Observation of the sling effect

Abstract

When cloud particles are small enough, they move with the turbulent air in the cloud. On the other hand, as particles become larger their inertia affects their motions, and they move differently than the air. These inertial dynamics impact cloud evolution and ultimately climate prediction, since clouds govern the Earth's energy balances. However, we lack a simple description of the dynamics. Falkovich et al describe theoretically a new dynamical mechanism called the 'sling effect' by which extreme events in the turbulent air cause idealized inertial cloud particles to break free from the airflow (Falkovich et al 2002 Nature 419 151). The sling effect thereafter causes particle trajectories to cross each other within isolated pockets in the flow, which increases the chance of collisions that forms larger particles. We combined experimental techniques that allow for precise control of a turbulent flow with three-dimensional tracking of multiple particles at unprecedented resolution. In this way, we could observe both the sling effect and crossing trajectories between real particles. We isolated the inertial sling dynamics from those caused by turbulent advection by conditionally averaging the data. We found the dynamics to be universal in terms of a local Stokes number that quantifies the local particle velocity gradients. We measured the probability density of this quantity, which shows that sharp

High-resolution measurement of cloud microphysics and turbulence at a mountaintop station

“…Specifically, techniques for Lagrangian tracking of particles in turbulent flows have provided new perspectives on the behavior of inertial particles in turbulence (Ayyalasomayajula et al, 2006;Gibert et al, 2012;Bewley et al, 2013). Of course, there are deviations from idealized flow conditions achievable in a laboratory setting: for example, boundary conditions vary in time so that quasi-stationary conditions must be sought.…”

Collision rates of small ellipsoids settling in turbulence

“…Hence, if two particles are accelerated by different vortices they can reach the same point in space and time but at different velocities (Abrahamson 1975;Falkovich, Fouxon & Stepanov 2002;Wilkinson, Mehlig & Bezuglyy 2006;Falkovich & Pumir 2007;Bec et al 2010). This so-called sling effect has recently been observed in experiments (Bewley, Saw & Bodenschatz 2013).…”

Inhomogeneous distribution of water droplets in cloud turbulence

“…Thus formation of inhomogeneities of particles by turbulence is small-scale phenomenon, often disregarded due to finite resolution of instruments. Since it is at those scales that droplets collide then the study of the inhomogeneities is necessary to predict rain formation.Much progress was obtained in the study of nonuniform distributions of inertial particles in the flow when gravity is negligible [1][2][3][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. In the regime where the inertia is not too large, which is where turbulence is most relevant in the rain formation process [3], the paircorrelation function of concentration was demonstrated to obey a power-law with negative exponent signifying singular (fractal) structure formed by particles in space.…”