Particles go with the flow

When dropping small particles into a turbulent fluid, we know from daily experience that the particles will be swept up in the swirling eddies and vortices of the fluid motion. But how will the particles be arranged in the flow, and does the addition of these particles smooth out the flow or make it more turbulent? These questions not only have industrial and technological implications, but are at the heart of our understanding of turbulence. It is also a puzzle that has resisted conventional fluid dynamics analysis. Now, in a paper published in Physical Review Letters, Tanaka and Eaton [1] of Stanford University have scrutinized a set of experimental measurements of particle motion in turbulent flows and find that a new dimensionless parameter, the particle momentum number Pa, can be used to assess whether particles will enhance or attenuate the turbulence.