Effect from Mechanical Stirring Time and Speed on Adsorption Performance of ZIF‐90 for n‐Hexane

Zeolite imidazolate frameworks (ZIFs) represent a class of metal‐organic frameworks (MOFs) for various potential applications due to their outstanding properties. However, to date, the creation of nanoframes with tunable structure faces a challenge. Herein, we develop a facile and efficient physical method that allows the preparation of ZIF‐90 with controllable surface area. In this study, the effect of various stirring time and speed in the acceleration of the precursor dissolution are revealed. The study shows that a moderate stirring speed (640 r · min^–1) and reaction time (6 h) are the optimal conditions to synthesize ZIF‐90 with a high adsorption capacity. More importantly, the maximum adsorption amount of n‐hexane is up to 211 mg · g^–1 by using this as‐prepared sample, which increases by 60 % in comparison with that of the minimum from other sample (133 mg · g^–1).     

Sound absorption behavior of flexible polyurethane foams including high molecular‐weight copolymer polyol

Noise pollution is an important issue for automotive industries. In this article, the high molecular‐weight copolymer polyol is blended in the polyol mixtures for fabricating flexible polyurethane foams to improve sound absorption efficiency. Changes of cavity size and material density of the foams are negligible by inclusion of copolymer polyol in the polyol mixture, but the closed pore ratio and specific airflow resistance increase for the copolymer polyol content higher than 20 wt% because of changes of phase separation behavior from drainage flow rate reduction that occurs with increased viscosity. Sound absorption efficiency increases with increasing copolymer polyol content up to 20 wt%, but it decreases beyond this point. The sound absorption property mainly results from the closed pore ratio, not from the cavity size. The compression strength increases with increasing copolymer polyol contents by increased amount of hard segments. Therefore, an optimum amount of high molecular‐weight polyol is recommended for enhanced sound absorption property.     

Three‐dimensional remeshed smoothed particle hydrodynamics for the simulation of isotropic turbulence

We present a remeshed particle‐mesh method for the simulation of three‐dimensional compressible turbulent flow. The method is related to the meshfree smoothed particle hydrodynamics method, but the present method introduces a mesh for efficient calculation of the pressure gradient, and laminar and turbulent diffusion. In addition, the mesh is used to remesh (reorganise uniformly) the particles to ensure a regular particle distribution and convergence of the method. The accuracy of the presented methodology is tested for a number of benchmark problems involving two‐ and three‐dimensional Taylor‐Green flow, thin double shear layer, and three‐dimensional isotropic turbulence. Two models were implemented, direct numerical simulations, and Smagorinsky model. Taking advantage of the Lagrangian advection, and the finite difference efficiency, the method is capable of providing quality simulations while maintaining its robustness and versatility.     

VELOCITY-ACCELERATION STRUCTURE FUNCTION IN TWO-DIMENSIONAL DECAYING TURBULENCE$^{\bf 1)}$

湍流场中二阶速度加速度结构函数 (velocity-acceleration structure function, VASF) 被认为与尺度间能量或者拟涡能的传递相关,其正负表明传递的方向. 三维湍流中,能量从大尺度向 小尺度传递,VASF 为负. 在二维湍流中,能量反向传递到大尺度,拟涡能正向传递到小尺度,因此理论上 VASF 无论在反向能量级串区还是在正向拟 涡能级串区均为正. 然而,相对于三维湍流中 VASF 的充分研究,二维湍流中 VASF 的正负性迄今尚无实验或数值模拟数据验证. 本文通过三维二维湍流中普适的公式推导,指出在空间非均匀湍流场中,VASF 除了尺度间传递,还受到非均匀项的影响. 一种常见的空间非均匀湍流场是在实验研究中常用的风洞或水洞中,湍流发生装置 (如栅格) 后的湍流. 该流场中,湍流强度随下游位置增大而逐渐衰减,这种衰减则带来空间上的非均匀性. 本文在基于竖直流动皂膜的二维衰减湍流场中,利用拉格朗日粒子追踪法测得在拟涡能级串区的 VASF,并分析各部分的影响. 结果表明,虽然尺度间传递项为正值,但由于衰减带来的非均匀项为负值,使得 VASF 的值为负,使之失去了表征拟涡能传递方向的意义. 因此,在类似风洞、水洞、水槽等衰减流场中对 VASF 的讨论不应忽略非均匀项. 最后对与 VASF 密切相关的弥散过程进行分析,发现后期弥散过程变慢是由于负的 VASF 导致.     

爆破动载作用下新喷射混凝土累积损伤效应的模型实验

利用模型实验的手段模拟了巷道的爆破掘进,并在实验中辅以声波测试,通过分析声波波速和声波波形的变化规律,研究了新喷射混凝土在多次爆破动载作用下的累积损伤效应。实验结果表明,爆破次数、与掌子面之间的距离和爆破药量均会影响喷射混凝土的累积损伤,这种影响体现在声波波速和波形2个方面:爆破次数增加、距离减小和药量增加,则声速降低值越大,波形变化越明显,即累积损伤值越大;爆破次数和距掌子面的距离均与累积损伤呈非线性关系,其中距离与爆破损伤之间的非线性关系可以用二次多项式进行较好的拟合;第一次爆破对声速和波形影响最大,造成的单次损伤值也最大;在小药量爆破的情况下,测试面的最大累积损伤值达到了0.1268,表明爆破近区喷射混凝土的损伤是需要关注的重点。     

Nonlinear Dynamics of a Flexible Spinning Disc Coupled to a Precompressed Spring

This paper analyses the nonlinear transverse vibrations of a rotating, clamped-free, flexible disc coupled to a precompressed spring. This is representative of a large class of loadings in rotating disc systems such as air jet and electromagnetic excitation commonly used in experiments. Such a loading induces a simultaneous critical speed resonance and parametric instability. The disc is modelled as a Von Kármán plate, and the equations of motion are discretised by a Galerkin projection onto a pair of 1:1 internally resonant modes. The large amplitude wave motions and their stabilities are studied using the averaging method and via numerical continuation techniques. The analysis is carried out in a co-rotating as well as a ground-fixed frame. Numerical simulations are used to verify the above analyses. The response predicted by these analyses is substantially different from that arising from a critical speed resonance or of a parametric instability alone. As many as five equilibrium solutions can coexist at supercritical speed. Two distinct regimes of large amplitude response appear to exist depending on the relationship between the strength of the parametric excitation and the damping. The existence of these regimes underscores the subtle competition between critical speed resonance and parametric instability that is likely to be observed in experiments near critical speed in such systems.Contributed by Prof. A.K. Bajaj.     

高速干铣削钛合金时涂层硬质合金刀具磨损机理研究

采用CVD涂层硬质合金可转位立铣刀对钛合金(Ti-6Al-4V)进行了高速干铣削试验,采用扫描电子显微镜(SEM)观察刀具的磨损形貌,通过能谱分析(EDS)分析失效刀具表面的元素分布,并对刀具的主要磨损机理进行了分析.研究结果表明:使用涂层硬质合金刀具高速干铣削Ti-6Al-4V时,刀具的失效机理主要为磨粒磨损、粘结磨损、氧化磨损、扩散磨损和热-机械疲劳磨损的综合作用.刀具刚参与切削时,刀具后刀面会产生粘结和由于摩擦引起的擦伤,粘结层在断续冲击作用下的脱落过程还会造成后刀面涂层的剥落;随着刀具进一步的磨损,涂层剥落、粘结磨损及磨粒磨损伴随整个刀具失效过程,且还会出现氧化磨损、扩散磨损和疲劳裂纹等.切削速度越高,新产生的钛合金切屑就越容易燃烧,使刀具粘结、氧化和扩散以及热-机械疲劳等磨损加剧.     

二维潮波模式底摩擦效应参数化方案同化研究

基于伴随同化方法对二维潮波模式底摩擦效应的线性与非线性参数化方案进行了比较研究。采用拉格朗日乘子法推导了底摩擦效应两种参数化方案的伴随表达式,并借助孪生实验对所述方法的参数反演能力进行了验证.通过同化高度计资料和验潮站资料,分别使用线性和非线性参数化方案,以及常数底摩擦系数与空间分布底摩擦系数设置的两两组合,对模拟结果进行了比较和分析。经过同化,模拟结果与观测数据的差异均有了明显下降。结果表明,如果将底摩擦系数设置为全区常数,则非线性参数化方案的模拟结果远优于线性方案;然而,在空间分布设置下,两者模拟得到的水位场与潮流场仅有细微差别,并尝试从底摩擦引起的能量耗散角度对其原因进行了分析。     

基于湍流类比的金属橡胶吸声特性定量分析

金属橡胶材料从表到里都具有大量互相贯通且混乱的孔隙, 经分析认为, 这种混乱性及一定的周期性与湍流中的不规则性和准周期性极其相似. 由于金属橡胶材料内部芯材结构的不规则性, 即便此时的雷诺数很小, 波在金属橡胶中的传播仍是以湍流流动为主. 因此, 引入Kolmogorov的关于湍流的局部各向同性概念, 同时对Kolmogorov关于湍流局部各向同性的两个假设进行类比. 从湍流物理模型出发, 借鉴了湍流的统计处理方法, 对金属橡胶材料的吸声特性进行定量分析, 得到金属橡胶材料的能量耗散率与其结构参数之间的表达式. 研究结果表明, 湍流统计方法的引入, 为基于金属橡胶材料的减振器、阻尼器、消声器等的优化设计提供了可靠的理论依据, 也为超轻金属多孔材料的工程应用提供了一种有效的定量分析方法.      

Assessment of High Speed Imaging Systems for 2D and 3D Deformation Measurements: Methodology Development and Validation

Ultra high-speed and moderate speed image acquisition platforms have been characterized, with special emphasis on the variability and accuracy of the measurements obtained when employed in either 2D or 3D computer vision systems for deformation and shape measurements. Specifically, the type of image distortions present in both single channel cameras (HS-CMOS) and multi-channel image intensified cameras (UHS-ICCD) are quantified as part of the overall study, and their effect on the accuracy of experimental measurements obtained using digital image correlation have been determined. Results indicate that established methods for noise suppression and recently developed models for distortion correction can be used effectively in situations where the primary intensity noise components are characterized by minimal cross-talk and stationary spatial distortions. Baseline uniaxial tension experiments demonstrate that image correlation measurements using high speed imaging systems are unbiased and consistent with independent deformation measurements over the same length scale, with point-to-point strain variations that are similar to results obtained from translation experiments. In this study, the point-to-point variability in strain using the image intensified system is on the order of 0.001, whereas the non-intensified system had variability of 0.0001. Results confirm that high speed imaging systems can be utilized for full field two and three-dimensional measurements using digital image correlation methods.