Effect of leading edge roughness on cavitation inception and development on thin hydrofoil

The cavitation incipience and development of water flow over a thin hydrofoil placed in the test section of high-speed cavitation tunnel were investigated.Hydrofoils with smooth and rough leading edge were tested for different upstream velocities and incidence angles.The observations clearly revealed that cavitation incipience is enhanced by roughness at incidence angle below 2°.This is in line with the former reports,according to whose roughness element decreases the wettability and traps a larger amount of gas.As a result,surface nucleation is enhanced with an increased risk of cavitation.Surprisingly,for higher incidence angles(>3°),it was found that cavitation incipience is significantly delayed by roughness while developed cavitation is almost the same for both smooth and rough hydrofoils.This unexpected incipience delay is related to the change in the boundary layer structure due to roughness.It was also reported a significant influence of roughness on the dynamic of developed cavitation and shedding of transient cavities.

Cavitation Inception in Turbulent Flows Around a Hydrofoil

The phenomenon of cavitation inception around a hydrofoil is studied experimentally. The flow velocities around the foil are measured by a laser doppler velocimetry （LDV）. The inception cavitation aspects are observed by using a high-speed video camera. In the experiment, the Reynolds number is fixed at a value of 7.0 × 10^5. The boundary layer around the foil undergoes turbulent flow under the experiment condition. The LDV measurement results show that the flow in the boundary layer around the foil doesn＇t separate from the surface. It is found that the cavitation inception in non-separated turbulent flow is related to the coherent structures in the boundary layer. It is clear that the turbulent bursting and the hairpin-shaped vortex structure accompany the incipient cavitation.

Study of tip vortex cavitation inception and vortex singing

Tip vortex cavitation(TVC)is an important cavitation phenomenon in marine propeller.The formation and evolution of tip vortex cavitation are hot topics consistently both in engineering application and mechanism research.In this paper some recent studies on tip vortex cavitation inception and the noise of tip vortex cavitation evolution are presented.The effects of both flow field and water qualities on tip vortex cavitation inception are considered by experiments and numerical simulations.The results show that besides the average minimum pressure in the vortex core the turbulence fluctuation and water qualities including air content and nuclei distribution have great influence on tip vortex cavitation inception.Based on the idea of first nucleus cavitating in tip vortex core new prediction formula for tip vortex cavitation inception is proposed.The synchronous technique of high speed video observation and noise measurement are adopted to study the development of tip vortex cavitation.S-type total noise characteristics are obtained when cavitation number from low to high.Vortex singing is found in the case where the tip vortex cavitation just before leaves the tip region.The excited mechanism of vortex singing is proposed by analyzing the wave propagation on the interface of vortex cavity.

ZERO-TENSILE STRENGTH FOR RESULT COMPARISON IN CAVITATION INCEPTION TESTS

The tensile strength of test liquid affects the cavitation inception numbers very seriously, and the zero-tensile strength as a criterion for detection of the cavitation inception numbers can be used, in order to reduce the influence of tensile strength. The scale effect on measuring zero-tensile strength of test liquid based on hydraulic methods must be considered. it is supposed that the zero-tensile strength can be defined by use of the comparative method, which unites the classical theory of cavitation inception with the modern one based on an identical criterion.

The effect of water quality on tip vortex cavitation inception

The inception of tip vortex cavitation is very sensitive to water quality. In order to quantify the effect of water quality on the inception of tip vortex cavitation, we develop a motion model to describe the migration and growth of nuclei in water. An analytical solution of migration of nuclei in a vortex flow is obtained so that the capture times of various nuclei can be given out directly. A criterion is built to determine the critical nucleus in a certain nuclei spectra distribution. Tensile strength of the critical nucleus is used to quantify the effect of water quality and correct the tip vortex cavitation inception number. Finally this change of cavitation inception number is compared with experimental results to validate our model.

CAVITATION INCEPTION WITNESSED BY SOUND PRESSURE LEVEL IN MODEL TEST AND PROTOTYPE OBSERVATION

The paper concerns with the relationship between cavitation inception andSound Pressure Level (SPL). A case study for the Xiaolangdi Hydro-project was taken as a typicalexample, which demonstrated the standard criterion of SPL indicating cavitation inception.Cavitation inception of an orifice plate used for energy dissipation in a tunnel spillway waswitnessed and verified both in model test and prototype observation. A design criterion was proposedfor a cavitation-free hydraulic structure which can be served as a reference for designers as wellas researchers. Consequently, scale effects were discussed with an aim at suggesting an appropriatefactor of safety in design.

BLADE SECTION DESIGN OF MARINE PROPELLERS WITH MAXIMUM CAVITATION INCEPTION SPEED

Kuiper and Jessup（1993）developed a design method for propellers in a wake based on the Eppler foil design method.The optimized section is transformed into the three-dimensional propeller flow using the approach of the effective blade sections.Effective blade sections are two-dimensional sections in two-dimensional flow which have the same chordwise loading distribution as the three-dimensional blade sections of a propeller.However,the design procedure is laborious in two aspects：finding an optimum blade section using the Eppler program requires much skill of the designer,and transforming the two-dimensional blade section into a propeller blade section in three-dimensional flow is complex.In this work,these two problems were coped with.A blade section design procedure was presented using an optimization technique and an alternative procedure for the effective blade section is developed using a lifting surface design method.To validate the method a benchmark model of a naval ship was used.This benchmark model was extended by new appendices and a reference propeller,and designed using conventional design methods.This reference propeller was optimized using the new design procedure and model tests were carried out.Special attention was given to the data of the model and the reference propeller,to make the configuration suitable for the Reynolds-Averaged Navier-Stokes（RANS）calculations.

COMPARISON OF RESULTS ON CAVITATION INCEPTION FOR CHECKING THE SCALE EFFECTS

Results on scale effects for cavitation inception at small and large Reynoldsnumbers were compared, respectively. The empirical relation for scale effects suggested by Dr.Andreas Keller was checked, which showed that the comparison of results on cavitation inception atsmall Reynolds numbers below 1 x 10~6 was confirmed very well, but at larger Reynolds numbers abovethe magnitude of 1 x 10~6 was not clearly confirmed.

CAVITATION INCEPTION IN LIQUIDS OF HIGHER VISCOSITY THAN WATER

1.INTRODUCTIONIn connection with experimental model investigations of cavitation phenomena at submergedbodies socalled scale effects can occur.In order to be able to predict the cavitation behaviour of aprototype from model tests,these scale effects must be known and be taken into account by simu-larity relations.In previous investigations test body families of different shapes and size were investigated inwater of different tensile strength for cavitation inception.

THE INFLUENCE OF PRESSURE FLUCTUATION AND PRURE GRADIENT ON CAVITATION INCEPTION OF IRREGULARITIES

An experimental investigation was conducted to study the cavitation inception of six isolated irregularities for negative and positive pressure gradients.The relationship between the root-mean-square of pressure fluctuation and the oscillating radius of gas bubble was determined in this paper.The results of the experiment show that the incipi- ent cavitation numbers for the same protrusions are higher and the intensities of pressure fluctuation are lower in negative gradient flows than that in positive pressure gradient flows.The peaks of pressure fluctuation are seven to twelve times that of the root-mean-square values of pressure fluctuation.

CAVITATION INCEPTION IN TURBULENT SHEAR LAYERS

In this paper the extensive calibration procedure of a hybrid model for predicting cavitation inception in free shear layers described.The hybrid model treats the cavitation process as two decoupled processes,a hydrodynamic one and a bubble dynamic one.The hydrodynamic part is modeled by a small-scale physical model,whereas the bubble dynamic part is solved mathematically.Besides a short explanation of the hybrid model,the procedure of fixing the free parameters of the mathematical formulation by given(measured)data from diverse test facilities is explained.

Scale Effects on Propeller Cavitating Hydrodynamic and Hydroacoustic Performances with Non-uniform Inflow

Considering the lack of theoretical models and ingredients necessary to explain the scaling of the results of propeller cavitation inception and cavitating hydroacoustics from model tests to full scale currently, and the insufficient reflection of the nuclei effects on cavitation in the numerical methods, the cavitating hydrodynamics and cavitation low frequency noise spectrum of three geometrically similar 7-bladed highly skewed propellers with non-uniform inflow are addressed. In this process, a numerical bridge from the multiphase viscous simulation of propeller cavitation hydrodynamics to its hydro-acoustics is built, and the scale effects on performances and the applicability of exist scaling law are analyzed. The effects of non-condensable gas（NCG） on cavitation inception are involved explicitly in the improved Sauer＇s cavitation model, and the cavity volume acceleration related to its characteristic length is used to produce the noise spectrum. Results show that, with the same cavitation number, the cavity extension on propeller blades increases with diameter associated with an earlier shift of the beginning point of thrust decline induced by cavitation, while the three decline slopes of thrust breakdown curves are found to be nearly the same. The power of the scaling law based on local Reynolds number around 0.9R section is determined as 0.11. As for the smallest propeller, the predominant tonal noise is located at blade passing frequency（BPF）, whereas 2BPF for the middle and both 2BPF and 3BPF for the largest, which shows the cavitating line spectrum is fully related to the interaction between non-uniform inflow and fluctuated cavity volume. The predicted spectrum level exceedance from the middle to the large propeller is 6.65 dB at BPF and 5.94 dB at 2BPF. Since it just differs less than 2 dB to the increment obtained by empirical scaling law, it is inferred that the scale effects on them are acceptable with a sufficient model scale, and so do the scaling law. The numerical implementation of cavitating hydrodynamics and hydro-acoustics prediction of propeller in big scale in wake has been completed.

叶片前缘椭圆比变化规律对离心泵初生空化性能的影响

In this research,a parametric control method of blade leading-edge geometry is proposed for a centrifugal pump based on previous study.Based on reasonable blade deconstruction scheme,the method adopts the change rate of leadingedge ellipse ratio from impeller shroud to hub(RaLE),the initial blade thickness(Y0)and thickness diffusion rate(Rt)as three control parameters for the blade leadingedge.The research method of combining theoretical analysis,experimental data and numerical simulation is adopted.The research indicated that according to the operating condition of centrifugal pump,the reasonable design of blade leading-edge ellipse ratio can improve its cavitation performance.This research not only proposed an effective and feasible parameter control method,but also laid a theoretical foundation for improving the inception cavitation performance of centrifugal pump through effective design.

The inception cavitating flows over an axisymmetric body with a blunt headform

The inception cavitating flows around a blunt body are studied based on flow visualizations and velocity field measureme- nts. The main purpose of the present work is to study the incipient cavity evolution and the interplay between the inception cavitation and the local turbulent flows. A high-speed video camera is used to visualize the cavitating flow structures, and the particle image velocimetry （PIV） technique is used to measure the velocity field, the vorticity, and the Reynolds stresses under non-cavitating and inception cavitating flow conditions. It is found that the appearance of visible cavities is preceded by the formation of a cluster of micro-bubbles not attached to the body surface and in a hairpin-shaped vortex structure. During its evolution, the cavity moves downstream with a lower speed. The effect of the incipient cavity is significant on the local vortical structures but slight on the time- averaged velocity distribution. The mean Reynolds stress distributions in the turbulent shear flow can be substantially altered by the incipient cavities. The presence of the incipient cavities can lead to the production of turbulent fluctuations.

3-D NUMERICAL SIMULATION OF FLOW THROUGH AN ORIFICE SPILLWAY TUNNEL

A Large Eddy Simulation (LES) approach based on the weakly compressible hydrodynamic equation with a single plase fluid model for the cavitation flow has been developed and employed in simulating 3 D unsteady viscous flow through an orifice type spillwy tunnel. The finite volume approach in space and the predictor corrector method in time have been used to the numerical discretization, and the 'Law of wall' is applied at the solid boundary. The velocity, pressure fields and the cavitation phenomenon are obtained, the computational results show that 3 D LES approach can give more realistic flow field prediction of the orifice type spillway tunnel.