Formation Of Supercavity Research Articles

Experimental investigation of high-temperature water vapour ventilated supercavitation

In the current landscape of underwater vehicle development, high-tech advances are increasingly being utilised, especially in the application of high-speed underwater equipment. Based on the concept of water ram engine operation, the formation of a ventilated supercavity by converting seawater into high-temperature water vapour has the potential to reduce the consumption of ventilating gases. However, the kinetic process that promotes the formation of a supercavity by ventilating it with high-temperature water vapour remains unexplored. This study attempts to bridge this knowledge gap by carefully investigating the cavitation flow around an axisymmetric object under various high-temperature water vapour ventilation conditions. Experiments conducted in a cavitation tunnel equipped with a high-temperature water vapour generator showed that the formation of a transparently ventilated supercavity requires a significant increase in the ventilation rate compared to the ventilation rate facilitated by room-temperature non-condensable gases. The injection of high-temperature water vapour produces perturbations along the cavity boundaries, thereby triggering interfacial instability, a phenomenon that is not present in super-cavities generated by non-condensable gases. Further observations revealed a correlation between the elevated ventilation temperature and elevated cavity boundary instability. In addition, the stability of a high-temperature water vapour ventilated supercavity (HWVVS) exhibits high sensitivity to external environmental conditions. The irrational pairing of the ambient cavitation number and ventilation volume poses a potential risk of supercavity collapse.

Investigation of ventilation demand variation in unsteady supercavitation

Understanding the air injection strategy of a ventilated supercavity is important for designing high-speed underwater vehicles wherein an artificial gas pocket is created behind a flow separation device to reduce skin friction. Our study systematically investigates the effect of flow unsteadiness on the ventilation requirements to form (CQf) and collapse (CQc) a supercavity. Imposing flow unsteadiness on the incoming flow has shown an increment in higher CQf at low free stream velocity and lower CQf at high free stream velocity. High-speed imaging reveals distinctly different behaviors in the recirculation region for low and high freestream velocity under unsteady flows. At low free stream velocities, the recirculation region formed downstream of a cavitator shifted vertically with flow unsteadiness, resulting in lower bubble collision and coalescence probability, which is critical for the supercavity formation process. The recirculation region negligibly changed with flow unsteadiness at high free stream velocity and less ventilation is required to form a supercavity compared to that of the steady incoming flow. Such a difference is attributed to the increased transverse Reynolds stress that aids bubble collision in a confined space of the recirculation region. CQc is found to heavily rely on the vertical component of the flow unsteadiness and the free stream velocity. Interfacial instability located upper rear of the supercavity develops noticeably with flow unsteadiness and additional bubbles formed by the distorted interface shed from the supercavity, resulting in an increased CQc. Further analysis on the quantification of such additional bubble leakage rate indicates that the development and amplitude of the interfacial instability accounts for the variation of CQc under a wide range of flow unsteadiness. Our study provides some insights on the design of a ventilation strategy for supercavitating vehicles in practice.

Ventilated supercavitation around a moving body in a still fluid: observation and drag measurement

This experimental study examines ventilated supercavity formation in a free-surface bounded environment where a body is in motion and the fluid is at rest. For a given torpedo-shaped body and water depth ($H$), depending on the cavitator diameter ($d_{c}$) and the submergence depth ($h_{s}$), four different cases are investigated according to the blockage ratio ($B=d_{c}/d_{h}$, where$d_{h}$is the hydraulic diameter) and the dimensionless submergence depth ($h^{\ast }=h_{s}/H$). Cases 1–4 are, respectively, no cavitator in fully submerged ($B=0$,$h^{\ast }=0.5$), small blockage in fully submerged ($B=1.5\,\%$,$h^{\ast }=0.5$), small blockage in shallowly submerged ($B=1.5\,\%$,$h^{\ast }=0.17$) and large blockage in fully submerged ($B=3\,\%$,$h^{\ast }=0.5$) cases. In case 1, no supercavitation is observed and only a bubbly flow (B) and a foamy cavity (FC) are observed. In non-zero blockage cases 2–4, various non-bubbly and non-foamy steady states are observed according to the cavitator-diameter-based Froude number ($Fr$), air-entrainment coefficient ($C_{q}$) and the cavitation number ($\unicode[STIX]{x1D70E}_{c}$). The ranges of$Fr$,$C_{q}$and$\unicode[STIX]{x1D70E}_{c}$are$Fr=2.6{-}18.2$,$C_{q}=0{-}6$,$\unicode[STIX]{x1D70E}_{c}=0{-}1$for cases 2 and 3, and$Fr=1.8{-}12.9$,$C_{q}=0{-}1.5$,$\unicode[STIX]{x1D70E}_{c}=0{-}1$for case 4. In cases 2 and 3, a twin-vortex supercavity (TV), a reentrant-jet supercavity (RJ), a half-supercavity with foamy cavity downstream (HSF), B and FC are observed. Supercavities in case 3 are not top–bottom symmetric. In case 4, a half-supercavity with a ring-type vortex shedding downstream (HSV), double-layer supercavities (RJ inside and TV outside (RJTV), TV inside and TV outside (TVTV), RJ inside and RJ outside (RJRJ)), B, FC and TV are observed. The cavitation numbers ($\unicode[STIX]{x1D70E}_{c}$) are approximately 0.9 for the B, FC and HSF, 0.25 for the HSV, and 0.1 for the TV, RJ, RJTV, TVTV and RJRJ supercavities. In cases 2–4, for a given$Fr$, there exists a minimum cavitation number in the formation of a supercavity while the minimum cavitation number decreases as the$Fr$increases. In cases 2 and 3, it is observed that a high$Fr$favours an RJ and a low$Fr$favours a TV. For the RJ supercavities in cases 2 and 3, the cavity width is always larger than the cavity height. In addition, the cavity length, height and width all increase (decrease) as the$\unicode[STIX]{x1D70E}_{c}$decreases (increases). The cavity length in case 3 is smaller than that in case 2. In both cases 2 and 3, the cavity length depends little on the$Fr$. In case 2, the cavity height and width increase as the$Fr$increases. In case 3, the cavity height and width show a weak dependence on the$Fr$. Compared to case 2, for the same$Fr$,$C_{q}$and$\unicode[STIX]{x1D70E}_{c}$, case 4 admits a double-layer supercavity instead of a single-layer supercavity. Connected with this behavioural observation, the body-frontal-area-based drag coefficient for a moving torpedo-shaped body with a supercavity is measured to be approximately 0.11 while that for a cavitator-free moving body without a supercavity is approximately 0.4.

On the synergistic interrelation between supercavity formation through vaporous and ventilated routes

A supercavity can be attained via two distinct routes: vaporous and ventilated supercavitation. A vaporous supercavity is one that is obtained by the coalescence of individual vapor bubbles formed by cavitation. On the other hand, a ventilated supercavity results from the coalescence of bubbles of non-condensable gas formed through gas injection at the rear part of the cavitator. A typical strategy of operation of a supercavitating underwater vehicle entails an interplay between these two modes viz. accelerating it to a high speed using ventilation, when a natural supercavity can be sustained. This fact necessitates a systematic study of the synergistic relationship between the process of supercavity formation under these two modes. Thus, in the current work, we have systematically carried out water tunnel experiments to study the effect of vaporous and ventilated modes of supercavitation on each other during supercavity formation. The results show a systematic dependence between the relevant parameters which include the formation natural cavitation number and the formation gas entrainment coefficient. Further, the effects of change in Froude number and blockage are also reported and discussed.

Study on Effects of The Shape of Cavitator on Supercavitation Flow Field Characteristics

The cavitator is the key part of the nose of the vehicle to induce the formation of supercavity, which has an important influence in the cavity formation rate, cavity shape and cavity stability. To study the influence of the shape on the supercavitation flew field characteristics, the cavity characteristics and the resistance characteristics of different shapes of cavitator under different working conditions are obtained by combining technical methods of numerical simulation and experimental research in water tunnel. The simulation results are contrast and analyzed with the test results. The analysis results show that : in terms of the cavity size, the inverted-conic cavitator can form the biggest cavity size, followed by the disk cavitator, and the truncated-conic cavitator is the least; in terms of the cavity formation speed, the inverted-conic cavitator has the fastest cavity formation speed, then is the truncated-conic cavitator, and the disk cavitator is the least; in terms of the drag characteristic, the truncated-conic cavitator has the maximum coefficient, disk cavitator is the next, the inverted-conic cavitator is the minimal. The research conclusion can provide reference and basis for the head shape design of supercavitating underwater ordnance and the design of hydrodynamic layout.

Supercavitating flow around high-speed underwater projectile near free surface induced by air entrainment

Cavitating flow near free surface is a complicated issue and may provide new inspiration on high-speed surface cruising. This study observes stable supercavitating flow as a new phenomenon in a launch experiment of axisymmetric projectile when the upper side of the projectile coincides with the free surface. A numerical approach is established using large eddy-simulation and volume-of-fluid methods, and good agreements are achieved between numerical and experimental results. Supercavity formation mechanism is revealed by analyzing the experiment photographs and the iso-surface of 90% water volume fraction in numerical results. The entrainment of a large amount of air into the cavity can cause the pressure inside the cavity to similarly increase with the pressure outside the cavity, which makes the actual cavitation number close to zero and is similar to supercavitation. Cases with various headforms of the projectile and cavitation numbers on the cavitating flow, as well as the drag reduction effects are further examined. Results indicate that the present strategy near the free surface could possibly be a new effective approach for high-speed cruising after vigorous design optimization in the future.

2차원 몰수체의 형상 변화에 따른 초월공동 수치해석

A cavitator plays an important role to generate the supercavity. Most previous numerical and experimental studies have been focused on the presence of cavitators alone. However, the body behind the cavitator causes a change in the wake flow and hence it affects generation and growth of the supercavity. In this paper, we present a boundary elementary method based on a potential flow analysis, and calculate characteristics of the supercavity formation depending on the change of the body shape of two-dimensional submerged objects. Various parameters such as cone angle of the cavitator, length of the forehead and diameter of the body are considered. The results show that the longer the forepart length, the longer the cavity is created under the same conditions, and also the change in the diameter of the body is the most influential factor on the growth of the supercavity. As a result, we suggest that it is necessary to carefully consider the influence of the body shape during the initial design stage of the supercavitating underwater vehicle.

Exploring the Relationship between Natural and Artificial Modes of Supercavitation

A supercavity can be attained via two distinct routes: vaporous and ventilated supercavitation. A vaporous supercavity is one that is obtained by the coalescence of individual vapor bubbles formed by cavitation. On the other hand, a ventilated supercavity results from the coalescence of bubbles of non-condensable gas formed through gas injection at the rear part of the cavitator. A typical strategy of operation of a supercavitating underwater vehicle entails an interplay between these two modes viz. accelerating it to a high speed using ventilation, when a natural supercavity can be sustained. This fact necessitates a systematic study of the synergistic relationship between the process of supercavity formation under these two modes. Thus, in the current work, we have systematically carried out water tunnel experiments to study the effect of vaporous and ventilated modes of supercavitation on each other during supercavity formation. The results show a systematic dependence between the relevant parameters which include the natural cavitation number and the gas entrainment coefficient.

A comparative study of natural and ventilated supercavitation across two closed-wall water tunnel facilities

Despite half a century of experimental investigation into both natural and ventilated supercavitation, there are still significant discrepancies among the results, in terms of supercavity geometry and ventilation demand, etc., under approximately similar conditions from different water tunnel facilities. To understand the influences of the flow facilities on the supercavitation experiments, a systematic comparison is conducted using the results from two closed-wall water tunnels, i.e. the Saint Anthony Falls high-speed water tunnel and the Chuangnam National University Closed Tunnel. For both ventilated and natural supercavitation, the experimental conditions from the two facilities are designed to match over a wide range of Froude number and blockage ratio, etc. For the ventilated supercavitation, the cavitation number for generating a ventilated supercavity and the hysteresis process for sustaining a supercavity show a proper match across the two facilities while holding the Froude number and blockage ratio constant. However, the ventilation demand to form a supercavity shows a noteworthy difference across the facilities even under the same Froude number and blockage ratio. Such a difference in the ventilation requirement is attributed to the mismatch of Reynolds number, the detailed geometry of the cavitator models as well as the test section which influences the pressure distribution along the span of the supercavity. Similarly, for natural supercavitation, both facilities yield a similar vaporous cavitation number for the supercavity formation under the same Froude number and blockage ratio, as well as similar choking behavior, i.e. cavitation number stays constant despite the decrease of test section pressure once a natural supercavity forms. The theoretical analysis of the choking phenomenon explains the trend of cavitation number under choking and its dependence on cavitator geometry, Froude number as well as the pressure loss in the water tunnel. A geometry comparison is conducted for both natural and ventilated supercavities in the two facilities under the same Froude number, blockage ratio and cavitation number. The comparison results show differences in the normalized cavity total length across different facilities as well as supercavity types despite the similarities in the supercavity maximum diameter and half-length. These differences were attributed to the variance in the pressure and flow distributions from the different facilities and across ventilated and natural supercavitation. The natural supercavities from the two facilities are further compared with the estimated natural supercavitation in unbounded flow under the same cavitation conditions. The comparison result highlights the limitation of the conventional theory in capturing the cavity geometries in actual experiments.

Air entrainment characteristics of artificial supercavities for free and constrained closure models

Artificial supercavitation is a promising technology for the drag reduction of high-speed underwater vehicles, whereby a cavity is generated through air entrainment to encompass the entire vehicle. Supercavitation experiments are conventionally conducted in closed wall water tunnels. In order to properly extend these results to the operation of underwater vehicles in the ocean, it is imperative to understand the underlying physics governing these processes. The current study focuses upon the factors that govern the ventilation demand of a supercavitating vehicle operating under artificial supercavitation mode. Systematic artificial supercavitation experiments are carried out in a high-speed water tunnel with two different experimental models and high-speed visualization and measurements of air entrainment are carried out at different stages of supercavity formation, sustenance and collapse. Our measurements suggest a marked difference between the two experimental models for formation, sustenance and critical air entrainments of supercavities. Such differences are explained based on the understanding of fundamental physical factors governing the process, such as bubble breakup, coalescence and the effect of the presence of a body inside the supercavity.

An experimental investigation into supercavity closure mechanisms

Substantial discrepancy in the conditions for attainment of different closure modes of a ventilated supercavity has existed widely in the published literature. In this study, supercavity closure is investigated with an objective to understand the physical mechanisms determining closure formation and transition between different closure modes and to reconcile the observations from prior studies under various flow settings. The experiments are conducted in a closed-wall recirculating water tunnel to image ventilated supercavity closure using high speed and high-resolution photography and simultaneously measure pressure inside the cavity. The flow conditions are varied systematically to cover a broad range of water velocity, ventilation flow rate and cavitator size, which correspond to different Froude numbers, air entrainment coefficients and blockage ratios, respectively. In addition to the classical closure modes reported in the literature (e.g. re-entrant jet, twin vortex, quad vortex, etc.), the study has revealed a number of new closure modes that occur during the transition between classical modes, or under very specific flow conditions. Closure maps are constructed to depict the flow regimes, i.e. the range of Froude number and air entrainment coefficient, for various closure modes at different blockage ratios. From the closure map at each blockage ratio, a critical ventilation flow rate, below which the supercavity collapses into foamy cavity upon reduction of Froude number, is identified. The air entrainment coefficients corresponding to such critical ventilation rate are found to be independent of blockage ratio. It has been observed that in the process of generating a supercavity by increasing ventilation flow rate, the cavitation number gradually reduces to a minimum value and stays fixed upon further increments in the ventilation rate. Once a supercavity is formed, the ventilation rate can be decreased to a much lower value with no change in cavitation number while still maintaining a supercavity. This process is accompanied by a change in closure modes, which generally goes from twin vortex, to quad vortex, and then to re-entrant jet. In addition, the blockage effect is shown to play an important role in promoting the occurrence of twin-vortex closure modes. Subsequently, a physical framework governing the variation of different closure modes is proposed, and is used to explain mode transition upon the change of flow conditions, including the blockage effect. This framework is further extended to shed light on the occurrence of closure modes for ventilated supercavitation experiments across different types of flow facilities, the natural supercavity closure and the pulsating supercavity reported in the literature. Finally, in combination with a recent numerical study, our research discusses the role of the internal flow physics on the observed features during supercavity formation and closure-mode transition, paving the way for future investigations in this direction.

수중 운동체 주변에 형성되는 환기 초공동(ventilated supercavitation) 현상 가시화

A laboratory experiment was carried out to observe and visualize ventilated supercavitation phenomena around a moving underwater body which is attached to a newly designed high-speed (Max. 20 m/s) carriage system in a wave tank. Compared to the existing many other experimental studies using cavitation tunnels, where the body is at rest and the fluid is in motion in a bounded or closed environment, the present experimental study deals with super-cavity formation in unbounded or free-surface bounded environments, where the body is in motion and the fluid is at rest. Main attention is paid to the effective visualization of the steady-state cavity formations around a moving body and, those cavity formations are reported pictorially according to the body speed, ventilated air-pressure, and with or without a cavitator.

Analytical and experimental study of hydrodynamic and hydroacoustic effects of air injection flow rate in ventilated supercavitation

Formation of supercavity around an underwater vehicle improves the hydrodynamic performance of fluid flow and causes vehicle to reach higher speeds. However, sound generated by the supercavitation is very effective in recognition of the vehicle and reduction in its controlled precision. In this study, compressible Navier–Stocks equations were solved by using the method of Large Eddy Simulation and combining them with the physical source and sink model to simulate supercavitation flow numerically. The qualitative and quantitative parameters from numerical results compared and confirmed with experimental results. Subsequently, by considering a suitable control surface around the simulated supercavity, the needed parameters for acoustic analysis were provided. Eventually, by solving the hybrid hydroacoustic model presented by Riahi et al. the far field noise of supercavity was estimated. In light of important role of air injection flow rate in the formation of ventilated supercavity, hydrodynamic and hydroacoustic results for three different lengths of supercavity stemmed by different air injections, were compared with each other. According to the obtained results, acoustic behavior of the supercavity was changed by increasing of air injection.