Conical Cavitators Research Articles

Numerical Investigations of Cavitation Nose Structure of a High-Speed Projectile Impact on Water-Entry Characteristics

In this study, a detailed analysis of the influences of cavitation nose structure of a high-speed projectile on the trajectory stability during the water-entry process was investigated numerically. The Zwart-Gerber-Belamri (Z-G-B) cavitation model and the Shear Stress Ttransport (SST)k-ω turbulence model based on the Reynolds Averaged Navier–Stokes (RANS) method were employed. The numerical methodology was validated by comparing the numerical simulation results with the experimental photograph of cavitation shape and the experimental underwater velocity. Based on the numerical methodology, the disk and the conical cavitation noses were selected to investigate the water-entry characteristics. The influences of cavitation nose angle and cavitation nose diameter of the projectile on the trajectory stability and flow characteristics were carried out in detail. The variation features of projectile trajectory, velocity attenuation and drag were conducted, respectively. In addition, the cavitation characteristics of water-entry is presented and analyzed. Results show that the trajectory stability can be improved by increasing the cavitation nose angle, but the drag reduction performance will be reduced simultaneously. Additionally, due to the weakening of drag reduction performance, the lower velocity of the projectile will cause the damage of the cavitation shape and the trajectory instability. Furthermore, the conical cavitation nose has preferable trajectory stability and drag reduction performance than the disk cavitation nose.

Super-Cavitating Flow Around Two-Dimensional Conical, Spherical, Disc and Stepped Disc Cavitators

A super-cavitating object is a high speed submerged object that is designed to initiate a cavitation bubble at the nose which extends past the aft end of the object, substantially reducing the skin friction drag that would be present if the sides of the object were in contact with the liquid in which the object is submerged. By reducing the drag force the thermal energy consumption to move faster can also be minimised. The super-cavitation behavioural changes with respect to Cavitators of various geometries have been studied by varying the inlet velocity. Two-dimensional computational fluid dynamics analysis has been carried out by applying k-ε turbulence model. The variation of drag coefficient, cavity length with respect to cavitation number and inlet velocity are analyzed. Results showed conical Cavitator with wedge angle of 30° has lesser drag coefficient and cavity length when compared to conical Cavitators with wedge angles 45° and 60°, spherical, disc and stepped disc Cavitators. Conical cavitator 60° and disc cavitator have the maximum cavity length but with higher drag coefficient. Also there is significant variation of supercavitation effect observed between inlet velocities of 32 m/s to 40 m/s.

The investigation of natural super-cavitation flow behind three-dimensional cavitators: Full cavitation model

In this study, natural super-cavitating flow around three different conical cavitators with wedge angles of 30°, 45° and 60° is investigated. We apply the k−ϵ turbulence model and the volume of fluid (VOF) technique to numerically study the three-dimensional cavitating flow around the cavitators. The turbulence approach is coupled with a mass transfer model which is implemented into the finite-volume package. Simulations are performed for different cavitation numbers. Finally, the effects of some important parameters such as the cavitation index, inlet velocity, Froude number and wedge angle of cavitators on the geometrical characteristics of the super-cavities are discussed. Our numerical results are in good agreement with the experimental data.

원뿔 캐비테이터의 초공동 크기 추정

A comparative method is applied to evaluate well-known formulas for estimating the size of supercavities of axisymmetric cavitators for the supercavitating underwater vehicle. Basic functional forms of these formulas are derived first for the cavity diameter from a momentum integral estimate and second for the cavity length from an asymptotic analysis of inviscid supercavity flows. The length and the diameter of axisymmetric supercavities estimated by each formula are compared, with available experimental data for a disk and a 45° conical cavitators, and also with computational results obtained by a CFD code, ‘fluent’, for conical cavitators of wide range of cone angles. Results for estimating the length and the diameter of the supercavities show in general a good agreement, which confirms the size of the supercavities for disk and conical cavitators can be estimated accurately by these simple formulas of an elementary function of cavitation number and drag coefficient of the cavitator. These formulas will be useful for from conceptual design of the cavitator to real-time control of the supercavitating underwater vehicle.

Investigation of cavitation around 3D hemispherical head-form body and conical cavitators using different turbulence and cavitation models

In this paper, cavitation and supercavitation around 3D hemispherical head-form body and a conical cavitator were simulated. Dynamic and unsteady behaviors of cavitation were solved using large eddy simulation (LES) and k-ω SST turbulence models, as well as Kunz and Sauer mass transfer models. In addition, the compressive volume of fluid (VOF) method is used to track the cavity interface. Simulation is performed under the framework of the OpenFOAM package. The main contribution of this work is to present a correlation between the cavity length and diameter for hemispherical head-form bodies for the first time. Moreover, we provide a detailed comparison between different turbulence and mass transfer models over a broad range of cavitation numbers, especially in small cavitation numbers, including σ=0.07, 0.05, 0.02 for two cases, which is not reported previously. Our numerical results are compared with the available experimental data and a broad set of analytic relations for the cavity characteristics such as cavity length and diameter with suitable accuracy. Discussions on boundary layer separation and re-entrant jet behavior, which play a significant role in the bubble shedding in the cavity closure region, are presented.

Detailed investigation of cavitation and supercavitation around different geometries using various turbulence and mass transfer models

In this paper cavitation around 3D hemispherical head-form body as well as disk and conical cavitators were studied numerically with particular emphasis on detailed comparisons of the various turbulence and mass transfer models. Dynamic and unsteady behaviors of cavitation were simulated using the large eddy simulation (LES) and k-ω SST turbulence models together with Kunz, Sauer and Zwart mass transfer models. In addition, the compressive volume of fluid (VOF) method is used to track the cavity interface. Simulation is performed under the framework of the OpenFOAM package. Discussions on the boundary layer separation, re-entrant jet, cavity cloud, vortices and pressure/volume fraction contours over a broad range of cavitation numbers, especially in very low cavitation numbers, are provided. Our numerical results compared fairly well with experimental data and a wide set of analytical relations. Our results indicate that the most accurate solutions will be obtained with the LES/ Kunz approach. Moreover, for the first time, we present a correlation between the cavity length and diameter for hemispherical head-form bodies.

원뿔 캐비테이터의 항력에 대한 수치해석

In this paper, a numerical analysis is carried out to study the drag of conical cavitators, supercavity generation devices for the high-speed underwater vehicle. The realizable  turbulence model and the Schnerr-Sauer cavitation model are applied to calculate steady-state supercavitating flows around cones of various cone angles. The calculated drags of the cones are decomposed of the pressure and the friction parts and their dependency on the geometry and the flow conditions have been analyzed. It is confirmed that the pressure drag coefficients of the cones can be estimated by a simple function of both the cone angle and the cavitation number while the friction drag coefficients approximately by well-known empirical formulas, e.g., Schults-Grunow's for the drag of the flat plate. Finally a practical method for estimating the total drags of supercavitating cones is suggested, which can be useful consequently for the design of conical cavitaors. Ke ywords : Supercavitating flows(초공동 유동), Conical cavitators(원뿔 캐비테이터), Drag(항력)

A Numerical Study of the Formation of a Conical Cavitation Bubble Structure at Low Ultrasonic Frequency

This paper presents a study of the formation of a conical bubble structure due to cavitation at 20kHz in water. This analysis is performed by using the numerical code SNOW-BL, which solves the interaction of finite amplitude pressure waves and a population of oscillating bubbles. Axisymmetric simulations show that a thin bubbly layer with inhomogeneous bubble density at the horn surface induces the strong focusing of the transmitted ultrasonic field and three-dimensional resonances of the layer. Waves at high amplitudes also exhibit nonlinear distortion due to the bubbles. Similarities are found with the self-stabilized conical bubble structure observed experimentally at the surface of a horn driven at 20kHz in water.

Modification of the ultrasound induced activity by the presence of an electrode in a sono-reactor working at two low frequencies (20 and 40 kHz). Part II: Mapping flow velocities by particle image velocimetry (PIV)

Sonoelectrochemical experiments differ from sonochemical ones by the introduction of electrodes in the sonicated reaction vessel. The aim of the study is to characterize the changes in the ultrasonic activity induced by the presence of an electrode located in front of the transducer. The scope of our investigations concerns two low frequency vibration modes: 20 and 40 kHz. For this purpose, two laser visualization techniques have been used. The first part of the study, described in a previous paper (Part I), deals with the laser tomography technique which provides an accurate picture of the reactor active zones, related to numerous cavitation events. The second part of the paper (Part II) will describe the particle image velocimetry (PIV) technique used to measure the velocity vector field in the fluid portion between the horn and the electrode. As for the previous study, two parameters were studied: the electrical power supplied to the transducer and the electrode/transducer distance. The velocity vector fields show a main flow in the reactor axis. This flow seems to correspond to the conical cavitation bubbles structure which is observed on the laser tomography pictures. When an electrode is introduced into the reactor, two additional symmetric transversal flows can be quantified on both sides of the electrode.

Observation of multibubble phenomena in an ultrasonic reactor

A cylindrical ultrasonic reactor radiated by a BLT (bolted Langevin transducer) with a large horn operated at 27.3 kHz was employed to investigate the behaviors of bubbles in the present research. The characteristics of this reactor were analyzed using experimental methods. It was found that traveling transient bubbles can eliminate the effect of standing wave on cleaning efficiency. Fresh tap water and degassed water were employed as test liquids. In tap water, a conical cavitation structure was observed as the input voltage of transducer was increased over 100 V. The detail of the conical cavitation structure was anatomized and its shape varied with the change of input voltage. In degassed water, ultrasonic fireworks were observed and special attention was paid to their subsequences, snowflake bubble clouds and water jet.