# A Review on the Erosion Mechanism in Cavitating Jets and Their Industrial Applications
## 1 Introduction  
Cavitation involves a phase change and vapor bubbles formation that occur in a fluid system when the absolute static pressure reaches or drops below the vapor pressure of the liquid at a given temperature [1]. The vapor bubbles collapse when they move to a high-pressure region [2]. Shock waves that are formed when the vapor bubbles collapse propagate at the speed of sound through the liquid [3]. Undesirable effects can thus be produced such as vibration, noise, and loss in hydraulic system performance.  
The cavitation principle has been studied for well over a century and the term cavitation was first used for describing voids in liquid flows [4]. [PERSON] [5] noticed the formation of a white cloud and the presence of a noise similar to that of boiling when he studied the flow of water in the throat of a converging-diverging glass tube in 1984. [PERSON] and [PERSON] [4] were the first to conduct a proper investigation and claimed that cavitation can erode solid materials for certain flow situations. By the end of the twentieth century, research on the cavitation process in hydro-mechanical systems focused on a better understanding of this mechanism [6].  
With the advent of erosion processes, the concept of cavitation jets became important to understand in order to control it when it is harmful and use it as a beneficial mechanism for certain applications. A theory was proposed to show the impact of water hammer effects on pumps and turbines. Water vapor bubbles were said to be eroding the turbine blades due to a water hammer effect that consists of high-pressure waves in a flow when it encounters an abrupt obstruction because of flow hindrance or other solid material [4]. [PERSON] amended [PERSON]'s theory and calculated the compressibility of vapor bubbles during the collapse of the cavity. The collapsing cavitation bubbles develop the pressure for the cavitation process that can transmit energy for the erosion of the metal body of machines [7].  
In the past two decades, the focus was on understanding the composition and behavior of cavitation [8], developing techniques to reduce or prevent the damage it causes [9], and investigating techniques that can benefit from this phenomenon such as drilling, tunneling, cleaning, etc. [10]. Cavitation's positive effects are also used in high-power ultrasonic applications such as for destroying kidney stones in human bodies with the help of shock waves that occur because of the cavitation bubbles. Other applications of cavitating jets are the improvement of the fuel spray quality and spray cooling using a cavitating nozzle in fuel injectors to enhance the spray breakdown process [11]. This breakdown results in finer droplets and enhances fuel evaporation [12; 13; 14].  
Self-resonating cavitating jets, also called oscillating pulsed jets, were proposed in order to generate high cavitation inception due to strong pressure oscillations [15] as compared to conventional jets. They can be modelled by the theories of transient flows and hydro-acoustics of self-sustained flows [16]. Self-resonating jets were used for different mechanical drilling bits to increase their drilling penetration rate and make the drilling process more efficient [17]. These nozzles can be used in deep-hole drilling and chips could be removed with an advanced mechanism. All of these jets are passively structured and have great potential for drilling processes [18].  
In this paper, the working principle of structured resonating jets and their benefits over conventional nozzles are discussed. Large-scale vortex dynamics and the resulting strong pressure oscillations found in self-resonating jets, proposed in the literature, are investigated. Under specific flow conditions, the methodology to generate higher cavitation inception and higher rock cutting ability is discussed for multiple industrial applications.