An experimental and computational investigation was conducted to study the interaction between bubbles generated by an underwater explosive (UNDEX) and a nearby steel plate structure. The experiments ...were performed for different standoff distances to investigate the interaction between the gas bubble and the rigid structure. High-speed photography was utilized to capture the underwater explosive gas bubble’s behavior, and a series of pressure transducers were used to record the emitted pressure histories. The numerical simulations were performed with the Dynamic System Mechanics Advanced Simulation software, which is a full y coupled Eulerian–Lagrangian fluid–structure interaction code. The numerical simulations were validated with the experiments in terms of the detonation pressure, structural surface pressures, and UNDEX gas bubble growth and collapse. Results show that the UNDEX standoff distance greatly influences the gas bubble’s shape, migration speed, bubble jetting behavior, and loading into its adjacent structure. Moreover, the impulse generated by the shock wave on the plate surface proved to be small in comparison to the impulse generated by the collapse of the UNDEX gas bubble. The magnitude of impulse depends on standoff distance, collapse symmetry, and relative collapse location.
Three-dimensional dynamics of a transient bubble inside a corner formed by two rigid curved parabolic plates (walls) is studied numerically using boundary integral method (BIM) based on the potential ...flow theory. The bubble dynamics, including the expansion and collapse phases until the jet impact, are investigated for different corner angles associated with different focal lengths k of the parabolas. However, for all the simulations, the dimensionless initial vertical standoff distance of the bubble’s center from the corner edge (h∗) is fixed at 4. The bubble remains almost spherical during expansion, except for parts of its surface that flattens near the walls. When the bubble is initiated at the bisector plane of the two intersecting walls, it oscillates symmetrically with respect to the bisector plane and becomes oblate during the late stages of the collapse phase. A high-speed liquid jet forms towards the end of bubble collapse, pointing to the corner. If the corner angle decreases, the bubble becomes more oblate along the bisector plane making the ensuing liquid jet wider and slower. In addition, a bubble initiated closer to one of the two walls is mainly influenced by the closer wall, oscillates non-symmetrically with respect to the bisector plane and the liquid jet formed in this case is inclined towards the closer wall due to the greater Bjerknes force of that wall.
•The bubble dynamics in a corner formed by curved plates is studied using a 3D BIM.•The effects of corner angle and initial position of the bubble are assessed.•The bubble’s oscillation is symmetric/asymmetric depending on its initial position.•In asymmetric case, the bubble’s liquid jet is inclined towards the closer wall.•For larger corner angles, the bubble behavior becomes independent of the angle.
The dynamics of a bubble near a corner formed by two flat rigid boundaries (walls), is studied experimentally using a spark-generated bubble. The expansion, collapse, rebound, re-collapse and ...migration of the bubble, along with jetting and protrusion, are captured using a high-speed camera. Our experimental observations reveal the behaviour of the bubble in terms of the corner angle and the dimensionless standoff distances to the near and far walls in terms of the maximum bubble radius. The bubble remains approximately spherical during expansion except for its surface becoming flattened when in close proximity to a wall. When a bubble is initiated at the bisector of the two walls, the bubble becomes oblate along the bisector during the late stages of collapse. A jet forms towards the end of collapse, pointing to the corner. The closer the bubble to the two walls, the more oblate along the bisector the bubble becomes, and the wider the jet. A bubble initiated near one of the two walls is mainly influenced by the nearer wall. The jet formed is pointing to the near wall but inclined towards the corner. After the jet penetrates through the bubble surface, the bubble becomes a bubble ring, and a bubble protrusion forms following the jet. The bubble ring collapses and subsequently disappears, while the protrusion firstly expands, and then collapses and migrates to the corner.
Microbubble dynamics subject to ultrasound are associated with important applications in biomedical ultrasonics, sonochemistry and cavitation cleaning. The viscous effects in this phenomenon is ...essential since the Reynolds number Re associated is about O(10). The flow field is characterized as being an irrotational flow in the bulk volume but with a thin vorticity layer at the bubble surface. This paper investigates the phenomenon using the boundary integral method based on the viscous potential flow theory. The viscous effects are incorporated into the model through including the normal viscous stress of the irrotational flow in the dynamic boundary condition at the bubble surface. The viscous correction pressure of Joseph & Wang (2004) is implemented to resolve the discrepancy between the non-zero shear stress of the irrotational flow at a free surface and the physical boundary condition of zero shear stress. The model agrees well with the Rayleigh–Plesset equation for a spherical bubble oscillating in a viscous liquid for several cycles of oscillation for Re=10. It correlates pretty closely with both the experimental data and the axisymmetric simulation based on the Navier-Stokes equations for transient bubble dynamics near a rigid boundary. We further analyze microbubble dynamics near a rigid boundary subject to ultrasound travelling perpendicular and parallel to the boundary, respectively, in parameter regions of clinical relevance. The viscous effects to acoustic microbubble dynamics are analyzed in terms of the jet velocity, bubble volume, centroid movement, Kelvin impulse and bubble energy.
•We observed with the dynamics of a spark-generated bubble inside a long, rigid, circular tube with two open ends submerged horizontally in a tank filled with water, in terms of the dimensionless ...tube radius T Bmax a = R R and the dimensionless eccentricity Bmax e = E R, where RT is the inner radius of the tube, E is the distance from the bubble centre at inception to the axis of symmetry of the tube, and RBmax is the maximum equivalent bubble radius (∼10 mm).•The expansion, collapse and rebound of the bubble were captured using a high-speed camera both for the case where a > 1 and a < 1, with and without eccentricity, respectively. Some new features of the bubble dynamics were observed as summarised in the.•A bubble initiated with eccentricity in a tube for which a < 1 was seen to migrate to the distal part of the tube at the end of collapse with formation of a jet also in that direction. This is distinct from the case of a bubble collapsing near a flat surface. A similar phenomenon has been observed previously in the case of a microbubble collapsing in a blood vessel under ultrasound excitation, but was attributed to the elasticity of the vessel wall. The present study suggests that it may in fact be due simply to the geometry of the system.•In all cases studied a cloud of microbubbles was observed shortly after the start of rebound of the bubble. We hypothesize that this is due to the very low pressure produced by the violent collapse of the bubble and confinement by the tube wall.
This paper is concerned with the dynamics of a spark-generated bubble inside a long, rigid, circular tube with two open ends submerged horizontally in a tank filled with water. The behaviour of the bubble was found to be sensitive to two geometrical parameters: the dimensionless tube radius α=RT/RBmax and the dimensionless eccentricity ε=E/RBmax, where RT is the inner radius of the tube, E is the distance from the bubble center at inception to the axis of symmetry of the tube, and RBmax is the maximum equivalent bubble radius (∼10 mm). The expansion, collapse and rebound of the bubble were captured using a high-speed camera both for the case where α > 1 and α < 1, with and without eccentricity, respectively. Some new features of the bubble dynamics were observed. In particular, a bubble initiated with eccentricity in a tube for which α < 1 was seen to migrate to the distal part of the tube at the end of collapse with formation of a jet also in that direction. This is distinct from the case of a bubble collapsing near a flat surface. A similar phenomenon has been observed previously in the case of a microbubble collapsing in a blood vessel under ultrasound excitation, but was attributed to the elasticity of the vessel wall. The present study suggests that it may in fact be due simply to the geometry of the system. A cloud of microbubbles was observed shortly after the start of rebound of the bubble. Our analysis shows that the microbubbles should be generated from nuclei in tap water with radii in the range of 10−8–10−6 m.
Bubble load in a noncontact underwater explosion can cause the ship hull global response and local response. In current literature, the ship hull is usually simplified as a hull girder to analyze its ...global response. However, literature dealt with the local response of a 3-D surface ship hull subjected to an underwater bubble were limited. This investigation develops a procedure which couples the finite element method with doubly asymptotic approximation (DAA) method to study the problem of transient responses of a ship hull structure subjected to an underwater explosion bubble. Using a 3-D ship model as examples, the global and local responses of the ship model in vertical, transverse and longitudinal directions are performed in detail. The acceleration, velocity and displacement time histories are presented. The characteristics of both the global and local responses of the ship model are discussed. The numerical results show that besides global whipping response, the ship hull also sustains severe local responses in different directions subjected to underwater explosion bubble jetting, which should be taken into consideration.
•We develop an UNDEX procedure which couples FEM with DAA.•We examine the interaction between a hull structure and a bubble on UNDEX.•A 4.5 m long ship model is given to analyze the responses of the ship structure.•Global and local responses of the ship model in different directions are performed.•Characteristics of both global and local responses of the ship model are discussed.
A bubble initiated near a rigid boundary may be almost in contact with the boundary because of its expansion and migration to the boundary, where a thin layer of water forms between the bubble and ...the boundary thereafter. This phenomenon is modelled using the weakly compressible theory coupled with the boundary integral method. The wall effects are modelled using the imaging method. The numerical instabilities caused by the near contact of the bubble surface with the boundary are handled by removing a thin layer of water between them and joining the bubble surface with its image to the boundary. Our computations correlate well with experiments for both the first and second cycles of oscillation. The time history of the energy of a bubble system follows a step function, reducing rapidly and significantly because of emission of shock waves at inception of a bubble and at the end of collapse but remaining approximately constant for the rest of the time. The bubble starts being in near contact with the boundary during the first cycle of oscillation when the dimensionless stand-off distance γ = s/Rm < 1, where s is the distance of the initial bubble centre from the boundary and Rm is the maximum bubble radius. This leads to (i) the direct impact of a high-speed liquid jet on the boundary once it penetrates through the bubble, (ii) the direct contact of the bubble at high temperature and high pressure with the boundary, and (iii) the direct impingement of shock waves on the boundary once emitted. These phenomena have clear potential to damage the boundary, which are believed to be part of the mechanisms of cavitation damage.
The growth and collapse of gaseous bubbles near a movable or deformable body are investigated numerically using the boundary element method and fluid–solid coupling technique. The fluid is treated as ...inviscid, incompressible and the flow irrotational. The unsteady Bernoulli equation is applied on the bubble surface as one of the boundary conditions of the Laplace’s equation for the potential. Good agreements between the numerical and experimental results demonstrate the robustness and accuracy of the present method. The translation and rotation of the rigid body due to the bubble evolution are captured by solving the six-degrees-of-freedom equations of motion for the rigid body. The fluid–solid coupling is achieved by matching the normal component of the velocity and the pressure at the fluid–solid interface. Compared to a fixed rigid body, the expansion of the bubble is not affected too much but much faster collapsing velocities during the collapsing phase of bubble can be observed when considering the motion of the rigid body. The rigid body is pushed away as the bubble grows and moved toward the bubble as the bubble collapses. The motion of two bubbles near a movable cylinder is also simulated. The large rotation of the cylinder and obvious deformation and distortion for the bubble in close proximity to a curved wall are observed in our codes. Finally, the growth and collapse of bubble near a deformable ellipsoid shell are also simulated using the combination of boundary element method (BEM) and finite element method (FEM) techniques. The oscillations of the ellipsoid shell can be observed during the growth and collapse of bubble, which much differs from the results obtained by only considering effects of a rigidly movable body on the bubble evolution.
The proposed elastic mesh technique (EMT) is a mesh regulation technique, which is based on the assumption that the segments of a mesh are elastic. EMT can be employed in conjunction with the ...boundary integral method (BIM) for the simulation of three-dimension bubble dynamics in which problems relating to severe mesh distortion as the bubble evolves are a common occurrence. With EMT, the mesh is advanced not by the material velocity, but the optimum shift velocity obtained by minimizing the total elastic energy stored in every segment of the mesh at each time step. In doing so, the prohibitively small time stepping associated with small meshes without EMT in order to maintain numerical stability is mitigated to a large extent. An important feature is that the EMT scheme accords the user the flexibility to implement a non-uniform optimum constitutive relation governing the elastic behavior of mesh segment and which can be further varied with time. Tests were performed for an underwater explosion bubble exhibiting the dynamics of strong jet development with and without EMT for comparison, and the consideration of incorporating EMT as a hybrid system serving as an alternative to the required mesh refinement which is computationally intensive. A full three-dimension simulation of explosion bubble(s) and in the presence of the free surface were further carried out to elucidate the associated flow physics.
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