Density-ratio effects on the capture of suspended particles in aquatic systems

Abstract

Particle capture, whereby suspended particles contact and adhere to a solid surface (a 'collector'), is important in a range of environmental processes, including suspension feeding by corals and 'filtering' by aquatic vegetation. Although aquatic particles are often considered as perfect tracers when estimating capture efficiency, the particle density ratio (ρ + ) -the ratio of the particle density to the fluid density -can significantly affect capture. In this paper, we use a numerical analysis of particle trajectories to quantify the influence of ρ + on particle capture by circular collectors in a parameter space relevant to aquatic systems. As it is generally believed that inertia augments the capture efficiency when the Stokes number (St) of the particles exceeds a critical value, we first estimate the critical Stokes number for aquatic-type particles and demonstrate its dependence on both ρ + and the Reynolds number (Re). Second, we analyse how efficiently circular collectors can capture neutrally buoyant (ρ + = 1), sediment-type (ρ + = 2.6) and weakly buoyant (ρ + = 0.9) aquatic particles. Our analysis shows that, for ρ + > 1, inertia can either augment or diminish capture efficiency, and inertial effects appear well before the critical Stokes number is reached. The role of particle inertia is maximised at Stokes numbers above the critical value and, for sediment-type particles, can result in as much as a fourfold increase in the rate of capture relative to perfect tracers of the same size. Similar but opposite effects are observed for weakly buoyant particles, where capture efficiency can decrease by 60 % relative to the capture of perfect tracers.

Lagrangian coherent structures and inertial particle dynamics

“…3 (d), (e) & (f). Since these are computed using the respective forward integrated flow maps, 15 0 , the ridges in contour represent material lines of exponential stretching when integrated forward in time. In other words, a pair of particles straddled along a ridge of iF-TLE are exponentially apart at t = 15.…”

Vortex-induced vibrations: a soft coral feeding strategy?

“…For a given Reynolds number, the particle density affects the capture efficiency depending on its size. Espinosa-Gayosso et al (2015) showed that for Re ∼ 100, a weakly buoyant particle slightly lighter than water (ρ = 0.9) would never beat the efficiency scored by a heavy sediment-type particle (ρ = 2.6) if it is big. They also showed that, conversely, the discrepancy between a weakly buoyant particle and a sediment-type particle disappears as long as the particle is small.…”

“…According to Espinosa-Gayosso et al. (2015), a sediment-type particle that is 2.6 times heavier than water achieves a capture efficiency equal to that of a perfect fluid tracer if the Stokes number is less than 0.1 at . For a typical simulation case at this Reynolds number, we have (e.g.…”

“…In this paper,ρ = 2, which we considered as the upper bound of densities of the existing food particles. According to Espinosa-Gayosso et al (2015), a sediment-type particle that is 2.6 times heavier than water achieves a capture efficiency equal to that of a perfect fluid tracer if the Stokes number Stk =ρR 2 Re/9 is less than 0.1 at Re ∼ 100. For a typical simulation case at this Reynolds number, we have Stk ∼ 0.06 (e.g.…”

On predicting particle capture rates in aquatic ecosystems

“…Capture due to diffusional deposition is typically negligible compared with direct interception, only becoming important for particles with diameters or less [ 27 ]. Inertial impaction tends to be significant only for large particles that are much denser than water (for example, suspended sediment particles with a size exceeding roughly 5% of the collector diameter, [ 28 ]). Direct interception is thus typically the dominant mechanism of contact between particles and collectors in aquatic systems [ 3 , 6 , 14 , 28 – 30 ], and is the focus of this study.…”