Cavity Collapse Research Articles

Frequency-dependent sonochemical degradation of perfluoroalkyl substances and numerical analysis of cavity dynamics

Ultrasonic breakdown of perfluorooctanoic acid and perfluorooctane sulfonic acid were evaluated at various ultrasonic frequency (575 kHz, 860 kHz, and 1140 kHz), pH (3–12), bulk water temperature (14.5–30 °C), and gases (Helium, Nitrogen, Oxygen, Ozone, Argon). Contrary to the result reported in the literature, we observed an increase in the rate kinetics of PFOA and PFOS decomposition in an air environment (i.e., without sparging any gases), at higher pH, and higher bulk water temperature. The rate kinetics of PFOA degradation in gases follows the order as Helium > Nitrogen > Argon > Oxygen > Ozone. The present work concludes that the presence of known/unknown chemical compounds formed during sonolysis, influence the interaction of PFOA and PFOS with the cavity-water interface. The cavity collapse simulation using Gilmore equation showed that an increase in acoustic pressure increases the compression ratio and bubble radial velocity of the collapsible cavity. This study suggests that the lower degradation rate of PFOS as compared to PFOA, over a range of ultrasound frequencies, is due to the lower number of active cavities collapsing at higher temperatures. The radius of active collapsible cavities, with maximum compression ratio and bubble radial velocity, was 3.2 µm, 2 µm, and 1.7 µm at an ultrasonic frequency of 575 kHz, 860 kHz, and 1140 kHz, respectively.

Curved surface effect on high-speed droplet impingement

Abstract

Jetting in finite-amplitude, free, capillary-gravity waves

Abstract

Study of backspatter using high-speed video of experimental gunshots

Backspatter is biological material that is ejected from the entry wound against the line of fire. This phenomenon was also observed in wound ballistic simulations using so called "reference cubes" (12 cm edge length, 10% gelatin, 4 °C, paint pad beneath the cover). High-speed video records from 102 experimental shots to these target models using full metal jacketed bullets in the calibers .32 auto, .38 special, 9 mm Luger and .357 Magnum were analyzed for chronology, morphological appearance and velocity of fluid ejection. Generally, a short tail splashing of surface material occurred when the bullet was penetrating the target. In 51 shots from distance (≥ 5 cm), regardless of caliber and shot range, a linear jet of fluid started in connection with the first collapse of the temporary cavity. The initial velocity of the jet was measured between 6 and 45 m/s. The jet was streaming on for about 60 to 100 ms with a stochastic deviation of ± 13° to the horizontal. Close range and contact shots showed earlier and faster (up to 330 m/s) backspatter depending on the cartridge and the gap between muzzle and target. Gaseous aerosol-like spray and cone-like spatter indicated an increasing influence of muzzle gases with decreasing shot range. Even under standardized experimental conditions, variations of backspatter were observed in near/contact shots.

LES investigation on vortex dynamics of transient sheet/cloud cavitating flow using different vortex identification methods

The sheet/cloud cavitating flow always contains complex multiscale vortex structures generated by the cavity cloud shedding and collapsing. In this study, the transient sheet/cloud cavitating flow around a Clark-Y hydrofoil is numerically investigated using the Large Eddy Simulation (LES) method coupled with the Zwart–Gerber–Belamri (ZGB) cavitation model. The simulation accurately reproduces the unsteady cavitation evolution process, and the predicted time-averaged lift and drag coefficients, total vapor volume variation and velocity distribution agree fairly well with the experimental measurements. The cavitation vortex dynamics are studied in detail with different vortex identification methods including the vorticity method, the [Formula: see text]-criterion method, the [Formula: see text] method, the [Formula: see text] method and the Liutex method. The vortex identification ability of the different methods in the transient sheet/cloud cavitating flow is also discussed. Generally, the Liutex method combines the advantages of the other methods and can accurately identify both the vortex position and strength. Further analysis of cavitation-vortex interactions demonstrates that the cavity cloud shedding and collapsing have a pronounced influence on the vortex structure.

Conditions of inertial-viscous transition and related jetting in large cavity collapse

Viscosity and surface tension of fluids regulates the collapse dynamics of a large cavity where collapse is initially inertial with the minimum radius ${r}_{m}\ensuremath{\sim}({t}_{0}\ensuremath{-}t{)}^{1/2}$, where ${t}_{0}\ensuremath{-}t$ is the time remaining for collapse. In low-viscosity fluids, an inertial-capillary transition occurs (exponent changes from 1/2 to 2/3), while it is inertial-viscous (exponent changes from 1/2 to 1) in viscous fluids at a late stage of collapse. The inertial-viscous transition occurs when the local capillary number is greater than 1 and the local Ohnesorge number is greater than 0.1.

Oblique droplet impact on superhydrophobic surfaces: Jets and bubbles

Previous studies revealed that the perpendicular impact of low-viscosity droplets on sufficiently lyophobic surfaces would stimulate a liquid jet during droplet recoiling, and in some cases, it is accompanied with the entrapment of an air bubble. However, whether such free-surface flow phenomena occur in oblique droplet impact and how surface inclinations influence the dynamics remain open questions. Herein, we perform a comparative investigation on the perpendicular and oblique impact dynamics of water droplets on superhydrophobic surfaces. We show that the inclinations of the target surface do not influence the jet emission and the bubble entrapment in droplet impact. The jetting phenomena, which are triggered by the collapse of the air cavity, can be classified into three types of scenarios, and the jet velocity scales with its size according to two power laws as suggested by the scaling analyses in the previous studies. On the other hand, the air bubble entrapment is caused by the asymmetric cavity collapse when the recoiling speed of the droplet top is close to that of the bottom, and the bubble size can be reduced by the rising jet from the cavity bottom. We also show that some entrapped bubbles on inclined surfaces would move to the droplet surface and eventually burst, shooting out another thin jet. The correlation between the jet velocity and the bubble size is compared with different empirical scaling laws in the literature.

Some pioneering research in laboratory simulation of scaled astrophysical phenomena by Russian physicists

The review about the problems of astrophysical phenomena pioneering researching by Russian recognized physicists – S.Anisimov, I.V.Minin, O.V.Minin, V.F.Minin and A.Velikovich - is carried out. For the first time, a mode of hypercumulative collapse of a conical cavity was proposed for the formation of an ultra-high-speed plasma jet. The description of “cumulative plasma jets” is presented. The main parameters of the cumulative plasma jet and the results of laboratory studies on the creation of a hyper cumulative plasma jet are described. It was shown that the proposed mechanism allows one to obtain record characteristics of such a jet, which have not been surpassed so far. Test data description of the numerical simulation of the inter action between plungers and a conic cavity on the explosive initiation of the D-D reaction with the aid of aluminum plungers is discussed. The mechanisms of the plunger surface acceleration are described. The generation of the Mach configuration of shock waves in the target was also used for the explosive production of wurtzite with a record yield of the mass of the synthesized material and to focusing of the shock waves in water using plane diffractive optics.

Numerical investigation of transient liquid nitrogen cavitating flows with special emphasis on force evolution and entropy features

This study aims to investigate the impacts of thermal effect on cavitation, entropy features and interactions among vorticity, force coefficients and entropy production rate (EPR) in a liquid nitrogen cavitating flow. Numerical results demonstrate that the increase in free-stream temperature intensifies thermal effect and weakens cavitation intensity. The EPR by fluctuating velocity gradient (ṠD'''') dominates the evolution of EPR, and the cavity shedding or collapse increases the velocity gradient, thereby increasing ṠD''''. The variation trend of EPR by wall region (ṠW) is consistent with ṠD''''. By contrast, EPR by temperature gradient (ṠC'''',ṠC¯''') always distributes on the liquid–vapour interface and increases with the development of the attached sheet cavity. Higher temperature acts as a disincentive to each item of EPR. The stable development of negative sheet vorticity at the leading edge increases the force coefficients nearly linearly. The counter clockwise vortex forms at the hydrofoil tail because the small-scale cloud cavity decreases the lift coefficient sharply. As the instability of attached sheet cavity is enhanced by re-entrant jet, the large vortex is dispersed into several clockwise vortexes, and positive vorticity occurs inside the cavity. Consequently, the force coefficients present a downward trend overall with intensive fluctuation. The distribution of EPR is similar to that of vorticity, and the opposite trend of entropy production integral and lift coefficient illustrates that the force can be used as a representation of energy loss to a certain extent.

Transient Process and Micro-mechanism of Hydrofoil Cavitation Collapse

Cavitation will cause abnormal flow, causing a series of problems such as vibration, noise, and erosion of solid surfaces. In severe cases, it may even destroy the entire system. Cavitation is a key problem to be solved for hydraulic machinery and underwater robots, and the attack angle is one of the most important factors affecting the cavitation. In order to systematically study the impact of the attack angle on the hydrofoil cavitation, the hydrofoils of NACA 4412 with different attack angles were selected to study the collapse process and hydraulic characteristics such as pressure, velocity, vortex, and turbulent kinetic energy during cavitation. The results showed that when the cavitation number was the same, the process of cavity collapse was greatly affected by the attack angle. The length of the cavity collapse area was positively correlated with the attack angle. As the attack angle increased, the volume of the falling bubbles increased, resulting in a larger pressure peak caused by the collapse of bubbles. Moreover, the pressure gradient near the collapse point changed more drastically, thereby affecting the growth of attached cavitation. The fluctuation range of vortex core and turbulent kinetic energy also increased with increasing the attack angle.

Inward collapse of the nasal cavity: Perinatal consolidation of the midface and cranial base in primates.

Living primates show a complex trend in reduction of nasal cavity spaces and structures due to moderate to severe constraint on interorbital breadth. Here we describe the ontogeny of the posterior end of the primate cartilaginous nasal capsule, the thimble shaped posterior nasal cupula (PNC), which surrounds the hind end of the olfactory region. We used a histologically sectioned sample of strepsirrhine primates and two non-primates (Tupaia belangeri, Rousettus leschenaulti), and histochemical and immunohistochemical methods to study the PNC in a perinatal sample. At birth, most strepsirrhines possess only fragments of PNC, and these lack a perichondrium. Fetal specimens of several species reveal a more complete PNC, but the cartilage exhibits uneven or weak reactivity to type II collagen antibodies. Moreover, there is relatively less matrix than in the septal cartilage, resulting in clustering of chondrocytes, some of which are in direct contact with adjacent connective tissues. In one primate (Varecia spp.) and both non-primates, the PNC has a perichondrium at birth. In older, infant Varecia and Rousettus, the perichondrium of the PNC is absent, and PNC fragmentation at its posterior pole has occurred in the former. Loss of the perichondrium for the PNC appears to precede resorption of the posterior end of the nasal capsule. These results suggest that the consolidation of the basicranial and facial skeletons happens ontogenetically earlier in primates than other mammals. We hypothesize that early loss of cartilage at the sphenoethmoidal articulation limits chondral mechanisms for nasal complexity, such as interstitial expansion or endochondral ossification.

An Energy-Based Analysis for Machining Novel AZ91 Magnesium Composite Foam Dispersed With Ceramic Microspheres

Abstract Metal syntactic foams are a novel grade of materials that find potential applications in the manufacture of lightweight structural components and biomedical applications. For these materials to be inducted into industrial applications, it becomes imperative to study their machining behavior. In this article, for the first time in the literature, machining characteristics of AZ91 magnesium foam reinforced with thin-walled hollow alumina ceramic microspheres being studied. Through cutting experiments, it is found that finer the size of hollow microspheres and higher their volume fraction, higher was the magnitude of cutting forces recorded. The failure mechanisms that constituted chip formation during cutting AZ91 foam has been explicated through a mechanistic cutting force model. The proposed force model takes into account key hollow alumina microsphere properties such as wall thickness-to-diameter ratio, average microsphere size, and volume fraction. The scanning electron microscopic (SEM) analysis showed two key modes of failure during cutting metallic foams. Microsphere bursts and fractures control matrix plastic deformation through an effective load transfer mechanism. The transverse matrix cracks, which are initiated as a result of induced shear stress, promote the propagation of longitudinal adhesive cracks. This rapid crack growth takes place along the direction of maximum energy release rate, thus weakening the interfacial strength and reducing effective load transfer. This leads to microsphere separation, and further matrix densification due to the collapse of microsphere cavities leads to chip separation. The developed mechanistic model was in better agreement with experimental results.

Evaluating Simple Ab Initio Models of the Hydrated Electron: The Role of Dynamical Fluctuations.

Despite its importance in electron transfer reactions and radiation chemistry, there has been disagreement over the fundamental nature of the hydrated electron, such as whether or not it resides in a cavity. Mixed quantum/classical simulations of the hydrated electron give different structures depending on the pseudopotential employed, and ab initio models of computational necessity use small numbers of water molecules and/or provide insufficient statistics to compare to experimental observables. A few years ago, Kumar et al. (J. Phys. Chem. A 2015, 119, 9148) proposed a minimalist ab initio model of the hydrated electron with only a small number of explicitly treated water molecules plus a polarizable continuum model (PCM). They found that the optimized geometry had four waters arranged tetrahedrally around a central cavity, and that the calculated vertical detachment energy and radius of gyration agreed well with experiment, results that were largely independent of the level of theory employed. The model, however, is based on a fixed structure at 0 K and does not explicitly incorporate entropic contributions or the thermal fluctuations that should be associated with the room-temperature hydrated electron. Thus, in this paper, we extend the model of Kumar et al. by running Born-Oppenheimer molecular dynamics (BOMD) of a small number of water molecules with an excess electron plus PCM at room temperature. We find that when thermal fluctuations are introduced, the level of theory chosen becomes critical enough when only four waters are used that one of the waters dissociates from the cluster with certain density functionals. Moreover, even with an optimally tuned range-separated hybrid functional, at room temperature the tetrahedral orientation of the 0 K first-shell waters is entirely lost and the central cavity collapses, a process driven by the fact that the explicit water molecules prefer to make H-bonds with each other more than with the excess electron. The resulting average structure is quite similar to that produced by a noncavity mixed quantum/classical model, so that the minimalist 4-water BOMD models suffer from problems similar to those of noncavity models, such as predicting the wrong sign of the hydrated electron's molar solvation volume. We also performed BOMD with 16 explicit water molecules plus an extra electron and PCM. We find that the inclusion of an entire second solvation shell of explicit water leads to little change in the outcome from when only four waters were used. In fact, the 16-water simulations behave much like those of water cluster anions, in which the electron localizes at the cluster surface, showing that PCM is not acceptable for use in minimalist models to describe the behavior of the bulk hydrated electron. For both the 4- and 16-water models, we investigate how the introduction of thermal motions alters the predicted absorption spectrum, vertical detachment energy, and resonance Raman spectrum of the simulated hydrated electron. We also present a set of structural criteria that can be used to numerically determine how cavity-like (or not) a particular hydrated electron model is. All of the results emphasize that the hydrated electron is a statistical object whose properties are inadequately captured using only a small number of explicit waters, and that a proper treatment of thermal fluctuations is critical to understanding the hydrated electron's chemical and physical behavior.

Automatic Approach for Fast Processing and Data Analysis of Seismic Ahead-Prospecting Method: A Case Study in Yunnan, China

The seismic ahead-prospecting method is useful to detect anomalous zones in front of the tunnel face. However, most existing seismic detection method is designed for drilling and blasting tunnel. The detection method should be improved to satisfy the rapid tunneling of Tunnel Boring Machines (TBMs). This study focuses on reducing the time spent on seismic data processing and result analysis. Therefore, to reduce the data processing time, an automatic initial model establishment method based on surrounding rock grade is proposed. To reduce the time spent on result analysis and avoid subjective judgment, a modified k-means++ method is adopted to interpret the detecting results and extracting anomalous zones. The efficacy of the developed method is demonstrated by field tests. The fractured zones such as cavity collapse and fissure are successfully predicted and identified.

Sinkholes threatening places of worship in the historic center of Naples

The historic center of Naples (southern Italy) is characterized by a variety of elements which, forming its cultural asset, have contributed to its recognition as a UNESCO World Heritage site. Among these, cathedrals, churches and chapels are particularly relevant due to their diffusion and differentiation in age. A singular feature of these places of worship is the presence of a frequently connected network of underground cavities. Such cavities, excavated for quarrying tuff blocks, are susceptible to sudden collapses generating sinkholes which in a number of cases have been responsible for the loss of human lives. On this basis and considering the importance of such elements as attractors of visitors, an analysis of susceptibility to sinkholes of cathedrals, churches and chapels has been here carried out. A sinkhole, cavities and places of worship inventory was prepared using available data and was used, in association to ongoing ground deformation data obtained by DInSAR. Results indicated that 40% of the inventoried sinkholes are a product of cavity collapses and in 45 cases cavities lie underneath places of worship. Geodetic data obtained by Differential Interferometric SAR technique indicate that approximately 1 km2 of the historic center of Naples is subject to ongoing subsiding deformation that in several cases is localized above cavities. The susceptibility map derived from available data indicate the presence of a zone where no cathedral, church and chapel can be subject to sinkholes (1) and 3 zones where they can be subject to sagging (2), potential future cavity collapses (3) and future cavity collapses anticipated by ongoing deformation (4); in detail, 34 places of worship are in zone 2, 57 in zone 3 and 9 in zone 4. Considering an increasing susceptibility level of the identified zones, the 9 places of worship located in zone 4 require a quick response in terms of characterization, stability analysis and real time monitoring have been identified.

Theoretical evaluation of chemical and physical feasibility of an in situ ultrasonic reactor for remediation of groundwater contaminated with per‐ and polyfluoroalkyl substances

AbstractPer‐ and polyfluoroalkyl substances (PFAS) have been found in air, water, and soil globally. Groundwater is used as a drinking water supply in many locations and PFAS detections in groundwater occur frequently. Cavitation occurs when ultrasound is applied to liquids and has been shown to be effective for PFAS degradation. Sonolysis is often used to describe the cavity collapse that occurs after cavity expansion. An ideal scenario implements sonolysis in a horizontal well, which allows passive capture of contaminated water while reducing the costs and hazards associated with ex situ remediation. Four PFAS‐contaminated site groundwaters were treated in the sonolytic reactor in the laboratory. Data from those experiments were used in a series of hypothetical case studies to evaluate the influence of chemical and physical properties on feasibility of installation of a novel in situ sonolytic reactor. When PFAS precursor species and perfluorobutanoic acid were included in the remediation goals they were found to drive the hydraulic retention times necessary for effective treatment compared to longer‐chain PFAS. Improved implementation efficiency occurred when the target treatment area was restricted to a concentrated source zone, rather than less concentrated areas of the downgradient plume.

An Experimental Study on the Water Hammer with Cavity Collapse under Multiple Interruptions

Pressurized pipeline system damage is primarily caused by the highly destructive water hammer force. Currently, research on water hammer-caused collapse is mostly based on single-point collapse cases, but water hammer research, which involves multipoint collapse, is insufficient. Here, we establish an experimental platform to realize water hammers with multipoint collapse. With different schemes, i.e., various initial flow rates and valve closing speeds, we observed the hydraulic transient process with a high-speed camera, analyzed its characteristics and explained experimental phenomena with theoretical knowledge. Using experimental data analysis, we summarized the influencing factors and laws of the cavity length and water hammer pressure. Flow and pressure data for the different schemes were recorded to provide basic simulation data. Water column separation experimental phenomena were observed: completely atomized, completely cavitated and partially cavitated, and both cavitated and atomized. At the pump outlet, three hydraulic transition states occurred simultaneously in the horizontal pipe section: completely atomized, completely cavitated, and both cavitated and atomized. Two hydraulic transition states occurred in the knee region: completely and partially cavitated, and without atomization. The experimental results reveal that the initial flow rate and valve closing speed greatly affect the water hammer pressure rise and cavity length. The higher the initial flow rate and valve closing speed are, the larger the water hammer pressure rise and cavity length are.

Cavity collapse near slot geometries

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Numerical simulation of the propagation of pressure waves in water during the collapse of a spherical air cavity

Numerical simulation of the collapse of a spherical gas cavity in a liquid under the influence of a pressure drop is performed. The unsteady motion of a viscous compressible fluid is described by the equations of continuity, momentum, and energy, supplemented by Menter’s SST model. The VoF approach was used to track the interface of the gas–liquid multiphase flow. The problem was solved using OpenFOAM software. Comparison of the obtained results with the known data of other authors was carried out. In experiments, the initial radius of the spherical cavity varied from 0.01 to 0.09 m, and the gas pressure varied from 0.05 to 0.5 MPa at a constant liquid pressure of 7 MPa. The dependences of the radius of the gas cavity, the velocity of the interface, and turbulent kinetic energy on time were obtained, and the shape of the cavity during the implosion process was calculated. The influence of the gas pressure and the radius of the spherical cavity at the initial time on the maximum pressure at the control point was analyzed.

Jet formation and deep seal phenomena associated with inclined oil entry of rotating steel spheres

The jet formation and deep seal phenomena following the inclined oil entry of rotating steel spheres were experimentally investigated. The results were compared with those obtained from vertical and non-rotating oil entry of the same spheres. It had been observed that the jet formation could be classified into two processes. First, a thin primary jet was formed immediately after deep seal. Second, the same jet became significantly thicker following the complete collapse of the air cavity at the oil surface. The inclined oil jet would gradually turn toward the vertical plane, while the angle between the primary jet and the quiescent oil surface was found to decrease when the Reynolds number of the spheres increased. The deep seal time was also independent of both linear and angular sphere velocities, while the vertical deep seal displacement increased with the Froude number.