Surface Of Hydrofoil Research Articles

Cavitation flow characteristics on the surface of hydrofoil with microjet structure

A hydrofoil physical model is established based on the surface microstructure to mitigate the detrimental effects of cavitation phenomena on hydrodynamic machinery, such as cavitation erosion or surface damage. Tangential microjet structures are arranged on the hydrofoil's surface, and the modified k-omega shear stress transport (SST k–ω) turbulence model is employed to simulate the hydrofoil numerically. This simulation aims to analyze the effects of different chordwise positions and widths of microjet structures on the cavitation flow and performance of hydrofoils. The mechanism of cavitation suppression is revealed by coupling the chordwise position and width of the microjet structures. The results indicated that the chordwise position of the microjet structures near the trailing edge of the hydrofoil has a minimal impact on the hydraulic properties. The optimal chordwise positions are 0.5c and 0.6c, with the deviation rate of the lift-drag ratio within 3%. The optimum jet width is 0.5 mm, and the cavitation suppression is approximately 15% of the prototype hydrofoil. The microjet structures with tangential jets suppress cavitation by creating obstruction and suppression of the re-entrant jet. The tangential jet ratio of 0.3 represents the most effective tangential jet hydrofoil scheme, and the addition of tangential jets produces a significant inhibitory effect on the shedding of large-scale cavitation.

Numerical simulation study of cavitation characteristics of jet hydrofoil surface

To mitigate the adverse effects of cavitation on the performance degradation and chemical corrosion of hydraulic machines, a jet hydrofoil physical model based on the structure of a guide vane model of a high-pressure mixed-flow hydraulic turbine was established. A density correction turbulence model based on filters was adopted to numerically simulate the cavitation characteristics on the surface of the jet hydrofoil. The results demonstrate that when the jet speed is 12 m/s and the jet angle is −15°, the scheme exhibits the best cavitation suppression effect among all combinations of jet angles and speeds. Cavities near the jet structure are prematurely collapsed after being pressurized, and the cavitation suppression effect in the low-pressure area at the trailing edge is also satisfactory. The time-averaged bubble volume fraction reaches its minimum at 2.17%, with the time-averaged bubble volume fraction being 27.3% of the prototype hydrofoil. At flow speeds of 2.4 m/s and 7.2 m/s, small-angle jet fluid accumulated will stick to the closed cavities on the surface of the hydrofoil and cause them to rupture, while the jet fluid at the jet angle of 60° and speeds of 7.2 m/s and 12 m/s will directly cut off the bubbles.

The acoustic signature of a propeller–hydrofoil system in the far field

Data from Large-Eddy Simulations on a grid consisting of 3.8 billion points are utilized to reconstruct the acoustic far field of a propeller–hydrofoil system, using the Ffowcs-Williams and Hawkings acoustic analogy. Results demonstrate that the lowest frequencies, around the shaft frequency, are dominated by the sound coming from the surface of the downstream hydrofoil, especially in its normal direction. In contrast, at the blade frequency and higher frequencies, the acoustic signature of the upstream propeller gains the lead, especially in the upstream and downstream directions. Minima of the sound pressure levels correlate in all cases with the spanwise direction of the hydrofoil. Despite the dramatic change of the flow conditions for growing incidence angles of the hydrofoil, characterized by a substantial rise of the turbulent fluctuations on its suction side, no similar increase in the acoustic pressure in the far field occurs at large incidence angles, even at the onset of separation phenomena, which are able to affect the highest frequencies only.

Using genetic algorithm to find the optimum piecewise superhydrophobic pattern maximizing the lift to drag ratio on a SD 7003 foil at different working conditions

Recently superhydrophobic surfaces are widely used to reduce drag, but there has been less focus on their influence on lift to drag ratio. In this paper, it is shown that using a superhydrophobic coating on the whole surface of a hydrofoil is not always beneficial to hydrodynamic performance (i.e. lift to drag ratio). Depending on angle of attack (AOA), Reynolds number (Re) or slip length (ls), some regions on the surface of hydrofoil should be left uncoated (optimum superhydrophobic pattern). To find the optimum superhydrophobic pattern, we developed a genetic algorithm (GA) combined with two dimensional RANS (Reynolds Averaged Navier Stokes) equations solver. To demonstrate the methodology a SD7003 foil surface is divided into 10 equal regions. Every region can either be superhydrophobic or uncoated, i.e. the problem is a binary optimization one. In the first step the initial population is evaluated by fluid solver and the individuals with higher fitness value are selected as parents by roulette wheel selection method. Combined (multi random points) cross-over is performed on the chromosome of two parents to generate the offspring. Parents and offspring are merged in a set and the individuals with higher fitness value are selected as the next generation. The results show that depending on angle of attack (0 < AOA <10ᵒ), Reynolds number (0.5×105<Re<2×105) and slip length (100 μm ≤ ls ≤ 400 μm), the optimum superhydrophobic pattern is different. It was found that the idea of piecewise superhydrophobic pattern works for the cases with detached boundary layers and in transition regime. According to results, at higher Re, AOA and ls, the optimum superhydrophobic pattern is the full superhydrophobic one.

Experimental analysis on dynamic/morphological quality of cavitation induced by different air injection rates and sites

Ventilated cavitating flow features resulting from the air injection at the hydrofoil surface are characterized based on experimental investigation. The experiments have been conducted in the cavitation tunnel at the Silesian University of Technology. The main focus of this work is to investigate how both the location of the injection hole at the surface of the hydrofoil (so-called injection site) and the injection rate have an impact on the cavitating flow in various flow conditions (i.e., different cavitation numbers). The Clark Y hydrofoil is fixed at an 8° angle of attack. In addition, three cavitation numbers, σ = 1.1, 1.25, and 1.6; five air injection rates, Q = 0, 0.25, 0.5, 0.75, and 1 l/min; and two injection sites at the surface of hydrofoil (Tap1-injection and Tap5-injection) are selected for the case studies. Furthermore, the level of dissolved air in water is kept constant at 11.7 mg/l. The unsteady measurements and high-speed imagining declare that, regardless of the injection rate, the injection site has a significant effect on the cavitation dynamic features and morphology. Moreover, it is shown that the effectiveness of air injection depends on the flow conditions.

Cavitating hydrofoil with a structural surface

A study of cavitating smooth hydrofoil and foil with a structural surface was carried out. The structural surface morphology was made by laser ablation technology with a step of 0.01 mm and wavelength of 1064 nm. High-speed visualization of cavitating hydrofoils at various of flow bulk velocity was performed. In the paper is presented a comparison of cavitation flow regimes for the smooth and the roughness hydrofoils at equal cavitation numbers. The maximal lengths of the attached cavities for different flow regimes are compared. The structural surface of hydrofoil allows delaying the nucleation of cavitation and reducing the attached cavity size at higher velocities of the incoming flow was obtained.

Numerical Simulation of Hydrofoil Cavitation in Pitching Motion

Based on CFD, the NACA4412 pitching motion hydrofoil is compared and analyzed for the lift/drag coefficient, the pressure and the gas volume fraction at different attack angles. The result is that the lift/drag coefficient curve of the dynamic hydrofoil is a closed curve. When the hydrofoil is pitching up, the lift/drag coefficient will be generally greater than that in the static state, which first increases and then decreases as attack angles increases. When the hydrofoil is down, the lift/drag coefficient will be generally smaller than that in the static state, which first decreases and then increases as attack angles decreases. Moreover, when the hydrofoil is pitching upwards, there will be a sudden change in pressure on the subsurface, resulting in an extreme value. In addition, there is more comprehensive cavitation on the upper surface of the dynamic hydrofoil, and local cavitation on the subsurface.

Interaction between surface blowing and re-entrant jet in active control of hydrofoil cavitation

Numerically simulations were performed to study the effect of active flow control (through steady blowing) on unsteady cavitation over a modified scaled-down Francis turbine (hydrofoil). Large eddy simulations (LES) using a one-equation subgrid-scale model and the Sauer & Schnerr cavitation model, all available in OpenFOAM, served as the main numerical tool for the study. An angle of attack of 9° and a cavitation number σ=1.56 were chosen for study. The dynamics of the re-entrant jet and its role in the shedding of the cavitation sheet were elucidated. For active flow control, wall jet injection through a slot on the top surface of hydrofoil was considered for two different injection velocities of Uinj=0.88U0 and Uinj=1.42U0, where U0 is the inflow velocity. Quantitative comparisons to previously published measured data were used for model validation. It was found that the interaction of the re-entrant jet with the wall jet causes large fluctuations in pressure, thereby counteracting the local rise in pressure due to the wall jet. Thus, the effectiveness of wall jet injection to mitigate cavitation is limited under the operating conditions studied here.

Cavitation control using Cylindrical Cavitating-bubble Generators (CCGs): Experiments on a benchmark CAV2003 hydrofoil

Cavitation often causes a destructive impact on the performance of hydraulic machinery, such as erosive wear, noise and vibrations of the framework and moving parts of marine propellers, pumps, hydraulic turbines and other equipments, which eventually leads to a degradation of overall system effectiveness. The paper reports on an experimental investigation of a passive method of flow control for different cavitation conditions: starting from the cavitation inception, including quasi-steady partial cavitation with shedding of small-scale vortical structures and finishing by unsteady cloud cavitation. The passive flow control was implemented using miniature vortex generators of a cylindrical type referred to as Cylindrical Cavitating-bubble Generators (CCGs) that were placed on the surface of a benchmark CAV2003 hydrofoil. First, we performed high-speed visualization of cavitation on the suction side of the original hydrofoil (without the control element) to find the cavitation inception point near the leading edge and to analyze the spatial structure and time evolution of partial cavities. In order to improve our understanding of the mechanism of cavitating flow unsteadiness and the effect of CCGs on the cavitation dynamics, we also applied a PIV technique to measure the mean flow velocity profiles and a hydroacoustic pressure transducer to record local pressure pulsations in the hydrofoil wake. As a result, this allowed us to determine the influence of CCGs on turbulent structure of the flow at different cavitation regimes and amplitude-frequency spectra of the pressure pulsations associated with attached cavity length oscillations for unsteady flow conditions. It was revealed that, in the case of unsteady cloud cavitation, CCGs were capable to mitigate large-scale cloud cavities. In addition, a substantial decrease in the amplitude of pressure pulsations was registered for the modified hydrofoil (with the control element). In general, CCGs appeared to be quite effective to hinder the cavitation development and to reduce the strength of side- and middle-entrant jets as the primary mechanisms of unsteady cloud cavitation.

Experimental investigation of the passive control of unsteady cloud cavitation using miniature vortex generators (MVGs)

The research is aimed at the study of a passive control method to control unsteady cloud cavitation that is characterized by regular shedding of large vapour structures from the solid surface of a cavitating immersible body. The unsteady cloud cavitation is an important subject of research because of its destructive impacts in various industrial applications, including ship propellers and rudders, pumping and hydraulic machinery systems. For this, we placed miniature vortex generators (MVGs) of a cylindrical type on the surface of a benchmark CAV2003 hydrofoil and investigated effects of these MVGs on the spatial structure of unsteady cavitation clouds. We analyzed the temporal and spatial cavity characteristics in comparison with those for the original hydrofoil (without MVGs) by means of high-speed imaging. In addition, we used a hydrophone to register the signal of pressure pulsations in time and thereby derive power spectra of the pressure pulsations. The results showed that the implemented cavitation control method is an effective tool to manage the unsteady behaviour of cloud cavitation and to mitigate the amplitude of pressure pulsations. It was revealed that, with this control approach, the large-scale cavitation clouds appear to be broken and only small-scale cavity structures are shed away from the hydrofoil surface. Moreover, a notable reduction in the cavitation-induced vibrations of the solid surface may be expected.

Numerical study of the impact of water injection holes arrangement on cavitation flow control.

A method of water injection to flow field using distributed holes on the suction surface of hydrofoil is presented in this article to control cavitation flow. Modified renormalization group k-ε turbulence model is coupled with full-cavitation model to calculate periodical cavitation patterns and the dynamic characteristics of the NACA66(MOD) hydrofoil. Water injection is found to be highly effective for cavitation suppression. The cavitation suppression effect of distributed regulation of jet holes and porosities along three-dimensional spanwise hydrofoil is also investigated. The appropriate porosities of single row spanwise jet holes and optimal jet position of double row jet holes are revealed for both cavitation suppression and good hydrodynamic performance. Double row jet holes setting in forward trapezoidal arrangement shows great potential for cavitation suppression and hydrodynamic performance. This research provides a method of water injection to flow field to actively control cavitation, which will facilitate development of engineering designs.

Effect of obstacle position on attached cavitation control through response surface methodology

The span-wise obstacle on the suction surface of a hydrofoil has been verified to be an effective passive control method for cloud cavitation. The position of obstacle significantly influences the performance of cavitation control. In this research, we investigated the effect of obstacle position on attached cavitation control on the suction surface of a NACA0015 hydrofoil through response surface methodology. The cavitation types covered from sheet cavitation to partial and transitional cavity oscillations. We derived regression equations and built response surfaces to illustrate the quantitative relationship between individual factors (obstacle position, cavitation number, and angle of attack) and cavitation dynamic response parameters (cavity length, acoustic intensity, and energy flux). Sheet cavitation was effectively suppressed because the obstacle increased the pressure at the near-wall region. However, the obstacle would induce a shear cavitation when its position was too close to the leading edge of the hydrofoil. Under partial cavity oscillation conditions, the obstacle was consistently performed well in cloud cavitation control. The cavitation dynamic response parameters significantly decreased. Under transitional cavity oscillation conditions, the obstacle cannot suppress the cavitation because the transitional cavity oscillation was likely a system-inherent instability. This research is beneficial for a comprehensive understanding of cavitation control mechanism using an obstacle and for further industrial application of obstacle in hydraulic machinery to control cavitation.

Experimental and numerical studies on the cavitation over flat hydrofoils with and without obstacle

To control the shedding of cavitation, an obstacle is placed on the surface of a flat hydrofoil. Both experimental and numerical studies are carried out. Images of cavitation evolution are recorded by a high-speed camera. 3-D simulations are performed to investigate the cavitating flows around the hydrofoil. The results show that the re-entrant jet plays an important role during the process of cavitation shedding. A kind of U-type shedding is identified during the evolution of the cloud cavitation. The length of the cavity is apparently reduced due to the placement of the obstacle. It is interesting to find that the cavitation shedding changes from the large-scale mode to a small-scale mode, as an obstacle is placed on the hydrofoil surface. As we can observe from both experimental and numerical results, the small-scale cavitation shedding dominates the cavitating flow dynamics, we thereby conclude that the placement of an obstacle is favorable for the inhibition of cavitation shedding.

DESIGN AND INVESTIGATION OF THE HYDRAULIC PERFORMANCE OF BIONIC HYDROFOIL BASED ON THE GEOMETRIC FEATURES OF STURGEONS

The airfoils designed by the National Advisory Committee for Aeronautics (NACA) are currently adopted in the blades of hydraulic machinery (pump, water turbine). However, the NACA series are designed on the basis of the aerodynamics theory, so their performance are inevitably affected when they are applied directly to hydraulic machinery. Thus, the mechanism affecting the hydrodynamic characteristics of a hydrofoil based on aquatic organisms must be further investigated. In this study, a sturgeon hydrofoil based on the water environment was designed by using a 3D laser scanner, and the hydrodynamic performance of the hydrofoil was integrated by numerical simulation. A k–? SST model was adopted at Reynolds numbers 1E6, 3E6, and 5E6 under 0°–27° angle of attack for the analysis of the lift and drag coefficients, tip vortices, and pressure distribution on the upper surface of the sturgeon hydrofoil as compared with those of the NACA0012 and NACA0015 hydrofoils. Results show that the lift coefficients of the sturgeon hydrofoil are greater than those of the NACA0012 and NACA0015 hydrofoils at different Reynolds numbers. The NACA0012 and NACA0015 hydrofoils generate and spread tip vortices easier than the sturgeon hydrofoil, but the sturgeon hydrofoil generates a larger lift force before reaching the stall angle of attack, especially in the maximum-thickness region on the upper surface. Conclusions obtained in the study have important implications for designing a bionic airfoil suitable for hydraulic machinery. Keywords: Bionics hydrofoil, Fluid flow, Tip vortices, Pressure distribution, Hydrodynamic performance

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.

Cavitation passive control on immersed bodies

This paper introduces a new idea of controlling cavitation around a hydrofoil through a passive cavitation controller called artificial cavitation bubble generator (ACG). Cyclic processes, namely, growth and implosion of bubbles around an immersed body, are the main reasons for the destruction and erosion of the said body. This paper aims to create a condition in which the cavitation bubbles reach a steady-state situation and prevent the occurrence of the cyclic processes. For this purpose, the ACG is placed on the surface of an immersed body, in particular, the suction surface of a 2D hydrofoil. A simulation was performed with an implicit finite volume scheme based on a SIMPLE algorithm associated with the multiphase and cavitation model. The modified k-e RNG turbulence model equipped with a modification of the turbulent viscosity was applied to overcome the turbulence closure problem. Numerical simulation of water flow over the hydrofoil equipped with the ACG shows that a low-pressure recirculation area is produced behind the ACG and artificially generates stationary cavitation bubbles. The location, shape, and size of this ACG are the crucial parameters in creating a proper control. Results show that the cavitation bubble is controlled well with a well-designed ACG.

Prediction and analysis of 3D hydrofoil and propeller under potential flow using panel method

Potential flow over an airfoil plays an important historical role in the theory of airfoil. The governing equation for potential flow is Laplace’s equation. One of Green’s identities can be used to write a solution to Laplace’s equation as a boundary integral. Using distributions of singularity solutions and determining their strength via the boundary conditions is the essence of panel method. This paper introduces a quick prediction method of three-dimensional hydrofoil and propeller performance based on panel method. The surface of hydrofoil and propeller is divided into numbers of quadrilateral panels. Combined sources with doublets singularities will be distributed on the corners of panels. Calculated blade pressure distributions of hydrofoil and propeller agree well with experimental data. Several sample calculations have been included using panel method.

Numerical investigation of time-dependent cloud cavitating flow around a hydrofoil

Time-dependent cloud cavitation around the 2-D Clark-Y hydrofoil was investigated in this paper based on an improved filter based model and a density correction method. The filter-scale in filter based model simulation was discussed and validated according to the grid size. Numerical results show that in the transition from sheet cavitation to cloud cavitation, the sheet cavity grows slowly to the maximum length during the re-entrant jet develops. The mild shedding bubble cluster convects downwards the hydrofoil and continues to grow up after detaching from the suction surface of hydrofoil, and a bubble cluster introduced at the rear part of hydrofoil. While the sheet cavity generates, the bubble cluster breakups.

Detached eddy simulation of unsteady cavitation and pressure fluctuation around 3-D NACA66 hydrofoil

The unsteady cavitating flow and pressure fluctuation around the 3-D NACA66 hydrofoil were simulated and validated based on detached eddy simulation turbulence model and a homogeneous cavitation model. Numerical results show that detached eddy simulation can predict the evolution of cavity inception, sheet cavitation growth, cloud cavitation shedding, and breakup, as well as the pressure fluctuation on the surface of hydrofoil. The sheet cavitation growth, detachment, cloud cavitation shedding are responsible for the features of the pressure fluctuation.

Computations of 2‐d cavitating flows using a dipole‐only BEM

A new boundary element procedure which employs dipole‐only singularities is developed to predict the flow past a two‐dimensional cavitating hydrofoil. In the present approach, the unknowns in the discretized simultaneous equation system consist of the dipole strengths at the control points on the wet surface of hydrofoil, the adjusted cavity increments at the nodal points on the cavity surface and the constant tangential speed on the cavity surface. As usual, the nonlinear iteration procedure is adopted; that is, the dynamic and kinematic boundary conditions of the cavity surface are specified on the correct cavity boundary which is a part of the flow solution and unknown before the problem is solved. Some numerical experiments have been carefully conducted. The results show that the present simpler approach successfully predicts the flow, compared to the methods in the literature.