Cavity Instability Research Articles

Observation of Superposition of Cavitation Instability in Liquid Rocket Inducer by Multi-point Pressure Measurement

Abstract The purpose of this study is to investigate the relationship between the growth and contraction processes of each blade cavity and cavitation instability, and to conduct multi-point pressure measurement tests. The experiments will be conducted using cavitation water flow tunnels installed at the Kakuda Space Center of the Japan Aerospace Exploration Agency (JAXA). In the multi-point pressure measurement experiment, a number of pressure sensors are installed in the inducer casing, and the cavity area can be estimated and the cavity length can be obtained by creating a pressure distribution between the blades during the depressurization experiments. This method makes it possible to continuously determine cavity length variation during inducer experiments and to investigate the cavity growth process. In addition, cavity length variation can be obtained with higher time resolution than in visualization experiment, allowing more detailed investigation of the unsteady cavity behaviour during cavitation instability. And two types of superposition cavitation instability were found in this study: one is Super-S R.C. and C.S., the other is Sync R.C. and C.S..

Characteristics of entrapped jet and cavity instability following the water entry of a superhydrophobic sphere

Following the impact of a superhydrophobic sphere on water, the formation of a stable-streamlined gas cavity undergoes several stages characterized by impact load slamming, cavity flow forming, cavity collapse, and unstable cavity. The former three stages have been extensively investigated. However, limited attention has been given to the post-collapse instability of the gas cavity. In the present study, a systematic experiment is conducted to investigate the evolution of the cavity subsequent to the water entry of a superhydrophobic sphere. The pattern of cavity collapse significantly influences the dynamic characteristics of the downward liquid jets entering the cavity. The liquid jets entrapped in the cavity behave as liquid films composed of tiny individual cavities generated with cavity collapse. Deep sealing with a relatively low Weber number consistently promotes the onset of jet penetration, whereas surface sealing results in droplet penetration. The repulsion of entrapped liquid films from the inner rim of the sphere plays a pivotal role in stimulating the periodic instability experienced by the descending gas cavity. The obtained conclusions shed light on the dynamics of the liquid films entrapped in a gas cavity and the cavity stability.

A Multivariate Time Series Analysis of Ground Deformation Using Persistent Scatterer Interferometry

Ground deformations in urban areas can be the result of a combination of multiple factors and pose several hazards to infrastructures and human lives. In order to monitor these phenomena, Interferometric Synthetic Aperture Radar (InSAR) techniques are applied. The obtained signals record the overlapping of the phenomena, and their separation is a relevant issue. In this framework, we explored a new multi-method approach based on the combination of Principal Component Analysis (PCA), Independent Component Analysis (ICA) and Hierarchal Clustering (HC) on the standardized results to distinguish the main trends and seasonal signals embedded in the time series of ground displacements, to understand spatial-temporal patterns, to correlate ground deformation phenomena with geological and anthropogenic factors, and to recognize the specific footprints of different ground deformation phenomena. This method allows us to classify the ground deformations at the site scale in the metropolitan area of Naples, which is affected by uplift cycles, subsidence, cavity instabilities and sinkholes. At the local scale, the results allow a kinematic classification using the extracted components and considering the effect of the radius of influence generated by each cavity, as it is performed from a theoretical point of view when the draw angle is considered. According to the results, among the classified cavities, 2% were assigned to subsidence and 11% to uplift kinematics, while the remaining were found to be stable. Furthermore, our results show that the centering of the Spatial-PCA (S-PCA) is representative of the region’s main trend, whereas Temporal-PCA (T-PCA) gives information about the displacement rates identified by each component.

Jet-supercavity interaction and instability mechanism analysis

In this research, an experimental system that couples the supercavity and the tail jet was established, aiming at exploring on the cavity instability mechanism. Both high pressure gas tank and solid rocket motor were used to generate the tail jet. The model shape and the jet rate were all varied to investigate different flow patterns. Experiments for the cold gas jet were first carried out, obtaining the variation of the flow pattern with the jet strength and the relative location. With strong jet intensity, the supercavity could lose instability by the re-entrained liquid jet, which was mainly generated by the back attack of the tail jet. Furthermore, the effect of the rearward facing step structure on the cavity instability was investigated, validating that the re-entrained jet could be successfully suppressed by this structure. However, when the cavity contacts with the wall surface, another kind of instability induced by cavity/body interaction could be induced. Subsequently, experiments were carried out for the hot jet generated by the solid rocket motor. A new kind of cavity instability at the moment of ignition was observed. The cavity shrinking and collapse processes were compared between different cases. When the ignition process finished, the hot jet transited into a continuous free expansion mode with weak entrainment of liquid. Finally, the cavity instability mechanisms were summarized and compared. Spectral analysis was carried out for different cases. An obvious larger Strouhal number exists for the pressure pulse induced instability.

Dissipative Kerr single soliton generation with extremely high probability via spectral mode depletion

Optical Kerr solitons generation based on microresonators is essential in nonlinear optics. Among various soliton generation processes, the single soliton generation plays a pivotal role since it ensures rigorous mode-locking on each comb line whose interval equals the free spectral range (FSR) of the microresonator. Current studies show that single soliton generation is challenging due to cavity instability. Here, we propose a new method to greatly improve single soliton generation probalility in the anomalous group velocity dispersion (GVD) regime in a micro-ring resonator based on silicon nitride. The improvement is realized by introducing mode depletion through an integrated coupled filter. It is convenient to introduce controllable single mode depletion in a micro-ring resonator by adjusting the response function of a coupled filter. We show that spectral mode depletion (SMD) can significantly boost the single soliton generation probability. The effect of SMD on the dynamics of optical Kerr solitons generation are also discussed. The proposed method offers a straightforward and simple way to facilitate robust single soliton generation, and will have an impact on the research development in optical Kerr soliton generation and on-chip optical frequency mode manipulation.Graphical abstract

Pillar safety in shallow salt caverns by using numerical simulations

Problems such as low recovery rates and frequent accidents occur in rock salt solution mining areas in China. Reasonable design parameters are essential for improving the economic benefits of solution mining areas and the safety of underground caverns. The long-term stability of salt caverns under different pillar widths was analyzed by considering the inclined shallow-buried deep salt layer with a buried depth of 300–400 m in the Huasu solution mining area, Anhui Province, China, as the research object. Physical and mechanical tests of the rock core were performed to obtain the corresponding mechanical parameters, and a three-dimensional geological model was established. The stability calculation was performed using FLAC3D software, and the safe spacing between the two adjacent cavities in the shallow buried salt layer was optimized to improve the recovery rate. The results demonstrate that when the pillar width is 1.2 times the cavity diameter, the plastic zone between the two cavities does not penetrate, and there is no dilatancy failure near the surrounding rock of the cavity. With an increase in the pillar width, the displacement around the cavity and the volume shrinkage rate gradually increases. Secondly, when the volume of brine accounts for 100 % of the cavity volume, it is beneficial to improve the stability of the cavity. A comparison with the simulation results of cavities with various salt dip angles demonstrates that the salt dip angle increases the risk of cavity instability, and the influence of the salt dip angle on cavity stability should be considered in the actual construction process. The results of this study are significant for ensuring the safety of shallow-buried salt cavern operations and the improvement of the recovery rate.

Mixing enhancement of supersonic flow induced by splitter plate cavity

Fast and efficient mixing of fuel and oxidizer in a RBCC combustor is crucial for the efficiency improvement of engine propulsion system. Whereas, the existence of compressibility effects of high-speed flow notably suppresses the mixing process. In present work, the strategy of splitter plate cavity is proposed to promote mixing. The influences of splitter plate cavity with different length to depth ratio (r) on mixing process are researched. Fine vortex structures are well captured and the newly found T-shaped structures and their evolution dynamics induced by the cavity instability are thoroughly analyzed. The results indicate that tremendous growth of the mixing layer in flow transition region is realized. For the dimensionless streamwise velocity distribution, the recognized appearance of multiple peaks at far flow field for splitter plate cavity with r=4 and 8 leads to great velocity gradient in local position, which can promote mixing process of upper and lower streams. The distributions of streamwise turbulence intensity, turbulent kinetic energy and transversely integrated turbulent kinetic energy suggest that vigorous dynamic behaviors of T-shaped structures can obviously increase local turbulence, especially for splitter plate cavity with r=8. The structure topology analysis results reveal that larger size of the evolutionary T-shaped structure and more intense rotation process of these structures can undoubtedly engulf more main stream into the mixing region to achieve mixing augmentation.

Proposal procedure to evaluate the seismic stability of underground cavities in seismic microzonation studies

Under seismic actions, underground cavities play a not negligible role for seismic site effect and, at the same time, they represent objects exposed to seismic risk. Thus, their possible instability up to the collapse could have effects on the above-ground structures and infrastructures, as revealed by recent and past earthquakes. In consideration of the fact that the presence of underground cavities is very common in many urban and historical areas in Italy, it represents issue of concern for our territory and the related seismic risk should be considered both for planning and management issues.Being a rather complex topic, the cavity instability under seismic actions is little covered in the technical literature, thus the present work represents a first attempt to frame the problem at different scales. An extensive analysis at national scale was firstly carried out to evaluate the relevance of the problem by using the data stored in the Italian database and catalogue of co-seismic effects and those reported in the database of Seismic Microzonation. This analysis allowed to create a database of Italian co-seismic cavities instabilities, to identify the most susceptible zones in Italy and to preliminarily evaluate some elements of screening. Afterwards, the problem was addressed on an urban scale by proposing a possible procedure to be adopted within the Italian Guidelines for Seismic Microzonation to identify areas potentially prone to seismic cavities collapse.Currently, the Italian Guidelines for Seismic Microzonation do not address this kind of co-seismic effect by means of specific guidelines as for other instabilities (i.e., liquefactions, landslides, active and capable faults, seismic densification), although underground cavities are regularly reported in the studies. Therefore, the proposed methodology represents a first attempt to define the requirements of Seismic Microzonation studies in areas potentially prone to this kind of co-seismic instability.According to the SM-Working Group (2008), the proposing procedure defines three levels of zonation, i) attention zones, ii) susceptible zones and iii) respect zones, corresponding to three different cavity seismic instability-prone zones for which, an increasing knowledge levels of the cavity and the geological/geotechnical conditions of the surrounding rock\soil from one hand, and increasing accuracy of the methods adopted to assess the seismic susceptibility of cavity from another, are expected.The proposed procedure includes for the level 1 of SM a preliminary screening of some predisposing and triggering factors to define the attention zones, while at level 3 of SM the introduction of a simplified approach based on the use of abacuses to preliminary quantify the cavity seismic instability, and an advanced approach to define the respect zones by means of specific numerical analysis.

Characteristic gas cavity patterns formed with gas injection into horizontally co-flowing water

The present study aims to reveal two-phase flow characteristics as gas is injected into co-flowing water. A computational fluid dynamics work was conducted using the volume of fluid model and the delayed detached eddy simulation. The numerical scheme was validated through experimental results. Effects of the upstream water velocity and the gas injection rate on flow patterns were investigated. Characteristics of gas cavities were compared under different operating conditions. In both experimental and numerical results, unique patterns of gas cavities are obtained. A large cavity is attached to the nozzle outlet, and simultaneously, bubbles are shed from the cavity front. With increasing gas injection rate, the attached cavity is pinched off as it extends in a streamwise direction. As the velocity of water increases, scattered pinch-off frequencies are evidenced and cavity instability is enhanced. Liquid wake downstream of the nozzle contributes significantly to the collapse of the gas cavity. A long and stable gas cavity is produced at high upstream water velocities and gas injection rates.

Inverse Simulation for Salt Cavern Wall Collapse during Cycling Loading: A Field Case Study

Salt cavities used as underground gas storage have received increased attention for serviceability and sustainability considerations during gas cycling loading. However, traditional stability analysis overlooks the thermal effects induced by the operation and cannot predict the potential risk. In a field case in Jintan, Jiangsu Province of China, the thermal response to gas injection-and-withdrawal process of the walls of Cavern L was explored. A Bonded-Block Model (BBM) based on a 3DEC platform is proposed in inverse simulation for the field case’s cavity collapse process. By comparing the field case with simulation results, the proposed thermal-mechanical coupling model is validated for predicting the risk of cavity instability. The simulation results show that the thermal effect induces the collapse of the surrounding rock, that is, thermal tensile stress causes thermal cracks on the surface of the cavity.

Stabilization of liquid instabilities with ionized gas jets.

Impinging gas jets can induce depressions in liquid surfaces, a phenomenon familiar to anyone who has observed the cavity produced by blowing air through a straw directly above a cup of juice. A dimple-like stable cavity on a liquid surface forms owing to the balance of forces among the gas jet impingement, gravity and surface tension1,2. With increasing gas jet speed, the cavity becomes unstable and shows oscillatory motion, bubbling (Rayleigh instability) and splashing (Kelvin-Helmholtz instability)3,4. However, despite its scientific and practical importance-particularly in regard to reducing cavity instability growth in certain gas-blown systems-little attention has been given to the hydrodynamic stability of a cavity in such gas-liquid systems so far. Here we demonstrate the stabilization of such instabilities by weakly ionized gas for the case of a gas jet impinging on water, based on shadowgraph experiments and computational two-phase fluid and plasma modelling. We focus on the interfacial dynamics relevant to electrohydrodynamic (EHD) gas flow, so-called electric wind, which is induced by the momentum transfer from accelerated charged particles to neutral gas under an electric field. A weakly ionized gas jet consisting of periodic pulsed ionization waves5, called plasma bullets, exerts more force via electrohydrodynamic flow on the water surface than a neutral gas jet alone, resulting in cavity expansion without destabilization. Furthermore, both the bidirectional electrohydrodynamic gas flow and electric field parallel to the gas-water interface produced by plasma interacting 'in the cavity' render the surface more stable. This case study demonstrates the dynamics of liquids subjected to a plasma-induced force, offering insights into physical processes and revealing an interdependence between weakly ionized gases and deformable dielectric matter, including plasma-liquid systems.

Onset of temporal dynamics within a low reynolds-number laminar fluidic oscillator

We investigate the onset of temporal dynamics of a fluidic oscillator (FO) in the laminar regime at very low Reynolds number (Re=Uhi/ν, where U and hi are the velocity and channel height at inlet, and ν is the kinematic viscosity of the fluid). Both two- and spanwise-periodic-three-dimensional simulations are performed using high-order spectral element methods to characterise the flow inside the FO cavity and the outcoming jet. While the flow remains steady and symmetric for sufficiently low Re, the two-dimensional FO undergoes a symmetry-breaking Hopf bifurcation at ReH2=75.7 that results in a space-time symmetric periodic solution. The remnant space-time symmetry is later broken in a pitchfork bifurcation of limit cycles at ReP2=294.2. Taking three-dimensionality into consideration results in an early spanwise-invariance disruption at about Re3≃68 of wavelength 13hi that retards the onset of the oscillation to ReH3=82.1. This effect is little representative of actual FO implementations, which are typically much narrower than the wavelength of the spanwise instability observed at these low Re. Contrary to what happens with FOs in the turbulent oscillatory regime, the oscillation frequency of the output jet in the laminar regime is found to decrease with Re. The mechanism that drives the oscillation, however, remains the same: as the high speed flow inside the cavity attaches to one of the internal walls through the Coandă effect, the feedback channels divert part of the momentum back, which pushes the incoming flow against the opposite wall. The alternate deviation of the flow inside the cavity to one or the other side results in the periodic flipping of the output jet. The pressure contribution to net momentum at the end of the feedback channels is found to be double that of the advective momentum flux at the early time-periodic regime but both become comparable as Re is increased. The jet sweeping angle amplitude is more pronounced for the two-dimensional FO as compared to three-dimensional at a fixed given Re, the Coandă effect being only partially fulfilled in the latter case. In both cases the sweep amplitude increases with Re. The instability of the output jet, which becomes slightly chaotic already at very low Re, is responsible for triggering the cavity instability that drives the oscillation slightly earlier than it would, should outside noise be suppressed altogether.

Entropy production analysis in two-phase cavitation flows with thermodynamic cavitation model

Entropy production is an important parameter for the design and optimization of turbo machines, which is short of intuitiveness in previous studies. The present article investigated the mechanisms of entropy production in cavitation flows around a hydrofoil. The objectives of this research are to (1) reveal the thermodynamic effects with advanced cavitation model, (2) investigate dynamic characteristics of different entropy production terms, (3) analyze the interactions between vorticity and local entropy production rate based on vorticity transport equation. Firstly, water from 298 K to 423 K is investigated around a NACA0015 hydrofoil to reveal thermodynamic effects on different temperatures. Then, in order to study the energy loss due to cavitation, the entropy production theory is firstly applied on two-phase cavitation flows with high temperature water based on homogenous model and its unsteady characteristics is revealed combining with cavity structure. The density corrected turbulence model (DCM) and advanced thermodynamic cavitation model adopted in this research are validate corresponding to experiment. The analysis of local entropy production rate (EPR) indicated that the entropy production rate induced by direct dissipation (EPDD) and turbulent dissipation (EPTD) has the characteristics of spatial and time delay compared with the evolution of cavity instabilities. Moreover, entropy production rate induced by temperature gradients and wall shear stress are slight, while entropy production rate induced by velocity gradients are main part toward total entropy production rate (TEPR).

Physicochemical Properties and Photochemical Reactions in Organic Crystals

Background:Molecular organic photochemistry is concerned with the description of physical and chemical processes generated upon the absorption of photons by organic molecules. Recently, it has become an important part of many areas of science: chemistry, biology, biochemistry, medicine, biophysics, material science, analytical chemistry, among others. Many synthetic chemists are using photochemical reactions in crystals to generate different types of organic compounds since this methodology represents a green chemistry approach.Objective & Method:Chemical reactions in crystals are quite different from reactions in solution. The range of organic solid state reactions and the degree of control which could be achieved under these conditions are quite wider and subtle. Therefore, for a large number of molecular crystals, the photochemical outcome is not the expected product based on topochemical principles. To explain these experimental results, several physicochemical factors in crystal structure have been proposed such as defects, reaction cavity, dynamic preformation or photoinduced lattice instability and steric compression control. In addition, several crystal engineering strategies have been developed to bring molecules into adequate orientations with reactive groups in good proximity to synthesize complex molecules that in many cases are not available by conventional methods. Some strategies involve structural modifications like intramolecular substitution with different functional groups to modify intermolecular interactions. Other strategies involve chemical techniques such as mixed crystal formation, charge transfer complexes, ionic and organometallic interactions. Furthermore, some examples of the single crystal to single crystal transformations have also been developed showing an elegant method to achieve regio and stereoselectivity in a photochemical reaction.Conclusion:The several examples given in this review paper have shown the wide scope of photochemical reactions in organic molecular crystals. There are several advantages of carrying photochemical reaction in the solid state. Production of materials unobtainable by the traditional solution phase reactions, improved specificity, reduction of impurities, and enhancement in the yields by the reduction of side reactions. These advantages and the multidisciplinary nature of solid-state photochemistry make this discipline quite likely to develop a lot in the future.

Numerical simulation and analysis of condensation shocks in cavitating flow

We analyse unsteady cavity dynamics, cavitation patterns and instability mechanisms governing partial cavitation in the flow past a sharp convergent–divergent wedge. Reproducing a recent reference experiment by numerical simulation, the investigated flow regime is characterised by large-scale cloud cavitation. In agreement with the experiments, we find that cloud shedding is dominated by the periodic occurrence of condensation shocks, propagating through the two-phase medium. The physical model is based on the homogeneous mixture approach, the assumption of thermodynamic equilibrium, and a closed-form barotropic equation of state. Compressibility of water and water vapour is taken into account. We deliberately suppress effects of molecular viscosity, in order to demonstrate that inertial effects dominate the flow evolution. We qualify the flow predictions, and validate the numerical approach by comparison with experiments. In agreement with the experiments, the vapour volume fraction within the partial cavity reaches values${>}80\,\%$for its spanwise average. Very good agreement is further obtained for the shedding Strouhal number, the cavity growth and collapse velocities, and for typical coherent flow structures. In accordance with the experiments, the simulations reproduce a condensation shock forming at the trailing part of the partial cavity. It is demonstrated that it satisfies locally Rankine–Hugoniot jump relations. Estimation of the shock propagation Mach number shows that the flow is supersonic. With a magnitude of only a few kPa, the pressure rise across the shock is much lower than for typical cavity collapse events. It is thus far too weak to cause cavitation erosion directly. However, by affecting the dynamics of the cavity, the flow aggressiveness can be significantly altered. Our results indicate that, in addition to classically observed re-entrant jets, condensation shocks feed an intrinsic instability mechanism of partial cavitation.

Cavity evolution and instability constraints of relativistic interiors

In this manuscript, we have identified the dynamical instability constraints of a self-gravitating cylindrical object within the framework of $f(R,T)$ theory of gravity. We have explored the modified field equations and corresponding dynamical equations for the systematic constructions of our analysis. We have imposed the linear perturbations on metric and material variables with some known static profile up to first order in the perturbation parameter. The role of expansion scalar is also examined in this scenario. The instability regimes have been discussed in the background of Newtonian and post-Newtonian limits. We found that the dark source terms due to the influence of modification in the gravity model is responsible for the instability of the system.

Evolution of Design and Construction Approaches in the Field of Tunnelling: The Results of Applying ADECO-RS When Constructing Large Underground Works in Urban Areas

In the last 25 years in Italy, over 1000km of tunnels have been constructed in urban and extra-urban areas. Despite being constructed in very complex and difficult grounds; generally these were completed (from the design stage to the construction and actual opening of activity) with industrial methods, and within the technical times and costs foreseen by the design laid out by the tenders. This extraordinary result has been made possible since the public administrations have adopted modern design and construction approaches in their contract specifications, based on strictly scientific criteria and which leave nothing to the typical vagueness of those design and construction systems from the twentieth century, which remedied cavity instability solely by means of partitioning the excavation face. Today we know that full-face advancement is all the more necessary the more difficult the excavation conditions are. Memory serves to illustrate the criteria that helped define modern design and construction concepts and an example of their application for constructing a great twin tunnel (258 m2 + 258 m2 in section) in the urban area of Rome.

REVIEW OF THE ENERGY LIMITERS APPROACH TO MODELING FAILURE OF RUBBER

ABSTRACTNonlinear theories of elasticity describe rubber deformation but not failure; however, in reality, rubbers do fail. In the present work, we review a new approach of energy limiters that allows for unifying hyperelasticity theories with failure descriptions, and we discuss results of this unification. First, we introduce the energy limiter concept, which allows the enforcement of failure descriptions in elasticity theories. The limiter provides the saturation value for the strain energy, hence indicating the maximal energy that may be stored and dissipated by an infinitesimal material volume. The limiter is a material constant that can be calibrated via macroscopic experiments. Second, we illustrate the new approach with examples in which failure initiation is predicted but its propagation is not tracked. Examples include the problems of crack initiation, cavity instability, and rupture of inflating membranes. In addition, the traditional strength-of-materials criteria are reassessed. Third, the theory is used for three-dimensional explicit finite element simulations of a high-velocity penetration of a stiff elastic body into a rubber plate. These simulations show that a high-velocity penetration of a flat projectile leads to a diffused nonlocal failure, which does not trigger the mesh sensitivity. To the contrary, a low-velocity penetration of a sharp projectile leads to a highly localized cracklike failure, which does trigger the mesh sensitivity. Calculation of the characteristic length of failure localization allows for setting the mesh size that provides regularization of the simulations. The fact that the calculation is based on results of solely macroscopic experiments is noteworthy.

Patterns of Failure in Canal Wall Down Mastoidectomy Cavity Instability

To evaluate patterns of failure for canal wall down mastoid cavities requiring surgical revision. Retrospective review. Academic tertiary referral center Adults and children that underwent revision of an unstable open mastoid cavity from 1995 to 2010. Review of demographic data, tympanomastoid pathology, and plausible risk factors for an unstable cavity. Available computed tomography (CT) scans were reviewed for indicators of suboptimal cavity shape. Spearman's correlation analysis was undertaken. Findings were classified as Type 1 (primary tympanomastoid pathology), Type 2 (cavity shape/size), or Type 3 (negative host environment). Frequency of risk factors and correlation. Approximately 130 cases were reviewed. Stapes erosion (49.2%), absent malleus (26.2%), cholesteatoma (44.6%), tympanic membrane perforation (34.6%), and fibrotic middle ear mucosa (20.8%) were common. CT scans often demonstrated an intact open mastoid tip (87.5%) and a high facial ridge (54.2%). Notable correlations were discovered between the facial ridge height proximally and the height distally (r = 0.46437, p = 0.0256) and tympanic membrane perforation and absent malleus (r = -0.17944, p = 0.0419). Approximately 68% of the subjects had at least 1 Class 1 risk factor present among cholesteatoma, tympanic membrane perforation/atelectasis, and extruded prosthesis. All CT scans reviewed demonstrated at least 1 class 2 factor. Although primary tympanomastoid pathology is quite common, some aspect of suboptimal mastoid cavity size and shape is pervasive. Correlation analysis suggests that surgeons tend to either lower the facial ridge completely or not at all and that an absent malleus seems to be associated with a tympanic membrane perforation.

Internal structure and dynamics of sheet cavitation

The internal structure and the dynamics of two-dimensional (2D) sheet cavitation on the suction side of a 2D foil section were investigated experimentally. Experiments were conducted in a cavitation tunnel and situations ranging from steady sheet cavitation to unsteady cloud cavitation were obtained by varying the foil incidence and the cavitation number. Using a novel endoscopic technique, coupled with x-ray attenuation measurements, the two-phase morphology and the void fraction within the sheet cavitation were investigated. Supplemental information on the instantaneous shape of the sheet cavity and its instability frequency were also obtained by visualization and pressure measurements, respectively. The investigations focused on (a) the void fraction distribution and (b) the frequency of the cavity oscillations. It was found that the void ratio can reach as much as 50% depending on the conditions of operation, and the Strouhal numbers are around 0.25 in the case of partial cavity instability and 0.12 in the case of transitional sheet cavitation. Finally, the visualization of the vapor structures within the sheet cavity for various stations along the chord gives a qualitative understanding of the process of vapor production and condensation.