Abstract
The acoustic streaming behaviour below an ultrasonic sonotrode in water was predicted by numerical simulation and validated by experimental studies. The flow was calculated by solving the transient Reynolds-Averaged Navier-Stokes equations with a source term representing ultrasonic excitation implemented from the predictions of a nonlinear acoustic model. Comparisons with the measured flow field from Particle Image Velocimetry (PIV) water experiments revealed good agreement in both velocity magnitude and direction at two power settings, supporting the validity of the model for acoustic streaming in the presence of cavitating bubbles. Turbulent features measured by PIV were also recovered by the model. The model was then applied to the technologically important area of ultrasonic treatment of liquid aluminium, to achieve the prediction of acoustic streaming for the very first time that accounts for nonlinear pressure propagation in the presence of acoustic cavitation in the melt. Simulations show a strong dependence of the acoustic streaming flow direction on the cavitating bubble volume fraction, reflecting PIV observations. This has implications for the technological use of ultrasound in liquid metal processing.
Highlights
The complexity of non-linear acoustics and cavitation phenomena, the opaqueness, high temperatures and chemical reactivity of metallic melts hinder the study of ultrasonic melt processing
The coupled set of equations have been solved stably using a finite volume method and the results compare favourably with velocity vector fields measured by particular image velocimetry in water at two ultrasonic powers
A net upward flow on the model centreline was predicted at low sonication power and observed by experiment, which has previously been difficult to predict accurately because the effect of cavitation on the acoustic streaming pattern has been neglected
Summary
The complexity of non-linear acoustics and cavitation phenomena, the opaqueness, high temperatures and chemical reactivity of metallic melts hinder the study of ultrasonic melt processing. To improve the understanding of ultrasonic process effects on liquid metals, an extensive research program was undertaken over the past five years [1] to implement a numerical model that can realistically predict acoustic pressures and acoustic streaming in the melt in the presence of cavitation, and to validate the model experimentally. More accurate efforts in quantifying acoustic pressures in liquid aluminium [14,15] employ non-linear equations as suggested by van Wijngaarden [16] following previous successful implementation in water [17,18] These models are computationally expensive, as the dynamics of bubbles in each computational cell have to be resolved [10,15]. Recent progress in the theory on non-linear sound propagation [26–29] resulted in an easier-to-solve nonlinear Helmholtz equation to quantify the acoustic pressure field Louisnard extended his nonlinear model to account for acoustic streaming in the presence of cavitation with two-dimensional results comparing well with experiment [11]. Transducer powers and the likely implications for liquid metal processing are discussed
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