Estimation on the Underwater Explosion Equivalent Based on the Threshold Monitoring Technique

Underwater nuclear explosions can be monitored in near real-time by the hydroacoustic network of the International Monitoring System (IMS) established by the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which could also be used to monitor underground and atmospheric nuclear explosions. The equivalent is an important parameter for the nuclear explosions’ monitoring. The traditional equivalent estimation method is to calculate the bubble pulsation period, which is difficult to obtain satisfactory results under the current conditions. In this paper, based on the passive sonar equation and the conversion process of acoustic energy parameters in the hydroacoustic station, the threshold monitoring technique used for underwater explosion equivalent estimation was studied, which was not limited to the measurement conditions and calculation results of the bubble pulsation period. Through the analysis of practical monitoring data, estimation on the underwater explosion equivalent based on the threshold monitoring technique was verified to be able to reach the accuracy upper boundary of current methods and expand the measurement range to further ocean space, along with the real-time monitoring capability of IMS hydroacoustic stations which could be estimated by this method.

Numerical Study of the Pulsation Process of Spark Bubbles under Three Boundary Conditions

In this study, a compressible three-phase homogeneous model was established using ABAQUS/Explicit. These models can numerically simulate the pulsation process of cavitation bubbles in the free field, near the flat plate target, and near the curved boundary target. At the same time, these models can numerically simulate the strong nonlinear interaction between the cavitation bubble and its nearby wall boundaries. The mutual flow of liquid and gas and fluid solid coupling were solved by the Euler domain in simulation. The results of the numerical simulation were verified by comparing them with the experimental results. In this study, we used electric spark bubbles to represent cavitation bubbles. A high-speed camera was used to record the pulsation process of cavitation bubbles. This study first verified the pulsation process of cavitation bubbles in the free field, because it was the simplest case. Then we verified the interaction process between cavitation bubbles and different wall boundaries. In order to further confirm the credibility of the numerical simulation results, for each wall surface, this study used two burst distances (10 mm and 25 mm) for simulation verification. The numerical model established in this study could effectively simulate the pulsation characteristics of cavitation bubbles, such as the formation of jets and annular bubbles. After verification, the simulated cavitation bubble was almost the same as the cavitation bubble captured by the high-speed camera in the experiment in terms of time, volume, and shape. In this study, a detailed velocity field of the cavitation bubble collapse stage was obtained, which laid down the foundation for the study of the strong nonlinear interaction between the cavitation bubble and the target plates of different shapes. Compared with the experimental results, we found that the numerical model established by the simulation could accurately simulate the bubble pulsation and jet formation processes. In the experiment, the interval time for the bubble pictures taken by the high-speed camera was 41.66 μs per frame. Using a numerical model, the bubble pulsation process can be simulated at an interval of 1 µs per frame. Therefore, the numerical model established by the simulation could show the movement characteristics of the cavitation bubble pulsation process in more detail.

A diffuse interface approach for disperse two-phase flows involving dual-scale kinematics of droplet deformation based on geometrical variables

The purpose of this contribution is to derive a reduced-order two-phase flow model in- cluding interface subscale modeling through geometrical variables based on Stationary Action Principle (SAP) and Second Principle of Thermodynamics in the spirit of [6, 14]. The derivation is conducted in the disperse phase regime for the sake of clarity but the resulting paradigm can be used in a more general framework. One key issue is the definition of the proper potential and kinetic energies in the Lagrangian of the system based on geometrical variables (Interface area density, mean and Gauss curvatures...), which will drive the subscale kinematics and dissipation, and their coupling with large scales of the flow. While [14] relied on bubble pulsation, that is normal deformation of the interface with shape preservation related to pressure changes, we aim here at tackling inclusion deformation at constant volume, thus describing self-sustained oscillations. In order to identify the proper energies, we use Direct Numerical Simulations (DNS) of oscillating droplets using ARCHER code and recently devel- oped library, Mercur(v)e, for mean geometrical variable evaluation and analysis preserving topological invariants. This study is combined with historical analytical studies conducted in the small perturba- tion regime and shows that the proper potential energy is related to the surface difference compared to the spherical minimal surface. A geometrical quasi-invariant is also identified and a natural definition of subscale momentum is proposed. The set of Partial Differential Equations (PDEs) including the conservation equations as well as dissipation source terms are eventually derived leading to an original two-scale diffuse interface model involving geometrical variables.

Cavitation-Induced Interface Instability of Droplet Between Plates

Interface instability of droplet and formation of the liquid jet caused by internal volume oscillation are directly related to liquid pumping and mixing of microfluidic devices. Complex morphology jet enables liquid shaping, which is advantageous for industrial applications and biomedical engineering. In this study, the interface instability of cylindrical droplet between plates is investigated. The problem is analyzed through numerical simulation and experimentation. In the experiment, a single-pulse laser is used to generate cavitation at the center of the cylindrical droplet between two polymethyl methacrylate plates, and the physical progress is captured by high-speed photography. A compressible two-phase solver in the open source code OpenFOAM is used to simulate the 3D progress of bubble pulsation and droplet jet in consideration of viscosity and surface tension. Numerical methods adopt large eddy simulation. Results show that the interface density gradient is not collinear with the pressure gradient due to the shock wave impact and the bubble pulsation, that is, the baroclinic effect is the main cause of the instability at the droplet interface. The mechanism of the radial jet formation in the first period of bubble pulsation is closely related to the interface instability. A pair of vortex rings is formed under the influence of instability, thereby causing a stacking phenomenon on the jet head and eventually being cut. Affecting factors of the instability of the droplet interface are discussed. A high instability intensity of the droplet interface can be caused by a large initial bubble energy and a small contact angle. The instability strength of the droplet interface and the mode of jet formation are very sensitive to the curvature of the initial droplet shape. Relevant results may provide a reference for further understanding of interface instability and related engineering applications.

Calibration of Numerical Simulation Methods for Underwater Explosion with Centrifugal Tests

The centrifugal underwater explosion tests and corresponding numerical simulations were carried out to study the laws of shock wave and bubble pulsation. A semiempirical method to determine JWL state equation parameters was given. The influence on numerical results caused by factors such as state equation of water, boundary condition, and mesh size was analyzed by comparing with the centrifugal underwater explosion test results. The results show that the similarity criterion is also suitable in numerical simulation; the shock wave peak pressure calculated by polynomial state equation is smaller than that of shock state equation. However, the maximum bubble radius and the pulsation period calculated by the two equations are almost the same. The maximum bubble radius is mainly affected by the boundary simulating the test model box, and the pulsation period is mainly affected by the artificial cutoff boundary. With the increase of standoff distance of measuring point, the mesh size required for the numerical calculation decreases; the size of the two-dimensional model is recommended to take 1/30 ∼ 1/10 explosion radius. In three-dimensional models, when mesh size is 2 times larger than explosion radius, the bubble motion change in the second pulsation period is not obvious. When mesh size is smaller than 1 time explosive radius, the full period of bubble pulsation can be well simulated, but calculation errors increase slowly and computation time greatly increases, so the three-dimensional mesh size is suggested to take the charge radius. Shock wave peak pressure is basically unaffected by gravity. As the gravity increases, the bubble maximum radius and the first pulsation period both decrease.