The behavior of spheres in non-steady translational flowhas been studied experimentally for values of Reynolds numberfrom 0.2 to 3000. The aim of the work was to improve ourqualitative understanding of particle transport in turbulentgaseous media, a process of extreme importance in powerplants and energy transfer mechanisms.
Particles, subjected to sinusoidal oscillations parallelto the direction of steady translation, were found to have changesin average drag coefficient depending upon their translationalReynolds number, the density ratio, and the dimensionlessfrequency and amplitude of the oscillations. When the Reynoldsnumber based on sphere diameter was less than 200, theoscillation had negligible effect on the average particle drag.
For Reynolds numbers exceeding 300, the coefficient ofthe mean drag was increased significantly in a particularfrequency range. For example, at a Reynolds number of3000, a 25 per cent increase in drag coefficient can be producedwith an amplitude of oscillation of only 2 per cent of the spherediameter, providing the frequency is near the frequency at whichvortices would be shed in a steady flow at the mean speed. Flowvisualization shows that over a wide range of frequencies, thevortex shedding frequency locks in to the oscillation frequency.Maximum effect at the natural frequency and lock-in show that anon-linear interaction between wake vortex shedding and theoscillation is responsible for the increase in drag.
