The influence of particle rotation on wake stability at particle Reynolds numbers, Re_P < 300 - Implications for turbulence modulation in two-phase flows

The influence of particle rotation on the stability of the grain wake is investigated experimentally for particle Reynolds numbers less than 300. Particle rotation may be present in most industrial and geophysical two-phase flows and imparts significant differences to the structure of the wake when compared to cases where no rotation is present. An oil-filled flume has been used to investigate the dimensions of the wake region and frequency of wake eddy shedding for isolated, spherical and spheroidal particles at rotation rates up to 10 revolutions/second. A parameter, β, is defined which expresses the ratio of the peripheral surface velocity of the particle to the relative velocity between the two phases. For both shaped particles, the wake is destroyed and absent at β>0.5, whilst the size of the wake region is significantly smaller for 0 < β < 0.5 when compared to the wake dimensions at zero rotation. At 0 < β < 0.5 for the spherical grain, the frequency of eddy shedding is modulated and equal to the rotation rate, w, whilst for spheroidal grains this frequency is 2w as alternate wakes are shed into the flow as the particle adopts two oblate and prolate orientations in each revolution. At greater rotation rates, an increasingly dominant shear layer is formed on the underside of the grain and generated by fluid being dragged over the rotating particle and sheared against the freestream flow beneath the particle. Where the wake region is destroyed through particle rotation (β>0.5), turbulence enhancement in the freestream may still be present due to the generation of this lower shear layer. This work suggests that a principal mechanism of turbulence enhancement by large grains in two-phase flows involves the influence of particle rotation. If particle rotation is present, turbulence enhancement may occur at much lower particle Reynolds numbers than previously assumed, and at higher rotation rates turbulence enhancement of the freestream flow may occur in the absence of the wake region. Previous numerical modelling of two-phase flows which invokes wake instability to explain turbulence enhancement in the absence of rotation, may therefore require modification.