Theoretical Analysis and Experiment on Flow Allocation Characteristics of Dual Discharging Axial Piston Pump

通过改变缸体结构、柱塞数、端盖油路、配流盘形状等,设计了双排油内外环并联配流结构的轴向柱塞泵,实现了单柱塞泵两路高压供油。针对单环柱塞数减少,腔; 内压力冲击增大,脉动变大等问题,对配流结构进行重新设计。在排油腰形槽和吸油腰形槽过渡区取消卸荷槽,利用加大配错角,在排油完毕未接通吸油时,腔内封; 闭体积增大,未排尽的高压油液压力降低;在吸油腰形槽和排油腰形槽过渡区,排油卸荷槽利用阶梯变化通流面积代替原连续变化的通流面积,削弱了卸荷槽几何形; 状要求。重新设计后的双排油配流结构,以45mL轴向柱塞泵结构为参考,对配流结构进行了理论分析,建立了双排油轴向柱塞泵仿真模型。以单柱塞腔内压力; 冲击、输出流量进行分析研究,得外环压力冲击小,与传统配流结构相比较双排油输出口压力脉动变化率变小,并试制双排油轴向柱塞泵。对试制泵进行压力脉动测; 试、容积效率测试和噪声测试,结果表明,与45mL轴向柱塞泵进行对比,压力脉动降低了约30%,噪声也降低,容积效率不低于0.92。该双排油轴向柱; 塞泵可以代替双联泵,使系统结构简化,能耗降低。When providing two-way independent high-pressure high-flow oil sources,; hydraulic system generally adopts two separate piston pumps or coaxial; ones in series, causing complex structure and high cost. Therefore,; single piston pump was proposed to achieve two-way high-pressure oil; supply. Axial piston pump was designed with dual discharging inter-outer; ring parallel allocation structure by changing cylinder structure,; piston number, cap circuit and valve plate shape. Flow allocation; structure was redesigned due to decreased single ring piston number,; increased pressure shock and fluctuation in the chamber. Relief notch; was cancelled in transition region from oil-discharging to oil-absorbing; waist slots. After that, mismatch angle was increased to enlarge closed; volume in chamber and reduce the pressure of unexhausted high-pressure; oil in the interval between oil extraction and absorption. In transition; region from oil-absorbing to oil-discharging waist slots, stepped flow; area was used to replace original continuous flow area to weaken; geometry requirements of relief notch. The optimized dual discharging; flow allocation structure was conducted with theoretical analysis to; establish dual discharging axial piston pump simulation model based on; 45mL axial piston pump structure. There was small pressure shock in; outer race by analyzing pressure shock and output flow in single piston; chamber. Compared with traditional flow allocation structure, dual; discharging oil output had smaller pressure fluctuation rate. Based on; this, the designed dual discharging axial piston pump was piloted. The; pilot dual discharging oil pump was compared with the original 45mL; pump through pressure fluctuation, volumetric efficiency and noise; tests. Result showed that the former had lower pressure fluctuation; (decreased by 30%) and noise level, while its volumetric efficiency was; not smaller than 92%. In general, the dual discharging axial piston pump; can replace duplex pump to simplify system structure and reduce energy; consumption. This new pump can also be used in closed circuit and; differential cylinder hydraulic systems to make the system simpler and; cost-effective.山西省自然科学基金项目; 国家自然科学基金项

A study on the transfer mechanism of helical turbulence

螺旋湍流指的是局部或平均螺旋度不为零的湍流流动状态，广泛存在于航空发动机内流、高超声速飞行器、惯性约束核聚变以及星系密度云演化等工程及自然现象过程中。上世纪六十年代螺旋度守恒性定理的发现，给湍流的理论研究提供了新的方向，几十年来主要在不可压缩均匀各向同性湍流等领域取得系列研究进展。相对于常规湍流而言，螺旋湍流有许多独特之处，如经典的-5/3标度发生改变，方程的非线性特征被弱化，飞行器气动阻力减小，发动机燃料混合效率提高等。近些年来，面向航空航天及聚变领域对湍流基础研究的重大需求，结合螺旋度对湍流性质的影响规律，有必要发展适用于可压缩、各向异性条件下的螺旋湍流理论。 &nbsp;&nbsp; 本文主要开展螺旋湍流的机理研究，采用理论推导及数值模拟的方法，重点关注可压缩及各向异性条件下相关螺旋湍流理论的研究，一方面是探索螺旋湍流内部物理过程，揭示湍流发生及发展规律，另一方面是为湍流建模，尤其是大涡模拟建模提供理论指导。为有效支撑所建立的湍流理论，采用直接数值模拟方法，建立了不可压缩均匀各向同性湍流、可压缩均匀各向同性湍流及流向旋转槽道湍流数据库。论文的创新点主要包括以下三个方面： (一).提出了螺旋度级串的双通道及尺度局部性理论 &nbsp;&nbsp;&nbsp; 为了描述一般各向异性流动中螺旋度跨尺度传输的特征，在传统螺旋度级串单通道的基础上，本文提出了螺旋度级串双通道的概念，并通过理论分析方法给出了双通道的理论表达式。经过严密的数学推导，证实所提出的螺旋度级串双通道在三维湍流中三个方向同时满足均匀性条件下，只考虑系综平均的层次上是完全等效的。因此，第二通道的提出，可弥补均匀湍流中螺旋度级串的高阶统计矩问题，同时可适用于非均匀湍流中螺旋度级串的研究。第二通道的提出，将二阶反对称张量引入到湍流级串理论研究中来，以矩阵张量几何为视角，研究湍流级串涉及的矩阵数学性质问题。 &nbsp;&nbsp;&nbsp; 结合上述提出的螺旋度双通道理论，我们进一步在物理空间研究螺旋度级串的尺度局部性问题。尺度局部性是经典湍流级串理论的基本假设，可证实湍流的小尺度统计特征具有普适性。而在物理空间研究湍流级串问题，需要引入准正则假设，具有一定的任意性。引入带宽滤波的分析方法，提取流场中两个特定的尺度，来研究特定尺度间的能量及螺旋度传输规律。数据库分析结果表明，能量及螺旋度的正级串和反级串均满足尺度局部性的特征，并定量给出正反级串的尺度局部性范围。 (二).建立了可压缩螺旋湍流的联合级串理论 &nbsp;&nbsp;&nbsp; 经典的湍流级串理论建立在动能及螺旋度是二次无粘守恒量的基础上，然而，在可压缩湍流中，动能及螺旋度的这种守恒性特征被压力破坏，成为将经典湍流理论向可压缩状态下推广的主要障碍。通过压力涡量密度协同谱分析方法及湍流数据库的验证，可以证实压力在统计意义上只在大尺度范围内起作用。意味着超出一定的临界尺度，动能及螺旋度可恢复其守恒性的特征，这个大尺度范围一般指含能区范围。另外，我们发现，动能及螺旋度级串对应的临界尺度有明显的差异，螺旋度级串的临界尺度更大，意味着螺旋度级串惯性区的上临界范围更大，证实前期基于螺旋度所建立的大涡模拟模型的网格适用范围更广。 (三).给出了可压缩湍流中三波交互涉及的手性传输规律 &nbsp;&nbsp;&nbsp; 可压缩湍流的三波交互问题，不仅涉及速度场胀压模态的引入，还涉及流场矢量与热力学量的耦合。本文以手性分解为视角，探索可压缩湍流中三波交互涉及的手性传输问题，重点关注压缩膨胀过程对手性传输的影响规律。拓展了经典螺旋波分解特征值的内涵，将亥姆霍兹分解和螺旋波分解统一起来，提出了广义螺旋波分解方法，为流场速度矢量提供了三个两两相互作用正交的基向量。研究结果表明，速度场的胀压模态对手性传输过程扮演着媒介的作用，对手性间的三波交互过程有着重要的影响。发现压缩促进手性平衡，而膨胀则促进手性非平衡，使得流场的螺旋度效应更加明显。</p

Scale locality of helicity cascade in physical space

Scale locality is a key concept in turbulent cascade theory and is also associated with reflection symmetry. Vortex stretching is proven to participate in the helicity cascade process while destroying the conservative characteristic of enstrophy transfer in three-dimensional flows. Numerical evidence indicates that a turbulent structure with scale L will also largely transfer its helicity to structures with scales of around 0.3L. However, the scale locality of the helicity cascade is slightly weaker than that of the energy cascade in physical space. The weaker scale locality suggests that more scales should be involved for turbulent modeling of helical turbulence. Published under license by AIP Publishing

可压缩螺旋湍流中手性转换规律的数值研究

探索螺旋度对湍流动力学过程的影响机制,是螺旋湍流研究领域中经久不衰的话题。而螺旋度自身的时空演化规律的研究,则是进一步发展螺旋湍流理论的先决条件。作为在三维湍流中仅有的两个二次无粘不变量之一,螺旋度不仅可以衡量涡的扭转、盘绕和打结等拓扑结构,还可以用来表征流动的手性破缺程度。早期发展的湍流理论,大多是基于手性对称的基本假设。然而,自然和工程实际流动中,即使整体意义上流动是手性对称的,流场局部的手性破缺仍然存在,而且对流场局部流动结构的生成演化会带来重要的影响。另外,在药物研制,农药制备,超分子化学材料等领域,手性的控制可显著改变分子的性质,即"手性材料"。手性转换是研究单手性流动演化的重要组成部分,手性间传输的幅值在惯性区与单手性跨尺度通量相当,在近耗散区,与粘性耗散的幅值相当。在考虑压缩性的影响下,手性间的转换规律会变得更加复杂。涨压模态作为一种媒介,间接参与手性间的传输。数值模拟发现,流动的压缩膨胀效应,可明显影响手性传输的方向,即压缩使流场向手性平衡的方向发展,膨胀促进手性的反向传输,加剧流场的手性破缺效应

可压缩螺旋湍流的联合级串理论

基于1941年由Kolomgorov等人提出不可压均匀各向同性湍流的能量级串理论[1],1973年Brissaud等人提出的不可压螺旋湍流的动能与螺旋度联合级串理论[2],2011年Aluie等人提出的可压缩均匀各向同性湍流的能量级串理论[3],我们综合动能与螺旋度级串,考虑压力,激波等复杂因素影响,针对三维湍流最复杂的状态建立可压缩螺旋湍流的动能与螺旋度联合级串理论。我们开展了网格量达的可压缩螺旋湍流的直接数值模拟研究,采用多尺度滤波方法研究可压缩湍流尺度间的相互作用伴随着能量与螺旋度传输问题。结合亥姆霍兹分解和螺旋波分解两种方法,我们创造性的提出了为流场提供三个两两相互正交的基向量的广义螺旋波分解方法,可用于将流场矢量投影到左手、右手及自由于性(胀压)模态。基于直接数值模拟的后处理结果,我们扫清了压力对可压缩湍流级串理论的障碍,认识到统计意义压力只在大尺度上发挥作用,进而可将不可压螺旋湍流的联合级串理论推广到可压缩湍流中来

Cross-chirality transfer of kinetic energy and helicity in compressible helical turbulence

As an underlying mechanism, cross-chirality transfer of kinetic energy and helicity plays an essential role in the turbulent dynamics, which is as important as cross-scale transfer especially in broken mirror-symmetry turbulence. The effects of helicity on the properties of turbulent flows in previous studies highlight the role of cross chirality, which may be developed into an efficient method of turbulence control. We numerically study the cross-chirality transfer of kinetic energy and helicity in this paper, particularly under the influence of compressibility in stationary helical homogeneous and isotropic turbulence. Through combining the Helmholtz decomposition and helical wave decomposition, a general helical wave decomposition is proposed to provide three orthogonal bases for velocity. Within the scope of chiral helicity, there also exists chiral kinetic energy based on chiral velocity. They are defined as the left- and right-chirality kinetic energy, and the remaining compressible component of velocity corresponds to free-chirality kinetic energy. Although there exists no difference in the definition of helicity in incompressible and compressible turbulence, its space-time evolution equation in compressible turbulence involves the compressible component of velocity. The compressibility has a great influence on the homochiral kinetic energy and helicity cascade, and it also plays an essential role in the chirality transfer process like cross-chirality kinetic energy and helicity transfer. The amplitude of cross-chirality kinetic energy transfer is comparable with cross-scale kinetic energy transfer at relatively large scales, and also with viscous dissipation at relatively small scales. The triple nonlinear interactions dominate the cross-chirality transfer relative to pairwise interaction of chiral modes, and it is less sensitive to compressibility. Relative to the compression, the expansion of fluid elements can lead to inverse chirality transfer and strengthen the lack of mirror symmetry. The only discrepancy of cross-chirality helicity transfer between incompressible and compressible turbulence lies in the medium role of the compressible component of velocity. The helicity transfer via the compressible component is weak, and even the kinetic energy of the compressible component relative to that of the other two chiral modes is highest. In addition, the inverse helicity transfer is always statistically associated with the compressible component of velocity via triple interaction extracted from nonlinear interactions

螺旋湍流的相关研究进展

螺旋度定义为速度与涡量的标量积,它是三维湍流中一个无粘不变量,是研究湍流系统演化的一个重要物理量。在螺旋度级串过程中,一个双通道级串过程被发现,两个通道分别由涡扭转和涡拉伸动力学过程所主导,这两个通道的统计特性之间存在明显的物理差异,包括概率密度分布、传输效率以及形态分布等。第二通道的发现也为能量逆级串现象提供了一个新的解释。在考虑到湍流镜像破缺的问题时,提出了螺旋湍流尺度间传输的局部性理论。在利用螺旋度建立大涡模拟模型方面,基于螺旋度传输耗散平衡为基础得到了新的线性涡粘模型以及引入亚格子螺度输运方程的一方程模型,并在槽道流、可压缩平板等经典流动中验证了新模型对转捩、湍流等问题具有优秀的预测效果

Dual channels of helicity cascade in turbulent flows

Helicity, as one of only two inviscid invariants in three-dimensional turbulence, plays an important role in the generation and evolution of turbulent flows. Through theoretical analyses, we find that there are two channels in the helicity cascade process, which differs dramatically from the traditional viewpoint. In this paper, we have conducted important research on the newly proposed dual-channel helicity cascade theory, including vortex dynamic processes, intermittent discrepancies, tensor geometries, etc. The first channel mainly originates from the vortex twisting process, and the second channel mainly originates from the vortex stretching process. Antisymmetric tensors are introduced to the derivations of dual-channel helicity cascade theory, and a complex rotation frame leads to a higher helicity transfer efficiency. By analysing data from direct numerical simulations of typical turbulent flows, we find that these two channels behave differently. The ensemble averages of helicity flux in different channels are equal in homogeneous and isotropic turbulence, while they are different in other types of turbulent flows. The intermittency of the second channel is stronger than that of the first channel. In addition, we find a novel mechanism of hindered or even inverse energy cascades, which could be attributed to the second-channel helicity flux

Effect of pressure on joint cascade of kinetic energy and helicity in compressible helical turbulence

Direct numerical simulations of three-dimensional compressible helical turbulence are carried out at a grid resolution of 1024(3) to investigate the effect of pressure, which is important for the joint cascade of kinetic energy and helicity in compressible helical turbulence. The principal finding is that the pressure term of the helicity equation [defined as Phi(H) = p partial derivative(i)(omega(i)/rho)] has a smaller effect on the helicity cascade in the aspect of amplitude and a smaller effective range, which leads to a longer inertial subrange of the helicity cascade, in contrast to a kinetic energy cascade. In addition, we also find that the effective range of Phi(H) is concentrated only in large scales statistically, which is similar to the effect of the pressure term of the kinetic energy equation (defined as Phi(E) = p partial derivative(i)u(i)). From the overall sense of the effect of Phi(E) and Phi(H) on the kinetic energy and the helicity, respectively, both of them play a role of dissipation especially in the compression region. We propose that high enough helicity can affect the process of energy transformation between kinetic energy and internal energy, which means that the absolute local helicity hinders the process of kinetic energy transferring to internal energy, and promotes internal energy transferring to kinetic energy. In addition,Phi(H) plays a source role both for positive and negative helicity. We also study the mechanism of cancellations between compression and rarefaction regions, and we find that the impact of a shocklet on the helicity cascade can be ignored statistically.</p

The Effect of Helicity on Kinetic Energy Cascade in Compressible Helical Turbulence

Helicity plays an important role in the nonlinear dynamic process of compressible helical turbulence. We carry out direct numerical simulations (DNS) of compressible helical turbulence with different mean helicity at the grid resolution of 512(3) and try to explore the physical mechanism of kinetic energy cascade under the effect of helicity. The filtering method of coarse-graining is employed for scale decomposition to get kinetic energy flux. We get some novel conclusions in contrast to pre-existing knowledge in incompressible helical turbulence. Firstly, helicity also hinders the process of compressible kinetic energy cascade and reduces viscous dissipation. Secondly, helicity weakens some exclusive features, such as coupling between compressible and solenoidal mode of fluctuating velocity, energy transformation between kinetic energy and internal energy, and shocklets which appear only in compressible turbulence and serves as the role of constraint.</p