Propeller Tip Vortex Research Articles

Alleviation of Propeller-Slipstream-Induced Unsteady Pylon Loading by a Flow-Permeable Leading Edge

The impingement of a propeller slipstream on a downstream surface causes unsteady loading, which may lead to vibrations responsible for structure-borne noise. A low-speed wind-tunnel experiment was performed to quantify the potential of a flow-permeable leading edge to alleviate the slipstream-induced unsteady loading. For this purpose, a tractor propeller was installed at the tip of a pylon featuring a replaceable leading-edge insert in the region of slipstream impingement. Tests were carried out with four flow-permeable inserts, with different hole diameters and cavity depths, and a baseline solid insert. Particle-image-velocimetry measurements showed that the flow through the permeable surface caused an increase in boundary-layer thickness on the pylon’s suction side. This led to a local drag increase and reduced lift, especially for angles of attack above 6 deg. Furthermore, it amplified the viscous interaction with the propeller tip-vortex cores, reducing the velocity fluctuations near the pylon surface by up to 35%. Consequently, lower tonal noise emissions from the pylon were measured in the far field. This suggests that the desired reduction in surface pressure fluctuations was achieved by application of the flow-permeable leading edge.

A Semi-Empirical Prediction Method for Broadband Hull-Pressure Fluctuations and Underwater Radiated Noise by Propeller Tip Vortex Cavitation †

A semi-empirical method is presented that predicts broadband hull-pressure fluctuations and underwater radiated noise due to propeller tip vortex cavitation. The method uses a hump-shaped pattern for the spectrum and predicts the centre frequency and level of this hump. The principal parameter is the vortex cavity size, which is predicted by a combination of a boundary element method and a semi-empirical vortex model. It is shown that such a model is capable of representing the variation of cavity size with cavitation number well. Using a database of model- and full-scale measured hull-pressure data, an empirical formulation for the maximum level and centre frequency has been developed that is a function of, among other parameters, the cavity size. Acceptable results are obtained when comparing predicted and measured hull-pressure and radiated noise spectra for various cases. The comparison also shows differences that require adjustments of parameters that need to be further investigated.

Numerical simulation of boundary-layer transition flow of a model propeller and the full-scale propeller for studying scale effects

Propeller scale effects were widely studied from boundary-layer flow, friction and pressure distributions, propeller performance and tip vortices. The γ-Reθ transition model and two turbulence models (STAR-CCM+) were applied for the model propeller simulation, and the influence of turbulence intensity on the boundary-layer flow was studied. The transition model was also applied for the full-scale propeller simulation, and numerical results were compared with that predicted by a commonly used turbulence model. Data from open water tests and velocity measurements of the model propeller were used for validation. Results show that scale effects of propeller performance are mainly caused by different boundary-layer flows: the boundary-layer flow of the full-scale propeller is basically turbulent, while the flows over the blade face and blade back of the model propeller are complex, different and sensitive to the Reynolds number. By studying the effect of varying Reynolds numbers on propeller performance, boundary-layer flow and tip vortices, why the Reynolds number of the model propeller in experiments needs to be greater than the critical Reynolds number was studied. Last, scale effects and propeller performance of the full-scale propeller predicted by numerical methods and the ITTC recommended scaling method are close.

Unsteady Pylon Loading Caused by Propeller-Slipstream Impingement for Tip-Mounted Propellers

An experimental analysis was performed of the unsteady aerodynamic loading caused by the impingement of a propeller slipstream on a downstream lifting surface. When installed on an aircraft, this unsteady loading results in vibrations that are transmitted to the fuselage and are perceived inside the cabin as structure-borne noise. A pylon-mounted tractor–propeller configuration was installed in a low-speed wind tunnel at Delft University of Technology. Surface-microphone and particle-image-velocimetry measurements were taken to quantify the pressure fluctuations on the pylon and visualize the impingement phenomena. It was confirmed that the propeller tip vortex is the dominant source of pressure fluctuations on the pylon. Along the path of the tip vortex on the pylon, a periodic pressure response occurs with strong harmonics. The amplitude of the pressure fluctuations increases with increasing thrust setting, whereas the unsteady lift coefficient displays a nonmonotonic dependency on the propeller thrust. The lowest integral unsteady loads were obtained for cases with approximately integer ratios between the pylon chord and the wavelength of the perturbation associated with the propeller tip vortices. This implies that structure-borne-noise reductions might be obtained by matching the pylon chord with an integer multiple of the axial separation between the propeller tip vortices.

Propeller tip vortex cavitation control and induced noise suppression by water injection

As noise emission from ships garners increasing interest, control of propeller cavitation, which generates vibration and radiates noise, gains critical importance. In the case of full-scale propellers, the tip vortex cavitation typically incepts first, and noise levels rise dramatically thereafter. We propose the mass injection method to reduce noise induced from the tip vortex cavitation and increase the cavity inception speed. Water injected from the propeller tip decreases the rotational speed of the tip flow, and restrains the growth of the tip vortex cavity. For the preliminary test, we investigate the flow around an elliptic wing in a cavitation tunnel, and measured the induced noise levels. The results show a clear decrease in the noise levels in the case of water injection from the wing tip compared to that of without injection. On the basis of a successful preliminary study, we apply this method to the rotating model propeller. Experimental investigations of the model tests in a large cavitation tunnel show that we can control the tip vortex cavitation of the propeller, and hence, increase the cavitation inception speed of the ship using water injection from the propeller tip.

SVA Potsdam 프로펠러 단독 및 캐비테이션 성능 수치해석

This paper presents numerical results of the performance of a marin propeller in cavitating and non-cavitating flow conditions. The geometry and experimental validation data of the propeller are provided in Potsdam Propeller Test Case(PPTC) in the framework of the second International Symposium on Marine Propulsors 2011(SMP'11) workshop. The PPTC includes open water tests, velocity field measurements and cavitation tests. The present numerical analysis was carried out by using the Reynolds averaged Navier-Stokes(RANS) method on a wall-resolved grid ensuring a y+=1, where the SST k-<TEX>${\omega}$</TEX> model was mainly used for turbulence closure. The influence of the turbulence model was investigated in the prediction of the wake field under a non-cavitating flow condition. The propeller tip vortex flows in both cavitating and non-cavitating conditions were captured through adaptation of additional grids. For the cavitation flows at three operation points, Schnerr-Sauer's cavitation model was used with a Volume-Of Fluid(VOF) approach to capture the two-phase flows. The present numerical results for the propeller wake and cavitation predictions including the open water performance showed a qualitatively reasonable agreement with the model test results.

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.

Compressive spherical beamforming for localization of incipient tip vortex cavitation.

Noises by incipient propeller tip vortex cavitation (TVC) are generally generated at regions near the propeller tip. Localization of these sparse noises is performed using compressive sensing (CS) with measurement data from cavitation tunnel experiments. Since initial TVC sound radiates in all directions as a monopole source, a sensing matrix for CS is formulated by adopting spherical beamforming. CS localization is examined with known source acoustic measurements, where the CS estimated source position coincides with the known source position. Afterwards, CS is applied to initial cavitation noise cases. The result of cavitation localization was detected near the upper downstream area of the propeller and showed less ambiguity compared to Bartlett spherical beamforming. Standard constraint in CS was modified by exploiting the physical features of cavitation to suppress remaining ambiguity. CS localization of TVC using the modified constraint is shown according to cavitation numbers and compared to high-speed camera images.

Cavitation tunnel analysis of radiated sound from the resonance of a propeller tip vortex cavity

The goal of this study is to test the hypothesis that the resonance of a tip vortex cavity is responsible for high-amplitude broadband pressure-fluctuations, typically between 40 and 70 Hz, for a full scale propeller. This is achieved with a model propeller in a cavitation tunnel. Simultaneous high-speed video shadowgraphy and sound measurements show that a steady tip-vortex cavity behind a propeller in a uniform inflow does not produce significant sound in the relevant range of 0.5–1.2 kHz. The addition of an upstream wake does result in high amplitude sound. It appears that the dominant sound frequency is directly related to the resonance of the tip vortex cavity. A model for the tip-vortex cavity-resonance frequency, using a Proctor vortex model, is able to give an accurate description of the dominant sound frequencies.

Study on Prediction of Underwater Radiated Noise from Propeller Tip Vortex Cavitation

The method to predict underwater radiated noise from tip vortex cavitation was studied. The growth of a single cavitation bubble in tip vortex was estimated by substituting the tip vortex to Rankine combined vortex. The ideal spectrum function for the sound pressure generated by a single cavitation bubble was used, also the empirical factor for the number of collapsed bubbles per unit time was introduced. The estimated noise data were compared with measured ship's ones and it was found out that this method can estimate noise data within 3dB difference.

Investigation on the wake evolution of contra-rotating propeller using RANS computation and SPIV measurement

The wake characteristics of Contra-Rotating Propeller (CRP) were investigated using numerical simulation and flow measurement. The numerical simulation was carried out with a commercial CFD code based on a Reynolds Averaged Navier-Stokes (RANS) equations solver, and the flow measurement was performed with Stereoscopic Particle Image Velocimetry (SPIV) system. The simulation results were validated through the comparison with the experiment results measured around the leading edge of rudder to investigate the effect of propeller operation under the conditions without propeller, with forward propeller alone, and with both forward and aft propellers. The evolution of CRP wake was analyzed through velocity and vorticity contours on three transverse planes and one longitudinal plane based on CFD results. The trajectories of propeller tip vortex core in the cases with and without aft propeller were also compared, and larger wake contraction with CRP was confirmed.

A novel approach for the isolation of the sound and pseudo-sound contributions from near-field pressure fluctuation measurements: analysis of the hydroacoustic and hydrodynamic perturbation in a propeller-rudder system

The main scope of the present work is to investigate the mechanisms underlying the hydroacoustic and hydrodynamic perturbations in a rudder operating in the wake of a free running marine propeller. The study consisted of detailed near-field pressure fluctuation measurements which were acquired on the face and back surfaces of the rudder, at different deflection angles. To this aim, a novel wavelet-filtering procedure was applied to separate and analyze distinctly the acoustic and hydrodynamic components of the recorded near-field pressure signals. The filtering procedure undertakes the separation of intermittent pressure peaks induced by the passage of eddy structures, interpreted as pseudo-sound, from homogenous background fluctuations, interpreted as sound. The use of wavelet in the filtering procedure allows to overcome the limitations of the earlier attempts based on frequency (wave number) band-pass filtering, retrieving the overall frequency content of both the acoustic and the hydrodynamic components and returning them as independent signals in the time domain. Acoustic and hydrodynamic pressure distributions were decomposed harmonically and compared to the corresponding topologies of the vorticity field, derived from earlier LDV measurements performed by Felli and Falchi (Exp Fluids 51(5):1385–1402, 2011). The study highlighted that the acoustic perturbation is mainly correlated with the unsteady load variations of the rudder and to the shear layer fluctuations of the propeller streamtube. Conversely, the dynamics of the propeller tip and hub vortices underlies the hydrodynamic perturbation.

추진기 날개 끝 형상변화에 따른 보오텍스 유동에 대한 수치해석

In order to control the tip vortex cavitation occurring around the tip of a rotating propeller blade, researches on the propeller cavitation and blade tip vortex flows have been increased. In this paper, the propeller tip vortex flow for a blunt and sharp tips was studied using an unsteady Reynolds-averaged Navier-Stokes equations solver based on a cell-centered finite volume method. In numerical open water test, torques, thrusts, pressure distributions and vortex flows were compared for various rotating speeds. To consider a hull wake, the nominal wake was specified in inlet boundary condition. Pressure distributions and vortex flows with the hull wake were investigated for various propeller rotating angles. From the results, it was confirmed that the blunt tip propeller delayed the tip vortex flow Ke ywords :P ropeller blade tip(추진기 날개 끝), Tip vortex flow(날개 끝 보오텍스 유동), Computational fluid dynamics(CFD, 전산유체역학)

Propeller tip and hub vortex dynamics in the interaction with a rudder

In the present paper, the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed in its wake are addressed; in particular, following the works by Felli et al. (Exp Fluids 6(1):1–11, 2006a, Exp Fluids 46(1):147–1641, 2009a, Proceedings of the 8th international symposium on particle image velocimetry: Piv09, Melbourne, 2009b), the attention is focused on the analysis of the evolution, instability, breakdown and recovering mechanisms of the propeller tip and hub vortices during the interaction with the rudder. To investigate these mechanisms in detail, a wide experimental activity consisting in time-resolved visualizations, velocity measurements by particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) along horizontal chordwise, vertical chordwise and transversal sections of the wake have been performed in the Cavitation Tunnel of the Italian Navy. Collected data allows to investigate the major flow features that distinguish the flow field around a rudder operating in the wake of a propeller, as, for example, the spiral breakdown of the vortex filaments, the rejoining mechanism of the tip vortices behind the rudder and the mechanisms governing the different spanwise misalignment of the vortex filaments in the pressure and suction sides of the appendage.

Mechanisms of evolution of the propeller wake in the transition and far fields

In the present study the mechanisms of evolution of propeller tip and hub vortices in the transitional region and the far field are investigated experimentally. The experiments involved detailed time-resolved visualizations and velocimetry measurements and were aimed at examining the effect of the spiral-to-spiral distance on the mechanisms of wake evolution and instability transition. In this regard, three propellers having the same blade geometry but different number of blades were considered. The study outlined dependence of the wake instability on the spiral-to-spiral distance and, in particular, a streamwise displacement of the transition region at the increasing inter-spiral distance. Furthermore, a multi-step grouping mechanism among tip vortices was highlighted and discussed. It is shown that such a phenomenon is driven by the mutual inductance between adjacent spirals whose characteristics change by changing the number of blades.

Numerical simulation of the structure of propeller's tip vortex and wake

By means of numerical simulation, the evolvement of tip vortex and vortical trailing wake of a propeller were studied in details. From upstream of leading edge up to downstream of trailing edge, the tip vortices region extends rapidly and becomes sizable, which reason is that the transport of vortices between trailing wake and tip vortex takes place. The trailing wake region or the tip vortex region, each consists of sub-regions. The direction of the vortices in one of the sub-regions is opposite to that of the other sub-region. Each sub-region is structured with vortex layers of varying strength. The trailing wake possesses significant thickness and the tip vortex region occupies sizable space. The present analysis also show that within certain distance downstream from tailing edge, although the size and the configuration of the vortices region varies with going backwards, the total vortices flux through each station almost remains constant.

Dynamic interaction of the cavitating propeller tip vortex with the rudder

Dynamic interaction of the cavitating propeller tip vortex with the rudderThe hydrodynamic interaction between the ship propeller and the rudder has many aspects. One of the most interesting is the interaction between the cavitating tip vortex shed from the propeller blades and the rudder. This interaction leads to strongly dynamic behaviour of the cavitating vortex, which in turn generates unusually high pressure pulses in its vicinity. Possibly accurate prediction of these pulses is one of the most important problems in the hydrodynamic design of a new ship. The paper presents a relatively simple computational model of the propeller cavitating tip vortex behaviour close to the rudder leading edge. The model is based on the traditional Rankine vortex and on the potential solution of the dynamics of the cylindrical sections of the cavitating kernel passing through the strongly variable pressure field in the vicinity of the rudder leading edge. The model reproduces numerically the experimentally observed process of initial compression of the vortex kernel in the high pressure region near the stagnation point at the rudder leading edge and subsequent explosive growth of the kernel in the low pressure region further downstream. Numerical simulation of this process enables computation of the additional pressure pulses generated due to this phenomenon and transmitted onto the hull surface. This new numerical model of the cavitating tip vortex is incorporated in the modified unsteady lifting surface program for prediction of propeller cavitation, which has been successfully used in the process of propeller design for several years and which recently has been extended to include the effects of propeller — rudder interaction. The results of calculations are compared with the experimental measurements and they demonstrate reasonable agreement between theory and physical reality.

On the mechanism of the bursting phenomena of propeller tip vortex cavitation

The bursting phenomenon of tip vortex cavitation of a propeller sometimes causes severe high-frequency vibration, but its mechanism has not yet been elucidated. In this study, we carried out model experiments by changing the propellers, wake distributions, thrust coefficients, and cavitation numbers parametrically, examined the bursting phenomenon with a high-speed video camera, and measured the pressure fluctuations caused by the phenomenon. We also measured flow distribution around the tip vortex. As a result, we found that in the bursting phenomenon, large pressure fluctuations occurred twice, and that they strongly depended on the wake distribution. Two means were suggested to suppress the bursting phenomenon, other than changing the wake distribution: stabilizing tip vortex cavitation or reducing the cavity volume. Numerical fluid simulations around a propeller in noncavitating, unsteady conditions were also conducted, and the strength of the tip vortex along the circumference and its derivative were examined. As a result, the phenomena were parameterized by the time derivative of the strength of the tip vortex, and if it was higher than a threshold value, the tip vortex cavitation burst. Therefore, it is possible to predict the occurrence of the bursting phenomenon by numerical analysis.

Bursting Phenomenon of Propeller Tip Vortex Cavitation and its Relationship with the Result of Numerical Simulation of Propeller in Unsteady Condition

Bursting phenomenon of tip vortex cavitation causes severe high-frequency vibration, but estimation of it by numerical computation is not yet realized. In this study the authors made model experiments to observe the bursting phenomena and measured the pressure fluctuations caused by that, changing two propellers, wake distributions, thrust coefficients and cavitation numbers parametrically. After analyzing frequency of it we found that there would be two methods to suppress the bursting phenomenon : stabilizing the tip vortex cavitation or reducing the volume of cavitation. We also did numerical fluid simulations around a propeller in non-cavitating, unsteady condition and compared the strength of tip vortex along the circumference and its derivatives. As a result the phenomena were parameterized by the time derivative of the strength of tip vortex, and if it was higher than a certain value, tip vortex cavitation bursted. Therefore there is the possibility to predict the occurrence of bursting phenomenon by numerical analysis.

High Angle-of-Attack Aerodynamics

High angle-of-attack (high-IX) aerodynamics has been a key element in airplane design beginning with the first attempts at unpowered manned flight using gliders in the 1 800s and extending to the present day with civil and military airplanes. High-IX aerodynamics is commonly perceived as arising on an airplane at a physically large angle of attack with its leading­ edge vortex separating from highly swept wings. However, the underlying flow is that of a heavily loaded lifting surface, operating at other than optimum lift-to-drag ratio. This off-design or high-lift condition often includes regions of separated flow. In this regard, high-IX embraces a broader range of aerodynamic flows than is widely appreciated. For example, high-IX can be defined as any situation where an airplane or any of its components encounters off-design, high-lift flight, and includes attached flows, uncontrolled separated flows, and organized flow sep­ aration in the form of vortices, shock waves, and combinations of these flow conditions. The high-IX regime encompasses local and global flow fields that can occur at subsonic through hypersonic speeds. These flow fields can be highly interactive and either static or dynamic in nature. Examples of localized high-IX flows include the interaction of the propeller tip vortex system with the airframe of a light general aviation airplane;