## 1 Introduction  
Cavitation is a harmful phenomenon for hydraulic machineries such as pumps, as severe impacts are produced at bubble collapse [1; 2]. However, cavitation impacts are utilized for mechanical surface treatment in the same way as shot peening, and this is named \"cavitation peening\" [3; 4]. The great advantage of cavitation peening is that shots are not used in the peening process, as cavitation impacts are used in cavitation peening [5]. Thus, the cavitation-peened surface is less rough compared with the shot-peened surface, and the fatigue strength of cavitation peening is better than that of shot-peening [6]. In conventional cavitation peening, cavitation is generated by injecting high-speed water jet into water [3; 4], and a submerged water jet with cavitation is called a \"cavitating jet\". The cavitation peening is utilized for the impacts of cavitation collapses, and it is different from water jet peening, in which water column impacts are used. To use the cavitating jet for peening, it is worth understanding the mechanism of the cavitating jet. In a cavitating jet, cavitation is produced inside and/or outside a nozzle when sufficient pressure difference is applied across the nozzle. As [PERSON] reviewed, vortex structures in turbulent jets [7], ring vortices (0-mode), a single helical vortex (1 st mode) and double-helical vortices (2 nd mode) were observed in a cavitating jet [8; 9; 10]. In the case of a developed cavitating jet, which has been used for practical applications such as cutting, material testing, drilling and peening [11; 12; 13; 14; 15; 16; 17], cavitation clouds are shed periodically [18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. As it was reported that the lifetime of the cloud is a key factor in the aggressive intensity of the cavitating jet [28], the investigation of cavitation cloud shedding is very important. On the other hand, from the viewpoint of cavitation inception, a turbulent jet with cavitation was investigated [29; 30; 31; 32]. In the present review, in order for the cavitating jet to be used for practical applications, the main subject is the developed cavitating jet, as shown in Figure 1a. In Figure 1, white bubbles are cavitation bubbles, as a used flash lamp was placed at the same side of a camera. As shown in Figure 1a, the cavitation clouds are clearly observed in the cavitating jet in water. Normally, a cavitating jet is produced by injecting a high-speed water jet into a water-filled chamber, as mentioned above. To apply a cavitating jet for components and/or plants which cannot be put in the chamber, [PERSON] developed a cavitating jet in air by injecting a high-speed water jet into a low-speed water jet, which was injected into air without the water-filled chamber using a concentric nozzle [33; 34; 35; 36]. A typical cavitating jet in air by injecting a high-speed water jet into a low-speed water jet is shown in Figure 1b. Cavitation clouds are also observed in the water column of low-speed water jets of cavitating jets in air, as shown in Figure 1b. Even though the cavitating jet was in air, cloud shedding was observed [37; 38; 35]. In addition, in the case of the cavitating jet in air at optimum conditions, the wavy pattern of the low-speed water jet was observed, as shown in Figure 1b [34; 35; 37; 38]. The frequency of the wavy pattern is equal to the shedding frequency of the cloud [35]. Even for the cavitating jet in air, unsteady behavior is very important. In cavitating jets both in water and in air, whereas the cloud shedding frequency is several hundred Hz [18; 35; 39; 40], only a few severe impacts occur per second [34; 41; 42; 43] when cavitation impacts are measured by special-made PVDF transducers [41; 42]. Thus, in order to enhance the aggressive intensity of the cavitating jet in water and air for practical applications of cavitating jets, the unsteady behavior of the cavitating jet should be investigated.