Rotational diffusion of particles in turbulence

Lay Abstract In aquatic environments, particles (e.g., sediment or microorganisms) tend to rotate as they move. For example, rotation is an important component of a microorganism's motion and search for food. Using two simple shapes, a sphere and a prolate ellipsoid (i.e., a football), we examined how shape affects rotation. To isolate the effects of rotation from the effects of settling, we made near neutrally buoyant particles (same density as water). We generated turbulence in a laboratory water tank and introduced hundreds of particles, one shape at a time. While the particles were tumbling in the flow, we measured their rotation by using stereoscopic particle image velocimetry (SPIV), a technique that uses a laser to light up tracers embedded in the particles and then takes photos in rapid succession to compute the point velocities inside the particle. Since every part of the body rotates at the same angular speed, we determined all components of the angular velocity from the velocities inside the particles. A series of SPIV images showed how rotation of the particle changes over time in the turbulence. Using the average of many different sets of images, we modeled the change in particle rotation over time as a random walk through the range of possible rotations. We found that ellipsoidal and spherical particles rotated with the same characteristic time, unaffected by shape difference. From this we concluded that the large scales of turbulence govern rotation, rather than the small scales; large eddies, which are unable to differentiate between spheres and ellipsoids, determine rotation.