Numerical prediction of the effective wake profiles of a high-speed underwater vehicle with contra-rotating propellers

A numerical method is proposed to predict the effective wake profiles of high speed underwater vehicles propelled by contra-rotating propellers (CRPs), in which the hydrodynamic effects of the CRPs are simulated by distributed body forces. First, Reynolds-averaged Navier-Stokes (RANS) simulations are conducted for identical body-force distributions in open-water and self-propulsion conditions. The effective wake profiles at the CRP disks are then obtained by subtracting the velocities induced by the body forces in the open water from those induced by the body forces in the self-propulsion condition. The effective wake profiles were then predicted for a generic underwater vehicle with an established CRP design. Next, the hydrodynamic performance of the CRPs in the effective wake was computed using an in-house vortex-lattice code. The potential-flow results agree well with those provided by the RANS simulation under the self-propulsion condition, indicating that the proposed method can predict the effective wake profiles for CRPs with reasonable accuracy. The influences of different wake components on the blade forces were investigated, determining that for CRPs, and especially for the aft propeller, the circumferential wake cannot be neglected in the design.     

Numerical Study of A Generic Tidal Turbine Using BEM Optimization Methods

Three blade-geometry optimization models derived along with assumptions from the blade element momentum(BEM) approach are studied by using a steady BEM code to improve a small horizontal-axis rotor of three blades that has been previously used in experiments. The base rotor blade has linear-radially varying chord length and pitch angle, while the other three models noted as Burton, Implicit and Hansen due to their references and characteristics yield blades of non-linearly varying chord length and pitch angle. The aim is to compare these rapid models and study how assumptions embedded in them affect performance and induction factors. It is found that the model that has the least assumptions(Hansen) and which considers the blade-profile drag in its optimization procedure yields the highest power coefficient, C_P, at the optimal tip speed ratio(TSR), about 7% higher than the base one and also higher C_P at high TSR. It produces an axial induction factor distribution along the blade that is closest to the 1 D optimal value of 1/3. All optimized tangential induction-factor distributions along the blade closely vary as inverse to the square of the radial distance, while being mildly higher than the base distribution. It shows that sufficient swirl is necessary to increase power but at a level causing not too much energy loss in unnecessary swirl of the wake. At high TSR, all optimized rotors adversely produce higher thrust than the base one, but the one with most embedded assumptions(Burton) produces the highest thrust. Details of all three optimization models are given along with the distributions of the power, thrust, blade hydrodynamic efficiency and induction factors.     

Application of polypyrrole actuators: feasibility of variable camber foils

A decade of research into electroactive polymer actuators is leading to the exploration of applications. These technologies are not ready to compete with the internal combustion engine and electric motors in high power propulsion systems but are suitable for intermittent or aperiodic applications with moderate cycle life requirements, providing an alternative to solenoids and direct drive electric motors. Polypyrrole, an emerging actuator material, is applied to drive hydrodynamic control surfaces and in particular to change the camber of a foil. The foil is intended for use in the propeller blade of an autonomous underwater vehicle. A scaled prototype is constructed which employs polypyrrole actuators imbedded within the blade itself to vary camber. The kinematics required to generate camber change are demonstrated, with >30/spl deg/ deflections of the trailing edge being observed from both bending bilayer and linear actuator designs. Forces developed in still conditions are five times lower than the 3.5 N estimated to be required to implement variable camber. The observed 70 kJ/m/sup 3/ polypyrrole work density however is more than sufficient to produce the desired actuation from within the limited blade volume, enabling an application that is not feasible using direct drive electric motors. A key challenge with the polypyrrole actuators is to increase force without sacrificing speed of actuation.     

水下滑翔器整体外形设计及水动力性能分析

对水下滑翔器的整体外形设计与水动力性能进行研究。在Slocum等几种典型水下滑翔器样机的基础上,对滑翔器的主体和附体进行一体化设计,得到阻力最小的新型水下滑翔器构型设计。利用CFD方法对水下滑翔器进行模拟仿真,通过分析对比五种主体构型,得到了比较合理的主体线型,然后用正交设计方法和曲线拟合法对附体进行了优选工作,最后得到了性能更优的整体载体外形。模拟仿真实验表明,滑翔器在8°左右攻角航行时,具有最大的升阻比;和Slocum等经典样机相比,新的载体具有更好的水动力性能。通过上述研究工作,也可以缩短水下滑翔器研制周期,降低设计成本,并为水下滑翔器的更优设计提供了有力的技术指导和参考。     

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.     

水下滑翔机器人运动分析与载体设计

水下滑翔机器人是一种新型水下机器人,具有噪声低、航行距离远、续航时间长、成本低等特点。分析了水下滑翔机器人的驱动机理和运动实现,给出了水下滑翔机器人典型运动的仿真结果,并以正在设计的一水下滑翔机试验样机为研究对象,描述了样机的整体结构布局,详细研究了浮力调节机构、俯仰调节机构和横滚调节机构的实现方法,并就样机中各执行机构的设计实现进行了论述。     

Effect of various surface-height-distribution properties on acoustic backscattering statistics

A problem of interest to underwater acousticians is understanding the relationship between ocean-bottom characteristics and acoustic backscattering statistics. This experimental work focused on examining surface roughness characteristics that cause backscattering strength statistics to deviate from the Rayleigh distribution. Several different scattering surfaces with known height distributions were designed for this study. The surfaces were modeled using a technique that allowed for different height-distribution functions and correlation lengths to be prescribed. Isotropic and anisotropic surfaces were fabricated having both Gaussian and non-Gaussian surface-height distributions. Many independent backscattering measurements were made for different aspects of each surface using a computer-controlled transducer-positioning system. Acoustic backscattering statistics were non-Rayleigh for the anisotropic surfaces when combining measurements from different aspects. Mean scattering strength was found to be dependent on both the surface-height distribution and correlation length. In addition, backscattering strength showed a dependence on the surface-height power distribution.     

Development and experiments of the Sea-Wing underwater glider

Underwater gliders,which glide through water columns by use of a pair of wings,are efficient long-distance,long-duration marine environment observatory platforms.The Sea-Wing underwater glider,developed by the Shenyang Institute of Automation,CAS,is designed for the application of deep-sea environment variables observation.The system components,the mechanical design,and the control system design of the Sea-Wing underwater glider are described in this paper.The pitch and roll adjusting models are derived based on the mechanical design,and the adjusting capabilities for the pitch and roll are analyzed according to the models.Field experiments have been carried out for validating the gliding motion and the ability of measuring ocean environment variables.Experimental results of the motion performances of the glider are presented.     

Stochastic Analysis of Short-Term Structural Responses and Fatigue Damages of A Submerged Tension Leg Platform Wind Turbine in Wind and Waves

In connection with the design of floating wind turbines, stochastic dynamic analysis is a critical task considering nonlinear wind and wave forces. To study the random structural responses of a newly designed submerged tension leg platform(STLP) wind turbine, a set of dynamic simulations and comparison analysis with the MIT/NREL TLP wind turbine are carried out. The signal filter method is used to evaluate the mean and standard deviations of the structural response. Furthermore, the extreme responses are estimated by using the mean upcrossing rate method. The fatigue damages for blade root, tower, and mooring line are also studied according to the simulated time-series. The results and comparison analysis show that the STLP gives small surge and pitch motions and mooring line tensions in operational sea states due to the small water-plane area. Additionally, in severe sea states, the STLP gives lower extreme values of platform pitch, slightly larger surge and heave motions and better towerbase and mooring line fatigue performances than those of the MIT/NREL TLP. It is found that the STLP wind turbine has good performances in structural responses and could be a potential type for exploiting the wind resources located in deep waters.     

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.     

The effects of blade twist and nacelle shape on the performance of horizontal axis tidal current turbines

The paper presents the effects of blade twist and nacelle shape on the performance of horizontal axis tidal current turbines using both analytical and numerical methods. Firstly, in the hydrodynamic design procedure, the optimal profiles of untwisted and twisted blades and their predicted theoretical turbine performance are obtained using the genetic algorithm method. Although both blade profiles produce desired rated rotational speed, the twisted blade achieves higher power and thrust performance. Secondly, numerical simulation is performed using sliding mesh technique to mimic rotating turbine in ANSYS FLUENT to validate the analytical results. The Reynolds-Averaged Navier-Stokes (RANS) approximation of the turbulence parameters is applied to obtain the flow field around the turbine. It is found that power and axial thrust force from BEMT (Blade Element Momentum Theory) method are under-predicted by 2% and 8% respectively, compared with numerical results. Afterwards, the downstream wake field of the turbine is investigated with two different nacelle shapes. It is found that the rotor performance is not significantly affected by the different nacelle shapes. However, the structural turbulence caused by the conventional nacelle is stronger than that by the NACA-profiled shape, and the former can cause detrimental effect on the performance of the downstream turbines in tidal farms.     

Numerical investigation on the gas entrainment of ventilated partial cavity based on a multiscale modelling approach

Ventilated cavitation which is acknowledged as an efficient drag reduction technology for underwater vehicle is characterised by the very disparate length and time scales, posing great difficulty in the application of this technology. A multiscale numerical approach which integrates a sub-grid air entrainment model into the two-fluid framework is proposed in this paper to resolve the complex flow field created by ventilated cavity. Simulations have been carried out for the partially ventilated cavity underneath flat plate, with special efforts putting on understanding the gas entrainment at the cavity tail and the bubble dispersion process downstream. The flow parameters including the void fraction, the bubble velocity and the bubble size distributions in and downstream of the ventilated cavity are fully investigated. Comparisons between the numerical results with the experimental data are in satisfactory agreement, demonstrating the potential of the proposed methodology. The ventilation rate effect on the cavity shape and bubbly flow parameters are further investigated, obtaining the law of bubble dispersion and the bubble size evolution. This research not only provide a useful method for the investigation on the multiscale multiphase flow, but also give insight on understanding the combined drag reduction mechanism resulted from large-scale cavity and microbubbles.     

海底管线检修潜水器垂直面控制及自航模试验

论述了纵倾控制律设计及自航模试验。首先选择一系列深度,对同一深度采用频域校正法单独设计控制律,使之对不同的速度和漂角具有足够的稳态精度和抗干扰性,这些控制律被集成统一为纵倾控制器,并根据潜深变化进行切换,对于其它深度采用同样的方法设计。控制器首先通过计算机仿真,然后进行自航模试验验证。设计的纵倾控制系统同时在其他试验项目中(水下管线跟踪和动力定位)发挥了重要的作用。     

地下气库(LPG)洞室围岩稳定性数值分析

地下气库的围岩稳定性关系到地下气库的安全与使用。影响地下洞室稳定的因素很多,解析法虽然可以得到精确解,但是只能针对圆形洞室。而数值计算方法可以计算各种情况下围岩的应力、变形、塑性分布等情况。利用有限元分析软件分析了洞室形状对围岩稳定性的影响,确定了合理的洞形;讨论了围岩应力应变随深度的变化规律,以及不同的支护类型对围岩变形的影响,确定了经济合理的支护类型。最终为该地质条件下的洞室围岩稳定设计提供了依据。     

Numerical and Experimental Analysis of A Vertical-Axis Eccentric-disc Variable-Pitch Turbine(VEVT)

A combined experimental and numerical investigation is carried out to study the performance of a vertical-axis eccentric-disc variable-pitch turbine(VEVT). A scheme of eccentric disc pitch control mechanism based on doubleblock mechanism is proposed. The eccentric control mechanism and the deflection angle control mechanism in the pitch control structure are designed and optimized according to the functional requirements of the turbine, and the three-dimensional model of the turbine is established. Kinematics analysis of the eccentric disc pitch control mechanism is carried out. Kinematics parameters and kinematics equations which can characterize its motion characteristics are derived. Kinematics analysis and simulation are carried out, and the motion law of the corresponding mechanical system is obtained. By analyzing the force and motion of blade of VEVT, the expressions of the important parameters such as deflection angle, attack angle and energy utilization coefficient are obtained. The lateral induced velocity coefficient is acquired by momentum theorem, the hydrodynamic parameters such as energy utilization coefficient are derived, and the hydrodynamic characteristics of VEVT are also obtained. The experimental results show that the turbine has good energy capture capability at different inflow velocities of different sizes and directions, which verifies that VEVT has good self-startup performance and high energy capture efficiency.     

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.     

全姿态水下移动平台模式切换姿态控制研究

为了适应复杂海洋环境中多样性的观探测任务需求,本文提出了一种融合Argo浮标、水下滑翔机（Glider）和自治式水下机器人（Autonomous Underwater Vehicle,AUV） 3种工作模式的全姿态水下移动平台（All-attitude Multimode Underwater Vehicle,AMUV）。首先,基于3种水下移动平台的工作原理,建立了AMUV的六自由度动力学模型;然后,针对动力学模型中的非线性耦合特性及模式切换过程中的驱动位形变化等问题,基于比例、积分、微分控制器（Proportional Integral Derivative,PID）与模糊控制概念,设计了不依赖于数学模型的自适应模糊PID姿态控制器,实现了AMUV多模式切换过程中的姿态控制;最后,开展多模式切换控制仿真实验,将自适应模糊PID控制器与传统PID控制器仿真结果进行对比,并设计了全模式任务工况,仿真结果表明,本文提出的控制器能够精确和稳定地控制AMUV进行多种工作模式的相互切换。     

The dynamic force on a two-dimensional planing plate of arbitrary camber

The thin wing methodology of Llewelyn (1964) is employed to calculate the pressure distribution of a two-dimensional cambered planing plate behind the stagnation line. A simplification of the jet flow in front of the stagnation line, solved exactly by Pierson and Leshnover (1949) and generalized to the case of arbitrary camber by Payne (1980) is then “patched on” to give the complete solution.Although the resulting pressure distributions can in principle be integrated analytically to find the normal force, pressure drag and pitching moment, the result is very cumbersome. It is simpler to integrate numerically on a computer. The paper presents examples obtained using this approach.     

Propulsive performance of three naturally occurring oscillating propeller planforms

A numerical method, the quasi-vortex-lattice method (QVLM), was applied to predict the propulsive performance of three naturally occurring oscillating propellers. These were cetacean flukes for a fin whale (Balaenoptera physalus); white-sided dolphin (Lagenorhynchus acutus); and white whale (Delphinapterus leucas). The fin whale's flukes had the highest aspect ratio (6.1) and moderate sweep angle (31°); the white-sided dolphin's flukes had the highest sweep angle (47°) and lowest aspect ratio (2.7); and the white whale's flukes had moderate aspect ratio (3.3) and the lowest sweep angle (28°). In the numerical simulations, the planforms were assumed to be rigid both in chordwise and spanwise directions, and to be oscillating harmonically in an irrotational, incompressible fluid. Calculation and comparisons of propulsive efficiency and thrust coefficient vs advance ratio for each of the planforms were made in three cases: varied heave amplitude; different pitching axis positions; and varied angular amplitude of pitch.