Unsteady RANS simulation of a surface piercing propeller in oblique flow

Conventional propellers might undergo severe cavitation at high speeds and this phenomenon not only affects the efficiency of the propeller, but also may result in serious damages in propulsion system. Due to their special geometries, surface piercing propellers (SPPs) overcome this problem and achieve high efficiencies in high speeds. Therefore, SPPs are one of the popular propulsors for high-speed crafts. The present research is aimed to pursue SPP's performance in the off-design conditions. URANS method was used to study the performance of the 841-B SPP (a case with some available experimental results; Olofsson, 1996) in several immersion ratios (I = 33%, 50%, 75% and 100%) and maneuvering conditions (incident angles of 0°, 10° and 20°). The free surface was simulated using VOF method. Off-design conditions might exert extra or less forces and torques on the propeller's blade. In the present research for 841-B SPP, it was found that a maneuver condition would increase the thrust and torque coefficient for some cases. The sliding mesh technique was utilized to simulate the 841-B SPP performance, which unlike the multiple reference frame (MRF) technique, this technique allows to capture the blades hit on the water surface in transient mode simulations.     

Numerical investigation of the immersion ratio effects on ventilation phenomenon and also the performance of a surface piercing propeller

Surface Piercing Propellers (SPP) show high efficiency at high advance speeds. Regarding operational conditions, this kind of propellers generate an air layer when entering the water due to the rotation of the propeller; this phenomenon is called ventilation. The ventilation phenomenon divided into some mechanism with respect to air cavity length on the propeller surface; among them are partially ventilation mechanism and fully ventilation mechanism which has great importance. In this study, using numerical simulation, we have investigated ventilation patterns and also the performance of a five-blade SPP propeller (SPP 5.74) at immersion ratio of 33, 40, 50and 70% respectively. We used Sliding Mesh Technique for modeling. Also, we applied the volume of fluid method to simulate the open surface pattern. To validate numerical results, the four-blade propeller, 841-B was simulated, and then the results of thrust and torque coefficients compared with Olofsson experimental results and validated accordingly. The findings indicate that the maximum value for thrust and torque coefficient would occur at immersion ratio of 70% and the maximum propeller efficiency occurs at immersion ratio of 33% and advance coefficient of 1.1; Moreover, the critical advance coefficient (at the partially and the fully ventilation boundary) increases by a reduction in immersion ratio, so that critical advance coefficients are 0.6 and 0.76, respectively at immersion ratios of 70 and 33%. Meanwhile, as advance coefficient increases, length of ventilation zone will decrease, and consequently the propeller will be laid on partial ventilation zone.     

Numerical analysis of a propeller during heave motion in cavitating flow

In practical maritime conditions, ship hulls experience heave motion due to the action of waves, which can further drive the ship’s propellers to oscillate relative to the surrounding water. In order to investigate the motion of a propeller working behind a surface vessel sailing in waves, a numerical simulation is conducted on a propeller impacted by heave motion in cavitating flow using the Reynolds-averaged Navier-Stokes (RANS) method. The coupling of the propeller’s rotation and translation is fulfilled using equations of motion defined for this purpose. The heave motion is simplified as a periodic motion based on a sinusoidal function. The numerical transmission of information from the unsteady flow field is achieved using the overset grid approach. In this manner, the unsteady thrust coefficient and torque coefficient of propellers in different periods of heave motion are analyzed. A comparative study is implemented on the unsteady cavitation performance and wake characteristics of propeller. With the propeller’s heave motion, the flow field non-uniformity constantly changes the load on the propeller during each revolution period and each heaving period, the propeller load and the wake field are closely related to the variation of heave motion period. The results obtained from the numerical simulation are expected to serve as a useful theoretical reference for the numerical analysis of a propeller in a heave motion.     

Effect of time step size and turbulence model on the open water hydrodynamic performance prediction of contra-rotating propellers

A growing interest has been devoted to the contra-rotating propellers (CRPs) due to their high propulsive efficiency, torque balance, low fuel consumption, low cavitations, low noise performance and low hull vibration. Compared with the single-screw system, it is more difficult for the open water performance prediction because forward and aft propellers interact with each other and generate a more complicated flow field around the CRPs system. The current work focuses on the open water performance prediction of contra-rotating propellers by RANS and sliding mesh method considering the effect of computational time step size and turbulence model. The validation study has been performed on two sets of contra-rotating propellers developed by David W Taylor Naval Ship R & D center. Compared with the experimental data, it shows that RANS with sliding mesh method and SST k-ω turbulence model has a good precision in the open water performance prediction of contra-rotating propellers, and small time step size can improve the level of accuracy for CRPs with the same blade number of forward and aft propellers, while a relatively large time step size is a better choice for CRPs with different blade numbers.     

Numerical Investigation of the Effect of Current on Wave Focusing

A numerical study on the acoustic radiation of a propeller interacting with non-uniform inflow has been conducted. Real geometry of a marine propeller DTMB 4118 is used in the calculation, and sliding mesh technique is adopted to deal with the rotational motion of the propeller. The performance of the DES (Detached Eddy Simulation) approach at capturing the unsteady forces and moments on the propeller is compared with experiment. Far-field sound radiation is predicted by the formation 1A developed by Farassat, an integral solution of FW-H (Ffowcs Williams-Hawkings) equation in time domain. The sound pressure and directivity patterns of the propeller operating in two specific velocity distributions are discussed.     

Numerical prediction of marine propeller noise in non-uniform inflow

A numerical study on the acoustic radiation of a propeller interacting with non-uniform inflow has been conducted.Real geometry of a marine propeller DTMB 4118 is used in the calculation,and sliding mesh technique is adopted to deal with the rotational motion of the propeller.The performance of the DES(Detached Eddy Simulation) approach at capturing the unsteady forces and moments on the propeller is compared with experiment.Far-field sound radiation is predicted by the formation 1A developed by Farassat,an integral solution of FW-H(Ffowcs Williams-Hawkings) equation in time domain.The sound pressure and directivity patterns of the propeller operating in two specific velocity distributions are discussed.     

Experimental analysis on the risk of vortex ventilation and the free surface ventilation of marine propellers

The paper presents a discussion of the ventilation inception and air drawing prediction of ships propellers, aiming to predict under what conditions ventilation will happen, and the actual physical mechanism of the ventilation.Three different types of ventilation inception mechanisms are included in our discussion: free surface vortex ventilation, ventilation by sucking down the free surface without forming a vortex as well as ventilation by propeller coming out of the water. Ventilation prediction is based on a series of model tests, where the propeller is tested in different levels of intermittent ventilation. The use of underwater video gives a visual understanding of the ventilation phenomena.Ventilation by vortex formation has analogies with other phenomena, such as the inlet vortex in pump sumps, ground vortex at the inlet of the aircraft engines and the Propeller Hull Vortex Cavitation (PHVC). The paper includes comparison between Propeller Hull Vortex Cavitation (PHVC) and Propeller Free Surface Vortex Ventilation (PFSVV) as well as comparison between PFSVV and vortex formations of aero engines during high power operation near a solid surface.Experimental data based on several different model tests shows the boundary between the vortex forming, non-vortex forming and free surface ventilation flow regimes. For comparison the following parameters, which determined the intensity of the hydrodynamic interaction between the propeller and free surface have been used: propeller load coefficient c_T, tip clearance ratio c/D, propeller submergence ratio h/R, ambient velocity V_i and flow cavitation/ventilation number σ_cav/σ_vent.     

A general equation for performance analysis of multi-component propulsor with propeller

An integral panel method (IPM) that treats the different components of multi-component propulsors as a whole is presented for efficient propulsor performance analysis. The IPM requires consider only one blade of the propeller in the performance analysis, which significantly reduces the number of computation grid. The control equations of the IPM are derived in detail for podded propulsors, contra-rotating propellers and hybrid contra-rotating shaft pod propulsors, and based on these derivations, a general control equation for multi-component propulsors with propeller is derived. Comparison between numerical results and experimental data show that the IPM provides good accuracy for the performance analysis of multi-component propulsors with propeller. In addition, the error sources of IPM are discussed, and the reasonableness of these errors is evaluated.     

Flow-induced vibration analysis of elastic propellers in a cyclic inflow: An experimental and numerical study

In this paper, the flow-induced vibrations of marine propellers in cyclic inflows are investigated both experimentally and numerically. A Laser-Doppler velocimetry (LDV) system is used to measure the axial flow velocity distributions produced by the seven-cycle wake screen in the water tunnel. A customized underwater slip ring and a single axis accelerometer sealed by silicon sealant are employed to measure the acceleration responses of rotating propeller blade. Numerical simulations of pressure fluctuations on the blades are performed using large eddy simulation (LES), while the forced vibrations of the propeller blades are obtained by a combined finite element and boundary element method. Experimental and numerical results are presented for two model propellers with the same geometries and different flexible properties, which show that the propeller blade vibrates at a frequency which is seven times as large as the axial passing frequency (APF) in the seven-cycle inflow. Moreover, the propeller blades are observed to resonance when the 7 APF excitation frequency is equal to the fundamental frequency of the propellers. The results indicate that both the inflow feature and the modal characteristic of blades contribute to flow-induced vibrations of elastic propellers.     

基于LES的ROV导管推进器梢隙流动数值研究

导管推进器是一种普遍应用于无人遥控潜水器（ROV）等潜器中的特种推进器。在桨叶与导管之间的梢隙中存在非常复杂的流动,本研究基于大涡模拟（LES）对导管推进器的梢隙流动进行了数值模拟分析。通过对时间步长的收敛性研究,建立两套基于不同网格类型的计算模型。将计算结果与试验进行对比,比较两种不同类型网格模拟结果的差异发现,切割体网格能够更好地捕捉到泄涡的细节,并结合梢隙流场的原理分析泄涡发展的过程,梢隙涡的驱动力是吸力面与压力面之间的压差。此外,随着进速系数增大,梢隙周向的涡管轴向分布范围减小,主泄涡发生位置延后,泄出涡的长度和数量都有所减少。     

RANS Simulation of Podded Propulsor Performances in Straight Forward Motion

The Computational Fluid Dynamics (CFD) approach is adopted to study the steady and unsteady performances of the podded propulsor by the Fluent software package. While the interactions of the propeller blades with the pod and strut are time-dependent by nature, the mixing plane model is employed firstly to predict the steady performance, where the interactions are time-averaged. Numerical experiments are carried out with systematically varied mesh sizes to investigate the dependence of the predicted force values on the mesh sizes. Furthermore, the sliding mesh model is employed to simulate the unsteady interactions between the blades, pod and strut. Based on the numerical results, the characteristics of unsteady hydrodynamic forces are discussed, and the applicability of the mixing plane model is investigated for puller-type podded propulsor.     

Design study for outboard propeller with spoiler

With a large number of recreational craft there is an interest in the development of efficient, high thrust outboard propellers. There has been some success with ‘cupped' propellers with a bent trailing edge. Along the same lines is a simpler idea of attaching a raised bar called an ‘Interceptor' or ‘Spoiler' on the propeller blade trailing edge. For the small diameter propeller, this simplifies the design to adjusting the height of the bar. This paper presents a three-part design study which examines the range of available outboard propellers, the optimum size of outboard propellers, and the capability of predicting the influence of the spoiler on the outboard propeller performance. The results indicate the feasibility of the outboard propeller with spoiler.     

Development of hybrid URANS-LES methods for flow simulation in the ship stern area

The paper presents the results of the application of a new hybrid URANS-LES method for the investigations of the ship wake behind the tanker KVLCC2. The switching between URANS and LES models is based on the ratio between the turbulence scale and the cell size of the mesh. Ship resistance, fields of the axial velocity and turbulent kinetic energy in the propeller plane are calculated and compared with measurements. Much attention is paid to the analysis of the unsteady velocities, their PDF distributions and spectra. Numerical analysis shows that the instantaneous velocities deviate substantially from their mean values which are usually used as the estimated velocities in modern engineering methodologies. The thrust variation in the unsteady wake is more than twice as large as that in the time averaged (frozen) wake. The results of the present study point out that the unsteadiness in the wake behind full ships can be very large and should be taken into account when propulsion and unsteady loadings are determined.     

Numerical and Experimental Analysis of A Ducted Propeller Designed by A Fully Automated Optimization Process Under Open Water Condition

A fully automated optimization process is provided for the design of ducted propellers under open water conditions, including 3D geometry modeling, meshing, optimization algorithm and CFD analysis techniques. The developed process allows the direct integration of a RANSE solver in the design stage. A practical ducted propeller design case study is carried out for validation. Numerical simulations and open water tests are fulfilled and proved that the optimum ducted propeller improves hydrodynamic performance as predicted.     

Numerical investigation on the hydrodynamic characteristics of a marine propeller operating in oblique inflow

The hydrodynamic characteristics of a marine propeller operating in oblique inflow are investigated by using CFD method. Two propellers with different geometries are selected as the study subjects. RANS simulation is carried out for the propellers working at a wide range of advance coefficients and incidence angles. The effects of axial inflow and lateral inflow are demonstrated with the hydrodynamic force on the propeller under different working conditions. Based on the obtained flow field details, the hydrodynamic mechanism of propeller operating in oblique inflow is analyzed further. The trailing vortex wake of propeller is highly affected by the lateral inflow, resulting in the deflected development path and the circumferentially non-uniform structure, as well as the enhanced axial velocity in slipstream. Different flow patterns are observed on the propeller blade with the variation of circumferential position. Combined with the computed hydrodynamic forces and pressure distribution on propeller, the mechanism resulting in the increase of propulsive loads and the generation of propeller side force is explored. Finally, a systematic analysis is carried out for the propulsive loads and propeller side force as a function of axial and lateral advance coefficients. The major terms that play a dominant role in the modeling of propulsive loads and propeller side force are determined through the sensitivity analysis. This study provides a deeper insight into the hydrodynamic characteristics of propeller operating in oblique inflow, which is useful to the investigation of propeller performance during ship maneuvers.     

带前缘结节叶片导管桨尾流不稳定性分析

导管桨的尾流不稳定性在其性能评价中非常重要,不但是其能否提供稳定推力的保证,而且也与螺旋桨的尾流噪声直接相关。为了改善导管桨的尾流,提高尾流稳定性,并优化导管桨的流场脉动,根据座头鲸鳍肢前缘结节的仿生原理,对导管桨叶片的导边进行改进,提出了两种仿生桨型,采用IDDES湍流模型对低进速系数下常规导管桨和仿生叶片导管桨进行数值模拟,探究叶片构型对导管桨性能和尾流不稳定性的影响。计算结果表明,前缘结节可以有效降低叶片受力波动的幅值和叶片所受合力的主频域峰值,具有较大结节的叶片对导管桨尾流有明显的优化作用,在尾流远场中扩大了流动稳定区,延后了尾流处涡破碎的发生,改善了能量谱密度的频域分布。进一步,大前缘结节叶片导管桨应用在低速工况下时,可以大量减少尾流泄涡区域的二次涡产生,这是由于前缘结节提升了相邻涡互感的强度,使得尾流更加稳定,而小结节叶片仿生桨型对导管桨尾流则无明显优化作用。研究方法和成果可为螺旋桨尤其是导管桨尾流不稳定性研究提供参考,不仅验证了前缘结节在导管桨叶片应用的合理性,而且揭示了其优化尾流稳定性的机理。     

Performance analysis of podded propulsors

In this study, the flow around the pod unit is analysed and the performance characteristics of the propeller on the pod are investigated. The main objective of the present work is to further improve the original numerical method developed before for the prediction of performance of podded propellers and to further validate the earlier developed numerical model with a specific emphasis on the hydrodynamic interaction amongst the propulsor components. While in the earlier numerical method, the axial induced velocities by pod and strut parts were included into the calculations on the propeller disc plane, in the present method the tangential induced velocities on the propeller disc plane are included in the calculations as well. The flow domain around the podded propeller is mainly divided into three parts; the axisymmetric pod part, the strut part and the propeller part. While the pod and strut parts are modelled by a low-order boundary element method (BEM), the propeller is represented by a vortex lattice method (VLM). Coupling of the BEM and the VLM is carried out in an iterative manner to incorporate the effect of the pod on the propeller, and vice versa. The present numerical method is applied to two different podded propellers with zero yaw angles in order to compare the results with those of experimental measurements. The present numerical method is also validated in the case of 15° of yaw angle for a podded propulsor. The effect of pod and strut on the propeller and vice versa are discussed.     

船舶螺旋桨尾流场的数值分析

利用基于速度势的低阶面元法计算船舶螺旋桨的尾流场。采用计算较为简捷的关于扰动速度势的基本积分微分方程，并采用双曲面形状的面凶以消除面元间的缝隙。Newton-Raphson迭代过程被用来在桨叶随边满足压力Kutta条件，使桨叶面上表面的压力在随边有良好的一致性。在计算面元的影响系数时，应用了Morino导出的解析计算公式，加快了数值计算的速度。从解面元法的基本积分方程得到的偶极强度和源汇强度，直接求得尾流场的速度分布。     

Effect of various winglets on the performance of marine propeller

The tip vortex cavitation (TVC) is an issue of increasing interest, because the TVC plays an important role in propeller radiated noise and cavitation erosion. The marine propeller with winglets, which is inspired by the winglets of airfoil, is numerically investigated in the present paper. The blade tip of newly designed propeller tilts toward the pressure side. The difference between six propellers is the change of the rake angle at r/R = 1.0. The pressure coefficient, TVC, axial velocity field and helicity are analyzed. The numerical results show that the winglets of newly designed propeller scarcely affect the efficiency of propeller. The thrust coefficient gradually decreases with the increase in rake angle. As for the suction side, the pressure coefficient (C_p) of winglets propellers is higher than the conventional propeller in general. In addition, the winglets are beneficial to generate less cavitation behavior when the rake angle is small. However, as the rake angle is further increased, the cavitation behavior of winglets propeller is also increased, even larger than the conventional propeller. Therefore, it can be deduced that the winglets can be used to effectively improve the TVC characteristics to some extent.