Resolving the three-dimensional structure of particles that are aerodynamically clustered by a turbulent flow

We report the first definitive experimental measurement of the four-dimensional (three spatial and one temporal) structure of particles that are aerodynamically clustered. High-speed tomographic imaging of a particle-laden turbulent flow was utilized to detect the temporal evolution of particle clusters at the exit of a long pipe. The measurements confirm that the particle clusters are coherent and ropelike in shape, rather than sheetlike, resolving a question that was not possible to address from previous two-dimensional measurements. These clusters are present right from the exit plane, where they are preferentially located near the jet edge, suggesting that they are generated inside the pipe close to the pipe wall.