Simulation and measurement of nonlinear behavior in a high-power test cell

High-power ultrasound has many diverse uses in process applications in industries ranging from food to pharmaceutical. Because cavitation is frequently a desirable effect within many high-power, low-frequency systems, these systems are commonly expected to feature highly nonlinear acoustic propagation because of the high input levels employed. This generation of harmonics significantly alters the field profile compared with that of a linear system, making accurate field modeling difficult. However, when the short propagation distances involved are considered, it is not unreasonable to assume that these systems may remain largely linear until the onset of cavitation, in terms of classical acoustic propagation. The purpose of this paper is to investigate the possible nonlinear effects within such systems before the onset of cavitation. A theoretical description of nonlinear propagation will be presented and the merits of common analytical models will be discussed. Following this, a numerical model of nonlinearity will be outlined and the advantages it presents for representing nonlinear effects in bounded fields will be discussed. Next, the driving equipment and transducers will be evaluated for linearity to disengage any effects from those formed in the transmission load. Finally, the linearity of the system will be measured using an acoustic hydrophone and compared with finite element analysis to confirm that nonlinear effects are not prevalent in such systems at the onset of cavitation.

I. Introduction

High-Power ultrasonic systems are commonly used in many applications such as food processing [1], water treatment [2], microbe inactivation [3], cleaning [4], and cutting [5]. It is a particularly application-focused technology, although recently a large degree of interest has developed in the research community pertaining to more efficient systems [6], in-process applications [7], and system qualification methods [8]. At the heart of many of these applications is the generation of cavitation within the high-power cell [9]. The phenomenon of cavitation produces a series of dramatic effects that have a powerful effect on the liquid media in which it is induced, effects such as extremely high local temperatures (3000K) and pressures (10 GPa) [10]. These effects create a local environment that can directly influence chemical reactions, material properties, and even chemical structures [11]. Cavitation itself is likely to occur within regions of a test cell that exhibit higher intensities. Therefore, accurate knowledge of spatial pressure distribution within such cells can be valuable when attempting to optimize the location of such regions.