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

TL;DR

This paper develops an extended thermoviscous acoustic theory for fluids in elastic cavities, incorporating thermal effects and boundary layers to enable efficient 3D simulations of acoustofluidic systems and analyze thermal influences on streaming.

Contribution

It introduces an effective thermoviscous model that includes thermal fields as boundary conditions, extending previous viscous theories and allowing for detailed 3D simulations of thermoviscous acoustofluidic phenomena.

Findings

Thermal fields significantly influence acoustic streaming velocities.

Small temperature gradients can induce high streaming velocities.

The model enables efficient numerical simulation of complex thermoviscous acoustic systems.

Abstract

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 3D 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 (1 K/mm) induce gradients in compressibility and density that may result in very high streaming velocities (1 mm/s) for moderate acoustic energy densities (100 J/m^3).