近自由面非定常空化流动特征与稳定性机理研究

近自由面非定常空化流动是水面航行器提高航速的重要障碍,主要表现为自由面/空化区相互作用,影响空泡的非稳态特征,空化流动的兴波效应也值得关注。以往研究大多针对定常问题,非定常的算法及实验验证手段的报道很少。本研究建立了水/气/汽三相并包含空化相变效应的大涡模拟数值方法,基于小尺寸水下发射水箱完成了典型的验证实验。计算得到的非定常空泡形态演化与实验结果吻合良好,验证了计算方法的精度和可靠性。在此基础上,分析了空泡形态非稳态演化的特征与自由面/空化相互作用规律,讨论了自由面对空泡流动的约束效果和非稳态空泡的兴波效应。进一步改变不同的浸没深度,发现在航行体距离自由表面非常接近的情况下,会发生通气云状空泡和通气超空泡现象。在通气效应和回射流变薄两种因素的影响下,空泡在距离自由表面更近的条件下能够变得更稳定,自由面可能对空泡发展和稳定性产生整体的影响,值得未来进一步深入研究并探索工程应用的前景

横向来流内单孔气泡形成的理论与实验研究

水下气泡的运动是水下航行器减阻和化工领域中的基础问题。本研究针对横向来流条件下气泡形成的现象进行了研究,主要包括循环水槽实验和理论模型的建立与求解。首先用开有通气孔的平板模型在循环水槽中进行实验,并配合高速摄像设备记录气泡各时刻的形态。然后依据力平衡、运动学方程和量纲分析建立了气泡膨胀阶段和分离阶段的理论模型,利用四阶Runge-Kutta算法并结合初始条件进行数值求解,得到气泡在各个时刻的质心位置、倾斜角度和气泡尺寸大小。最后将数值计算结果与实验结果进行对比,结果表明两者具有良好的一致性,均方根误差为5.38%,进而证实了理论模型的可靠性

挡流条对轴对称航行体空化演化过程的影响研究

空泡的准周期性非稳态演化过程中,空泡的不断脱落和溃灭等会对航行体产生不利影响。本文讨论了一种在航行体前端附近设置挡流条来增加非稳态演化过程中空泡的稳定性的方式,并基于分离式霍普金森压杆发射实验研究了挡流条对空泡初生、发展、脱落及溃灭的全过程的影响。试验表明挡流条会改变空泡的演化过程,如使肩部空泡缩短,提前形成回射流,并在一定程度上增加非稳态演化过程中空泡的稳定性

绕回转体浅水空泡流的实验与数值模拟研究

在浅水条件下,航行体附近壁面和自由面同时影响空泡的非定常演化,其耦合效应非常复杂。本研究针对浅水发射回转体的非定常空化流动现象进行了研究,主要研究方法包括发射实验及CFD数值模拟。实验中通过高速摄像记录了空泡的非定常,包括不同位置空泡长度和形状的发展规律。数值计算中基于商业软件FLUENT建立了水/气/汽三相并包含空化相变效应的大涡模拟方法,以获得自由面-空泡-壁面相互作用结果。典型工况计算结果与实验吻合良好,验证了数值方法的可靠性与精度。结果表明自由液面和壁面在一定距离内均能够影响回转体表面三维空泡内部回射流的位置,改变不同垂向截面位置空泡脱落的时序,引起空泡演化过程中流形流态的变化。其中自由液面会提高空泡的稳定性,靠近自由液面的空泡整体形状在周期内趋于稳定。而壁面会导致空泡厚度和长度增加,同时在距离壁面一定范围时,航行体表面空泡会发生局部脱落。在此基础上,模型上下表面空泡也会存在相互影响,体现了自由面和壁面两种因素的耦合效应

Passive control of cavitating flow around an axisymmetric projectile by using a trip bar

Quasi-periodical evolutions such as shedding and collapsing of unsteady cloud cavitating flow, induce strong pressure fluctuations, what may deteriorate maneuvering stability and corrode surfaces of underwater vehicles. This paper analyzed effects on cavitation stability of a trip bar arranged on high-speed underwater projectile. Small scale water tank experiment and large eddy simulation using the open source software OpenFOAM were used, and the results agree well with each other. Results also indicate that trip bar can obstruct downstream re-entrant jet and pressure wave propagation caused by collapse, resulting in a relatively stable sheet cavity between trip bar and shoulder of projectiles

Free surface and near-wall effects on the cloud cavitating flow over an axisymmetric projectile

For high-speed underwater vehicles, cavitation can be one of the most important speed barriers. The cavitating flow can be even complex when an underwater vehicle is sailing close to the free surface/wall. In this study, the mutual effects of the free surface and near-wall on the cloud cavitating flow that surrounds an axisymmetric projectile are investigated by water tank experiment and the computational fluids dynamics (CFD). The Split-Hopkinson pressure bar (SHPB) technology is applied in the experiment to accelerate the projectile in a short time. In our numerical approach, the volume of fluid (VOF) method, the large eddy simulation (LES) turbulence model, and the Zwart-Gerber-Belamri (ZGB) cavitation model are used. The cloud cavitating flow under the free surface/wall effect is investigated by changing the distance between the projectile and free surface (d(f)) or wall (d(w)). Both the experiment and the simulation show that the unstable cavity evolution includes four stages induced by the re-entrant jet: cavity growth, re-entrant jet, cavity shedding and collapsing. The results further show that as d(f) decreases, the cavity length is shorter and the cavity becomes more stable; as d(w) decreases, the cavity length is longer and the cavity becomes less stable. By understanding the free surface/wall effect on the stability of the cloud cavitating flow, the parameters space spanned by d(f) and d(w) can be divided into several distinct flow regimes, in which the entire shedding cavity (symmetry/asymmetry), steady and non-shedding cavity, partial cavity shedding, and ventilated cavity are identified

The utility model claims a mechanical arm error analyzing method

本发明涉及一种机械臂误差分析方法，在机械臂运动模型的基础上，分析各运动学参数误差对机械臂末端位置精度的敏感性和影响规律，为机械臂的设计、加工、装配时的精度分配提供依据。其分析步骤为：(1)利用指数积方法建立机械臂的运动学模型；(2)根据运动学模型建立机械臂末端位置误差模型；(3)分析各关节误差和初始位姿误差对机械臂末端位置精度的敏感性；(4)分析关节各项误差随各关节角变化时对机械臂末端位置精度的影响规律；(5)根据各误差的敏感性和影响规律，指导机械臂的精度分配。本发明具有计算精确，操作方便，分析全面等优点

近自由面垂直水翼非定常空泡流动实验及数值研究

本文针对近自由面垂直水翼表面产生的空化现象进行了研究,方法包括水箱发射实验及CFD数值模拟。通过实验可以观察到空泡变化过程,空泡长度及自由液面的影响。计算中利用了商业软件FLUENT对水翼模型进行了数值仿真,从而对于该水翼模型在运动中产生的空化现象进行分析研究,并获得自由面-空泡相互作用结果。结果表明自由面能够影响水翼三维空泡主回射流的位置,改变不同垂向截面位置空泡脱落的时序。进一步对比了实验观察图片与数值模拟中获得的典型空泡运动特征,以对模拟仿真的精度进行验证和评估,并为下一步的分析研究奠定了基础

一种松软地面爬行机器人

The present invention belongs to the field of robots, and relates to a soft ground crawling robot. Front wheels are connected on both sides of the front end of a body shell; a universal wheel is arranged on the rear end; rotary stepping motors are installed on both sides of a supporting assembly; an output shaft of the rotary stepping motor on each side passes through the body shell and then is connected with the front wheel on the same side; a swinging stepping motor is installed on the supporting assembly; the output shaft is connected with a connecting plate; the connecting plate is connected with the body shell through a connecting shaft; the body shell is driven to swing by the swinging stepping motor; lower end covers are rotatably connected on both sides of the body shell; a wheel bracket is connected to the rotary stepping motor on each side; and the wheel bracket on each side is connected with the lower end cover on the same side. The crawling robot of the present invention can crawl on all soft ground while moving in a plane, and overcome the phenomenon that some existing crawling robots cannot walk and work on the soft ground such as sand due to sinking

近自由面垂直水翼非定常空泡流动实验及数值研究

本文针对近自由面垂直水翼表面产生的空化现象进行了研究,方法包括水箱发射实验及CFD数值模拟。通过实验可以观察到空泡变化过程,空泡长度及自由液面的影响。计算中利用了商业软件FLUENT对水翼模型进行了数值仿真,从而对于该水翼模型在运动中产生的空化现象进行分析研究,并获得自由面-空泡相互作用结果。结果表明自由面能够影响水翼三维空泡主回射流的位置,改变不同垂向截面位置空泡脱落的时序。进一步对比了实验观察图片与数值模拟中获得的典型空泡运动特征,以对模拟仿真的精度进行验证和评估,并为下一步的分析研究奠定了基础