Non-uniform Wake Research Articles

Numerical and experimental study of the cavitation performance of a composite propeller

Cavitation, which is harmful and unavoidable, has troubled people for a long time and it is urgent to be solved. A composite propeller has the potential ability to improve the cavitation performance because of its distinctive hydro-elastic characteristic, but the related research is limited. In this paper, the cavitation performance of a composite propeller under uniform and non-uniform wake flow is investigated numerically and experimentally. A cavitation coupling simulation method is presented for predicting the cavitation performance of a composite propeller. Cavitation tests are conducted at a cavitation tunnel. The calculated and tested results of cavitation hydrodynamic performance and cavitation patterns are discussed and analyzed. The results show that they are quite consistent, more importantly, cavitation has a significant impact on the hydro-elastic response of a composite propeller.

Fluid-structure interaction analysis of the propeller-shafting system in a non-uniform wake

The hydroelastic analysis of the propeller-shafting system in the non-uniform wake is a key step for designing a modern propeller-shafting system. The hydroelastic responses of the propeller-shafting system in the wake of ships are predicted by applying a three-dimensional time-frequency combined panel approach in conjunction with the finite element method. A fully non-penetration boundary condition applied on the deformed blade surface is conducted. The correction of both the incoming flow velocities and the normal vectors imposed on the blade surface of the transient vibration and the equilibrium positions is considered. The added-mass and -damping matrices due to strongly coupled fluid-structure interaction are derived. By comparing the present findings to numerical solutions calculated using the commercial tool Ansys Mechanical, the accuracy of the suggested technique is verified. It is observed that the added damping is higher, and the amplitudes of bearing forces are smaller, by applying the fully non-penetration boundary condition compared with the results obtained by imposing the other two simplified non-penetration conditions. This indicates that the designers need to imply the fully non-penetration condition on the deformed surfaces to predict the hydroelastic dynamics of the propeller-shafting system, especially in the case of high skew propellers. In addition, the amplitudes of the exciting forces after considering the fluid and propeller-shaft system interaction can be larger or smaller than those ignoring the interaction. The results depend on the phase difference and the mode shape.

Numerical study on the flow-induced noise from waterjet-propelled ship regarding a flexible boundary

In hydroacoustics, it is generally acknowledged that a half-space is bounded by a flexible boundary with non-constant reflection coefficients. However, limited research has focused on the impact of the flexible boundary on noise characteristics. In this paper, a hybrid method combining the unsteady Reynolds-averaged Navier-Stokes (URANS) equations with the Ffowcs-Williams–Hawkings (FW–H) equation is presented. Firstly, the hybrid method coupled with the mirror source method (MS method) is applied to predict noise in a half-space with a rigid boundary for validation. Secondly, the proposed method is extended to predict the underwater noise generated by a medium-speed, mono-hull ship. Specifically, the differences between the noise fields in a half-space and free-space are considered and investigated comprehensively. The results reveal that: (1) the hybrid method performs well in predicting the overall noise induced by complex-shaped configurations in a half-space bounded by different boundaries, and (2) the half-space boundary can influence the noise field due to constructive\\destructive interferences. Additionally, (3) the quadrupole noise generated in the non-uniform wake of a waterjet-propelled ship should be considered. The proposed method has provided an efficient approach to predict the flow-induced noise radiated from a waterjet-propelled ship regarding a flexible boundary.

Global cavitation and hydrodynamic characteristics of a composite propeller in non-uniform wake

Global cavitation and hydrodynamic characteristics of a composite propeller in non-uniform wake

On wake dynamics of a propeller operating under nonuniform behind-hull condition

The wake dynamics of the benchmark propeller INSEAN E779A operating in the nonuniform wake of a typical ship hull are investigated with large-eddy simulations. The flow features and evolution of vortex structures in the propeller wake are analyzed to evaluate the influences of the nonuniform inflow and to identify the instability mechanisms. Unlike the uniform inflow condition, the azimuthal-variant inflow velocity field results in an asymmetric and faster destabilization of the wake. Local mutual interaction among the neighboring tip vortices in the upper region of the wake, which shows a unique bridging-reconnection mechanism of vortex filaments, initiates the decomposition and destabilization of tip vortices and induces instability in the hub vortex. The power spectrum density (PSD) of kinetic energy is employed to analyze the transition characteristics of the tip vortices quantitatively. The hub vortex core circularity and trajectory envelope are calculated to quantify the long- and short-wave instabilities.

The Scale Effect Study on the Transient Fluid–Structure Coupling Performance of Composite Propellers

In order to predict the fluid–structure coupling performance of a full-scale composite propeller under a wake flow field behind a full ship, a bidirectional transient fluid–structure coupling algorithm was established. The performance includes the transient fluid–structure coupling deformation, structural natural frequency, and unsteady hydrodynamic performance. The results showed that the circumferential non-uniform wake flow field can cause periodic pulsation of the propeller’s hydrodynamic force. The average values of thrust and torque coefficient increase, while the pulsation ratio decreases with the increase in scale. The maximum deformation ratio of fluid–structure coupling is linearly related to the scale ratio. Due to the influence of fluid-added stiffness, the maximum deformation ratio needs to be modified by 3% based on the cantilever plate deflection formula. The first five natural frequencies of dry mode and wet mode decrease with the increasing scale, and the wet natural frequencies of each order decrease by 60~68% compared with dry mode. The fluid–structure coupling hydrodynamic performance still show a periodic pulsation with the phase angle, and its average value increases linearly with the scale ratio, while the pulsation ratio decreases with the power relationship of the scale ratio.

Analysis of the effect of vortex reduction structures on submarine tonal noise via frequency-domain method employing thickness noise source

In this study, the acoustic effect of unfavorable flow, such as horseshoe vortex (HSV) generated around the appendages, on propeller radiated noise was analyzed. To analyze this, a frequency-domain noise model is developed, which can predict the noise of an underwater propeller placed in a non-uniform wake. To clearly analyze the acoustic effects of HSV around appendages, vortex reduction structures (VRS) are installed at the junctions of a submarine where HSV is generated intensively, and the acoustic effects before and after the installation of the VRS are compared. In a hydrodynamic analysis of submarines, adverse pressure gradients are resolved and the HSV is suppressed after installing the VRS. In a hydroacoustic analysis, directivity and contribution analysis by radius and order are performed. In the directivity analysis, when the VRS are installed, the first blade-passing frequency (BPF) is reduced by 5∼12 dB, while the second BPF is reduced by up to 20 dB. Through the contribution analysis involving radius, it was found that installing VRS on the rudder is effective in reducing the first BPF noise. By contrast, in the second BPF, the VRS installed on the sail was more effective in noise reduction than that on the rudder.

Comparison of the hydrodynamic performance of front and rear-stator pump-jet propulsors in an oblique wake under the cavitation condition

Pump-jet propulsors, which exhibit high efficiency and low noise, are widely used in underwater vehicles. In a marine environment, a nonuniform wake and an oblique flow affect a propulsor's hydrodynamic force. Therefore, the hydrodynamic performance of front and rear-stator pump-jet propulsors under complex operational conditions was compared in this study. The unsteady hydrodynamic performance was predicted using a shear stress transport k–ω model. When the calculation converged, the Schnerr–Sauer cavitation model was used to simulate the cavitation performance. After an analysis of grid independence and a verification of the calculation model's reliability, the hydrodynamic performance of two pump-jet propulsors in an oblique wake was simulated under the non-cavitation and cavitation conditions. The results indicate that under the non-cavitation condition, the performance of the front-stator pump-jet propulsor worsens more slowly with an increase in the inflow angle. Moreover, the rear-stator pump-jet propulsor stops exhibiting high efficiency when the flow angle (β) is 30°. When cavitation occurs, the performance of the two pump-jet propulsors worsens and their hydrodynamic coefficients decrease rapidly. When β = 30°, the front and rear-stator pump-jet propulsors lose 60% and 40% of their thrust, respectively. The results of this study indicate that cavitation generation has a greater impact on the performance of a front-stator pump-jet propulsor.

Numerical Study on Multiple-Blade-Rate Unsteady Propeller Forces for Underwater Vehicles

_ The unsteady propeller forces of an underwater vehicle were numerically simulated using computational fluid dynamics to investigate the effects of the axial location of the stern planes. A benchmark study was undertaken using a three-bladed propeller; experimental results of the nominal inflow wake profile were analyzed and the unsteady propeller forces were measured. The numerical method was applied to predict the unsteady propeller forces in the SUBOFF model’s wake by varying the axial locations of the stern planes. Several remarks were made on the primary harmonics of the hull’s wakes and blade-rate propeller forces. Introduction The hydroacoustic noise, which matches multiples of the number of propeller blades and its rotational speed, known as “blade-rate (BR) noise,” has been increasingly used to manage hydroacoustics for naval vessels. BR noise can be caused by alternating blade loads owing to fluctuations in the angle of attack of the blades because marine propellers are operated in the nonuniform wake of ships’ hulls. The unsteady blade load produces unsteady propeller forces that are transmitted via the propeller shaft and bearing, thus producing undesirable vibration and noise. Although the resultant BR noise is a common issue for marine vessels, in particular, submarines and other underwater vehicles deployed for undersea defense systems and oceanographic survey systems require strict specifications for the acoustic signature. Therefore, the unsteady propeller forces must be improved for reduced detectability, because the vehicles should be able to operate without being discovered while sonar detection technology continues to improve.

Empirical mode decomposition of ship hull pressure fluctuation induced by cavitating propeller

We report for the first time a cavitation-induced pressure fluctuation decomposition developed from empirical mode decomposition (EMD) [Huang et al., Proc. R. Soc. London, Ser. A 454, 903–995 (1998)]. The idea is to decompose the nonlinear and non-stationary time series data into a finite and usually small number of “intrinsic mode functions” based on the local properties of the signal, which admit a well-behaved Fourier transform. With this transform, we can obtain frequency characteristics that give sharp identifications of imbedded structures. The cavitation evolution and excited pressure fluctuation around a cavitating propeller in the nonuniform wake are investigated using high-speed imaging and pressure sensors. By the EMD method, we separate the pressure fluctuations induced by different types of cavitation. The high frequency components of the pressure fluctuations are mainly caused by the collapse of sheet cavitation, followed by the shrinking and growth of sheet cavitation. Furthermore, the tip vortex cavitation leads to higher frequency but contributes less to pressure fluctuations. The periodical motion of the propeller contributes to the first blade frequency, and the pressure fluctuations induced by cavitation are superimposed on it.

Numerical Investigation of Unsteady Cavitation Flow around E779A Propeller in a Nonuniform Wake with an Insight on How Cavitation Influences Vortex

In the current study, the turbulent cavitation flow around a marine propeller in a nonuniform wake is simulated with the shear stress transport (k−ω SST) turbulence model combining Zwart–Gerber–Belamri (ZGB) cavitation model. The predicted cavity evolution shows a fairly well agreement with the available experimental results. Important mechanisms of propeller cavitation flow, including side‐entrant jet and cavitation‐vortex interaction, are analyzed in this paper. Vorticity is found to be mainly located in cavitation regions and the propeller wake during propeller rotating. The unsteady behavior of cavitation and side‐entrant jet can both promote local vorticity generation and flow unsteadiness. In addition, it is indicated with the relative vorticity transport equation that the stretching term plays a major role in vorticity transportation, while baroclinic torque and Coriolis force term mainly influence the vorticity distribution along the liquid‐vapor interface.

Numerical Study on the Unsteady Characteristics of the Propeller Cavitation in Uniform and Nonuniform Wake Flows

Propeller cavitation is a problematic issue because of its negative effects, such as performances losses, noise, vibration and erosion. Numerical methodology is an effective and efficient technical tool for the study of propeller cavitation, however, it is hard to capture tip-vortex cavitation in the previous work by using common turbulence models based on turbulent-viscosity hypothesis. In this work, the Reynolds-Averaged Naiver-Stokes (RANS) approach, adopting the Reynolds stress turbulence model (RSM), is taken to study the unsteady characteristics of the cavitation on the four-bladed INSEAN E779A model propeller. The numerical simulation was carried out using the commercial CFD software ANSYS Fluent 14.0. One kind of uniform wake flow and two kinds of nonuniform wake flows are considered here. The results in the uniform flow show a good agreement with previous experimental results on both the sheet cavitation and the tip vortex cavitation and prove the ability of the RSM on capturing the tip vortex cavitation. Two kinds of nonuniform wake flows are designed based on the previous experimental researches and the unsteady characteristics of the propeller cavitation are analyzed by comparing the results in the uniform and two nonuniform wake flows together.

Hydrodynamic and Hydroacoustic Computational Prediction of Conventional and Highly Skewed Marine Propellers Operating in Non-uniform Ship Wake

Despite their high manufacturing cost and structural deficiencies especially in tip regions, highly skewed propellers are preferred in the marine industry, where underwater noise is a significant design criterion. However, hydrodynamic performances should also be considered before a decision to use these propellers is made. This study investigates the trade-off between hydrodynamic and hydroacoustic performances by comparing conventional and highly skewed Seiun Maru marine propellers for a noncavitating case. Many papers in the literature focus solely on hydroacoustic calculations for the open-water case. However, propulsive characteristics are significantly different when propeller-hull interactions take place. Changes in propulsion performance also reflect on the hydroacoustic performances of the propeller. In this study, propeller-hull interactions were considered to calculate the noise spectra. Rather than solving the full case, which is computationally demanding, an indirect approach was adopted; axial velocities from the nominal ship wake were introduced as the inlet condition of the numerical approach. A hybrid method based on the acoustic analogy was used in coupling computational fluid dynamics techniques with acoustic propagation methods, implementing the Ffowcs Williams-Hawkings (FW-H) equation. The hydrodynamic performances of both propellers were presented as a preliminary study. Propeller-hull interactions were included in calculations after observing good accordance between our results, experiments, and quasi-continuous method for the open-water case. With the use of the time-dependent flow field data of the propeller behind a non-uniform ship wake as an input, simulation results were used to solve the FW-H equation to extract acoustic pressure and sound pressure levels for several hydrophones located in the near field. Noise spectra results confirm that the highest values of the sound pressure levels are in the low-frequency range and the first harmonics calculated by the present method are in good accordance with the theoretical values. Results also show that a highly skewed propeller generates less noise even in noncavitating cases despite a small reduction in hydrodynamic efficiency.

Experimental investigation of vortex generator influences on propeller cavitation and hull pressure fluctuations

In this paper, the transient turbulent cavitating flow around a marine propeller behind a ship was investigated experimentally with emphasis on how vortex generator (VG) influences propeller cavitation and hull pressure fluctuations. The experiments were carried out in China Large Cavitation Channel with the closed test section covering 10.5 m in length and cross-section of 2.2 m2 ϗ 2.0 m2. The experiment recorded instantaneous cavitation photos of the propeller and pressure fluctuations on the hull surface. The results demonstrate that without vortex generator, the sheet cavitation inception begins at −50° (310°), and then from −50° (310°) to 30°, the extent and area of sheet cavitation increase. When vortex generator installed on the hull, the sheet cavitation inception occurs in advance at −60° (300°), and the sheet cavitation expands to larger rotation range due to the vortex generator. It is shown that the vortex generator with proper geometry and installed location may contribute the highly nonuniform wake to be more uniform, and can make the change of propeller cavitation to be milder, which decrease the pressure fluctuations in this study. Further analysis indicates that the vortex generator may promote the energy distribution to be more uniform in physical and spectral space in some degree.

Computational hydrodynamic analysis of a highly skewed marine propeller

The objective of the current paper is to study the flow around Seiun Maru Highly Skewed (HSP) marine propeller by assessment of blade forces and moments under non-cavitating case. The calculations are performed in open water (steady case) and non-uniform ship wake (Unsteady case). The governing equations based on Reynolds Averaged Navier-Stokes Equation (RANSE) are solved using Finite Volume Method. Ansys Fluent 14.0 is used to implement the simulation. For the steady case, Moving Reference Frame (MRF) is selected while sliding mesh technique is adopted for the unsteady case. Calculated open water performances in terms of thrust and torque coefficients fit very well with experimental data for a wide range of advance ratio. In the unsteady calculations, axial velocities, deduced from the nominal wake, are introduced in the Ansys fluent code. To locate suitably the non-uniform wake in the propeller front plane, three positions of inlet wake have been taken into account to determine their effects on the accuracy of the results. Obtained results show that computed performances are improved compared to panel method when the inlet is close to the propeller.

Study on the axial exciting force characteristics of marine propellers considered the effect of the shaft and blade elasticity

Marine propellers usually operate in a spatially nonuniform wake and then the propeller exciting forces are produced. These exciting forces will cause serious hull vibration and noise radiation. So, there are many researches on the exciting forces of propellers. However, the effects of the shaft and blade elasticity are ignored in most studies. Therefore, firstly, considered the effects of the shaft and blade elasticity, a fluid–structure interaction dynamic model of the fluid-propeller-shaft system is established by coupled BEM and FEM. Then, based on this model, the characteristics of axial exciting force and theirs transmission mechanism to the hull via the shaft are studied. The research results show that first, there are two kinds of vibration modes for blade bending vibration: global mode and local mode. The elastic coupling effect between the blade and the shafting only affects the global mode, but hardly affects the local mode of the blade. Secondly, during the transmission of axial exciting force to the hull through the blades and shafting, only the global mode of the blades can amplify it, while the local mode cannot. These studies could provide a guideline for the optimal design of the propeller-shaft system to make the exciting force transmitted to the ship hull via the shafting be the smallest.

Numerical modelling of unsteady cavitation and induced noise around a marine propeller

The objective of this paper is to numerically investigate the cavitating flow around a marine propeller and to explore the intrinsic relationships between the sheet cavitation and its radiation noise. The k-ω SST turbulence model with the turbulence viscosity correction and the Zwart cavitation model are introduced to the simulation of cavitating flow around a propeller in a non-uniform wake. The loading noise and cavitation noise have been predicted based on the theory for acoustic fan source and the sound radiation theory for spherical bubble respectively. The periodic cavitation development has been captured, and the periodic large pressure fluctuation around the blade has been analyzed with the dominant frequencies in accordance to the first order of the blade passing frequency. For the non-cavitation case, the high sound pressure levels mainly concentrate in the low frequency stage and decrease from the low-order to the high-order blade passing frequency. While for the sheet cavitation case, the sound pressure level at the high-order of blade passing frequencies are enhanced. The sound pressure induced by the cavitation development is periodically varied accompanied with the periodic pulsating cavity evolution, and the acoustic energy mainly focus on the low order of blade passing frequencies.

Obtaining mathematical functions of the propeller thrust and torque coefficients fluctuations at non-uniform wake flow including geometry effects

The main purpose of this paper is to obtain mathematical functions of the propeller thrust and torque coefficients fluctuations at non-uniform flow. Two types of inflow wakes from Seiun-Maru ship and INSEAN 7000 DWT Tanker are investigated for the B-series propeller. The computed results include the thrust and torque coefficients fluctuations and their mathematical functions under the influence of two different wakes for one blade and all blades of the studied propellers. To this accomplishment, variation of thrusts and torques coefficients under different pitch ratio, expanded area ratio and number of blades are investigated using classical mathematical methods in one cycle and the Fourier coefficients with ten terms are presented for each case. The results of thrust and torque coefficients at different propeller geometries are presented and discussed.

Continuous-Time State-Space Unsteady Aerodynamic Modeling for Efficient Loads Analysis

The present paper proposes a continuous-time state-space formulation of the unsteady vortex lattice method, which is derived through a discretization of the governing advection equation for transport of vorticity in the wake. A continuous-time system is obtained by only discretizing the advection equation in space, while retaining the derivative with respect to time. The discretization in space is based on the discontinuous Galerkin method. The present method can be applied to any arbitrary nonuniform wake discretization and can be extended to higher-order panel methods or a nonflat wake shape. The method is extended to compressible flows by applying the Prandtl–Glauert transformation. The time-dependent terms in the small disturbance potential equation are neglected. Thus, incompressible flow solution procedures are applied with minimum modifications to unsteady compressible problems. The benefits are demonstrated by applying the model to the gust analysis of a general aircraft wing, varying the time step, and introducing a nonuniform wake discretization, resulting in a reduced model size for a given accuracy. The resulting continuous-time state-space model can be used for efficient loads analysis of general aircraft wings including the effects of compressibility and allows for easy integration with structural or flight dynamic models for efficient aero(servo)elastic analyses.

Numerical Prediction of Tip Vortex Cavitation for Marine Propellers in Non-uniform Wake

Tip vortex cavitation is the first type of cavitation to take place around most marine propellers. But the numerical prediction of tip vortex cavitation is one of the challenges for propeller wake because of turbulence dissipation during the numerical simulation. Several parameters of computational mesh and numerical algorithm are tested by mean of the predicted length of tip vortex cavtiation to validate a developed method. The predicted length of tip vortex cavtiation is on the increase about 0.4 propeller diameters using the developed numerical method. The predicted length of tip vortex cavtiation by RNG k – e model is about 3 times of that by SST k – ω model. Therefore, based on the validation of the present approach, the cavitating flows generated by two rotating propellers under a non-uniform inflow are calculated further. The distributions of axial velocity, total pressure and vapor volume fraction in the transversal planes across tip vortex region are shown to be useful in analyzing the feature of the cavitating flow. The strongest kernel of tip vortex cavitation is not at the position most close to blade tip but slightly far away from the region. During the growth of tip vortex cavitation extension, it appears short and thick, and then it becomes long and thin. The pressure fluctuations at the positions inside tip vortex region also validates the conclusion. A key finding of the study is that the grids constructed especially for tip vortex flows by using separated computational domain is capable of decreasing the turbulence dissipation and correctly capturing the feature of propeller tip vortex cavitation under uniform and non-uniform inflows. The turbulence model and advanced grids is important to predict tip vortex cavitation.