90°弯管内冰浆流动特性数值模拟

基于欧拉法建立了冰浆流动的混合模型,对冰浆在90°弯管内的流动特性进行了数值模拟研究,获得了弯管内冰浆的流场和冰晶颗粒的运动轨迹,探讨了弯管管径、曲率半径及冰浆的流速、浓度参数对弯管内冰浆压降的影响,并对弯管压降的模拟结果与实验结果进行对比,两者吻合较好。结果表明:在计算参数范围内,弯管压降随冰浆流速、浓度及弯管曲率半径的增大而增大,随弯管管径的增大而减小;冰浆在弯管内流动形成二次流现象,两个漩涡区域出现在弯管截面两侧;漩涡导致部分冰晶颗粒的运动轨迹发生改变,并使其向弯管下方的负压区聚集,增大了冰晶颗粒的碰撞几率。     

Particle Size Characterization of an Extra Fine Milled Product

The present research aims to characterize the particle size distribution of sub micron particles suspended in a liquid. The particles milled are an organic poorly water soluble crystalline product. To characterize the size of these particles, different techniques have been tested: imaging techniques (SEM, CryoTEM), static light scattering techniques, dynamic light scattering techniques, centrifugation and flow field flow fractionation. The results indicate that the studied milled particles have a primary particle size close to 180nm and there is strong evidence of larger particles which are very likely aggregates. This is clearly seen from the Cryo TEM results. All the above mentioned techniques should in principle be able to measure samples of dispersion containing particles of ca 180 nm but several are disturbed by the presence of large aggregates. It is difficult to estimate the amount of aggregate present, but most of the time one is interested in what the primary particle size distribution is. It is clear that no single piece of equipment is capable of exactly determining the particle size distribution of our samples, but the static light scattering with low shear on mixing does give a good representation of what is seen with the image analysis by cryo TEM.     

基于代数滑移模型的光催化反应器内催化剂颗粒分布特性研究

针对光催化制氢反应器内液固两相流的特点,运用代数滑移模型,对反应器内的催化剂颗粒-水两相流动进行了数值模拟。采用分块的结构化网格,并运用多重网格方法进行求解。通过与前人实验中压力梯度结果的比较,对模型进行了验证,并最终得到了反应器内催化剂颗粒的典型分布及相应的类型图。     

Unconditionally convergent nonlinear solver for hyperbolic conservation laws with S-shaped flux functions

This paper addresses the convergence properties of implicit numerical solution algorithms for nonlinear hyperbolic transport problems. It is shown that the Newton–Raphson (NR) method converges for any time step size, if the flux function is convex, concave, or linear, which is, in general, the case for CFD problems. In some problems, e.g., multiphase flow in porous media, the nonlinear flux function is S-shaped (not uniformly convex or concave); as a result, a standard NR iteration can diverge for large time steps, even if an implicit discretization scheme is used to solve the nonlinear system of equations. In practice, when such convergence difficulties are encountered, the current time step is cut, previous iterations are discarded, a smaller time step size is tried, and the NR process is repeated. The criteria for time step cutting and selection are usually based on heuristics that limit the allowable change in the solution over a time step and/or NR iteration. Here, we propose a simple modification to the NR iteration scheme for conservation laws with S-shaped flux functions that converges for any time step size. The new scheme allows one to choose the time step size based on accuracy consideration only without worrying about the convergence behavior of the nonlinear solver. The proposed method can be implemented in an existing simulator, e.g., for CO₂ sequestration or reservoir flow modeling, quite easily. The numerical analysis is confirmed with simulation studies using various test cases of nonlinear multiphase transport in porous media. The analysis and numerical experiments demonstrate that the modified scheme allows for the use of arbitrarily large time steps for this class of problems.     

Denoising of quadrature ultrasound Doppler signal from bi-directional flow based on matching pursuit

Denoising of Doppler signal is a preliminary and important step in medical ultrasound imaging. To denoise quadrature Doppler signal from bi-directional flow, we propose a novel method based on matching pursuit in this paper. The proposed method is an iterative decomposition algorithm which decomposes the original Doppler signal into a linear expansion of atoms in a time-frequency dictionary. The time-frequency dictionary is similar to Fourier transform domain and the atoms are similar to orthogonal bases in Fourier transform. In each step of the iteration, the atom which gives the largest inner product with the analyzed signal is selected from the dictionary, and the contribution of this atom is subtracted from the Doppler signal. This process is repeated on the residue until the SNR reaches the maximum. The linear expansion of the selected atoms is the denoised signal. Simulations were conducted on a simulation model with a sampling rate of 12.8 kHz. When the original SNRs are 0 dB, 2 dB, 4 dB, 6 dB, 8 dB, 10 dB, the proposed method can improve the SNR for 7.9 dB, 7.8 dB, 7.5 dB, 7.3 dB, 7.05 dB, 6.8 dB respectively, reduce the root mean square error (RMSE) of the mean frequency waveform to 0.0441 kHz, 0.0303 kHz, 0.0245 kHz, 0.0215 kHz, 0.0161 kHz, 0.0125 kHz respectively, and suppress the RMSE of the spectral width waveform to 0.1774 kHz, 0.0591 kHz, 0.0486 kHz, 0.0170 kHz, 0.0145 kHz, 0.0117 kHz respectively. Preliminary in vivo evaluation was also carried out on a healthy 33-year-old male using B-K medical A/S 3535 ultrasound scanner, and the results showed that the proposed method can effectively enhance the Doppler spectrogram.     

Reply to comment on “On the controversy around Daganzo’s requiem for and Aw-Rascle’s resurrection of second-order traffic flow models” by H.M. Zhang

This contribution compares several different approaches allowing one to derive macroscopic traffic equation directly from microscopic car-following models. While it is shown that some conventional approaches lead to theoretical problems, it is proposed to use an approach reminding of smoothed particle hydrodynamics to avoid gradient expansions. The derivation circumvents approximations and, therefore, demonstrates the large range of validity of macroscopic traffic equations, without the need of averaging over many vehicles. It also gives an expression for the “traffic pressure”, which generalizes previously used formulas. Furthermore, the method avoids theoretical inconsistencies of macroscopic traffic models, which have been criticized in the past by Daganzo and others.     

Vibration and instability analysis of tubular nano- and micro-beams conveying fluid using nonlocal elastic theory

A nonlocal Euler–Bernoulli elastic beam model is developed for the vibration and instability of tubular micro- and nano-beams conveying fluid using the theory of nonlocal elasticity. Based on the Newtonian method, the equation of motion is derived, in which the effect of small length scale is incorporated. With this nonlocal beam model, the natural frequencies and critical flow velocities for the case of simply supported system and for the case of cantilevered system are obtained. The effect of small length scale (i.e., the nonlocal parameter) on the properties of vibrations is discussed. It is demonstrated that the natural frequencies are generally decreased with increasing values of nonlocal parameter, both for the supported and cantilevered systems. More significantly, the effect of small length scale on the critical flow velocities is visible for fluid-conveying beams with nano-scale length; however, this effect may be neglected for micro-beams conveying fluid.     

叶片开缝的离心风机流场研究

本文提出在离心叶轮轮盖与叶片之间开缝,使得叶片压力面侧的高压气体射流到吸力面侧的低速尾迹区,从而直接给叶轮内的低速流体提供能量,减弱了叶轮内由于二次流所导致的射流-尾迹结构,并可用于消除或解决部分负荷时常发生的离心叶轮积灰问题.对某离心风机整机的数值模拟表明,在设计流量和小流量情况下,叶轮开缝后使叶片表面分离区域减小,整个流道速度分布更为均匀,改善了叶轮内部流场的流动状况,提高了离心叶轮性能和整机性能.研究表明,本方法对于提高离心式流体机械性能是有效的.     

Magnetohydrodynamic Stagnation Point Flow of a Maxwell Nanofluid with Variable Conductivity

This article reports the simultaneous properties of variable conductivity and chemical reaction in stagnation point flow of magneto Maxwell nanofluid.The Buongiorno's theory has been established to picture the inducement of Brownian and thermophrotic diffusions effects.Additionally,the aspect of heat sink/source is reported.The homotopic analysis method(HAM) has been worked out for the solution of nonlinear ODEs.The behavior of inferential variables on the velocity,temperature,concentration and local Nusselt number for Maxwell nanofluid are sketched and discussed.The attained outcomes specify that both the temperature and concentration of Maxwell fluid display analogous behavior,while the depiction of Brownian motion is quite conflicting on both temperature and concentration fields.It is further noted that the influence of variable thermal conductivity on temperature field is similar to that of Brownian motion parameter.Moreover,for the confirmation of our study comparison tables are reported.