheric cooling causes a temperature reduction, the formation of ice, and brine rejection, which induces an increase in salinity. It is therefore evident how these currents are an integral part of the thermohaline circulation and their water properties are of global importance. The understanding of the mechanism involved in such flows is of crucial importance for a proper modeling and for the development of risk mitigation measures. Hence, in the last decades, the dynamics of the gravity currents flowing over horizontal surfaces have been extensively investigated by theoretical models, high-resolution numerical simulations, and laboratory experiments. However, only recently the dynamics of these currents over rough bottoms have received attention. Despite all the research efforts achieved by the technological advancement in computational power and laboratory instruments, there is still a lack of knowledge about the dynamics of the gravity currents over rough surfaces. The main contribution of this study is to characterize the inner velocity of the lock released gravity currents and to assess the effect of the bed roughness on them. The research is conducted by performing laboratory experiments using the optical non-intrusive experimental technique of Particle Image Velocimetry (PIV). The laboratory experiments are conducted in 6m long Perspex tank, with horizontal bed and rectangular cross section of 25× 30 cm^2. A removable gate is placed at a distance 50cm from the left wall dividing the tank into two volumes. The two reservoirs are filled up to the same water depth. The left side of the tank is filled with salty water with initial density ρ1, while the rest of the tank is filled with an ambient fluid of density ρ0<ρ1 (∆ρ=5±0.5 cm s^(-2)). LEGO® Bricks are used as bottom roughness elements with a constant quadratic section and a constant spacing from each other. Three experiments are performed by varying the roughness elements dimension, maintaining constant all other variables; additionally a reference test with smooth bed condition is performed. The analysis of the instantaneous maps of the flow velocity shows that the bed roughness reduces the front and the streamwise velocity due to the introduction of an extra drag; moreover the speed of the current decreases as the height of the roughness increases. The frontal region of the current loses definition due to the roughness elements, indeed the high velocity regions within the dense flow are mostly positioned above the roughness level, while a recirculation patterns are observed between the elements that separate the main current from the bottom area.


Year: 2022

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