The motion of a small sphere in a rotating velocity field: A possible mechanism for suspending particles in turbulence

Abstract

The diffusion and transport of particles suspended in turbulent flows depend on the interaction between the particles and the turbulence. To investigate a possible particle‐turbulence interaction, we observed particle trajectories in a rotating fluid with a solid body velocity profile, as might occur in the core of a vortex or an eddy. The experiments were conducted with a series of single small spheres (with low particle Reynolds numbers) sinking or rising in a horizontal cylinder rotating about its central axis at a constant speed. Each sphere was found to follow a nearly circular orbit in a vertical plane perpendicular to the cylinder axis. The orbit center lay very near the horizontal plane through the axis of the cylinder and was at the point in the fluid where the velocity of the fluid was equal and opposite to the terminal velocity of the particle. The particle trajectories also evolved slowly, spiraling either inward or outward. A theoretical description of the particle motion shows that the two principal forces on the particle, fluid drag and gravity‐buoyancy, account for the circular motion. A force due to particle inertia (a centrifugal force from the center of its orbit), a small position‐dependent wall effect on drag, a very small force (also affected by the walls), and a very small centrifugal buoyancy force account for the long‐term inward or outward spirals. This kind of systematic interaction in which particles seek and remain in fluid closely opposing their own motion could have a role in the suspension of small particles in some turbulent flows.

The use (and misuse) of sediment traps in coral reef environments: theory, observations, and suggested protocols

“…Although small d material may not be observed on the seabed during the sampling period, it is likely suspended in waters over the reef, with the potential consequence of decreasing photosynthetically active radiation (PAR) and desorbing nutrients and/or contributing toxicants. Interestingly, the influence of sediment trap design and energetics on sediment trap collection rates were first addressed about three decades ago (e.g., Tooby et al 1977;Gardner 1980a, b;Butman et al 1986), yet sediment traps are still commonly used in shallow coral reef environments that typically are much more energetic than the deep sea. Many authors have suggested that sediment traps should not be employed in u f greater than 20 cm s -1 , yet typical current speeds in many coral reef environments often exceed 20 cm s -1 (e.g., Wolanski 1994;Lugo-Fernandez et al 1998;Storlazzi et al 2004).…”

“…2. Sediment traps should be deployed with their trap mouths as the highest point in the flow so the trap mouths are not downstream of turbulent wakes caused by the structure (e.g., posts, rebar) used to secure the trap in its location (e.g., Tooby et al 1977;Gardner 1980a, b;Butman 1986;Butman et al 1986;and, Gust et al 1996). 3.…”

“…Tooby et al (1977), Hargrave and Burns (1979), Gardner (1980a, b), Butman (1986), and Butman et al (1986) performed important research on sediment trap dynamics using theoretical calculations, laboratory tests, and field experiments. Tooby et al (1977), Gardner (1980a, b), Butman's (1986), and more recently Gust et al (1996) flume experiments suggested that particles in these scenarios are collected by a process of fluid exchange, rather than falling freely into a trap (Fig. 3).…”

Characteristics of clustered particles in skimming flows on a stepped spillway

“…When a bubble is trapped in a turbulent structure, the centrifugal pressure gradient moves the bubble inside the vortex core (Tooby et al 1977), where bubble-bubble interactions may further take place in a highly aerated flow (Sene et al 1994). The present results tended to indicate that the clustered bubble chord times were close to the typical bubble chord time.…”

“…The effects of coherent structures on solid particles were superbly illustrated by Tooby et al (1977) and further studies included Nielsen (1993) and Nielsen and Zeng (1998), while Elperin et al (2007) investigated the mechanisms of formation of small-scale aerosol in-homogeneities in the atmospheric turbulent flow.…”