A numerical study of thermoviscous effects in ultrasound-induced acoustic streaming in microchannels

TL;DR

This study numerically investigates thermoviscous effects on ultrasound-induced acoustic streaming in microchannels, highlighting the influence of channel height and validating results with analytical solutions.

Contribution

It introduces improved numerical methods and detailed analysis of thermoviscous effects on acoustic streaming in microfluidic channels.

Findings

Channel height influences streaming flow magnitude.

Numerical validation against analytical solutions achieved.

Enhanced numerical scheme improves performance and accuracy.

Abstract

We present a numerical study of thermoviscous effects on the acoustic streaming flow generated by an ultrasound standing-wave resonance in a long straight microfluidic channel containing a Newtonian fluid. These effects enter primarily through the temperature and density dependence of the fluid viscosity. The resulting magnitude of the streaming flow is calculated and characterized numerically, and we find that even for thin acoustic boundary layers, the channel height affects the magnitude of the streaming flow. For the special case of a sufficiently large channel height we have successfully validated our numerics with analytical results from 2011 by Rednikov and Sadhal for a single planar wall. We analyze the time-averaged energy transport in the system and the time-averaged second-order temperature perturbation of the fluid. Finally, we have made three main changes in our previously published numerical scheme to improve the numerical performance: (i) The time-averaged products of first-order variables in the time-averaged second-order equations have been recast as flux densities instead of as body forces. (ii) The order of the finite element basis functions has been increased in an optimal manner. (iii) Based on the International Association for the Properties of Water and Steam (IAPWS 1995, 2008, and 2011), we provide accurate polynomial fits in temperature for all relevant thermodynamic and transport parameters of water in the temperature range from 10 C to 50 C.