Periodic Reporting for period 1 - WE-EXPERTH (Parallel Water Entry of Hydrophilic/Hydrophobic Projectiles: Experimental and Theoretical Aspects)
Periodo di rendicontazione: 2022-08-01 al 2024-07-31
A solid object entering a fluid like water represents a classic case of fluid-structure interaction. The practical importance of water entry research extends beyond objects impacting a liquid surface, and it is also relevant for fixed or floating structures that are exposed to the incoming flow or surface waves. When an object impacts the water surface, it initially pushes the upper fluid layer radially outward, then accelerates the surrounding fluid downward as it descends into the water. This process displaces water with air, creating an air-filled cavity in the object's wake. During this dynamic interaction, several complex and nonlinear flow phenomena arise, including air-entraining cavities, splash crowns, cavity pinch-off, vortex shedding, etc. These phenomena influence the kinematics of the object's motion. The study of water entry has garnered extensive attention from academic and technical researchers due to its wide range of applications in various scientific and non-scientific fields. These applications include industrial processes such as inkjet printing, film coatings, and sprayed adhesives; naval architecture and marine engineering tasks like ship slamming and launching; aerospace engineering challenges such as seaplane landings and spacecraft water entry; sports applications like rowing oars and high diving; and natural phenomena observed in predaceous diving beetles, basilisk lizards, and diving seabirds.
Most studies on water entry focus on single water entry scenarios. However, the study of dual or multiple water entry—particularly parallel water entry, which is the focus of this research—is crucial in contexts such as the impact of parallel oars in rowing, lifeboat water entry, synchronized diving, and the interaction of ocean waves with adjacent offshore structures like oil rigs or wind turbine towers. In these situations, differences in size, shape, or surface characteristics add complexity to the problem. Parallel water entry is a highly nonlinear and unsteady process, with significant deviations from single water entry. The interactions affect air-entrainment cavities, cavity pinch-off, splash curtains, and jets, as well as the trajectories of the projectiles, making a detailed investigation both a scientific and an engineering necessity.
The main objectives of this project include studying cavity dynamics and the objects' kinetics/interaction in the context of parallel water entry of two spheres. This research demonstrates the significant impact of neighboring objects on the air cavity and splash sheet shape, spheres' trajectories, and even descent velocities. The results provide valuable information concerning the physics of the water entry process and can serve as reference cases for numerical studies of parallel water entry phenomena.
Most studies on water entry focus on single water entry scenarios. However, the study of dual or multiple water entry—particularly parallel water entry, which is the focus of this research—is crucial in contexts such as the impact of parallel oars in rowing, lifeboat water entry, synchronized diving, and the interaction of ocean waves with adjacent offshore structures like oil rigs or wind turbine towers. In these situations, differences in size, shape, or surface characteristics add complexity to the problem. Parallel water entry is a highly nonlinear and unsteady process, with significant deviations from single water entry. The interactions affect air-entrainment cavities, cavity pinch-off, splash curtains, and jets, as well as the trajectories of the projectiles, making a detailed investigation both a scientific and an engineering necessity.
The main objectives of this project include studying cavity dynamics and the objects' kinetics/interaction in the context of parallel water entry of two spheres. This research demonstrates the significant impact of neighboring objects on the air cavity and splash sheet shape, spheres' trajectories, and even descent velocities. The results provide valuable information concerning the physics of the water entry process and can serve as reference cases for numerical studies of parallel water entry phenomena.
In this research project, two spheres with a central distance ranging from 1.0 to 5.0 sphere diameters are simultaneously released from heights ranging from 15 to 95 cm. Experiments are conducted using hydrophobic and hydrophilic spheres with diameters of 20 mm, as well as differently-sized spheres with diameters of 12 mm and 20 mm. The original spheres have a smooth surface and exhibit hydrophilic (HPI) behavior. To create hydrophobic (SHP) spheres, some are coated with a superhydrophobic layer. The experiments are conducted in an acrylic water tank filled with demineralized water.
The release mechanism consists of a machined plate with a row of small holes to ensure specific center-to-center distances of the spheres. The plate is hinged to a fixed rod, and is held in a horizontal position by an electromagnet. Upon deactivation of the magnet, a compressed spring propels the plate downwards and the spheres are released. The entire release mechanism is mounted on an horizontal beam to adjust the release height. To capture the water entry process, two high-speed cameras (HSC) are used.
Particle Image Velocimetry (PIV) measurements are used to capture the flow field of the water around the spheres. PIV uses a thin laser sheet to illuminate small particles in the water so that the particle movement can be captured by an HSC. A spezialiced setup is used to minimize reflections and to allow for high quality particle images.
The experimental analysis was performed in two main phases. The first involved using equally-sized sphere pairs and high-speed photography techniques, while the second phase additionally employed PIV measurements for equally-sized spheres and studied the interaction of differently-sized sphere pairs.
Results for SHP/SHP cases show that even at the maximum lateral distance, a slightly asymmetric cavity forms. However, deep-seal pinching, as in single water entry, occurs at a single point. As the lateral distance decreases, the spheres significantly influence each other's behavior, forming highly asymmetric air cavities and oblique jets above the water surface. In the case of an SHP/HPI pairing, a secondary pinch-off cavity of the SHP sphere can occur, especially at low lateral distances. Additionally, at higher impact velocities and minimum lateral distance (spheres in direct contact), a smaller cavity detaches from the SHP sphere's cavity, attaches to the HPI sphere, and moves with it. These different regimes result in varying descent velocities for the spheres.
In the case of an SHP/HPI pairing, the vortex shedding behind the HPI sphere significantly influences the air cavity produced by the SHP sphere. The HSC and PIV analysis demonstrates that a vortex ring forms behind the HPI sphere and first causes some waviness in the cavity interface. This vortex ring is shed and migrates towards the cavity wall causing an indentation which grows over time and finally completely severs the air cavity (2nd pinch-off). Correlations for this second pinch-off time could be derived from the HSC recordings of the conducted experiments. The findings for the case of differently-sized spheres, reveal that a second pinch-off can also be observed in pairings where the smaller sphere is hydrophilic.
The results and discussion related to the first phase of the study are published in doi.org/10.1063/5.0167494
The outcomes and analysis related to the second phase are currently being prepared for another journal publication. The title will be "Parallel water entry of pairs of hydrophobic-hydrophilic spheres: particle image velocimetry and high-speed camera analysis".
The release mechanism consists of a machined plate with a row of small holes to ensure specific center-to-center distances of the spheres. The plate is hinged to a fixed rod, and is held in a horizontal position by an electromagnet. Upon deactivation of the magnet, a compressed spring propels the plate downwards and the spheres are released. The entire release mechanism is mounted on an horizontal beam to adjust the release height. To capture the water entry process, two high-speed cameras (HSC) are used.
Particle Image Velocimetry (PIV) measurements are used to capture the flow field of the water around the spheres. PIV uses a thin laser sheet to illuminate small particles in the water so that the particle movement can be captured by an HSC. A spezialiced setup is used to minimize reflections and to allow for high quality particle images.
The experimental analysis was performed in two main phases. The first involved using equally-sized sphere pairs and high-speed photography techniques, while the second phase additionally employed PIV measurements for equally-sized spheres and studied the interaction of differently-sized sphere pairs.
Results for SHP/SHP cases show that even at the maximum lateral distance, a slightly asymmetric cavity forms. However, deep-seal pinching, as in single water entry, occurs at a single point. As the lateral distance decreases, the spheres significantly influence each other's behavior, forming highly asymmetric air cavities and oblique jets above the water surface. In the case of an SHP/HPI pairing, a secondary pinch-off cavity of the SHP sphere can occur, especially at low lateral distances. Additionally, at higher impact velocities and minimum lateral distance (spheres in direct contact), a smaller cavity detaches from the SHP sphere's cavity, attaches to the HPI sphere, and moves with it. These different regimes result in varying descent velocities for the spheres.
In the case of an SHP/HPI pairing, the vortex shedding behind the HPI sphere significantly influences the air cavity produced by the SHP sphere. The HSC and PIV analysis demonstrates that a vortex ring forms behind the HPI sphere and first causes some waviness in the cavity interface. This vortex ring is shed and migrates towards the cavity wall causing an indentation which grows over time and finally completely severs the air cavity (2nd pinch-off). Correlations for this second pinch-off time could be derived from the HSC recordings of the conducted experiments. The findings for the case of differently-sized spheres, reveal that a second pinch-off can also be observed in pairings where the smaller sphere is hydrophilic.
The results and discussion related to the first phase of the study are published in doi.org/10.1063/5.0167494
The outcomes and analysis related to the second phase are currently being prepared for another journal publication. The title will be "Parallel water entry of pairs of hydrophobic-hydrophilic spheres: particle image velocimetry and high-speed camera analysis".
The effect of vortex shedding generated behind the HPI sphere on air cavity indentation and the occurrence of a second pinch-off were analyzed for the first time in a scientific context.
The tests conducted in this project summarized a wide range of results regarding the second pinching and the threshold for indentation occurrence on the cavity wall in SHP-HPI water entry scenarios. Consequently, a diagram of cavity types, based on the parameter space of the spheres' lateral distance and impact Weber numbers, was introduced for the first time in a scientific context.
The horizontal path deviation observed in SHP-HPI water entry scenarios was also presented for the first time in a scientific context.
Additionally, a formula for estimating the first and second pinch-off depths, and the variation of the second pinch-off depth relative to the first, was introduced for the first time in a scientific context.
The tests conducted in this project summarized a wide range of results regarding the second pinching and the threshold for indentation occurrence on the cavity wall in SHP-HPI water entry scenarios. Consequently, a diagram of cavity types, based on the parameter space of the spheres' lateral distance and impact Weber numbers, was introduced for the first time in a scientific context.
The horizontal path deviation observed in SHP-HPI water entry scenarios was also presented for the first time in a scientific context.
Additionally, a formula for estimating the first and second pinch-off depths, and the variation of the second pinch-off depth relative to the first, was introduced for the first time in a scientific context.