定常结构动力方程增维精细积分法求解的注记

精细积分法应用于求解线性定常结构的动力方程,能得到逼近于精确解的数值解,而增维精细积分法在求解非齐次的动力方程时,通过化非齐次方程为齐次方程来求解,避免了矩阵求逆的困难。该方法的提出,使得在用增维精细积分法求解定常结构动力方程时,避免每一个时间步都做20次矩阵加、乘迭代。该方法对程序的实现十分有利,而且对大型问题的长时间仿真可明显提高计算效率,算例显示了该方法的有效性。

汽车模型风洞射流剪切层的非定常结构特性

针对1:15的3/4开口式整车模型风洞不同收集口角度和喷口速度工况,采用热线风速仪对射流剪切层内的流场进行了多个截面瞬时速度的测量,研究此类风洞的喷口射流剪切层定常与非定常结构.对比分析不同工况下平均速度,速度脉动以及剪切层厚度的变化,得到了各种工况射流剪切层的结构特征、厚度发展特征及其中的脉动分布情况,明确了该风洞射流剪切层所特有的流动结构.

定常结构FFT算法

本文系统地讨论了定常结构FFT算法和结构,给出了构造定常结构的方法及扩展方法.定常结构FFT的每级流图均具有相同的流图结构,因而易于编程,在FFT硬件处理器中使控制器得到简化,同时使阵列处理器和流水线处理器易于实现模块化.

非线性动力方程的增维精细积分法

对线性定常结构的动力系统提出的精细积分法 ,能得到在数值上逼近于精确解的结果。但是对于非齐次动力方程却涉及到矩阵求逆的困难 ,而且通常与时间有关的非齐次项不能进入精细积分的细化过程。采用增维的方法 ,将非齐次动力方程化为齐次方程 ,在实施精细积分的过程中不必进行矩阵求逆。这种处理方法对于程序实现和提高数值计算的稳定性十分有利 ,而且在大型问题中可明显提高计算效率 ,数值算例显示本文方法是有效的。

亚声速段横向射流对Coil性能影响的数值研究

在化学氧碘激光的混合喷管内发生的是一个气体动力学、化学反应动力学以及光学等相互耦合的复杂过程,每个过程都对COIL性能有着至关重要的影响.利用三维CFD技术,通过求解层流Navier-Stokes方程与组分输运方程,对简化后的化学氧碘激光RADICL模型进行数值模拟与分析,对COIL的气动和增益特性进行探讨.在不同的射流穿透条件下,计算COIL混合喷管中的混合与化学反应过程,发现穿透深度决定了增益的分布特性以及过度穿透条件下的非定常结构.

实现非定常物性围护结构与基于状态空间法建筑热过程耦合计算方法研究

双层皮玻璃幕墙、种植屋面、蓄热相变墙体等新型围护结构具有良好的保温隔热、维持舒适的室内热环境的特征,其节能潜力得到了广泛的认可。随着新型围护结构方面的研究日益增多,此类技术也日益完善,在建筑中得到一些应用,因此,在建筑能耗模拟过程中需要考虑到新型围护结构。新型围护结构热(质)传递等物理过程均为非定常物性的动态热(质)传递过程,常采用有限差分法进行求解。应用有限差分法实现采用新型围护结构建筑的热过程模拟计算,存在计算量大、计算时间长等问题。为提高计算速度、保证计算稳定性,本文提出了实现建筑本体中非定常物性围护结构模拟与基于状态空间法的建筑热过程耦合计算的方法,并将此算法在建筑能耗模拟DeST软件中进行实现和案例计算及校核。

跨音速三维操纵面嗡鸣数值方法研究

在非结构化动态网格耦合欧拉方程求解跨音速非定常流场的基础上,考虑了翼面-操纵面系统的缝隙间网格运动问题。采用中心有限体积法进行空间离散和双时间方法进行时间推进求解非定常欧拉方程。通过与气动力方程的联立求解,在时域内用四步龙格-库塔方法求解结构运动方程。数值分析研究了NA SA三维操纵面嗡鸣标模算例——全翼展操纵面NA SP(N ationalA eroSpace P lane)基本型机翼-操纵面嗡鸣现象,并和风洞实验结果进行了对比,取得了一致的结果,说明了该方法的正确性。

A CFD Based Investigation of the Unsteady Hydrodynamic Coefficients of 3-D Fins in Viscous Flow

The motion of the fins and control surfaces of underwater vehicles in a fluid is an interesting and challenging research subject.Typically the effect of fin oscillations on the fluid flow around such a body is highly unsteady, generating vortices and requiring detailed analysis of fluid-structure interactions.An understanding of the complexities of such flows is of interest to engineers developing vehicles capable of high dynamic performance in their propulsion and maneuvering.In the present study, a CFD based RANS simulation of a 3-D fin body moving in a viscous fluid was developed.It investigated hydrodynamic performance by evaluating the hydrodynamic coefficients (lift, drag and moment) at two different oscillating frequencies.A parametric analysis of the factors that affect the hydrodynamic performance of the fin body was done, along with a comparison of results from experiments.The results of the simulation were found in close agreement with experimental results and this validated the simulation as an effective tool for evaluation of the unsteady hydrodynamic coefficients of 3-D fins.This work can be further be used for analysis of the stability and maneuverability of fin actuated underwater vehicles.

Cold Welding of Au Nanostructures at Room Temperature

The common Au nanostructures(nanospheres,nanorods and nanosheets)were prepared by the seed growth method to explore the cold welding phenomenon of these non-single crystal nanostructures at room temperature.Systematic studies show that the concentration of surfactant cetyltrimethylammonium bromide(CTAB)and drying conditions are important factors to determine the evolution and final configuration of nanostructures during welding.The key factor of cold welding is the concentration of surfactant as low as 0.3 mm/L,and the welding should be carried out under the condition of slow evaporation and sufficient relaxation time,rather than rapid drying process.At the same time,the structural evolution during the welding process of gold rod head and tail is simulated by combining the electronic microscope characterization and density functional theory,which reveals that the stability of the welding nanostructure is better than that of the dispersed nanostructure.In the slow evaporation process of Au nanostructures with the same crystal structure,the low surfactant attached to the surface of the nanoparticles increases the attraction between the nanoparticles,which makes the nanoparticles close to each other adhere due to the interaction,and improves the physical properties of the intersection due to the diffusion,epitaxy and surface relaxation of the metal surface atoms.The results provide a research basis for the physical property analysis of nanostructures and the construction of defect devices.

Experimental Analysis of Flow Structure in Contra-Rotating Axial Flow Pump Designed with Different Rotational Speed Concept

As a high specific speed pump, the contra-rotating axial flow pump distinguishes itself in a rear rotor rotating in the opposite direction of the front rotor, which remarkably contributes to the energy conversion, the reduction of the pump size, better hydraulic and cavitation performances. However, with two rotors rotating reversely, the significant interaction between blade rows was observed in our prototype contra-rotating rotors, which highly affected the pump performance compared with the conventional axial flow pumps. Consequently, a new type of rear rotor was designed by the rotational speed optimization methodology with some additional considerations, aiming at better cavitation performance, the reduction of blade rows interaction and the secondary flow suppression. The new rear rotor showed a satisfactory performance at the design flow rate but an unfavorable positive slope of the head - flow rate curve in the partial flow rate range less than 40% of the design flow rate, which should be avoided for the reliability of pump-pipe systems. In the present research, to understand the internal flow field of new rear rotor and its relation to the performances at the partial flow rates, the velocity distributions at the inlets and outlets of the rotors are firstly investigated. Then, the boundary layer flows on rotor surfaces, which clearly reflect the secondary flow inside the rotors, are analyzed through the limiting streamline observations using the multi-color oil-film method. Finally, the unsteady numerical simulations are carded out to understand the complicated internal flow structures in the rotors.

Numerical simulation of unsteady cavitating flows around a transient pitching hydrofoil

The objective of this paper is to improve the understanding of the influence of multiphase flow on the turbulent closure model, the interplay between vorticity fields and cavity dynamics around a pitching hydrofoil. The effects of pitching rate on the sub- cavitating and cavitating response of the pitching hydrofoil are also investigated. In particular, we focus on the interactions between cavity inception, growth, and shedding and the vortex flow structures, and their impacts on the hydrofoil performance. The calculations are 2-D and performed by solving the incompressible, multiphase Unsteady Reynolds Averaged Navier Stokes （URANS） equations via the commercial CFD code CFX. The k-co SST （Shear Stress Transport） turbulence model is used along with the transport equation-based cavitation models. The density correction function is considered to reduce the eddy viscosity according to the computed local fluid mixture density. The calculation results are validated with experiments conducted by Ducoin et al. （see Computational and experimental investigation of flow over a transient pitching hydrofoil, Eur J Mech/B Fluids, 2009, 28：728-743 and An experimental analysis of fluid structure interaction of a flexible hydrofoil in vari- ous flow regimes including cavitating flow, Eur J Mech B/fluids, 2012, 36： 63-74）. Results are shown for a NACA66 hydro- foil subject to slow （quasi static, t2=6~/s, ＆＊ =0.18） and fast （dynamic, ＆=63~/s, dr＂ =1.89） pitching motions from a =0~ to a =15~. Both subcavitaing （or =8.0） and cavitating （cr=3.0） flows are considered. For subcavitating flow （or=8.0）, low frequency fluctuations have been observed when the leading edge vortex shedding occurs during stall, and delay of stall is ob- served with increasing pitching velocity. For cavitating flow （tr=3.0）, small leading edge cavities are observed with the slow pitching case, which significantly modified the vortex dynamics at high angles of attack, leading to high frequency fluctuations of the hydrodynamic coefficients and different stall behaviors compared to the subcavitating flow at the same pitching rate. On the other hand, for the fast pitching case at or=3.0, large-scale sheet/cloud cavitation is observed, the cavity behavior is un- steady and has a strong impact on the hydrodynamic response, which leads to high amplitude fluctuations of the hydrodynamic coefficients, as well as significant changes in the stall and post-stall behavior. The numerical results also show that the local density modification helps to reduce turbulent eddy viscosity in the cavitating region, which significantly modifies the cavity lengths and shedding frequencies, particularly for the fast pitching case. In general, compared with the experimental visualiza- tions, the numerical results with local density correction have been found to agree well with experimental measurements and observations for both slow and fast transient pitching cases.

Unsteady flow structures in the tip region for a centrifugal compressor impeller before rotating stall

To get an insight into the occurrence and the mechanism of flow unsteadiness in the tip region of centrifugal compressor impellers, the flow in Krain’s impeller is investigated by using both steady and unsteady RANS solver techniques. It is found that the flow unsteadiness on the pressure side is much stronger than that on the suction side. The periodical frequency of the unsteady flow is around half of the blade passing frequency. The originating mechanism of the flow unsteadiness is illustrated with the time-dependent tip leakage flow and blade loading at the tip region. Due to the blockage caused by the joint effects of broken-downed tip leakage vortex, separated fluids and tip leakage flow at downstream, a low pressure region is formed on the pressure side, consequently the blade loadings is altered. In turn, the changed blade loadings will alter the intensity of tip leakage flow. Such alternative behavior finally results in the periodic process. By comparing the calculated flow field in the cases of single-passage and four-passage models, it is confirmed that the investigated flow unsteadiness is confined in each single passage, as no phase differences are found in the model of four passages. This is different from the situation in axial compressor when the rotating instability is encountered. The flow unsteadiness only occurs at the working conditions with small mass flow rates, and the oscillation intensity will be enhanced with the decrease of mass flow rate. When the mass flow rate is too small, the flow unsteadiness in a single passage may trigger rotating stall, as the disturbance propagates in the circumferential direction.

Unsteady analysis of six-DOF motion of a 6:1 prolate spheroid in viscous fluid

Free-moving simulations of airplanes, submarines and other automobiles under extreme and emergency conditions are becoming increasingly important from operational and tactical perspectives. Such simulations are fairly challenging due to the extreme unsteady motions and high Re(Reynolds) numbers. The aim of this study is to perform a six-DOF motion simulation of a 6:1prolate spheroid that is falling in a fluid field. Prior to conducting the six-DOF simulation, some verification simulations were performed. First, a laminar flow past an inclined prolate spheroid at a Re number of 1000 and incidence angle of 45. with a tetrahedral mesh was simulated to verify the relevant targeted discrete method for an unstructured mesh. Second, to verify the LES(large eddy simulation) models and dependent parameters for the DDES(delayed detached eddy simulation), a turbulent flow past a sphere was performed at a subcritical Re number of 10000. Third, a steady maneuvering problem about a prolate spheroid pitching up from 0. to 30. incidence at a uniform angular velocity was established based on a dynamic tetrahedral mesh with changing topology and the ALE(arbitrary Lagrangian-Eulerian) method of fluid-structure coupling at a Re number of 4.2 × 10~6.Finally, two six-DOF motions of an inclined 6:1 prolate spheroid at an initial incidence of 45. were simulated at different Re numbers of 10000 and 4.2 × 10~6.