Interdisciplinary studies of dust-drag fluid instabilities in protoplanetary discs
In the cotext of dust-infused turbulent proto-planetary discs, fluid instabilities are considered a critical mechanism that cause accretion and help to form planetary precursors, known as planetesimals. The formation of planetesimals cannot be directly observed in protoplanetary discs, and so this topic relies upon expertise in theoretical and experimental flow dynamics.
Theoretical predictions
Simulation of a flow instability that is driven by the collective aerodynamic drag of dust particles in a dilute fluid. From (Lambrechts et al., 2016).
The way that solids bind together at different length scales in protoplanetary discs is hypothesized to be controlled by hydrodynamic instabilities arising from particle-drag dynamics. Simiplified simulations were run to isolate the mechanism of the aerodynamic particle clustering and to make predictions that could be tested experimentally.
Experiments in a system with properties equivalent to the simulation parameters revealed a spontaneous clustering dynamic and collective particle drag reduction effects, that are characteristic of the predicted particle-drag induced fluid instabilities.
This work was conducted in collaboration with The Max Planck Institute for Dynamics and Self-organization, Germany, Lund University, Sweden, and the Technical University of Braunschweig, Germany.
References
2019
Observation of aerodynamic instability in the flow of a particle stream in a dilute gas
Holly L. Capelo, Jan Moláček, Michiel Lambrechts, and 5 more authors
Forming macroscopic solid bodies in circumstellar discs requires local dust concentration levels significantly higher than the mean. Interactions of the dust particles with the gas must serve to augment local particle densities, and facilitate growth past barriers in the metre size range. Amongst a number of mechanisms that can amplify the local density of solids, aerodynamic streaming instability (SI) is one of the most promising. This work tests the physical assumptions of models that lead to SI in protoplanetary discs (PPDs). We conduct laboratory experiments in which we track the three-dimensional motion of spherical solid particles fluidised in a low-pressure, laminar, incompressible, gas stream. The particle sizes span the Stokes- Epstein drag regime transition and the overall dust-to-gas mass density ratio, ɛ, is close to unity. A recently published study establishes the similarity of the laboratory flow to a simplified PPD model flow. We study velocity statistics and perform time-series analysis of the advected flow to obtain experimental results suggesting an instability due to particle- gas interaction: (i) there exist variations in particle concentration in the direction of the mean relative motion between the gas and the particles, that is the direction of the mean drag forces; (ii) the particles have a tendency to “catch up” to one another when they are in proximity; (iii) particle clumping occurs on very small scales, which implies local enhancements above the background ɛ by factors of several tens; (iv) the presence of these density enhancements occurs for a mean ɛ approaching or greater than 1; (v) we find evidence for collective particle drag reduction when the local particle number density becomes high and when the background gas pressure is high so that the drag is in the continuum regime. The experiments presented here are precedent-setting for observing SI under controlled conditions and may lead to a deeper understanding of how it operates in nature.
2016
Spontaneous concentrations of solids through two-way drag forces between gas and sedimenting particles
M. Lambrechts, A. Johansen, H. L. Capelo, and 2 more authors
