Incubation pit analysis and calculation of the hydrodynamic impact pressure from the implosion of an acoustic cavitation bubble

Abstract

An experimental study to evaluate cavitation bubble dynamics is conducted. The aim is to predict the magnitude and statistical distribution of hydrodynamic impact pressure generated from the implosion of various individual acoustic cavitation bubbles near to a rigid boundary, considering geometrical features of the pitted area. A steel sample was subjected to cavitation impacts by an ultrasonic transducer with a 5mm diameter probe. The pitted surface was then examined using high-precision 3D optical interferometer techniques. Only the incubation period where surface is plastically deformed without material loss is taken into account. The exposure time was adjusted in the range of 3-60 s to avoid pit overlapping and a special procedure for pit analysis and characterisation was then followed. Moreover, a high-speed camera device was deployed to capture the implosion mechanisms of cavitation bubbles near to the surface. The geometrical characteristics of single incubation pits as well as pit clusters were studied and their deformation patterns were compared. Consequently, a reverse engineering approach was applied in order the hydrodynamic impact pressure from the implosion of an individual cavitation bubble to be determined. The characteristic parameters of the cavitation implosion process such as hydrodynamic impact pressure and liquid micro-jet impact velocity as well as the hydrodynamic severity of the cavitation impacts were quantified. It was found that the length of the hypotenuse of the orthographic projections from the center of the pit, which basically represents the deformed area of the pit, increases with the hydrodynamic impact aggressiveness in a linear rate. Majority of the hydrodynamic impacts were in the range of 0.4-1 GPa while the corresponding micro-jet velocities were found to be in the range of 200-700 m/s. Outcomes of this study, contribute to further understanding the cavitation intensity from the implosion of acoustically generated bubbles and could certainly represent a significant step towards developing more accurate cavitation models.

The ability of using the cavitation phenomenon as a tool to modify the surface characteristics in micro- and in nano-level

“…here the damage process is shifted to much longer times. Others reported similar findings with other techniques (laser and ultrasound generated cavitation) [40,41].…”

Evaluation of Cavitation Erosion Behavior of Commercial Steel Grades Used in the Design of Fluid Machinery

“…Figure 11 presents data normalized to the maximum value of the accumulated strain energy with the volume loss according to Eqs. [3] and [4] with the data taken from Table II and Figure 8 in the case of brittle behavior (Figure 11(a)) and in the case of elastoplastic behavior (Figure 11(b)). The AISI 52100 steel exhibits the best cavitation erosion performance among the tested steel samples, i.e., for the same time exposure to cavitation damage (i.e., 8 hours) much more accumulated strain energy is required in order to remove the same amount of material.…”

“…[1,2] When cavitation bubbles implode close to a surface, powerful micro-jets of velocity in the range of 300 to 1000 m/s with hydrodynamic impact pressures of more than 1 GPa are produced. [3,4] These micro-jets in combination with the pressure waves emitted during the implosion of cavitation bubbles promote the formation of incubation pits of various sizes and shapes across solid surfaces. [5][6][7] The cyclic nature of the surface stress leads to the coalescence of the incubation pits and consequently to the typical cavitation erosion damage (i.e., formation of deep craters and cracks).…”

Effect of Input Power and Temperature on the Cavitation Intensity During the Ultrasonic Treatment of Molten Aluminium

“…Bubbles then grow during the rarefaction phase and they collapse in the compression phase of the wave. In the vicinity of collapsing bubbles, extreme temperatures (>10000 K) [2], pressures (>400 MPa) [3], and cooling rates (>1011 K/s) [4] occur. To establish a condition where cavitation occurs, a peak to peak amplitude of about 10 μm at 20 kHz, corresponding to acoustic pressures of greater than 0.5 MPa at frequencies of 20 kHz, is required [5][6].…”