Cavity induced vibration of flexible hydrofoils

The objective of this work is to investigate the influence of cavity-induced vibrations on the dynamic response and stability of a NACA66 hydrofoil at 8° angle of attack at Re=750 000 via combined experimental measurements and numerical simulations. The rectangular, cantilevered hydrofoil is assumed to be rigid in the chordwise direction, while the spanwise bending and twisting deformations are represented using a two-degrees-of-freedom structural model. The multiphase flow is modeled with an incompressible, unsteady Reynolds Averaged Navier–Stokes solver with the k–ω Shear Stress Transport (SST) turbulence closure model, while the phase evolutions are modeled with a mass-transport equation based cavitation model. The numerical predictions are compared with experimental measurements across a range of cavitation numbers for a rigid and a flexible hydrofoil with the same undeformed geometries. The results showed that foil flexibility can lead to: (1) focusing – locking – of the frequency content of the vibrations to the nearest sub-harmonics of the foil׳s wetted natural frequencies, and (2) broadening of the frequency content of the vibrations in the unstable cavitation regime, where amplifications are observed in the sub-harmonics of the foil natural frequencies. Cavitation was also observed to cause frequency modulation, as the fluid density, and hence fluid induced (inertial, damping, and disturbing) forces fluctuated with unsteady cavitation.     

Design approach for cavitation tolerant hydrofoils and blades

Cavitation inception and growth on conventional shape hydrofoils and blades leads initially to a jump of their flow-induced noise, further to an amplification of flow-induced vibration with frequently assisted erosion and finally, to a lift/thrust decrease combined with the drag increase. These undesirable cavitation effects can be mitigated or even suppressed for stable partial cavities experiencing no tail pulsations. A design approach enhancing performance of cavitating hydrofoils/blades by maintaining stable partial cavities is described. Experimental data manifesting an increase of hydrofoil lift with reduction of its drag and of force pulsations by such design are provided. Application of this design approach to propeller/turbine blades and advantages of its employment for blades operating in non-uniform incoming flows are analyzed. The possibility of an increase of the lift to drag ratio and of a reduction of the cavity volume oscillation in gust flows for blade sections is numerically manifested.     

Parametric excitations and lock-in of flexible hydrofoils in two-phase flows

This work introduces a reduced-order method to study the parametric excitations and lock-in of flexible hydrofoils caused by unsteady two-phase (cavitating) flow. The reduced-order method is based on a 1-DOF structural model coupled with a van der Pol wake oscillator with empirically derived relations for the variation in lift, cavity-length, and cavity-shedding frequency as a function of a non-dimensional cavitation parameter. The results are compared with several available data from both numerical simulations and experimental measurements. The frequency content of both the predicted and measured vibrations suggested that, in addition to the primary cavity-shedding frequency and the hydrofoil natural frequencies, unsteady two-phase flows may excite additional modulated frequencies due to time-varying fluid-added mass effects. The results show that these frequency modulations might cause the flexible hydrofoil to undergo higher-order resonances, as well as parametric resonances. While the maximum deformations for the primary and higher-order resonances were observed to damp out, parametric resonances might persist even with realistic fluid damping coefficients (4–12%). It was observed that with higher effective foil flexibility, the cavity-shedding frequencies may be significantly modified from the rigid foil trends, and may instead lock-in with the system natural frequencies.     

Comparison of mass transfer models for the numerical prediction of sheet cavitation around a hydrofoil

Cavitating flows, which can occur in a variety of practical cases, can be modelled with a wide range of methods. One strategy consists of using the RANS (Reynolds Averaged Navier Stokes) equations and an additional transport equation for the liquid volume fraction, where mass transfer rate due to cavitation is modelled by a mass transfer model. In this study, we compare three widespread mass transfer models available in literature for the prediction of sheet cavitation around a hydrofoil. These models share the common feature of employing empirical coefficients, to tune the models of condensation and evaporation processes, that can influence the accuracy and stability of the numerical predictions. In order to compare the different mass transfer models fairly and congruently, the empirical coefficients of the different models are first well tuned using an optimization strategy. The resulting well tuned mass transfer models are then compared considering the flow around the NACA66(MOD) and NACA009 hydrofoils. The numerical predictions based on the three different tuned mass transfer models are very close to each other and in agreement with the experimental data. Moreover, the optimization strategy seems to be stable and accurate, and could be extended to additional mass transfer models and further flow problems.     

On the pressure of cavitation bubbles

Shock wave emission upon the collapse of a cavitation bubble attached to a rigid wall is investigated using high-speed photography with 200 million frames/s and 5 ns exposure time. At a distance of 68 μm from the bubble wall, the shock pressure is 1.3 ± 0.3 GPa. The shock pressure decays proportionally to r^−1.5 with increasing distance from the bubble. An estimation of the peak pressure at the bubble wall reveals a pressure of about 8 GPa. A major part of the shock wave energy is dissipated within the first 100 μm from the bubble wall.     

The influence of surface roughness on cloud cavitation flow around hydrofoils

The aim of this study is to investigate experimentally the effect of surface roughness on cloud cavitation around Clark-Y hydrofoils. High-speed video and particle image velocimetry(PIV) were used to obtain cavitation patterns images(Prog. Aerosp. Sci. 37: 551–581, 2001), as well as velocity and vorticity fields. Results are presented for cloud cavitating conditions around a Clark-Y hydrofoil fixed at angle of attack of α = 8?for moderate Reynolds number of Re = 5.6 × 10~5. The results show that roughness had a great influence on the pattern, velocity and vorticity distribution of cloud cavitation. For cavitating flow around a smooth hydrofoil(A) and a rough hydrofoil(B), cloud cavitation occurred in the form of finger-like cavities and attached subulate cavities, respectively. The period of cloud cavitation around hydrofoil A was shorter than for hydrofoil B.Surface roughness had a great influence on the process of cloud cavitation. The development of cloud cavitation around hydrofoil A consisted of two stages:(1) Attached cavities developed along the surface to the trailing edge;(2) A reentrant jet developed, resulting in shedding and collapse of cluster bubbles or vortex structure. Meanwhile, its development for hydrofoil B included three stages:(1) Attached cavities developed along the surface to the trailing edge, with accumulation and rotation of bubbles at the trailing edge of the hydrofoil affecting the flow field;(2) Development of a reentrant jet resulted in the first shedding of cavities. Interaction and movement of flows from the pressure side and suction side brought liquid water from the pressure side to the suction side of the hydrofoil, finally forming a reentrant jet. The jet kept moving along the surface to the leading edge of the hydrofoil, resulting in large-scale shedding of cloud bubbles. Several vortices appeared and dissipated during the process;(3) Cavities grew and shed again.     

CFG桩复合地基沉降影响因素分析

对绕2D水翼无分离流边界层内的初生空化形态进行了实验研究. 采用高速摄像机观测 了空化初生结构的形态，应用2D-LDV测量了空化初生时翼型周围的流动速度分布，并对实 验结果进行了分析. 结果表明：绕水翼无分离边界层内，初生空化结构中空泡伴随 着近壁湍流拟序结构的发生而出现，在初生空化条件下，形成空化涡结构，大量的微空泡产生于发夹 涡结构中，并在涡结构的猝发过程中出现生成---长大---溃灭---反弹---再溃灭的过程. 初生空 化涡结构具有空泡和近壁拟序结构双重特性.     

Modeling the indentation, penetration and cavitation of elastic membranes

An energy argument is used to predict the penetration of an elastic membrane by a rigid cylindrical indenter. We study a sequence of axisymmetric equilibrium deformations beginning with the indentation of a solid disc, followed by penetration of the membrane at a critical value of the indenter displacement, and ending in a state of cavitation in which the membrane is detached from the indenter and a central traction-free hole is maintained by boundary data alone.     

Comparison of slamming and whipping loads by fully coupled hydroelastic analysis and experimental measurement

This paper proposes a numerical method for analyzing whipping using a fully coupled hydroelastic model. The numerical analysis method utilizes a 3-D Rankine panel method, 1-D/3-D finite element methods, and a 2-D generalized Wagner model, which are strongly coupled in the time domain. The computational results were compared with those of a model test of an 18 000-TEU containership. The slamming pressures and whipping responses to regular waves for bow flare and stern slamming were compared. Furthermore, the slamming pressure was decomposed into its dynamic and static components. The numerical and experimental models produced similar results. In addition, the effects of the discretization and geometric approximation of the 2-D slamming sections were investigated.     

On the possibility of shock-induced cavitation in submerged cylindrical shell systems

A circular cylindrical shell loaded by one or two fluids and responding to an external shock wave is analyzed in the context of the possible inception of shock-induced cavitation. Several scenarios of fluid contact are considered including a submerged evacuated shell and a submerged fluid-filled shell for three different combinations of the parameters of the internal and external fluids. A semi-analytical shell-shock interaction model is employed in order to predict the regions of the fluids where cavitation is likely to occur, and the respective cavitation development is hypothesized about. The most interesting and practically important finding is that when fluid is present both inside and outside the shell, there exist conditions when cavitation is expected to occur in both the internal and external fluid, resulting in a particularly complex and violent structural re-loading occurring upon the collapse of the respective cavitation regions. The inception of cavitation in the internal fluid alone and in the external fluid alone is also possible. The findings are summarized in a manner that is suitable for use at the pre-design stage as a guide for preliminary assessment of the possibility of shock-induced cavitation in fluid-interacting industrial systems.     

A coupled WC-TL SPH method for simulation of hydroelastic problems

A coupled weakly compressible (WC) and total Lagrangian (TL) smoothed particle hydrodynamics (SPH) method is developed for simulating hydroelastic problems. The fluid phase is simulated using WCSPH method, while the structural dynamics are solved using TLSPH method. Fluid and solid components of the method are validated separately. A sloshing water tank problem is solved to test the WCSPH method while oscillation of a thin plate and large deformation of a cantilever beam are simulated to test the TLSPH method. After validating each component, the coupled WC-TL SPH scheme is used to simulate two benchmark hydroelastic problems. The first test case shows the evolution of water column with an elastic boundary gate, and the second one investigates the breaking water column impact on elastic structures. The agreement between WC-TL SPH results and literature data shows the ability of the proposed method in simulating hydroelastic phenomena.     

Combined experimental observation and numerical simulation of the cloud cavitation with U-type flow structures on hydrofoils

Sheet/cloud cavitation is an important topic that is a very common type of cavitation in turbo-machinery and marine propeller. Up to now we still have limited understanding of the cavitation shedding dynamics and cloud cavity formation and development. The present study used experimental and numerical studies to gain a better understanding of the complex physics involved in this problem. A series of experimental observations around hydrofoils are carried out in the cavitation tunnel of the China Ship Scientific Research Center (CSSRC) to illustrate the spatial–temporal evolution of the cloud cavity in detail. The results demonstrate that U-type flow structures are common in cloud cavities and can be divided into three stages and the closure line in a sheet cavity often has a convex–concave profile. Reentrant flows occur in the convex region with the jet direction normal to the contour edge so the shedding is mainly caused by the converging reentrant flows. Further analysis demonstrated that there was a striking difference with the cavity growth suppressed substantially in the twisted hydrofoil case if compared with straight hydrofoil and the effect of side entrant jets might make the cavity more uniform across the span. Numerical simulations were used to simulate the formation and development of the cloud cavity. The results show that the strong adverse pressure gradient in the stagnation region at the downstream end of the attached cavity forces the re-entrant flows into the vapor structure with a radially-diverging re-entrant jet and a pair of side-entrant jets, which causes the cavity shedding. Further analyzes of the local flow fields show that the interactions between the circulating flow and the shedding vapor cloud may be the main reason for the formation of the U-type cloud cavity structures.     

The application of three-dimensional hydroelastic analysis of ship structures in Pekeris hydro-acoustic waveguide environment简

The hydroelastic analysis and sonoelastic analysis methods are incorporated with the Green＇s function of the Pekeris ocean hydro-acoustic waveguide model to produce a three-dimensional sonoelastic analysis method for ships in the ocean hydro-acoustic environment. The seabed condition is represented by a penetrable boundary of prescribed density and sound speed. This method is employed in this paper to predict the vibration and acoustic radiation of a 1 500 t Small Water Area Twin Hull （SWATH） ship in shallow sea acoustic environment. The wet resonant frequencies and radiation sound source levels are predicted and compared with the measured results of the ship in trial.     

A cavitation model for computations of unsteady cavitating flows

A local vortical cavitation(LVC) model for the computation of unsteady cavitation is proposed.The model is derived from the Rayleigh–Plesset equations,and takes into account the relations between the cavitation bubble radius and local vortical effects.Calculations of unsteady cloud cavitating fows around a Clark-Y hydrofoil are performed to assess the predictive capability of the LVC model using well-documented experimental data.Compared with the conventional Zwart's model,better agreement is observed between the predictions of the LVC model and experimental data,including measurements of time-averaged fl w structures,instantaneous cavity shapes and the frequency of the cloud cavity shedding process.Based on the predictions of the LVC model,it is demonstrated that the evaporation process largely concentrates in the core region of the leading edge vorticity in accordance with the growth in the attached cavity,and the condensation process concentrates in the core region of the trailing edge vorticity,which corresponds to the spread of the rear component of the attached cavity.When the attached cavity breaks up and moves downstream,the condensation area fully transports to the wake region,which is in accordance with the dissipation of the detached cavity.Furthermore,using vorticity transport equations,we also fin that the periodic formation,breakup,and shedding of the sheet/cloud cavities,along with the associated baroclinic torque,are important mechanisms for vorticity production and modification When the attached cavity grows,the liquid–vapour interface that moves towards the trailing edge enhances the vorticity in the attached cav-ity closure region.As the re-entrant jet moves upstream,the wavy/bubbly cavity interface enhances the vorticity near the trailing edge.At the end of the cycle,the break-up of the stable attached cavity is the main reason for the vorticity enhancement near the suction surface.     

Study of cavitation phenomena based on a technique for visualizing bubbles in a liquid pressurized chamber

In this paper, the influence of nozzle geometry on cavitation and near-nozzle spray behavior under liquid pressurized ambient is studied. For this purpose, eight steel drilled plates, with different diameters and degrees of conicity of their holes, are analyzed. A special near-nozzle field visualization technique, using a test rig pressurized with fuel, is used. Due to the difference in refractive index between liquid and vapor phase, bubbles present at the outlet of the orifice are visualized. The pressure conditions at which bubbles start appearing at the orifice outlet are compared with those at which choked flow appears. The results showed that pressure conditions for inception of cavitation obtained in the visualization tests differs from those seen for choked flow (5–8% in terms of cavitation number). In addition to this, the images taken are analyzed to get the angle of the jet formed by fuel bubbles, showing that it increases significantly for those conditions more prone to cavitate. Furthermore, comparison of bubbles generation when increasing or decreasing backpressure indicates the presence of hysteresis in cavitation inception phenomena.     

Experimental assessment of performance characteristics for pitching flexible propulsors

Using a flexible hydrodynamic foil that pitches to produce thrust, the most pertinent aspects of a fish-like propulsion system are replicated in a controlled environment. The pitching and flexing combination creates a hydroelastic coupling in which the fluid and flexible foil simultaneously affect each other's behavior. The project investigated relationships for the propulsors’ thrust and efficiency performance to gain a better understanding of the dynamic interaction with the surrounding fluid. The analysis was conducted through reduction of the measured force and torque data. The experiments took place in a large recirculating water channel, using full span flexible propulsor models and a higher Reynolds number than previous flexible propulsor experiments. The propulsor pitched about a fixed axis at its quarter chord, with a six-axis load cell measuring the forces and torques on the shaft. The efficiency of the propulsor and the Coefficients of Thrust and Lift are presented as functions of both Strouhal Number and Stiffness Coefficient. The ensemble data will facilitate the engineering of fish-like propulsion systems for future application of this technology.     

绕三维水翼云状空化现象的实验研究

为了研究云状空化阶段空穴发展和脱落的机理,采用实验的方法对绕三维水翼云状空化流动进行了研究.实验在高速水洞中进行,采用高速摄像技术研究了不同空化阶段的空穴形态,并测量了翼型所受的升阻力,并对上述数据进行了频谱分析.结果发现:在云状空化阶段,观测到空穴的产生-发展-脱落-溃灭的准周期性变化;并捕捉到空泡脱落时附着在翼型前...     

Visualisation and les simulation of cavitation cloud formation and collapse in an axisymmetric geometry

Visualisation and Large Eddy Simulations (LES) of cavitation inside the apparatus previously developed by Franc (2011) for surface erosion acceleration tests and material response monitoring are presented. The experimental flow configuration is a steady-state closed loop flow circuit where pressurised water, flowing through a cylindrical feed nozzle, is forced to turn 90° and then, move radially between two flat plates towards the exit of the device. High speed images show that cavitation is forming at the round exit of the feed nozzle. The cavitation cloud then grows in the radial direction until it reaches a maximum distance where it collapses. Due to the complexity of the flow field, direct observation of the flow structures was not possible, however vortex shedding is inferred from relevant simulations performed for the same conditions. Despite the axisymmetric geometry utilized, instantaneous pictures of cavitation indicate variations in the circumferential direction. Image post-processing has been used to characterize in more detail the phenomenon. In particular, the mean cavitation appearance and the cavity length have been estimated, showing good correlation with the erosion zone. This also coincides with the locations of the maximum values of the standard deviation of cavitation presence. The dominant frequency of the ‘large-scale’ cavitation clouds has been estimated through FFT. Cloud collapse frequencies vary almost linearly between 200 and 2000 Hz as function of the cavitation number and the downstream pressure. It seems that the increase of the Reynolds number leads to a reduction of the collapse frequency; it is believed that this effect is due to the agglomeration of vortex cavities, which causes a decrease of the apparent frequency. The results presented here can be utilized for validation of relevant cavitation erosion models which are currently under development.     

Numerical simulation of cavitation effects behind obstacles and in an automotive fuel jet pump

By means of computational fluid dynamics (CFD) this study examines cavitation effects behind obstacles and within an automotive fuel jet pump. Especially with regard to gasoline such effects are serious issues for applications of jet pumps in automotive fuel systems. The cavitation phenomena are captured by a model based on a void region approach within the volume-of-fluid method (VOF) including the k--model of turbulence. A first-order and a second-order scheme are compared, and the potential of the numerical method is evaluated by considering benchmark cases.