水动阻力随动补偿系统设计与仿真分析

针对中性浮力实验中水动阻力的补偿问题,提出了一种参数自整定模糊PI控制方法,即采用传统的PID控制与模糊控制相结合的新型控制方法。将方法应用于水动阻力补偿系统的控制闭环中,并利用MATLAB软件中的模糊控制工具箱进行了系统的辅助设计与仿真。仿真结果表明,参数自整定模糊Pl控制可改善水动阻力补偿系统的动态性能,该方法是水动阻力实时补偿的有效方法。

水上飞机滑行阶段静水阻力性能的一种估算方法

根据水上飞机起飞滑行阶段的受力特点,借鉴高速艇阻力性能预报和分析方法,提出考虑水上飞机机翼气动布局影响的半理论半经验公式方法,并采用该方法对水上飞机在静水中中高速滑行时的阻力性能进行估算。以某船身式水上飞机模型为算例,计算其航行姿态、气动阻力、水动阻力和总阻力。将计算结果和模型试验数据进行比对,结果表明,计算模型的气动阻力和水动阻力的计算值随着宽度弗劳德数变化的基本规律与试验结果基本一致。

船身式水上飞机中高速静水滑行阻力估算

根据水上飞机起飞滑行阶段特殊的多介质环境,参考滑行艇水面滑行阶段的阻力特性分析方法,提出一种考虑气动布局影响的估算船身式水上飞机中高速静水滑行阻力的半理论半经验公式;用此公式计算得到某船身式水上飞机中高速滑行阶段的水动阻力及俯仰角随体积弗劳德数变化曲线,并分析水动阻力及俯仰角变化的原因;根据得到的水动阻力及俯仰角,计算某船身式水上飞机起飞滑跑距离,计算结果与试验结果一致,间接验证了所提出阻力计算公式的有效性,同时也为计算船身式水上飞机起飞滑跑距离提供一种思路.

梭型车客渡船水动力特性数值预报研究

针对镇扬车客渡船不同载荷状态下阻力性能,基于粘性流体理论,开展梭型车客渡船水动力特性数值预报研究。首先,利用随体动网格技术开展了A航区和B航区6种状态下车客渡船阻力性能的数值预报;然后,利用三因次法换算至实船阻力,计算不同载况下车客渡船的有效功率;最后,分析了梭型船体表面压力及尾流场分布规律。结果表明:所采用的数值预报方法可准确模拟车客渡船阻力及流场分布规律;利用三因次法将船模数据换算为实船阻力准确性更高;梭型结构的下潜体相对于常规方形底,具有分流减阻效果。

基于ADAMS的水下人体模型仿真

将着服装人体简化为11刚体模型,参考Morison公式引入水动阻力,基于多体动力学软件ADAMS建立了航天员水下动力学模型。进行了试验并根据试验记录对模型进行了修正。通过模型仿真与试验录像处理得到的运动学数据进行比较来验证模型的有效性。在水下环境和空间环境的的仿真数据基础上,讨论了配重,水动阻力和浮漂反馈对肩关节和膝关节力矩的影响。

Simulation of Hydrodynamic Performance of Drag and Double Reverse Propeller Podded Propulsors

The unsteady performance of drag and double reverse propeller podded propulsors in open water was numerically simulated using a computational fluid dynamics （CFD） method. A moving mesh method was used to more realistically simulate propulsor working conditions, and the thrust, torque, and lateral force coefficients of both propulsors were compared and analyzed. Forces acting on different parts of the propulsors along with the flow field distribution of steady and unsteady results at different advance coefficients were compared. Moreover, the change of the lateral force and the difference between the abovementioned two methods were mainly analyzed. It was shown that the thrust and torque results of both methods were similar, with the lateral force results having the highest deviation

Resistance and Seakeeping Numerical Performance Analyses of a Semi-Small Waterplane Area Twin Hull at Medium to High Speeds

The hydrodynamic analysis of a new semi-small waterplane area twin hull （SWATH） suitable for various applications such as small and medium size passenger ferries is presented. This may be an attractive crossover configuration resulting from the merging of two classical shapes： a conventional SWATH and a fast catamaran. The final hull design exhibits a wedge-like waterline shape with the maximum beam at the stem; the hull ends with a very narrow entrance angle, has a prominent bulbous bow typical of SWATH vessels, and features full stern to arrange waterjet propellers. Our analysis aims to perform a preliminary assessment of the hydrodynamic performance of a hull with such a complex shape both in terms of resistance of the hull in calm water and seakeeping capability in regular head waves and compare the performance with that of a conventional SWATH. The analysis is performed using a boundary element method that was preliminarily validated on a conventional SWATH vessel.

The Effect of Netting Solidity Ratio and Inclined Angle on the Hydrodynamic Characteristics of Knotless Polyethylene Netting

Knotless polyethylene(PE) netting has been widely used in aquaculture cages and fishing gears, especially in Japan. In this study, the hydrodynamic coefficient of six knotless PE netting panels with different solidity ratios were assessed in a flume tank under various attack angles of netting from 0?(parallel to flow) to 90?(perpendicular to flow) and current speeds from 40 cm s^(-1) to 130 cm s^(-1). It was found that the drag coefficient was related to Reynolds number, solidity ratio and attack angle of netting. The solidity ratio was positively related with drag coefficient for netting panel perpendicular to flow, whereas when setting the netting panel parallel to the flow the opposite result was obtained. For netting panels placed at an angle to the flow, the lift coefficient reached the maximum at an attack angle of 50? and then decreased as the attack angle further increased. The solidity ratio had a dual influence on drag coefficient of inclined netting panels. Compared to result in the literature, the normal drag coefficient of knotless PE netting measured in this study is larger than that of nylon netting or Dyneema netting.

Flow Past an Accumulator Unit of an Underwater Energy Storage System： Three Touching Balloons in a Floral Configuration

An LES simulation of flow over an accumulator unit of an underwater compressed air energy storage facility was conducted. The accumulator unit consists of three touching underwater balloons arranged in a floral configuration. The structure of the flow was examined via three dimensional iso surfaces of the Q criterion. Vortical cores were observed on the leeward surface of the balloons. The swirling tube flows generated by these vortical cores were depicted through three dimensional path lines. The flow dynamics were visualized via time series snapshots of two dimensional vorticity contours perpendicular to the flow direction; revealing the turbulent swinging motions of the aforementioned shedding-swirling tube flows. The time history of the hydrodynamic loading was presented in terms of lift and drag coefficients. Drag coefficient of each individual balloon in the floral configuration was smaller than that of a single balloon. It was found that the total drag coefficient of the floral unit of three touching balloons, i.e. summation of the drag coefficients of the balloons, is not too much larger than that of a single balloon whereas it provides three times the storage capacity. In addition to its practical significance in designing appropriate foundation and supports, the instantaneous hydrodynamic loading was used to determine the frequency of the turbulent swirling-swinging motions of the shedding vortex tubes; the Strouhal number was found to be larger than that of a single sphere at the same Reynolds number.

Estimation of Added Resistance for Large Blunt Ship in Waves

Under the background of the energy saving and emission reduction, more and more attention has been placed on investigating the energy efficiency of ships. The added resistance has been noted for being crucial in predicting the decrease of speed on a ship operating at sea. Furthermore, it is also significant to investigate the added resistance for a ship functioning in short waves of large modern ships. The researcher presents an estimation formula for the calculation of an added resistance study in short waves derived from the reflection law. An improved method has been proposed to calculate the added resistance due to ship motions, which applies the radiated energy theory along with the strip method. This procedure is based on an extended integral equation （EIE） method, which was used for solving the hydrodynamic coefficients without effects of the irregular frequency. Next, a combined method was recommended for the estimation of added resistance for a ship in the whole wave length range. The comparison data with other experiments indicate the method presented in the paper provides satisfactory results for large blunt ship.

Effects of Roughness Elements Distribution on Overland Flow Resistance

Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resistance strongly impact the overland flow process and watershed floods. In view of the universal existence of natural vegetation, such as Chlorophytum malayense(CM) or Ophiopogon bodinieri(OB), and the sand-gravel bed of the river channel, it is important to understand the role of different types of roughness elements in flow resistance. This study was performed to investigate and compare through flume experiments the behaviors of overland flow resistance by the reaction of multi-scale configuration of different roughness elements. The result showed that the resistance coefficient gradually reduced versus the increase of flow rate in unit width and tended to be a constant when q = 3.0 l/s.m, Fr = 1.0, and Re = 4000 for slopes of 6 to 10 degrees. The gap of the vegetated rough bed and the gravel rough bed is limited to the same as the gap of the two types of vegetation, CM and OB. It was noted that the vegetation contributed to the increase in form resistance negatively and may lead to the mean resistance on decrease. To classify the flow pattern, the laminar flows were described by DarcyWeisbach's equation. In the study the f-Re equation of vegetated bed was developed with f ?5000 Re.The friction coefficient for laminar flows can be regarded as the critical value for identifying the transformation point of the flow pattern.

Effect of attack angle change on hydrodynamic character of supercavitating bodies

A series of experiments has been done in a moderate-velocity cavitation tunnel to investigate the effects of attack angle change on hydrodynamic characters of supercavitation. Hydrodynamic characters of the aft section at various attack angles were compared. The investigation shows that hydrodynamic forces of the aft section are dependent of supercavity shapes at different attack angles,and the magnitude of hydrodynamic forces of the aft section varies with the change of attack angle. When the aft section is in the fully wetted case,the drag coefficient changes little. Lift and moment coefficients both increase with the increased attack angle,and the increase magnitude is not large. When the aft body planing is on the cavity boundary,the drag coefficient of nonzero attack angle is larger than that of zero attack angle,and the maximal lift and moment coefficients both vary obviously with the increased attack angle. In the case that the body is fully enveloped by cavity,the drag coefficient,lift coefficient and moment coefficient are nearly constant with the change of attack angles.

新型仿鱼尾推进装置设计与水动力学仿真分析

由电机驱动的仿生机器鱼在水下推进时主要依靠尾鳍摆动,大多数样机的驱动电机会在鱼尾的摆动过程中出现反转的现象,以致不能输出高转速的稳定性能。针对这些问题,对仿生鱼尾进行了推进装置结构优化,用一种新型的双斜面偏转关节带动鱼尾驱动。该模型的双电机驱动避免了频繁正反转带来的效率低下问题,能长时间正转保持额定功率的输出,提高了关节的功率密度。在此模型上进行了水动阻力的计算和仿真实验,更有效地模拟了水下环境。

A prediction of drag reduction by entrapped gases in hydrophobic transverse grooves

The drag reduction effect of super-hydrophobic surface induced by the entrapped gas is unstable due to the gradual disappearance of the trapped gas.In this paper,a hydrophobic transverse grooved surface was designed to sustain gas in valleys.A detail numerical simulation was presented to investigate the flow field near the proposed surface.When water flowed over this surface,the entrapped gas was blocked by the ridges and the solid-liquid interface was replaced by the liquid-gas interface due to the entrapped gas,furthermore the micro-vortex formed in the groove.Because there was an effective slippage between water and solid induced by the entrapped gas,the velocity gradient of boundary layer decreased,which contributed to a remarkable drag reduction effect.Additionally,considering the extra undesired pressure drag reduction which negatively impacted the drag reduction effect of this method,the total drag coefficient including the viscous drag coefficient and the pressure coefficient was analyzed.An effective drag reduction rate of about 15%was achieved and the effect of this method was confirmed by experiments conducted in a high-speed water tunnel when grooves were optimized.

Experimental study of the flow and heat transfer of a gas–water mixture through a packed channel

Waste heat recovery from the flue gas of gasfired boilers was studied experimentally by measuring the flow and heat transfer of air and water through six kinds of packing with saturated humid air as the simulated flue gas.The experiments measured the effects of inlet air temperature, inlet air velocity and circulating water flow rate on the flow and heat transfer. The results show that higher inlet air temperatures and lower inlet air velocities lower the flow resistance and increase the heat transfer coefficient. The stainless steel packing had better surface wettability and larger thermal conductivity than the plastic packing, which enhanced the heat transfer between the water and the saturated moist air. When both the flow resistance reduction and the heat transfer enhancement were considered, the experimental results gave an optimal packing-specific surface area. A packed heat exchanger tower was designed for waste heat recovery from the flue gas of gas-fired boilers based on the experimental results which had better flow and heat transfer characteristics with lower pump and fan power consumption, more stable system operation and less thermal fluctuations compared with a non-packed heat transfer system with atomized water.