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The crater formation on granular particle beds is important for engineering applications, chemical and process industries as well as for an explanation of related natural phenomena. In this article, experimental studies on the formation of a crater and the subsequent movement of granular particles are carried out. Granular beds consisting of mono-dispersed or poly-dispersed spherical glass-beads are subjected to an air-jet impingement. The impinging air-jet causes creation of craters of various sizes and shapes (such as saucer shape, parabolic shape, parabolic shape with an intermediate region, U shape, and craters with conical slants with a curved bottom surface). The experimental observations reveal two predominant regimes, categorized based on the crater stability, namely, a stable regime or an unstable regime. The mechanisms for the crater formation such as viscous erosion, diffused gas eruption, bearing capacity failure, and diffusion driven flow or combination of them are identified. It is observed that the steady-state depth of a crater increases linearly with an increase in the air-jet flow-rate. The temporal growth of crater depth shows logarithmic variation for a given flow rate. A regime map of the observed crater shapes is presented.

This article reports experimental insights into the physics of water entry of hydrophobic spheres. In the set of experiments, parameters such as sphere density, diameter, and impact velocity are varied. The trajectory of the sphere after impact, the dynamics of trapped air-cavity, including the cavity formation, and the retraction analysis are given. Furthermore, analysis of the Worthington-jet, the cavity ripple, and early bubble shedding after the air-cavity detachment is carried out. At the location of cavity closure, radial expansion and contraction behavior are reported for the case of the shallow seal (near the air–water interface), while for the deep seal, only one such behavior is observed. Further, five cavity shapes are recorded based on the cavity retraction behavior (i.e., shallow, deep seal), namely, conical shape, slender-cone shape, telescopic shape, spearhead shape, and the thick spearhead shape. The radial dynamics and radial surface energy analysis are reported at various locations on these cavity shapes to find that the thick spearhead cavities hold the most cross-sectional surface energy. The slender-cone shaped cavity generates the fastest Worthington-jet, followed by the telescopic shaped cavities. The thick spearhead shaped cavities are reported to have the longest intact Worthington-jets, followed by the spearhead shaped cavities. Finally, a new regime map is presented for single ripple and multiple ripple behaviors at the time of retraction in the wake of descending spheres. A bubble shedding behavior has also been characterized as the most frequent bubble shedding for shallow seal and associated longer bubble length compared to the other cases.

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Experiments have examined the phenomenon of direct contact condensation when steam is injected vertically into the subcooled water pool. The investigation is carried out by varying the steam mass flow rate and submergence depth of the steam injection pipe in the range of 10-50 kg/hr and 1-13 cm, respectively. The behavior of the bubble that appeared at the pipe outlet, transient heat transfer coefficient, pressure variation in the steam injection pipe, and its associated frequency have been analyzed. The images captured by high speed-camera showed different bubble shapes. The overall cycle time of bubble evolution has decreased with an increase in the mass flow rate and increased with an increase in the pipe submergence depth. The time averaged heat transfer coefficient increased with an increase in the mass flow rate and decreased with the rise of the pipe submergence depth. The pressure drop within the steam injection pipe shows the parabolic variation with an increase in the mass flow rate and is slightly influenced by the submergence depth due to changes in interfacial structures within the pipe. The peak frequency associated with the pressure has increased with an increase in the mass flow rate and decreased with an increase in the pipe submergence depth at higher mass flow rates. The FFT (Fast Fourier Transform) of interfacial area of the larger bubble at the pipe outlet shows that the first peak frequency lies between 0.5-5 Hz, and the second peak frequency lies in the range of 25-30 Hz.