Sowmitra Singh 1 e-mail: sowmitra@dynaflow-inc.com Jin-Keun Choi Georges L. Chahine Dynaflow, Inc., Jessup, MD 20794 Characterization of Cavitation Fields From Measured Pressure Signals of Cavitating Jets and Ultrasonic Horns Cavitation pressure fields under a cavitating jet and an ultrasonic horn were recorded for different conditions using high frequency response pressure transducers. This was aimed at characterizing the impulsive pressures generated by cavitation at different intensities. The pressure signals were analyzed and statistics of the amplitudes and widths of the impulsive pressure peaks were extracted. Plots of number densities and cumulative numbers of peaks as functions of peak amplitude, peak width, and the power of the ultrasonic horn or the jet were generated. The analysis revealed the dominance of pulses with smaller amplitudes and larger durations at lower cavitation intensities and the increase of the amplitudes and reduc- tion of the pulse widths at higher intensities. The ratio of the most probable peak amplitude to peak width was computed. A representative Gaussian curve was then generated for each signal using a characteristic peak amplitude and the corresponding most probable peak du- ration/width. This resulted in a proposed statistical representation of a cavitation field, useful to characterize cavitation fields of various intensities. [DOI: 10.1115/1.4024263] 1 Introduction Prediction of cavitation erosion on propellers, ship structures, and in general on any structure subjected to cavitation is of great interest to many industries. However, this task is often difficult and selection of new materials or material protection coatings that are cavitation erosion resistant is instead most often based on lab- oratory testing using accelerated erosion methods. These aim at comparing within short time periods the resistance of a new mate- rial relative to other standard materials. Erosion in the real field occurs over long durations of exposure, while accelerated erosion tests, by definition, involve subjecting the material to an erosion field that is significantly more “intense” than the actual cavitation that the studied material will be subjected to. The validity of such an approach is however not obvious, as it has been observed that the relative resistance of two materials can be different at different “intensities” of cavitation [1,2]. However, the definition of cavita- tion “intensity” is not universal. One classical definition [3,4] using an integral quantity is based on a concept similar to that of the acoustic intensity. This expresses the cavitation intensity at a selected point on the material subjected to cavitation as, ð1=qcÞ P N 1 P 2 i Dt i where N is the total number per unit time of im- pulsive loads of amplitude P i and duration Dt i ; q is the liquid den- sity, c is the sound speed in the liquid and qc is the liquid acoustic impedance. With this definition, an infinite number of configura- tions can result in the same intensity; namely one can achieve in an accelerated erosive test the same intensity by either increasing the amplitude of the impulsive loads, P i , for the same N, or increasing the frequency, N, for the same P i . It is however, obvious, from the material response viewpoint that these two acceleration types may not result in the same outcome. For instance, increasing the frequency a lot while using an amplitude well below the material limit strength will not result in any signifi- cant damage, while a few blows well above the material limit strength will. In this paper, we aim at developing an understand- ing of the distribution of impulsive loads characteristics of the two popular accelerated erosion testing methods: the cavitating jet method (e.g., ASTM G134 standard [5]) and the ultrasonic method (i.e., ASTM G32 standard [5]) in order to develop the knowledge base needed to conduct intelligent well-controlled tests. Such a characterization should also enable a better descrip- tion of the degree or level of advancement of cavitation in a given flow field. Any cavitation field, irrespective of its configuration and origin can be described as being comprised of numerous individual cavi- tation events; each event corresponding to the explosive volume growth and violent collapse of single or multiple bubbles or bub- ble clouds. These events are known to be accompanied with sharp pressure peaks and impulsive loads on nearby structures are well- known since the early works of Besant in 1859 [6] and Lord Ray- leigh in 1917 [7] on isolated bubbles. More recently, the dynamics of bubble clouds have been shown to result in extremely high pressures and intense erosion. This is presently recognized as the most aggressive or erosive form of cavitation and has been the subject of many studies starting with the pioneering works of Mørch [8], d’Agostino and Brennen [9], and Chahine [10]. These studies have shown that bubble collective effects in the cloud result in much enhanced bubble collapse pressures exerted over longer periods of time. Cloud cavitation has been extensively studied since then; see for example Refs. [11–13]. In order to study cavitation erosion in a controlled environment and in an accelerated manner, several laboratory techniques to generate cavitation have been devised by the community. These techniques involve the utilization of ultrasonic vibration to gener- ate the cavitation, cavitation flow loops with strong separating flows, rotating disks, vortex generators, and submerged cavitating jets. Ultrasonic horns are used to generate cavitation on sample material surfaces and have significant applications in material cavitation erosion resistance testing [14–16]. Ultrasonic horns typ- ically have a fixed vibration frequency but variable vibratory am- plitude. The advantage of these vibratory systems is that they can generate cavitation erosion in a quiescent liquid. The cavitating jet technique is also popular due to its flexibility of adjusting in a wide range the cavitation intensity [17–21]. Some of the afore described techniques were standardized and resulted in American 1 Corresponding author. Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received November 19, 2012; final manuscript received April 12, 2013; published online June 6, 2013. Assoc. Editor: Olivier Coutier-Delgosha. Journal of Fluids Engineering SEPTEMBER 2013, Vol. 135 / 091302-1 Copyright V C 2013 by ASME Downloaded From: http://fluidsengineering.asmedigitalcollection.asme.org/ on 10/04/2013 Terms of Use: http://asme.org/terms