ENME Webcast Archives

LEADERS IN MECHANICAL ENGINEERING LECTURE SERIES - SPRING 2007

Origins of Irreversibility in Fluid Dynamics: Chaotic Particle Interactions and Chaotic Mixing

Lecturer: Jerry Gollub - Haverford College/University of Pennsylvania

Original Air Date: Friday, March 16th at 2:00pm

Abstract: Although much of the microscopic world is governed by time-reversible equations of motion, the macroscopic world is Irreversible. Precisely how does this happen in fluid dynamics? To elucidate it, we can consider the slow flow of a Newtonian fluid at very low Reynolds number, which is known to be reversible. Now consider fluids containing solid particles that are too large to exhibit Brownian motion, i.e. suspensions. The accepted wisdom is that they are also governed by reversible equations, but we have shown experimentally that the particles in a sheared suspension do not return to their starting positions if the fluid sheared beyond a certain threshold. Instead they appear to undergo a random walk after one or more cycles. I will compare the experiments with numerical simulations demonstrating that the particles interact chaotically with each other, and that the strength of the chaos grows dramatically above the threshold for irreversibility. The comparison illuminates the connections between chaos, reversibility, and predictability.[1] Finally, I will give a second example showing how irreversibility is responsible for the mixing or inter-penetration of two stirred fluids.

[1] This work was done jointly with D.J. Pine, J. Brady, and A. Leshansky.