Cavitation Pressure Research Articles

Impact response of flowing-fluid filled square vessels

The concept of cavitation drag reduction is widely known in many fields. However, the existence of cavitation is not beneficial to the protection of structures. In this study, an attempt was made to improve the protection efficiency of fluid-filled vessels by adopting the reverse approach of increasing drag forces by reducing cavitation: The vessels were filled with flowing fluids to reduce the pressure and change the shape of cavitation during penetration. Ballistic tests, numerical simulations, and theoretical analyses were conducted to evaluate the impact response of flowing-water filled square vessels impacted by a spherical projectile. The impact responses of flowing-water filled square vessels were analyzed and compared with static-water filled vessels, in terms of the deformation and failure of target panels as the vessel's walls, sidewall pressure time histories, and cavitation processes. Moreover, the mechanism of increasing drag forces due to the fluid flow was elucidated. The effects of fluid flow on the cavitation and pressure distribution were determined. This study elucidated the active attenuation mechanism of fluid flow on mitigating the damage of rear panel. The results show that fluid flow can increase the drag forces, leading to a higher protection efficiency of flowing-water filled vessels than static-water filled vessels. Owing to the effect of fluid flow, sidewall pressures are evidently lower than those in static fluid cases. Fluid flow can change pressure distributions in the vicinity of cavitation, which can alter not only the cavitation shape but also the action direction of cavitation jetting. The active attenuation effect of fluid flow predominantly contributes to mitigating the pressure accumulation on the inner surface of rear panel during penetration, resulting in lower pressures and subsequently less damage to the structures. This study sheds light on the benefits of flowing-fluid filling on increasing the drag forces as well as mitigating the cavitation pressures.

Effects of dissolved gas on surface pressure of a hydrofoil under cavitation condition

Abstract Dissolved gas in water is known as one of the factors which affect cavitation. The present study focuses on the effect of dissolved gas on cavitation around a hydrofoil, more specifically on the hysteresis of cavitation and surface pressure of the hydrofoil. The pressure measurement on a hydrofoil of Clark Y-11.7% was carried out under various dissolved gas conditions. Pressure in the test section was reduced stepwise from an atmospheric pressure to that in a super-cavitation condition, then was increased stepwise to the atmospheric pressure condition. Amount of dissolved oxygen (DO) was used as a parameter representing the degree of air content, and three DO conditions were set at low DO: under 30%, middle DO: around 50%, and high DO: over 70% of saturation under atmospheric pressure. Also, two angles of attack, 8 and 20 degrees were selected. Although there is no significant difference of the surface pressure and its hysteresis at the angle of attack of 8 degrees, dissolved gas seems to affect the behavior of cavitation. At the angle of attack of 20 degrees, it was found that the appearance of cavity was changed by DO conditions, and it would affect surface pressure of the foil. Under high DO condition, when the cavity oscillated with large amplitude, dissolved gas seemed to enhance the growth of the cavity, and the cavity tended to grow as covering the suction surface of the foil; it would make the time-averaged surface pressure of high DO condition lower than that of the other DO condition.

Numerical Investigation on Blockage-Related Cavitation Surge and Pressure Gain of a Mixed-Flow Pump With Influence of Blade Leading Edge Shape on Suction Performance

Abstract The blade leading edge is a design variable that can affect the local flow patterns and pressure peaks, implying a direct effect on the cavitation performance. This study was conducted to analyze the effect of the blade leading edge shape on the cavitation and noncavitation states. A total of four sets, including the square shape, were selected under the definition of ellipse ratio, and the main focus was on the cavitation state rather than the noncavitation state. In the noncavitation state, the square set denoted a remarkable negative influence, while the other three sets obtained almost the same performance despite different ellipse ratios. In the cavitation state, the square set obtained a relatively low net positive suction head required, related to the inlet flow pattern with the cloud cavity. The other three sets contained the sheet cavity, and their suction performance tended to improve as the cavity blockage decreased. As a parallel focus, an in-depth analysis of cavitation surge and pressure gain was presented with the head drop slope for the other three ellipse sets. The numerical results included the off-design flow rate points and were validated through an experimental test.

A comprehensive study of hole-to-hole interaction for multi-hole orifice (MHO) in hydrodynamic cavitation process

The hydrodynamic cavitation (HC) as an influential advanced oxidation technology is commonly used for treating organic wastewater. In the present research, a CFD model is used to study the hydrodynamic cavitation in multi-hole orifice (MHO). Accordingly, the effect of hole-to-hole interaction in MHO and its influence on the bubble generation and the flow turbulence downstream of the orifice are analyzed. During the cavitation, the central hole exhibits the highest flow rate, i.e., strong jet, over other jets for the dimensionless hole's pitch circle, δ, equal to 50 and 75%. This causes the occurrence of the pitchfork bifurcation phenomenon, and thereby a high level of turbulence intensity behind MHO especially for δ=50%. The value of δ also affects the streamlines curvature at the hole entrance. This curvature influences on the bubble generation due to a reduction in the pressure gradient. The bubble distribution is not uniform at the entrance of each holes because of non-uniform flow separation at the tip holes. The minimum cavitation number and pressure loss are obtained at δ equal to 50%. The MHO thickness, tD=0.5, provides more guidance to the outlet jet and thereby a decrease of the jet's interaction. Also, the results show the significant impact of MHO thickness on the issued jet, which in turn affects the turbulence intensity (i.e., bubble life). Alternatively, increasing the number of holes rather than using larger holes leads to more bubble formation from each hole when the opening area ratio remains constant. Finally, the change in operating conditions of the inlet pressure and the working temperature is studied and the results are discussed.

Hydrodynamic ram analysis in high-speed projectile penetrating into water-filled vessels

Hydrodynamic ram (HRAM) is a phenomenon that occurs when a high-speed projectile penetrates a fluid-filled tank, where the energy transferred to the liquid could result in a catastrophic failure and excessive structural damage. It is essential to take physical comprehension of the hydrodynamic effects that occur during an HRAM event in the civilian domain as well as for the military aircraft design, which would in fact contribute to design better structures with respect to this particular threat. For this present work, the HRAM due to cavity evolution in confined vessels under ballistic impact by projectiles was investigated through theoretical analysis combined with experimental results. Taking the confinement effects due to the limited size of the fluid-filled vessel and corresponding structure deformation into account, a model is proposed to describe the pressure between the liquid and vessel wall along the penetration trajectory based on cavity dynamics analysis. By studying relevant parameters, changes of wall pressure, non-dimensional disturbance range and energy of two extreme containers, (namely, non-pressure resistance containers and rigid containers without deformation) were examined. The results show that the cavitation pressure has less intense peak but much greater temporal extent compared with the pressure generated during the shock stage, thereby the cavity pressure impulse induced by cavity evolution is a non-negligible factor affecting the deformation and failure of a fluid-filled vessel. The aim of the present work is to reveal the mechanism of the pressure acting on the container wall caused by cavity evolution rather than the initial shock wave in an HRAM event, which will help to study the nature of hydrodynamic ram and understand the role of the cavitation pressure in the damage or deformation of liquid-filled vessels under projectile impact.

Numerical Study of Mitigating Cloud Cavitation Shedding Using Biomimetic Protuberant Stripes

Abstract Cloud cavitation usually appears on impeller blades of hydraulic machinery. When unsteady cloud cavitation travels downstream to a high-pressure region and collapses, performance drops and cavitation erosion appears. It is important to develop effective methods to mitigate the undesirable effects. By million years of natural selection, bird feather has presented excellent flow performance. As typical characteristic structure of bird feather, a number of barbs are supported by a central hollow shaft. Under inspiration of bird feathers, we presented a method of passive cavitation control. This method of passive cavitation control uses biomimetic protuberant stripes (PSs) mounted on a NACA66 hydrofoil. The effects of various biomimetic PS arrangements on the cavitation pattern, pressure fluctuation, and hydrodynamic load are numerically studied. Then, the cavitation control mechanism of the biomimetic PS is analyzed in detail. We observe that the biomimetic PSs not only inhibit the shedding of large-scale cloud cavitation but also reduce the cavitation size. Moreover, analysis shows that turbulence velocity fluctuation may be mitigated and boundary layer thickness is reduced with biomimetic PSs, which enhances the flow intensity in the main flow direction.

Study of the Extraction Kinetics and Calculation of Effective Diffusivity and Mass Transfer Coefficient in Negative Pressure Cavitation Extraction of Paclitaxel from Taxus chinensis

Study of the Extraction Kinetics and Calculation of Effective Diffusivity and Mass Transfer Coefficient in Negative Pressure Cavitation Extraction of Paclitaxel from Taxus chinensis

The role of cavitation in drying cementitious materials

This study examines the role of cavitation in drying cementitious materials. Classical nucleation theory is used to understand and study the parameters that influence cavitation pressures. The water desorption isotherms indicate that both heterogeneous and homogeneous cavitation occur during the drying of saturated cement pastes and the commonly observed large moisture loss at 0.3 pv/psat can be explained by homogeneous cavitation. The homogeneous cavitation is suggested to occur in the gel porosity of the inner product that is accesible by the interlayer spaces. The findings of this study indicate that it is important to consider the effects of cavitation for characterization of pore sizes from moisture desorption isotherms.

Investigation on Unsteady Cavitation Flow and Excited Pressure Fluctuations in Regulating Valve

A multi-field synchronous measurement system for the cavitation flow in a regulating valve was established. The system combines a high-speed full-flow field display system with a pressure measurement system to realize the simultaneous acquisition of cavitation shapes and pressure pulsations. Cavitation flow occurs near the throttle orifice, which is obviously a quasi-periodic behavior. The unsteady cavitation flow mainly includes three stages: the growth of the attached cavity, the fracture and shedding of the attached cavity and the growth and collapse of the free cavity. The time evolution of the cavitation behaviors is highly related with excited pressure fluctuations. With the increasing attached cavity area, the corresponding pressure in the flow field decreases slowly. When the attached cavity falls off and develops downstream, the cavity area decreases gradually, and the pressure increases gradually. When the free cavity shrinks and collapses, the pressure in the flow field reaches the peak value. The pressure pulsation and the change of cavity area have the same dominant frequency, around 2000 Hz, at the monitoring point in the upstream, throat and expansion monitoring points. Furthermore, with increasing inlet pressure, the mean and variance values of cavitation area become larger, and the excited pressure fluctuation at each measuring point becomes more intense. The mean value of pulsating pressure at the throat gradually increases, while the pressure in the expansion section presents a downward trend. The variance of pressure pulsation and the maximum pressure also increase gradually with the increase in inlet pressure. The change of cavitation area and the pressure pulsation in the regulating valve complement each other. The results in this paper could provide experimental guidance on optimizing the structure of the valve, inhibiting cavitation occurrence and prolonging the service life of the valve.

Negative pressure cavitation fractional precipitation for the purification of paclitaxel from Taxus chinensis

Negative pressure cavitation fractional precipitation for the purification of paclitaxel from Taxus chinensis

Investigation on cavitation initiation in jet pump cavitation reactors with special emphasis on two mechanisms of cavitation initiation

Hydrodynamic cavitation is used in various engineering fields, such as water treatment and chemical processing. Cavitation initiation is critical to assessing the cavitation effect in cavitation reactors, so cavitation initiation should be studied to improve cavitation in various fields. This paper investigates the mechanisms and characteristics of cavitation initiation in jet pump cavitation reactors (JPCRs) to intensify the cavitation. A criterion derived for cavitation initiation focuses on the fluctuating pressures caused by the velocity gradients. Experiments show that the criterion predicts low pressure cavitation and strong shear dominant cavitation in a JPCR. Then, the characteristics of these two cavitation mechanisms are investigated for various geometric parameters and operating conditions and a cavitation initiation regime map is developed for JPCR. The results show that cavitation occurs more easily, and the two cavitation mechanisms become increasingly more difficult to distinguish in the JPCR as the inlet pressure increases. The velocity gradient and loss coefficient increase with a decrease in suction pressure, which promotes cavitation. The low pressure cavitation and strong shear dominant cavitation are both more likely when the area ratio is relatively small. The induced pressure spectra further show that the low frequency component dominates during the formation of the low pressure cavitation, while the high frequency component dominates during the formation of the strong shear dominant cavitation. This study clarifies the cavitation mechanisms with special emphasis on the fluctuating pressure and the corresponding characteristics in the JPCR to provide guidance for JPCR applications in industry.

Examination of Viscosity Effect on Cavitating Flow inside Poppet Valves Based on a Numerical Study

The higher susceptibility to cavitation in poppet valves due to the lower viscosity of water than the traditionally used mineral oil poses a challenge in fluid transmission technology. To reveal the underlying mechanism of cavitating flow physics associated with the variation in viscosity effect, the current paper examines both the water and oil cavitating flow dynamics inside poppet valves with varied structures through a numerical study. The simulation results are validated with a comparison to previous experimental data in terms of cavitation morphology and pressure distribution. According to the predicted cavitation distribution, three kinds of cavitation occurred at separated positions in both water- and oil-flow cases. The vortex cavitation, which in the oil-flow case displays a remarkable paired structure with favorable coherence, is featured with a scattered dispersion in the water-flow case, while the profound attached cavitation at the poppet trailing edge in the water-flow case almost disappears in the oil-flow case. Furthermore, the attached cavitation within the chamfered groove has higher stability in the oil-flow case, compared to the thorough detachment behavior featured with profound 3-dimensionality in the water-flow case. According to the potential core and vortex evolution, the strong 3-dimensionality due to the violent laminar-turbulent transition in the water-flow case together with the produced puff pattern of the potential core, to a large extent, interrupts the periodic behavior of cavitation, which is essentially preserved in the oil-flow case featured with favorable coherence.

Reynolds Model versus JFO Theory in Steadily Loaded Journal Bearings

Cavitation has a potential effect on the performance of full circle journal bearings. This paper studied the effects of cavitation on steadily loaded journal bearings, with the purpose of analyzing the necessity of adopting a mass-conserving model for ordinary journal bearings. The Christopherson’s method and Elrod cavitation algorithm were implemented to represent the non-mass-conserving Reynolds model and the mass-conserving Jakobsson-Floberg-Olsson (JFO) theory, respectively. The difference in the oil film reformation boundaries predicted by the two methods was focused on. The typical performance parameters including oil film pressure, load-carrying capacity, attitude angle, friction force, and leakage were comprehensively compared. The results show that the load-carrying capacity is improved by the decrease in cavitation pressure, and the effect is significant in lightly loaded cavitated bearings. In non-cavitated cases and the cavitated cases with intermediate and heavy loads, the difference between the Reynolds model and the JFO theory can be effectively ignored, but the accuracy of the leakage predicted using the Reynolds model should be carefully evaluated.

Experimental and numerical study on cavitation under elastohydrodynamic lubrication point contact

As a common phenomenon in elastohydrodynamic lubrication, cavitation has an effect on the completeness of the oil film in the contact area. Many studies have therefore been conducted on cavitation. Experimental researches on cavitation usually rely on optical interference observation, which offers a limited resolution and observation range. In this paper, an infrared thermal camera is used to observe the cavity bubbles on a ball-on-disc setup under sliding/rolling conditions. The results show that the cavity length increases with an increases of the entrainment speed and the viscosity of the lubricants. These observations are explained by a numerical model based on Elrod's algorithm. Effects of entrainment speed and lubricant viscosity on the breakup of cavitation bubbles and the cavitation states are investigated. Both the simulation and experimental results show that a negative pressure area is present behind the Hertzian contact area. The ambient pressure plays a role in maintaining cavitation state 1. The cavitation pressure is close to the vacuum pressure when the entrainment speed is low and to the ambient pressure instead when the entrainment speed is high.

Liquid Cavitation during Nitrogen Sorption on Soils

The nitrogen sorption isotherm is conventionally used to deduce the specific surface area of porous materials. However, it often exhibits a sharp drop around 0.5 relative pressure. A theory explicitly accounting for intermolecular-scale pressure, instead of classical theories of constant disjoining pressure in condensed liquid, is constructed and used to determine cavitation during desorption. Intermolecular-scale liquid pressure distribution is quantified using a recently developed soil sorptive potential framework, showing compressive liquid nitrogen pressure decaying nonlinearly with increasing distance to the particle surface. A range of cavitation pressure is predicted by classical nucleation theory and the van der Waals equation of state. Cavitation is shown to be triggered when nitrogen’s global minimum liquid pressure falls within the cavitation threshold. It is shown that this criterion is valid for all tested soils. Computed minimum liquid pressure always occurs at 0.5 relative pressure, which is in accordance with experimental isotherm data and further indicates the validity of the cavitation onset criterion.

Water cavitation from ambient to high temperatures

Predicting cavitation has proved a formidable task, particularly for water. Despite the experimental difficulty of controlling the sample purity, there is nowadays substantial consensus on the remarkable tensile strength of water, on the order of −120 MPa at ambient conditions. Recent progress significantly advanced our predictive capability which, however, still considerably depends on elaborate fitting procedures based on the input of external data. Here a self-contained model is discussed which is shown able to accurately reproduce cavitation data for water over the most extended range of temperatures for which accurate experiments are available. The computations are based on a diffuse interface model which, as only inputs, requires a reliable equation of state for the bulk free energy and the interfacial tension. A rare event technique, namely the string method, is used to evaluate the free-energy barrier as the base for determining the nucleation rate and the cavitation pressure. The data allow discussing the role of the Tolman length in determining the nucleation barrier, confirming that, when the size of the cavitation nuclei exceed the thickness of the interfacial layer, the Tolman correction effectively improves the predictions of the plain Classical Nucleation Theory.

Effects of Wavy Leading-Edge Protuberance on Hydrofoil Performance and Its Flow Mechanism

This paper examines the effects on a Clark-y three-dimensional hydrofoil of wavy leading-edge protuberances in a quantitative and qualitative way. The simulation is accompanied by a hybrid RANS-LES model in conjunction with Zwart-Gerber–Belamri model. Detailed discussions of the stable no-cavitating, unsteady cavitating flow fields and the control mechanics are involved. The force characteristics, complicated flow behaviors, cavitation–streamwise vortex interactions, and the cavitating flow instability are all presented. The results demonstrate that protuberances acting as vortex generators produce a continuous influx of boundary-layer vorticity, significantly enhancing the momentum transfer of streamwise vortices and therefore improving the hydrodynamics of the hydrofoil. Significant interactions are described, including the encouragement impact of cavitation evolution on the fragmentation of streamwise vorticities as well as the compartmentation effect of streamwise vorticities binding the cavitation inception inside the troughs. The variations in cavitation pressure are mainly due to the acceleration in steam volume. In summary, it is vital for new hydrofoils or propeller designs to understand in depth the effects of leading-edge protuberances on flow control.

Spatio-temporal evolution mechanism of cavitation vortex ropes in a swirling flow

Cavitation vortex rope widely occurs in hydraulic machinery, leading to the decrease in performance characteristic and increase in pressure fluctuation. The objective of this study was to explore the generation and transformation of cavitation vortex ropes in a swirling flow. A visual swirling-flow generator platform was designed to investigate their spatiotemporal evolution mechanism. A flow pattern observation system with a high-speed camera was built to capture the vortex rope forms, and pressure fluctuation experiments were carried out to present fluctuating characteristics of corresponding cavitation vortex ropes. Cavitation vortex rope forms and pressure fluctuation characteristic under different operating conditions were exhibited. Four types of stable cavitation vortex ropes (broken, dual, single, and subulate) were observed. Regional distribution of vortex ropes under different Reynolds and cavitation numbers was characterized, which showed that broken and subulate vortex ropes account in large Reynolds and cavitation ranges. Pressure fluctuation analyses revealed dominant characteristic frequencies were 2.13, 1.98, 1.74, and 1.93 times the rotational frequency of the runner for the broken, dual, single, and subulate cavitation vortex ropes, respectively. In addition, two unstable transitions were identified during the conversion process. One is an unstable transitional triple-vortex rope during from a dual- to single-vortex rope process, and the other is an unstable subulate-vortex rope between the occurrence of the single- and stable subulate-vortex ropes. The present study could give a deep understanding of the generation of cavitation vortex ropes and provide some references to improve the hydraulic instabilities induced by cavitation vortex ropes in hydraulic machinery.

Innovation Ultrasonic Assistant Soldering in Electronics

Innovative approaches in ultrasonic assistant soldering consist consists of increasing the activity of cavitation and accelerating diffusion processes at the interface between the solder and the soldering material. Besides that, it improves the effectiveness of cavitation processes in melts by saturating gas cavities with diamete rs that are smaller than the resonant sizes of cavitation germs. Gas saturation of liquids and melts raises level of cavitation pressure by 20 25%25%, that intensifies US processing of cleaning, soldering and metallization. Modelling diffusion process showed that the US activation increased the concentration of diffusing elements of Zn and Al in the interface depth by 15 20% on average, and the combined activation by the US and electric field increased it by 30 45%. Furthermore, as the energy quantity adsorbed by melt increases, increased amplitude and frequency of US vibrations induces concentration rise. The heat energy was also boosted by combining the activation of the melt–soldered material system with US vibrations energy and high current pulses. This allows for a faster increase in soldering temperature, as well as improved solder wettability.

Cavitation of Water in Soil

Despite increasing awareness that cavitation can be a potential mechanism for drainage of water from soil, comprehensive understanding of its effects on soil water retention (SWR) remains largely elusive. A major obstacle is that pore water pressure is commonly conceptualized and measured as a mean value spatially averaged over the entire representative soil volume, including solid, liquid, and gas phases. Water pressure defined as such does not represent the intermolecular pressure that dictates the cavitation phase change process. Classical theories for predicting water cavitation pressure are critically assessed using recent theory for soil sorptive potential and synthesized to determine cavitation of water in soil. Classical cavitation mechanisms of metastable nucleation that occur on a scale of less than tens of nanometers are unlikely to occur in soil water. Cavitation from entrapped bubbles on rough surfaces of soil particles on a scale of hundreds of nanometers, however, is identified as responsible for soil water cavitation. Specifically, cavitation can occur for soils with pore sizes ranging from 10−8 to 10−4 m with suction of 0.1–15 MPa but is most prominent in soils with pore sizes from 10−7 to 10−5 m and suction of 0.1–2.2 MPa (silty soils). A framework for soil water cavitation is proposed, parametrically analyzed, and experimentally validated against measured SWR data obtained using axis translation (AT) and hygrometer-based methods. Results indicate that AT can artificially suppress cavitation that otherwise may be a viable water drainage mechanism for soils in the natural environment.