Particle Tracking in High-Reynolds-Number Turbulence of Simple and Complex Fluids

Cornell University, Ithaca NY

Investigators

Abstract

We are conducting experiments to understand how turbulent flows mix and disperse particles. We will track small particles such as water droplets by means of very high speed cameras that can follow their trajectories. The particles will be illuminated by laser sheets and a novel "sled" will carry the camera along the side of the wind tunnel so that the complex motion of the particles can be observed. The resultant digital information will be used to dermine the details of the particle trajectories.These "Lagrangian" measurements differ from the more conventional "Eulerian " measurements in the same way that a small bug sitting on a particle in a turbulent river flow would have a different perspective than an observer looking at the flow from a bridge . The Lagrangian measurement is analogous to the bug in the flow and gives a much richer picture of the physics. Lagrangian measurements are in their infancy and they are needed to understand issues ranging from raindrop formation to pollution dispersion and industrial coagulation and mixing processes. A particular focus will be on particles that are heavier than the surrounding fluid, such as water drops in air. The results will have a number of broad impacts . The rate of rain drop formation in clouds is poorly understood. Conventional theory estimates that they take much longer to form from the nucleating particles than actually occurs in nature. It is thought that the high turbulence in clouds may force the particles to come together in regions of local shear , thereby enhancing collision rates. Understanding this problem will have an impact on the way we model cloud dynamics and this in turn will affect our modelling of global climate change. Another impact is understanding how particles such as pollutants or biological matter disperse in the atmosphere or in water systems. Here the turbulence causes the particles to become more dilute because they separate, but the rate at which they separate is still not well understood. The Lagrangian measurements will help solve both the cloud and the dispersion problems. They will also help us to understand the basic physics of turbulence.