Computational and Empirical Investigation of Propeller Tip Vortex Cavitation Noise

In this study,non-cavitating and cavitating flow around the benchmark DTMB 4119 model propeller are solved using both viscous and potential based solvers.Cavitating and non-cavitating propeller radiated noises are then predicted by using a hybrid method in which RANS(Reynolds-averaged Navier-Stokes)and FWH(Ffowcs Williams Hawkings)equations are solved together in open water conditions.Sheet cavitation on the propeller blades is modelled by using a VOF(Volume of Fiuld)method equipped with Schnerr-Sauer cavitation model.Nevertheless,tip vortex cavitation noise is estimated by using two different semi-empirical techniques,namely Tip Vortex Index(TVI,based on potential flow theory)and Tip Vortex Contribution(TVC).As the reference distance between noise source and receiver is not defined in open water case for TVI technique,one of the outputs of this study is to propose a reference distance for TVI technique by coupling two semi-empirical techniques and ITTC distance normalization.At the defined distance,the starting point of the tip vortex cavitation is determined for different advance ratios and cavitation numbers using potential flow solver.Also,it is examined that whether the hybrid method and potential flow solver give the same noise results at the inception point of tip vortex cavitation.Results show that TVI method based on potential flow theory is reliable and can practically be used to replace the hybrid method(RANS with FWH approach)when tip vortex cavitation starts.

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.

Dynamics of cavitation–structure interaction

Cavitation–structure interaction has become one of the major issues for most engineering applications. The present work reviews recent progress made toward developing experimental and numerical investigation for unsteady turbulent cavitating flow and cavitation–structure interaction. The goal of our overall efforts is to（1） summarize the progress made in the experimental and numerical modeling and approaches for unsteady cavitating flow and cavitation–structure interaction,（2） discuss the global multiphase structures for different cavitation regimes, with special emphasis on the unsteady development of cloud cavitation and corresponding cavitating flow-induced vibrations,with a high-speed visualization system and a structural vibration measurement system, as well as a simultaneous sampling system,（3） improve the understanding of the hydroelastic response in cavitating flows via combined physical and numerical analysis, with particular emphasis on the interaction between unsteady cavitation development and structural deformations. Issues including unsteady cavitating flow structures and cavitation–structure interaction mechanism are discussed.

Calculation of tip vortex cavitation flows around three-dimensional hydrofoils and propellers using a nonlinear k-ε turbulence model

Simulations of tip vortex wetted flows and cavitating flows are carried out by using a RANS model. Two types of turbule- nce models, with and without the Boussinesq turbulent-viscosity hypothesis, are adopted in comparing with experimental results regarding the vorticity, the strain rate and the Reynolds shear stress distributions in the vortex region. The numerical results imply that the spatial phase shift between the mean strain rate and the Reynolds stresses can be accurately modeled by the nonlinear κ-ε turbulence model, the tip vortex cavitation region can only be predicted using the nonlinear κ-ε turbulence model. The mecha- nism of the over-dissipation due to the turbulence model is analyzed in terms of the turbulence production, which is one of the dominant source terms in the transport equations of energy.

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.

Large eddy simulation of cloud cavitation and wake vortex cavitation around a trailing-truncated hydrofoil

The cavitation has received considerable attention for decades because of its negative influence on the performance and the safety of the hydraulic machinery.In this study,a large eddy simulation is carried out to numerically investigate the unsteady cavitating flow around a trailing-truncated NACA 0009 hydrofoil for determining the underlying physical mechanisms.Two types of cavitation morphologies are identified:The large-scale bubble cluster and the von Kármán vortex cavity,named as the cloud cavitation and the wake vortex cavitation,respectively.It is shown that the velocity profiles obtained over the hydrofoil suction surface are in good agreement with the experimental data,indicating the accuracy of the current simulation.The dynamic evolution of the sheet/cloud cavity is also well reproduced,covering the sheet cavity breakup,the sheet/cloud transformation,and the collapse of the cloudy bubble cluster.The wake-vortex cavitation is caused by the blunt geometry at the hydrofoil trailing edge,where pairs of vortex cavities are induced.Both the cloud and vortex cavities significantly affect the lift oscillation,which makes it difficult to decompose the components.The fundamental shedding mechanisms of the wake vortex cavitation are discussed based on the finite-time Lyapunov exponent field.Specifically,the suction-side bubble grows and squeezes the giant pressure bubble away from the trailing edge.After the pressure bubble detaches,a new counterclockwise vortex or a new bubble appears at the pressure side,thus lifting the ridge towards the suction trailing edge and generating a strong vortex eye that pinches off the trailing portion of the suction-side bubble.

EXPERIMENTAL ESTIMATION OF A SCALING EXPONENT FOR TIP VORTEX CAVITATION VIA ITS INCEPTION TEST IN FULL-AND MODEL-SHIP

Tip vortex cavitation noise of marine propeller became primary concems to reduce hazardous environmental impacts from commercial ship or to keep the underwater surveillance of naval ships. The investigations of the tip vortex and its induced noise are normally conducted through the model test in a water cavitation tunnel. However the Reynolds number of model-test is much smaller than that of the full-scale, which subsequently results in the difference of tip vortex cavitation inception. Hence, the scaling law between model- and full-scales needs to be identified prior to the prediction and assessment of propeller noise in full scale. From previous researches, it is generally known that the incipient caivtation number of tip vortex can be represented as a power of the Reynolds number. However, the power exponent for scaling, which is the main focus of this research, has not been clearly studied yet. This paper deals with the estimation of scaling exponent based on tip vortex cavitation inception test in both full- and model-scale ships. Acoustical measurements as well as several kind of signal processing technique for an inception criterion suggest the scaling exponent as 0.30. The scaling value proposed in this study shows slight difference to the one of most recent research. Besides, extrapolation of model-ship noise measurement using the proposed one predicts the full-scale noise measurement with an acceptable discrepancy.

Novel scaling law for estimating propeller tip vortex cavitation noise from model experiment

The tip vortex cavitation（TVC） noise of marine propellers is of interest due to the environmental impacts from commercial ships as well as for the survivability of naval ships. Due to complicated flow and noise field around a marine propeller, a theoretical approach to the estimation of TVC noise is practically unrealizable. Thus, estimation of prototype TVC noise level is realized through extrapolation of the model TVC noise level measured in a cavitation tunnel. In this study, for the prediction of prototype TVC noise level from a model test, a novel scaling law reflecting the physical basis of TVC is derived from the Rayleigh-Plesset equation, the Rankine vortex model, the lifting surface theory, and other physical assumptions. Model and prototype noise data were provided by Samsung Heavy Industries（SHI） for verification. In applying the novel scaling law, similitude of the spectra of nuclei is applied to assume the same nuclei distribution in the tip vortex line of the model and the prototype. It was found that the prototype TVC noise level predicted by the novel scaling law has better agreement with the prototype TVC noise measurement than the prototype TVC noise level predicted by the modified ITTC noise estimation rule.

Application of signal processing techniques to the detection of tip vortex cavitation noise in marine propeller

The tip vortex cavitation and its relevant noise has been the subject of extensive researches up to now. In most cases of experimental approaches, the accurate and objective decision of cavitation inception is primary, which is the main topic of this paper. Although the conventional power spectrum is normally adopted as a signal processing tool for the analysis of cavitation noise, a faithful exploration cannot be made especially for the cavitation inception. Alternatively, the periodic occurrence of bursting noise induced from tip vortex cavitation gives a diagnostic proof that the repeating frequency of the bursting contents can be exploited as an indication of the inception. This study, hence, employed the Short-Time Fourier Transform （STFT） analysis and the Detection of Envelope Modulation On Noise （DEMON） specmma analysis, both which are appropriate for finding such a repeating frequency. Through the acoustical measurement in a water tunnel, the two signal processing techniques show a satisfactory result in detecting the inception of tip vortex cavitation.

VISCOUS SCALE EFFECTS ON PROPELLER TIP VORTEX CAVITATION

Viscous scale effects on propeller TVC were investigated by testing a series of three geosim propeller models in the large cavitation Tunnel of CSSRC without and with two different turbulence stimulators. Tests included flow visualization by oil film method and cavitation observation for five different stages of development of propeller TVC: desinent, unattached, attached, developed and fully developed TVC. The main findings are: 1)there existed a size effect of the boundary-layer transition on propeller models which could be analyzed by using the critical roughness Reynolds number and a newly defined quasi-critical Reynolds number, 2)the preliminary results of the blackboardpaint used as a tripping device was encouraging, 3)the Reynolds number exponent n of TVC scaling rules was found to be dependent upon the blade surface condition, the stage of development of TVC and the thrust loading of propeller models.

Prediction of the precessing vortex core in the Francis-99 draft tube under off-design conditions by using Liutex/Rortex method

The turbulent flow in the draft tube of a Francis turbine is very complicated while working under off-design conditions. Although the off-design conditions were widely studied, the vortex core line in the draft tube of a Francis turbine with splitter blades is not well understood, especially the vortex rope property. This letter presents a prediction of the behavior of the vortex rope in the draft tube of the Francis-99 turbine obtained by the computational fluid dynamics (CFD), where the Liutex/Rortex method, as the most recent vortex definition, is applied to analyze the periodical precession of the vortex rope in the draft tube cone. The advantage of this Liutex/Rortex method is shown by its enhanced ability to represent the vortex rope structurewith the vortex-core lines. Furthermore, since it seems to be very hard to define a sharp boundary surface for the whole vortex structure, it is advantageousfocusing only on the vortex core line,by which different vortex structures can be clearly differentiated. The evolution of the vortex core and the process of the vortex breakdown in the draft tube are revealed, which might help to comprehend the development of the turbulent flow in the draft tube.

Cavitation shedding dynamics around a hydrofoil simulated using a filter-based density corrected model

The unsteady turbulent cloud cavitation around a NACA66 hydrofoil was simulated using the filter-based density corrected model(FBDCM). The cloud cavitation was treated as a homogeneous liquid-vapor mixture and the effects of turbulent eddy viscosity were reduced in cavitation regions near the hydrofoil and in the wake. The numerical results(in terms of the vapor shedding structure and transient pressure pulsation due to cavitation evolution) agree well with the available experimental data, showing the validity of the FBDCM method. Furthermore, the interaction of vortex and cavitation was analyzed based on the vorticity transport equation, revealing that the cavitation evolution has a strong connection with vortex dynamics. A detailed analysis shows that the cavitation could promote the vortex production and flow unsteadiness by the dilatation and baroclinic torque terms in the vorticity transport equation.

High-fidelity numerical simulation of unsteady cavitating flow around a hydrofoil

Cavitation is a widespread and detrimental phenomenon in hydraulic machinery, therefore, it requires to be accurately predicted. In this study, large eddy simulation (LES), scale-adaptive simulation (SAS) and grid-adaptive simulation (GAS) are employed to investigate the unsteady cavitating flow around a NACA0009 hydrofoil. The prediction accuracy of GAS, SAS, both using the shear-stress transport (SST) k — ω model as baseline turbulence model, is validated by comparing with experimental and LES results. The cavity behaviors and turbulence fields are analyzed systematically. Results show that the GAS gives a more reasonable turbulent viscosity and accurately predicts the periodic evolution of typical vortical structures of cavitating flow, such as tip leakage vortex cavitation, tip separation vortex cavitation, leading-edge cavitation, and trailing-edge vortex. The time-averaged cavity volume, volume fluctuation amplitude, and characteristic frequencies of cavities predicted by the GAS are very closed to the LES, while the SAS fails to accurately capture these cavity characteristics. Furthermore, the local trace criterion is applied to extract the vortical structures and to analyze the swirling patterns of the tip leakage vortex. Multi-scale vortical structures in LES are well identified by local trace criterion. The prediction accuracy of the SAS method for small-scale vortical structures, such as the vortex shedding on the suction side and the vortex rope around the tip leakage vortex, is obviously insufficient, while the GAS has a higher accuracy in predicting vortex shedding. The tip leakage vortex and induced vortex extracted from GAS are also closer to that of LES in both swirling patterns and scale.