Theory of pressure acoustics with thermoviscous boundary layers and streaming in elastic cavities

We present an effective thermoviscous theory of acoustofluidics including pressure acoustics, thermoviscous boundary layers, and streaming for fluids embedded in elastic cavities. By including thermal fields, we thus extend the effective viscous theory by Bach and Bruus [J. Acoust. Soc. Am. 144, 766 (2018)]. The acoustic temperature field and the thermoviscous boundary layers are incorporated analytically as effective boundary conditions and time-averaged body forces on the thermoacoustic bulk fields. Because it avoids resolving the thin boundary layers, the effective model allows for numerical simulation of both thermoviscous acoustic and time-averaged fields in three-dimensional models of acoustofluidic systems. We show how the acoustic streaming depends strongly on steady and oscillating thermal fields through the temperature dependency of the material parameters, in particular the viscosity and the compressibility, affecting both the boundary conditions and spawning additional body forces in the bulk. We also show how even small steady temperature gradients (similar to 1 K/mm) induce gradients in compressibility and density that may result in very high streaming velocities (similar to 1 mm/s) for moderate acoustic energy densities (similar to 100J/m(3)). (C) 2021 Acoustical Society of America

Automated summary

This study presents an effective thermoviscous theory of acoustofluidics, including pressure acoustics, thermoviscous boundary layers, and streaming phenomena in fluids embedded in elastic cavities. By incorporating thermal fields, the effective model extends previous viscous theories and allows for numerical simulations of acoustofluidic systems. The research shows how acoustic streaming is influenced by thermal fields and material parameters, affecting both boundary conditions and body forces in the bulk.