Settling and Transport of Actively Buoyant Particles

Firebrands are small pieces of burning material that are ejected into the air by wildfires and carried downstream by wind. Predicting where firebrands land is important because they can ignite new fires far downstream of the main fire edge. This project focuses on how the motion of burning particles, and especially their settling rate, are affected by changes in the air density near the hot particles. The hot firebrands heat the cooler surrounding air, which changes their effective buoyancy, allowing them to travel unexpectedly long distances. The project will measure experimentally the settling of particles and measure their transport in a turbulent flow. Buoyancy manipulation can occur in many other situations. For example, many marine organisms can change their buoyancy to position themselves in their stratified environment. Thus, the results of this work will lead to improved models for a wide range of problems. The project will provide opportunities for undergraduate and graduate students to participate in the research, which will help encourage a future workforce in science and engineering. The objectives of this award are to measure and characterize the settling and transport behavior of actively buoyant particles. Unlike inert particles, the effective density of actively buoyant particles can change depending on the local flow around them, leading to buoyancy forces that are dynamic and coupled to the flow. Motivated by spot ignition of wildfires, the active buoyancy in this project will arise from a reservoir of a buoyancy-enhancing scalar attached to the particle that can diffuse and advect into the surrounding fluid. The primary control parameter to be varied is the particle Grashof number, which quantifies the relative importance of the dynamic buoyancy forces and the viscous forces. The results will be expressed in nondimensional form and situated in the full parameter space of the problem to make them as useful as possible for follow-on research. They will also have immediate implications for modeling firebrand transport in wildfires. Current operational models for predicting firebrand transport and assessing spotting risk make many simplifying assumptions about the nature of firebrands that may not be valid, including the neglect of active buoyancy, and that have been shown to lead to qualitatively incorrect results. Thus, this research addresses a key knowledge gap in this important problem. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.