Cole, B. C. (2016). Size-Dependence of Rotation Rates of Spheres and Disks in Turbulence. Retrieved from https://doi.org/10.14418/wes01.2.138
Time-resolved positions and orientations of anisotropic inertial-range particles are experimentally measured using simultaneous stereoscopic images from four high-speed cameras. Tetrads (four slender rods in tetrahedral symmetry) are used to probe the energy cascade across a range of scales and triads (three rods in triangular planar symmetry) are measured to probe for the first time the rotational behavior of inertial-range disks. It is shown analytically that small tetrads and triads rotate like spheres and disks, respectively, and this small-particle prediction is experimentally extended well into the inertial range. The measured rotation rates of tetrads are shown to qualitatively follow canonical inertial range scaling predictions derived using transverse velocity structure functions. An experimentally derived scaling constant is predicted from the data. Furthermore, a size-dependence of the strength of preferential alignment of disks with their rotation rates is suggested based on comparison with prior experimental and computational work on small particles.
Measuring of the rotation rate of rigid bodies in turbulence is a direct way of probing the energy cascade at the scale of the particle. Therefore, very accurate measurements of particle rotation rates can in principle be used to find higher moments of velocity structure functions, as has previously been done using multiple-point velocity correlations. The measurements presented in this thesis are the first step along this path, and it is suggested finally that rotation rate measurements are a more intuitive and direct way to approach the subject of turbulent energy scaling for researchers and students alike.