Cavitation Instabilities Research Articles

Numerical investigation of passive cavitation control using a slot on a three-dimensional hydrofoil

Purpose The purpose of this paper is to study the mechanism and suppression of instabilities induced by cavitating flow around a three-dimensional hydrofoil with a particular focus on cavitation control with a slot. Design/methodology/approach The transient cavitating flow around a Clark-Y hydrofoil was investigated using a transport-equation-based cavitation model and the stress-blended eddy simulation model was used to capture the flow turbulence. A homogeneous Rayleigh–Plesset cavitation model was used to model the transient cavitation process and the results were validated with test data. A slot was applied to the hydrofoil to suppress cavitation instabilities, and various slot widths and exit locations were applied to the blade and the cavitation behavior, as well as drag/lift forces, were simulated and compared to investigate the effects of slot geometries on cavitation suppression. Findings The large eddy simulation based turbulence model was able to capture the interactions between the cavitation and turbulence. Moreover, the simulation revealed that the re-entrant jet was responsible for the periodic shedding of cavities. The results indicated that a slot was able to mitigate or even suppress cavitation-induced instabilities. A jet flow was generated at the slot exit and disturbed the re-entrant jet. If the slot geometry was properly designed, the jet could block the re-entrant jet and suppress the unsteady cavitation behavior. Originality/value This study provides unique insights into the complicated transient cavitation flows around a three-dimensional hydrofoil and introduces an effective passive cavitation control technique useful to researchers and engineers in the areas of fluid dynamics and turbomachinery.

Real-time microscopic and rheometric observations of strain-driven cavitation instability underlying micro-crack formation in asphalt binders

ABSTRACT We observed and analysed the early stages of formation and growth of instabilities in binders that lead to crack formation. A monotonic mechanical strain was applied to a thin, confined film of binder between two transparent plates; meanwhile, the bulk of the binder was observed using dark field microscopy to capture its behaviour as the strain increased. During the application, the formation of small cavities that rapidly grew was observed, which corresponded with notable points on a similar test that measured the load in the specimen. A three-stage model is used to describe the nucleation of the first cavity, of subsequent cavities and the growth of existing cavities that closely track the engineering stress–strain behaviour. In addition, it was observed that a cavity forms from a single microstructural feature, but not every feature forms a cavity. A theoretical model for this formation is presented. The growth of these cavities and their choice of nucleation sites suggests that although the density and distribution of the microstructure may not be critical to the formation of cavities that lead to failure, the presence and material properties of these microstructural entities compared to the matrix may be the factor that dictates damage nucleation.

Visual experiment and numerical simulation of cavitation instability in a high-speed inducer

Cavitation instabilities in a high-speed inducer at a design flow rate were investigated for different cavitation numbers in numerical simulations and visual experiments. On the basis of a shear stress transport k–ω turbulence model and Zwart–Gerber–Belamri cavitation model, the transient cavitating flow in a high-speed centrifugal pump with an inducer is numerically simulated using ANSYS-CFX 15.0 software. Visual experiments were carried out to capture the evolution of cavitating flow in the inducer by using a high-speed camera. The performance and cavitation characteristic curves from numerical simulation agree with those from experiment. With a decreasing cavitation number, the cavitation development in the high-speed inducer goes through incipient cavitation, developing cavitation, critical cavitation, and deteriorated cavitation and presents vortex cavitation, sheet cavitation, cloud cavitation, backflow cavitation, and a cavitation surge. The region having a high vapor volume fraction basically coincides with the region of low local pressure at the same cavitation number. The position of largish blade loading on the inducer changes with the development of cavitation. A cavitation surge as one type of cavitation instability appears in the inducer at lower cavitation numbers. The drop or rise of the head coefficient is affected by an increasing or decreasing cavitation area in the cycle of a cavitation surge.

Shear-band-to-crack transition in bulk metallic glasses under quasi-static and dynamic shearing

The intrinsic fracture mechanism of bulk metallic glasses (BMGs) is often obscured with the involvement of normal stress. To decouple the normal-stress effect, a double-notched shearing technique is developed to investigate the mechanical properties of BMGs. Real-time shear banding and fracture behavior under quasi-static and dynamic shearing are captured by using high-speed photographing, which confirms multiple shear banding at low strain rates and single shear banding at high strain rates. More importantly, the transition from shear band to crack could be observed with little influence of normal stress. Such transition is found to be induced by cavitation mechanism and shows obvious rate dependence; that is originated from regularly distributed controllable cavities under quasi-static loading, while from a series of concentrated cavitation under dynamic loading. Referring to studies under compression and tension, we clarify that strain rate controls cavitation concentration level and normal stress controls cavitation instabilities, respectively.

Numerical study of unsteady cavitating flows with RANS and DES models

Unsteady cavitating flow with high Reynolds number and significant instability commonly exists in fluid machinery and engineering system. The high-resolution approaches, such as direct numerical simulation and large eddy simulation, are not practical for engineering issues due to the significant cost in the computational resource. The objective of this paper is to provide the approach with Detached-Eddy Simulation (DES) model based on the Reynolds-averaged Navier–Stokes (RANS) equations for predicting unsteady cavitating flows. The credibility of the approach is validated by a set of numerical examples of its application: the unsteady cavitating flows around the two-dimensional (2D) Clark-Y hydrofoil and the three-dimensional (3D) blunt body. It is found that the calculated cavity shapes, cavity lengths and unsteady characteristics by DES model agree well with the experimental measurements and observations. Further analysis indicates that the turbulent eddy viscosity around the cavity and wake region is well predicted by the DES model, which results in the development of large-scale vortexes, and further cavitation instability. The DES model, which exhibits a significantly unsteady 3D behavior, is a more comprehensive turbulence model for unsteady cavitating flows.

Investigation of the Pressure Fluctuation Alleviation in a Hydraulic Turbine by Runner Modification

Cavitation and system instability are the most common problems occurring in hydraulic power systems, especially operated under part-load conditions. Previous research show that when the hydraulic turbine operates apart from designed conditions, such as part-load conditions, a helical vortex rope occurs from the runner exit, and usually generates severe pressure oscillations. Cavitation usually appears and turns the rope to become a two-phase cavitation rope. The occurrence of cavitation rope is believed to be the main reason of the severe pressure oscillations. Based on a new multiphase flow simulation method re-developed in ANSYS CFX, this paper did the runner modification by using grooves and made the investigation of pressure fluctuation alleviation in a hydraulic turbine at different cavitation numbers. The behavior of cavitation rope and the pressure fluctuations induced by cavitation under typical part load conditions with modified runner were analyzed in present research. The results show that besides the pressure fluctuation induced by the rope rotating, cavitation induced a new pressure fluctuation with a lower frequency. For the embedded analysis the results show that the modified runner can decrease the vortex rope eccentricity and increase the vortex core pressure and finally alleviate the two types of pressure fluctuations. Also, the turbine efficiency has a little rise when equipped with the modified runner.

Suction Performance and Cavitation Instabilities of Turbopumps with Three Different Inducer Design

Suction Performance and Cavitation Instabilities of Turbopumps with Three Different Inducer Design

Control effect of micro vortex generators on attached cavitation instability

The control effect of micro-vortex generators (VGs) on the instability of attached cavitation was investigated in a series of experiments. The micro-VGs, located at the leading edge of a NACA0015 hydrofoil, were used to alter the near-wall flow and control the attached cavitation dynamics. The effect of the nondimensional height of micro-VGs on the nondimensional cavity length was quantitatively evaluated by regression equations through response surface methodology. The micro-VGs increased the nondimensional cavity length. The counter-rotating streamwise vortices induced by micro-VGs had a rectifying effect on the near-wall flow and withstood the flow disturbance in the spanwise direction. Additionally, the micro-VGs partially suppressed Rayleigh–Taylor instability and Kelvin–Helmholtz instability arising from reverse flow underneath the cavity. Under a partial cavity oscillation (PCO) condition, the growth of sheet cavitation was highly two-dimensional in the spanwise direction, and the cloud cavity shedding had a strict periodicity with a smaller Strouhal number (St) than for the smooth hydrofoil. The shedding cloud cavity was captured in a single spanwise vortex core, which was advected toward the trailing edge of the hydrofoil. The transition from PCO to transitional cavity oscillation (TCO) occurred when the cavity length was larger than 0.8 of chord length. Under the TCO condition, the concave cavity closure line of sheet cavitation on the hydrofoil showed perfect symmetry and the St was nearly constant. As a result of our investigation, the micro-VGs have high potential to manipulate and control the attached cavitation dynamics.

Numerical investigation of the cavitation instability in a central jet pump with a large area ratio at normal cavitating conditions

Cavitation in central jet pumps has gained much attention for its effect on transportation efficiency and the safe and stable operation of systems. In order to investigate the mechanism of the cavitation instability in central jet pumps with large area ratios at normal cavitating conditions, numerical simulation has been carried out with Stress Blended Eddy Simulation turbulence model and Zwart-Gerber-Belamri cavitation model. The unsteady simulation predicts the periodic fluctuation of total cavity volume and the time evolution of cavities. Numerical results show that the most important factor for the cavitation instability in central jet pumps is the cavity accumulation induced by the pairing process of cavities. The cavitation instability at conditions with different cavitation intensities is a little different. Moreover, the cavitation period differs a lot at different flow ratios because of different cavitation dynamic behaviors.

The characteristics of unsteady cavitation around a sphere

The unsteady cavitation typically includes three processes: the growth of cavitation, the instability and shedding of cavitation, and the collapse of cloud cavitation and the regeneration of attached cavitation. In this paper, the unsteady features of the cavitating flow past a sphere are investigated. A detached eddy simulation turbulence model and a transport equation cavitation model are used to model the cavitating flow. The numerical results give the unsteady process of cavitation at different cavitation numbers (0.36 < σ < 1.0), which cover the cavitation state from inception to supercavitation. When the cavitation number 0.8 < σ < 1.0, the flow instability belongs to the single frequency mode; when the cavitation number σ < 0.8, the flow instability becomes the dual-frequency mode. We analyzed the Strouhal (St) number based on the length of the cavitation and found that the St numbers are stable around 0.5 and 0.2 in the dual-frequency mode. In this mode, the high frequency corresponds to the frequency of the large-scale cavity shedding caused by repeated re-entrant jets. The low frequency, caused by the combination of wake flow and cavitation, is close to the natural frequency of the sphere. In addition, the cavity leading edge position and cavity morphology are analyzed in details. Some of the numerical results are compared with existing experimental data.

Stabilization of cloud cavitation instabilities using Cylindrical Cavitating-bubble Generators (CCGs)

Unsteady cloud cavitation phenomenon is an important subject due to its undesirable effects in various applications such as ship propeller, rudder and hydraulic machinery systems. We present an efficient passive control method to control the cavitation instabilities which may be caused by the shedding of cavity structures in the vicinity of the solid surface of an immersible body. We proposed a passive control method so called Cylindrical Cavitating-bubble Generators (CCGs) on the surface of a benchmark hydrofoil and analyzed the effects of this passive controller on the dynamics of the unsteady cloud cavitation. First we modeled the unsteady cavitating flow around the hydrofoil without CCGs using a hybrid URANS model which was implemented in an open source code. Next, we studied the effect of CCGs on the mechanism of the unsteady cloud cavitation. The results show that using this method, the unsteady cavity structure was changed to a quasi-stable cavity structure compared with the cloud cavity shedding in the case without CCGs. We observed that the instability behavior of the unsteady cloud cavitation was mitigated and only small-scale cavity may be shedded from the hydrofoil in the free stream flow away from the hydrofoil surface. A notable reduction in cavitation-induced vibration and high wall-pressure peaks on the solid surface was observed.

Multimodal partial cavity shedding on a two-dimensional hydrofoil and its relation to the presence of bubbly shocks

The shedding dynamics of partial cavitation forming on a NACA0015 hydrofoil is known from the previous studies to be multimodal, exhibiting abrupt changes in Strouhal number as the cavitation number is reduced (Arndt et al. in Instability of partial cavitation: a numerical/experimental approach. National Academies Press, Washington, D.C., Retrieved from the University of Minnesota Digital Conservancy. http://hdl.handle.net/11299/49781 , 2000). The present study aims to understand the underlying shedding mechanisms responsible for this abrupt change in dynamics. As was observed in Ganesh et al. (J Fluid Mech 802:37–78, 2016), the shedding process can result from both re-entrant liquid flow and the formation of propagating bubbly shock waves within the cavity. Time-resolved X-ray densitometry and high-speed videography are combined with time synchronous measurements of acoustic noise produced by the cavity to examine the formation and shedding of the partial cavitation on a NACA0015 hydrofoil. From the experiments on the hydrofoil, it was observed that the mean cavity length increased with decreasing cavitation number, as expected. At higher pressures, stable partial cavities formed that shed smaller vapor clouds mainly due to the re-entrant liquid flow in the cavity closure. With a reduction in cavitation number, the partial cavity grew in length, and shedding often occurred when the cavity was pinched-off from its leading edge. Once a large region of vapor was shed, the pressure pulse caused by its subsequent collapse could suppress the growth of the newly forming partial cavity. This feedback led to complex, multistep cavity shedding dynamics. At still lower cavitation numbers, bubbly shocks were found to be responsible for the strongest periodic shedding of vapor clouds. Spectral analysis of the acoustic signal revealed a multimodal nature, which was more pronounced at lower cavitation numbers, and similar dynamic behavior was also found for the time varying local void fraction measurements in the vicinity of cavity closure. The occurrence of strongest multimodal behavior was also characterized by the presence of bubbly shock waves as the dominant mechanism of shedding. From the measurements and analysis, it is concluded that at least four different types of shedding modes occurred over a range of cavitation numbers at a fixed attack angle (10°).

Effect of Partition on the Mechanical Behaviors of Soft Adhesive Layers

A soft adhesive layer bonded between two rigid substrates, which are being pulled apart, may exhibit diverse instability phenomena before failure, such as cavitation, fingering, and fringe instability. In this study, by subdividing the soft layers into different numbers of disconnected smaller parts, we achieve desired instability modes and mechanical responses of the layer. The partition process not only retains the monotonicity on the tensile curve but also tunes the modulus and stretchability of the adhesive layer. Meanwhile, cavitation in layers of large aspect ratios is suppressed, and the hysteresis during cyclic loading is reduced. This study provides a guideline for the structural design of soft joints and adhesive layers.

Experimental investigation on suction performance and cavitation instabilities of turbopumps with two different inducers

In the present study, the suction performance and the cavitation instabilities in turbo-pumps with two different inducers designed with different design incidence angle are experimentally investigated in the wide range of operating flow rate. Compared to the Inducer A, Inducer B is designed with larger incidence angle, therefore it has slightly larger inlet and outlet angles. The identical centrifugal impeller is used as a main impeller. As a result, the total head of pump with inducer B is confirmed to be larger than that with inducer B especially at large flow rates, while the shaft power is almost the same, resulting in the better efficiency with the inducer B. The suction performance is better with inducer B at large flow rates. Two kinds of instabilities, the cavitating whirling vortex and the surges are mainly observed for the both inducers, but they are limited at low flow rates. The occurrence ranges of these cavitation instabilities are wider with inducer B.

Three dimensional numerical analysis of inducer about suppression of cavitation instabilities by asymmetric slits on blades

Cavitation is the phenomenon in which liquid evaporates and vapor bubbles occur in the low pressure region such as a pressure of liquid is roughly less than saturated vapor pressure. The cavitation causes a bad in uence such as vibration, noise, and a decline in performance on the turbomachineries. Moreover, the unsteady cavitation brings the oscillations in the turbomachinery which is called cavitation instabilities. In our laboratory, the suppression technique of the cavitation instabilities by using asymmetric slits on each blade has been investigated as a simple method. From the numerical results, it was con rmed that the cavitation instabilities are completely suppressed by a certain arrangement of asymmetric slits. In this study, the three-dimensional numerical analysis of the inducer was conducted. Two inducers which have asymmetric slits were used for this calculation. Moreover, to consider the asymmetric slit for effectiveness of cavitation instabilities, transient analysis are conducted. Although arranging the asymmetric slit cannot completely suppress the phenomenon, the cavity volume becomes smaller than without slit in all blades. This indicates that arranging asymmetric slits has the possibility to weaken the amplitude of cavitation instabilities.

Experimental and unsteady numerical research of a high-specific-speed pump for part-load cavitation instability

Net positive suction head peak is a well-known cavitation instability phenomenon in high-specific-speed pumps. Both non-cavitating performance and cavitating performance of a high-specific-speed pump were investigated by experiments and numerical simulations. According to the cavitating performance results, net positive suction head peak is found at 80% of nominal flow. The head curves of non-cavitating performance also have saddle-type instabilities near 70%–80% of nominal flow. Water vapor volume fraction distributions show that cavitation region at net positive suction head peak flow only covers 3% of the blade length when head drops 6%. It proves that net positive suction head peak is not caused by huge amounts of cavitation bubbles, which indicates that net positive suction head peak does not represent excessive cavitation. The velocity vector and pressure distribution plots reveal that net positive suction head peak is related to recirculation near the trailing edge. With inlet pressure decreasing, the flow pattern is sensitive to the cavitation bubbles, and recirculation region from the pressure side to the suction side becomes larger and larger.

Unsteady simulation of cavitating turbulent flow for low head Francis turbine

Cavitation is undesirable phenomenon and it becomes more severe under off-design conditions and lower tail race level. Transient simulations has been performed using Shear Stress Transport (SST) turbulence model in CFX solver. The Rayleigh Plesset mass transfer model is applied for the numerical predictions of cavitating flow through Francis turbine. An attempt has been carried out to analysis of cavitating flow at low head Francis turbine under different operating conditions having varyingsuction heads. Summarizing, it has been found that the performance loss and cavitation rate are found maximum under over load conditions. A high amplitude of pressure at low frequency has been found at part load operating conditions of the turbine which may cause fatigue damage to the turbine over time. Cavitation intensity, performance loss and instability increase with increase of suction head.

The suppression of cavitation surge in a double-suction centrifugal pump by using branch-type and slit-type accumulations installed at a pump outlet pipe

Recently, cavitation surge was one of cavitation instabilities in high performance turbopumps and it damaged to a hydraulic system. In the present research, the experiments were carried out in order to investigate the influences of the branch-type and slit-type accumulators installed at a discharge pipe on cavitation surge. The experiments were conducted by the closed-type water tunnels. As a result, the occurrence region of cavitation surge in the V-shaped suction performance curve was reduced by the branch-type accumulator and the pressure amplitudes due to cavitation surge were decreased by the slit-type accumulator.

Unsteady forces and moments acting on a cantilevered rectangular hydrofoil with tip clearance in cavitating conditions

It is known that the lift and drag forces of hydrofoil increase then decrease with the decrease of cavitation number, i.e. the development of cavitation, in many cases. In our previous study, the measurement of lift and drag forces of cavitating Clark Y-11.7% hydrofoil was conducted under the assumption of two-dimensional flow, which showed the similar tendency to above common knowledge. However, since there was a tip clearance, the bending moment should present to some content due to the flow asymmetry. In the present study, by using hydrofoil supported by a cantilever equipped with totally 8 strain gauges, the lift and drag forces and their moments are separately measured. It is found that the time-averaged moments around midspan due to lift and drag forces change with the decrease of cavitation number as well as the lift and drag forces change. This means that the working points of lift and drag forces move in the spanwise direction, indicating the importance of the three-dimensionality of cavitation development including that due to tip clearance flow. Unsteady lift and drag forces and their moments also show the significant fluctuations due to periodic behaviour of cavitation instabilities.

Self-similarly expanding regions of phase change yield cavitational instabilities and model deep earthquakes

The dynamical fields that emanate from self-similarly expanding ellipsoidal regions undergoing phase change (change in density, i.e., volume collapse, and change in moduli) under pre-stress, constitute the dynamic generalization of the seminal Eshelby inhomogeneity problem (as an equivalent inclusion problem), and they consist of pressure, shear, and M waves emitted by the surface of the expanding ellipsoid and yielding Rayleigh waves in the crack limit. They may constitute the model of Deep Focus Earthquakes (DFEs) occurring under very high pressures and due to phase change. Two fundamental theorems of physics govern the phenomenon: the Cauchy-Kowalewskaya theorem, which, based on dimensional analysis and analytic properties alone, dictates that there is zero particle velocity in the interior, and Noether's theorem, that extremizes (minimizes for stability) the energy spent to move the phase boundary so that it does not become a sink (or source) of energy, and which determines the self-similar shape (axes expansion speeds). The expression from Noether's theorem indicates that the expanding region can be planar, thus breaking the symmetry of the input and the phenomenon manifests itself as a newly discovered one of a “dynamic collapse/ cavitation instability”, where very large strain energy condensed in the very thin region can escape out. In the presence of shear, the flattened very thin ellipsoid (or band) will be oriented in space so that the energy due to phase change under pre-stress is able to escape out at minimum loss condensed in the core of dislocations gliding out on the planes where the maximum configurational force (Peach-Koehler) is applied on them. Phase change occurring planarly produces in a flattened expanding ellipdoid, a new defect present in the DFEs. The radiation patterns are obtained in terms of the equivalent to the phase change six eigenstrain components, which also contain effects due to planarity through the Dynamic Eshelby Tensor for the flattened ellipsoid. Some models in the literature of DFEs are evaluated and excluded on the basis of not having the energy to move the boundary of phase discontinuity. Noether's theorem is valid in anisotropy and nonlinear elasticity, and the phenomenon is independent of scales, valid from the nano to the very large ones, and applicable in general to other dynamic phenomena of stress induced martensitic transformations, shear banding, and amorphization.