## 1 Introduction  
Cavitation, when it occurs, is a cause of many detrimental effects on marine propellers and turbomachinery. Apart from performance degradation, it may trigger erosion, cause vibration problems, or result in high-intensity noise emission on a broad frequency range. Different types of cavitation involve complex features such as bubbly flow, bubble coalescence, nucleation, free-surface instability, and turbulence. These features do not necessarily obey a single but multiple length and time scales, which renders their reliable prediction, either experimentally or numerically, a major undertaking. For instance, when investigating the performance of marine propellers, depending on their operating conditions, many different forms of cavitation can take place simultaneously, such as steady attached sheet cavitation near the blade's leading edge, cloud cavitation in the propeller's wake, and vortex cavitation originating from the blade tips and the propeller hub. Even in the case of steady uniform inflow conditions, we can observe unsteady behavior of the cavitation. In particular, the shedding behavior of sheet cavitation (Figure 1) and its subsequent transformation to cloudy and bubbly cavities are important in the generation of the aforementioned erosion and noise issues, since upon collapsing these cavitation types can yield strong pressure fluctuations and even shock waves in the bubbly mixture or in the liquid phase [1,2]. A sketch of the shedding cycle of two-dimensional sheet cavitation is shown in Figure 1. The first frames depict the growth of the attached sheet cavity following the break-off of the previous sheet. The growth will cease after a certain length of the sheet cavity, and a strong re-entrant jet is formed at the closure of the sheet cavity, as depicted in Frame 3. The following frame shows the impingement of the cavity interface by the jet, as the jet reaches the cavity near the leading edge. The rear part of the sheet cavity breaks off and reduces to a bubble cloud, which is convected downstream as shown in Frame 5. As the cloud travels downstream, parts of it may undergo a violent collapsing stage, possibly causing noise and material erosion. Meanwhile, as seen in Frame 6, the newly formed sheet cavity on the foil behind the LE will begin to grow, initiating a subsequent break-off cycle. The dynamics of the sheet cavitation are mainly determined by the re-entrant jet, although a bubbly shock wave can in certain conditions affect this development [1]. If the sheet cavity is thick, the break-off cycle is relatively periodic with approximately constant shedding frequency. In these cases, a Strouhal number \(St=fl_{ref}/U_{ref}\) can be determined. The length scale of the Strouhal number is chosen as the maximum length of the cavity. This can be argued to be a typical length scale of the cavity since the shedding frequency depends on the distance the re-entrant jet has to travel. The choice of the velocity scale is somewhat problematic, but can be taken as the free-stream velocity \(U_{ref}=U_{\infty}\), or based on an approximation of the re-entrant jet velocity \(U_{ref}=U_{\infty}\sqrt{1+\sigma}\), where \(\sigma\) is the cavitation number, which acknowledges the presence of a cavity structure in a better way than \(U_{\infty}\) alone [4]. The frequency, \(f\), is based on global shedding of the cavity clouds, determined with a visual inspection of the cavity shedding periodicity, with the help of time histories of the measured lift and drag forces. Typically, the Strouhal numbers for sheet-cavitation shedding vary as \(0.20\dots 0.50\), depending on the investigated conditions and the definition for the reference quantities [4, 5, 6, 7]. In frames 5-6 of Figure 1 we observe that the detached cavity clouds convect downstream toward a higher-pressure region. Upon reaching the regions of sufficiently higher ambient pressure, the detached  
Figure 1: Sketch of the shedding mechanism of sheet cavitation. The figure illustrates one break-off cycle, adapted from [3].