Understanding the Coupled Dynamics of Particles and Wall Turbulence
This work focuses on understanding the coupled interactions between large and heavy solid particles, on a particle bed, and a gaseous (air) carrier phase turbulent boundary layer developing over the bed. Here, large refers to particles with diameter greater than the dissipation scale and heavy refers to particles with density much larger than the carrier phase. This work will lead to a deeper understanding (and subsequently better predictive models) of particle-carrier phase interactions impacting a variety of fields from the geophysical sciences to traditional branches of engineering. Studying their coupled interaction requires synchronous measurements of both the carrier and particle phase dynamics at high spatial and temporal resolution. To tackle this challenge, the proposed work will use advanced optical diagnostic techniques to study these interactions, thus providing new insight into their physics. The proposed work is broadly divided into two parts. Part I – The incipient mobilization of particles by the carrier phase is currently predicted by various measures of the mean shear of the carrier phase velocity field. However, there is increasing evidence that particle mobilization is an inherently unsteady process better correlated with the unsteady carrier phase eddies. The proposed work seeks to systematically quantify and understand these unsteady aspects of particle mobilization, particularly as a function of the energy and scale size of the carrier phase eddies. The proposed approach is to introduce flow scales of controlled energy and scale size into a turbulent boundary layer developing over a particle bed, while methodically characterizing the subsequent initiation of particle mobilization. The properties of the particles, namely the diameter and density, will be varied. As the carrier phase is fixed (air), the proposed approach will then describe the processes of particle mobilization as a function of not only the carrier phase eddy energy and size but also the particle Reynolds and Stokes numbers. Part II – Once particles are mobilized, they form a saltating layer adjacent to the particle bed and become two-way coupled with the carrier phase flow. This interaction, thus far has been reported as modifications to the carrier phase turbulence statistics. However, the exact nature of this interaction has yet to be studied in any further detail. Specifically, the scale dependence or the energy transfer mechanism of this coupled interaction has yet to be described. To study this interaction, it is proposed to carry out careful measurements of the carrier phase turbulent boundary layer in the presence of a saltation layer. In addition, during the course of both parts of the proposed work, detailed, simultaneous measurements of both phases will be carried out, in a time-resolved manner, to describe the scale dependent characteristics of the underlying physics. This will involve establishing an instantaneous shear velocity that initiates particle mobilization as a function of particle properties as well as carrier phase eddy scale and energy. While studying the interactions during mobilization and after a saltating layer is formed, the goal will be to establish scale dependent energy transfer pathways between the carrier and particle phases. To this end, the primary measurement technique used to characterize the carrier phase will be particle image velocimetry (PIV), while the particle phase velocity fields will be measured using particle tracking velocimetry (PTV). These PIV/PTV measurements will use multiple cameras at multi-scale, providing a detailed description of both phases of the flow at high spatial and temporal resolution. Together these techniques will then provide unique multi-scale, multi-phase measurement sets that will capture the detailed interactions of the particle and carrier phase, leading to new insights into the physics of these interactions.