Influence of particle's buoyancy on turbidity currents from continuous influx

We investigate experimentally the dynamics of particle-laden gravity currents originating from a continuous influx in a nearly steady regime. We seek to characterize the influence of suspended particles on the inner structure of the current, depending on their settling/rising properties. The compositional fluid for the current is obtained by adding salt (NaCl) to the ambient fluid of the density 0, the density difference of the current with the ambient fluid is noted 0. In the case of particle-laden gravity currents, we add plastic particles (diameter 500 m and density 1050 kg/m3) in a semi-dilute concentration ( ~ 1%). We can also change the settling/rising properties of the particles by choosing the density of the fluid in the current, the density ratio varying between R=0.98 (rising particles) and R=1.02 (settling particles). Once prepared, the compositional fluid, stored and continuously stirred in a large reservoir, is injected in a nearly two-dimensional long tank by a pump at fixed flow rate Q, long durations of injection are possible by using either a very long tank, or an open channel configuration. Two setups have been used, associated to turbulent regimes, with typical Reynolds number of the order of 103 to 104 respectively. Within the body of the current, the density/concentration profiles are estimated by light attenuation technique 1. Our results show that the front velocity Uf is well controlled by a characteristic velocity based on the buoyancy of the current and the flow rate per unit width, as shown in Figure 1(a). Depending on the height of the injection inlet h0, the current can exhibit different hydraulic features near the front, depending on the value of the Froude number at the inlet F0, defined as the ratio of Uf over (0/0 . g . h0)1/2, with g the gravity constant, being supercritical (F0>1) or subcritical (F0), extracted at 1.8m from the inlet for rising (red)-neutral (magenta)-settling (blue) particles. Density profile for a density current is indicated with a dashed (black) line. Shaded areas indicate fluctuations around the mean profiles. References1 Schneider et al. Investigation of particle laden gravity currents using the light attenuation technique. Exp. Fluids 64, 23 (2023).