Biomimetic reduction of wake deficit using tail articulation at low Reynolds number

The field of biomimetics seeks to distill biological principles from nature and implement them in engineering systems in an effort to improve various performance metrics. In this paper, a biology-based approach is used to address the problem of radiated propulsor noise in underwater vehicles using active control. This approach is one of "tail articulation" of a stator blade, which is carried out using a suitable strategy that effectively alters the flow field impinging on a rotor downstream and in turn changes the radiated noise characteristics of the rotor blades. A reduced-order two-dimensional noise model is developed by characterizing the impact of the articulation as a point circulation input, which is then used to develop an active control strategy. An experimental investigation of such a control strategy is also carried out in this paper using a simple benchtop open-channel water tunnel at Re=4000 and stepper motor controlled articulation. Tail articulations using sinusoidal and transient motion were able to reduce the wake deficit behind the stator by as much as 40-60%. The implications of the proposed method for reducing blade tonal noise in autonomous underwater vehicles are briefly discussed at the end of the paper.     

Wake studies of a 1/30th scale horizontal axis marine current turbine

A 0.4 m diameter (1:30th scale) horizontal axis marine current turbine (MCT) was tested in a circulating water channel. The turbine performance and wake characteristics were determined over a range of flow speeds and rotor thrust coefficients. Measurements of the water surface elevation profiles indicated increasing variation and surface turbulence with increasing flow speeds. Blockage-type effects (where the measured point velocity was greater than the inflow velocity) occurred around the sides of the rotor for all flow speeds. Although the effects were exaggerated at model scale, it is expected that reasonable variations in water level and flow velocity could also occur over a full scale MCT array.     

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。     

Numerical flow and performance analysis of a water-jet axial flow pump

The purpose of the present study is to investigate the performance and three-dimensional flow fields in a water-jet pump. TASCflow is employed to simulate the rotator-stator coupling flow field. A standard k-ε turbulence model combined with standard wall functions is used. In order to investigate the effect of a rear stator on flow fields, the flows in two water-jet pumps with and without a rear stator are studied. Computational fluid dynamics (CFD)-predicted overall performances are in good agreement with the experimental results. Then the flow fields, such as the pressure distribution on the blade surfaces, and the axial and tangential velocity distribution, especially the radial loading distribution, are investigated at different flow rates. In addition, the effects of a rear stator and different spacings between the rotor and the stator on the overall performance and the flow fields of the water-jet pump are also investigated.     

Unsteady hydromechanics of a steering podded propeller unit

Unsteady forces, torques and bending moments were predicted for a model podded propulsor unit at various azimuth angles. Predictions in time history include propeller shaft thrust, propulsor unit thrust, normal forces to the propeller shaft bearing, total forces acting on the propulsor unit, propeller shaft torque, blade spindle torque, in-plane and out-of-plane bending moments, and propulsor unit stock shaft torque and bending moments. Analysis was performed for averaged forces and their fluctuations as well. A time-domain unsteady multi-body panel method code, PROPELLA, was further developed for this prediction work. Predictions were compared with a set of time averaged in-house experimental data for a puller-type podded propulsor configuration in the first quadrant operation. Unsteady fluctuations of forces were predicted numerically. Analysis was made for the bending moment on propeller blades, shaft and the propulsor unit stock shaft for azimuth angles from 0° to 45°. It indicates that the magnitude and fluctuation of the forces are significant and they are essential for structural strength and design optimization. The predicted bending moment and global forces on the propulsor unit provide some useful data for ship maneuvering motion and simulation in off-design conditions.     

Influence of Ice Size Parameter Variation on Hydrodynamic Performance of Podded Propulsor

During ice-breaking navigation, a massive amount of crushed ice blocks with different sizes is accumulated under the hull of an ice-going ship. This ice slides into the flow field in the forward side of the podded propulsor, affecting the surrounding flow field and aggravating the non-uniformity of the propeller wake. A pulsating load is formed on the propeller, which affects the hydrodynamic performance of the podded propulsor. To study the changes in the propeller hydrodynamic performance during the ice podded propulsor interaction, the overlapping grid technique is used to simulate the unsteady hydrodynamic performance of the podded propulsor at different propeller rotation angles and different ice block sizes. Hence, the hydrodynamic blade behavior during propeller rotation under the interaction between the ice and podded propulsor is discussed. The unsteady propeller loads and surrounding flow fields obtained for ice blocks with different sizes interacting with the podded propulsor are analyzed in detail. The variation in the hydrodynamic performance during the circular motion of a propeller and the influence of ice size variation on the propeller thrust and torque are determined. The calculation results have certain reference significance for experiment-based research, theoretical calculations and numerical simulation concerning ice podded propulsor interaction.     

Performance analysis of ducted marine propellers. Part I – Decelerating duct

The paper analyses the flow around a marine propeller ducted with a so-called decelerating nozzle both through the axial momentum theory and the nonlinear semi-analytical actuator disk model. While the well-known and widely diffused axial momentum theory can be successfully employed only to qualitatively investigate the characteristics of the flow around a ducted propeller, the nonlinear and semi-analytical method can instead evaluate the thrust exerted by the duct for different values of the overall thrust and advance coefficients. There are several advantages characterising the more advanced actuator disk method. Specifically, the wake convergence and rotation may be fully taken into account, the shape of the duct and of the radial distribution of the load can be of general type, and, finally, the mutual interaction between the duct and the propeller may be readily dealt with. The methods are employed to investigate the effects of the decelerating nozzle on the efficiency and on the cavitation condition of the propeller. In particular, the influence of some duct geometrical parameters on the device performance is thoroughly analysed providing useful insights on the operating principles of this kind of propulsive systems.     

Performance assessment of a concept propulsor: the thrust-balanced propeller

This paper presents the results of a numerical performance analysis to demonstrate the worthiness of a recently patented new concept propulsor, the so-called “thrust-balanced propeller (TBP)”. The main advantage of this unconventional propulsor is its inherent ability to reduce the unsteady effect of blade forces and moments when it is operating in a non-uniform wake flow. The propulsor comprises a pair of diametrically opposed blades that are connected to one another and mounted so as to be rotatable together through a limited angle about their spindle axis. A quasi-hydrodynamic approach is described and applied to perform the numerical analysis using a state-of-the-art lifting surface procedure for conventional propellers. Performance comparisons with a conventional fixed-pitch propeller are made for the blade forces and moments, efficiency, cavitation extents and fluctuating hull pressures. Bearing in mind the quasi-static nature of the analyses, the results present favourable performance characteristics for the thrust-balanced propeller and support the worthiness of the concept. However, the concept needs to be proved through physical model tests, which are planned to take in a cavitation tunnel.     

Savonius型转轮三角阵列减流性能研究

为了助力海洋牧场减流防护工程, 研究Savonius型转轮阵列减流性能。作者建立Savonius型转轮三角阵列尾流场数值模型, 并通过水池实验验证准确性, 基于可靠数值模型探究转轮阵列尾涡减流机理, 研究三角阵列结构参数L_X、L_Y, 以及动力参数TSR、初始流速、旋向对整体减流性能的影响规律。结果表明,下游转轮产生的涡流呈现非对称分布, 并且产生更多涡流的转轮拥有更好的减流效果。另外, L_X为3D和L_Y为2D时减流性能最佳。最后对比发现, 在叶尖速比为0.9~1.1减流效果更好; 初始流速大小不影响减流效果; 下游转子对称分布时, 随着上游转子改变旋转方向, 减流效果出现明显差异。     

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.     

A novel maneuverable propeller for improving maneuverability and propulsive performance of underwater vehicles

The existing propulsor that can perform both propulsion and maneuvering along axis of rotation is propeller/rotor for a helicopter. Helicopter propellers when maneuvering increase or decrease their blades’ pitch cyclically to create imbalanced thrust and hence maneuvering force/torque. A “maneuverable propeller” was developed and its performance on both maneuvering and propulsion is assessed. The “maneuverable propeller” is an alternative of the existing helicopter rotors. The novelty of this propulsor is that the imbalanced thrust force/torque is created by cyclically increasing or decreasing the angular speed of their blades relatively to the hubs/shafts, to provide the desired maneuvering torque. This maneuverable propeller is hence defined as the Cyclic Blade Variable Rotational Speed Propeller (CBVRP). One of the best advantages is that the maneuvering torque created by the “maneuverable propeller” is much higher, about 5 times of the shaft torque of the same propeller at thrust only mode. The “maneuverable propeller” has wide applications for both surface ships and underwater vehicles that require high maneuverability for cruising inside the narrow passage.     

Aerodynamic loads on a typical tension leg platform

The wind load effects on tension leg platforms have been recognized to be a significant environmental loading. An accurate assessment of the aerodynamic loads is, therefore, a prerequisite for the design of an economic and a reliable structure. The design codes and specifications recommend the use of a projected area approach that is thought to be conservative. The code recommendations fail to quantify aerodynamically induced forces in directions different to the mean wind flow. The interference and shielding effects suggested in some specifications provide only a simplistic view. Physical modeling as reported in this paper, therefore, continues to serve as the most accurate and practical means of predicting aerodynamic loads.The mean aerodynamic force and moment coefficients of a typical tension leg platform for various approach wind directions were measured on a scale model exposed to simulated flow conditions in a boundary layer wind tunnel. Major components on the upper deck of the model were designed for easy removal so that measurements could be obtained for different platform configurations. A parametric study was conducted to determine shielding and interference effects, i.e. the manner in which aerodynamic coefficients are influenced by the location and orientation of the ancillary structures on the platform, e.g. living quarters, flare boom, derricks, etc. The present paper addresses the wind tunnel modeling procedures and automated data acquisition and reduction methods. The aerodynamic force and moment coefficients with respect to the body and flow axes were reduced from the experimental measurements for azimuth angles of 0 to 360 degrees at 15-degree intervals. A total of eight configurations were monitored ranging from a platform configuration that included all the ancillary structures to the case where every deck component was removed. The aerodynamic coefficients obtained from the classification society recommended procedures provided conservative estimates in comparison with the measured values for all configurations. The results also illustrate that the interference effects among various ancillary structures on the platform are significant.     

Bioinspired delayed stall propulsor

This paper provides an overview of a bioinspired delay stall propulsor (BDSP) concept that employs delayed stall unsteady lift enhancement to increase the lift on propeller blades without adding any complexity to the propulsor. This BDSP concept can provide greatly increased propeller thrust for a given propeller diameter, leading to both increased speed and/or maneuverability. Alternately, this technology offers reduced radiated noise while maintaining current thrust levels through reduction in both propulsor rotation speed and acoustic cancellation. Preliminary two-dimensional simulations have shown a potential 36% reduction in rotational speed at constant thrust, leading to an estimated 4-dB reduction in the total radiated acoustic power. It is believed that the BDSP concept will be simple to manufacture, rugged, and easy to retrofit into existing marine propulsors. This technology has direct application to torpedoes, unmanned underwater vehicles, maneuvering thrusters, submarines, and other propeller-driven devices.     

Experimental analysis of the flow field around horizontal axis tidal turbines by use of scale mesh disk rotor simulators

Understanding the flow field around horizontal axis marine current turbines is important if this new energy generation technology is to advance. The aim of this work is to identify and provide an understanding of the principal parameters that govern the downstream wake structure and its recovery to the free-stream velocity profile. This will allow large farms or arrays of devices to be installed whilst maximising device and array efficiency. Wake characteristics of small-scale mesh disk rotor simulators have been measured in a 21 m tilting flume at the University of Southampton. The results indicate that wake velocities are reduced in the near wake region (close behind the rotor disk) for increasing levels of disk thrust. Further downstream all normalised wake velocity values converge, enforcing that, as for wind turbines, far wake recovery is a function of the ambient flow turbulence. Varying the disk proximity to the water surface/bed introduces differential mass flow rates above and below the rotor disk that can cause the wake to persist much further downstream. Finally, the introduction of increased sea bed roughness whilst increasing the depth-averaged ambient turbulence actually decreases downstream wake velocities. Results presented demonstrate that there are a number of interdependent variables that affect the rate of wake recovery and will have a significant impact on the spacing of marine current turbines within an array.     

A three-dimensional wake impingement model and applications on tandem oscillating foils

Potential flow based vortex numerical methods have been widely used in aerodynamics and hydrodynamics. In these methods, vortices shed from lifting bodies are traced by using vortex filaments or dipole panels. When the wake elements encounter a downstream body, such as a rudder behind a propeller or a stator behind a rotor, a treatment is necessary to divert the wake elements to pass by the body. This treatment is vital to make wake simulations realistic and to satisfy the non-penetration condition during wake body interaction. It also helps to avoid pure numerical disturbances such as when a vortex filament or an edge of a dipole panel passes through the collection point of a body element; this is a singularity for induced velocity and it will introduce a large numerical disturbance. This necessary treatment for three-dimensional problems with geometrical complexity has not been found to date. In this study, a wake impingement model was developed to divert wake elements to slip over the body surface, model the vortex/body interaction, and predict forces on fluctuating components. The model was also tested on configurations of oscillating foils in tandem with an existing panel method code. Simulation results with the wake impingement model are shown to be in closer agreement with limited published experimental data than those without the model. With the established wake impingement model, force fluctuations on the after body due to the wake vortex impingement were investigated based on a series of simulations. The series varied several parameters including distance between two foils, oscillating frequency, span, rear foil pitch angle, swap angle and vertical position.     

Simulating fundamental and higher harmonic VIV of slender structures

Vortex-induced vibrations (VIV) of slender marine structures are complicated response processes, where a number of distinct frequency components might be simultaneously active. These are categorized as fundamental and higher harmonics respectively, where the latter can have a tremendous impact on the fatigue life. The present work proposes a method for time domain simulation of such multi-frequency response, by introducing a higher harmonic load term to a pre-existing semi-empirical hydrodynamic force model. Forced motion tests of a circular cylinder were simulated to experimentally and qualitatively validate the fluid-structure energy transfer. Next, the model was used to predict the response of a tension dominated riser in uniform current, for 22 velocities in the range 0.3 m/s to 2.4 m/s. The empirical input was chosen to give an average best fit with respect to the cross-flow strain measurements, which allowed dominating frequencies, fatigue damage, higher harmonics and response variability to be predicted with a high level of realism. Same set of empirical coefficients was subsequently used to predict VIV of three additional flexible pipe experiments in uniform flow, with significant differences in structural properties. The results were satisfactory for all cases, but could be improved by moderate changes to the empirical input.     

海洋大型底栖动物群落次级生产力估算模型研究进展及应用评价

大型底栖动物是海洋生态系统的重要组成部分,在生态系统物流和能流中占有重要地位。定量估算海洋大型底栖动物次级生产力在其功能研究中具有重要意义。本文综述了海洋大型底栖动物次级生产力估算模型研究的3个阶段:起步阶段(1979-1990)、发展与完善(1990-2001)、成熟并广泛应用(2001-至今)。国内关于海洋大型底栖动物次级生产力估算基本都采用Brey(1990)经验公式,采用经验公式的方式可分为3类:逐种计算、按站位计算和按类群计算,逐种和按类群计算最符合Brey(1990)给出的参考步骤。不同方法估算同一海域次级生产力结果不同,同一估算方法估算不同生境不同群落次级生产力结果偏差也不同。今后的工作应注重种群次级生产力研究,积累大量基础数据,以便建立适宜我国特定海域的大型底栖动物次级生产力估算模型。     

Numerical simulations of sloshing flows with an elastic baffle using a SPH-SPIM coupled method

In this work, sloshing flows were successfully simulated by using a coupled numerical scheme between smoothed particle hydrodynamics (SPH) and smoothed point interpolation method (S-PIM) (SPH-SPIM coupled method). SPH is a Lagrangian particle method to solve flow fields while S-PIM is developed to deal with the structure dynamics. A coupling scheme is proposed, the key of which is that the fluid and solid fields are not necessary to be discretized by the same resolution. The stability, accuracy, convergence and conservation of the SPH-SPIM coupled method were validated by the case of hydrostatic water column on an elastic plate. Then, a wave impact problem was simulated to verify that the present SPH method worked well for sloshing flows. Finally, two sloshing problems with an elastic baffle were simulated, which validated the accuracy and stability of the method in predicting the fluid-structure interaction (FSI) features during the process of sloshing. It has been found that both the shape of the free surface and the large deformation of the elastic baffle can be well captured by the present method, which shows the potential of the present method to be a good candidate for simulating sloshing problems.     

海洋环境荷载下输液立管的静、动力特性研究

考虑管内流动流体和管外海洋环境荷载共同作用 ,建立海洋立管侧向运动微分方程。用Hermite插值函数离散 ,在微机上编写海洋立管静、动力分析程序 ,通过计算分析研究管内流体对立管侧向变形和应力的作用 ;另外 ,探讨管内流体的流动速度和立管顶端的预张力对立管动力特性的影响。结果表明 ,立管变形和应力均随管内流体流动速度增加而增大 ,同时内流速度的增大会降低立管的固有频率 ,但适当增大立管顶端预张力会抵消内流流速增加引起的固有频率下降。     

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。