Effects of impact energy on the crown formation and underwater cavity of free-falling thick disks

Abstract

Abstract A series of laboratory experiments was conducted to investigate the effects of impact energy on the crown formation and cavity dynamics of free-falling thick disks in stagnant water. To simulate the effects of impact energy on the dynamics of free-falling thick disks, four different release heights, hr, and six different disk masses were considered. A threshold limit of hr = 4do, where do is the disk diameter, was obtained as different patterns of cavity dynamics and crown characteristics were observed during the impact and descending of solid disks. The evolution of crown characteristics such as crown shape, diameter, and height with time were measured, and the pinch-off time and location of pinch-off were extracted from image analysis. Experimental results revealed that the crown characteristics at the pinch-off varied linearly with the impact Froude number and they were larger than solid spheres. A threshold value of non-dimensional impact energy to create a full seal crown was obtained which occurred when impact energy was more than 36% of the initial energy. The variations of crown diameter at the pinch-off indicated relatively larger crown diameters for hr/do > 4 whereas the crown height was invariant with release height. The results indicated a relatively shorter pinch-off depth in gravity-driven disks in comparison with the force-driven disks and gravity driven sphere in stagnant water. The energy losses due to impact, crown formation, and pinch-off were calculated for all tests and an adverse correlation was found between normalized energy losses and impact Froude number. Experimental observations indicated that disks with smaller impact energy dissipated more energy due to the impact. The time variations of the frontal position and velocity of disks showed that the threshold release height highly affected the trajectory, falling speed, and the duration of the descending process.