Relationship between vortex and sound in under‐expanded supersonic impinging jets

The study of an under‐expanded supersonic jet impinging on a flat plate by using large‐eddy simulation is reported. A third‐order upwind compact difference and a fourth‐order symmetric compact scheme are employed to discretize the nondimensional axisymmetric compressible Favre‐filtered Navier–Stokes equations in space, whereas the third‐order Runge–Kutta method with the total variation diminishing property is adopted to deal with the temporal discretization. The numerical simulation successfully captures the shock wave and vortex structures with different scales in the flow field. Waves with high and low frequencies traveling forward and reflecting back, and sound sources in different locations can be observed. By comparison with the frequency of the impinging tone from the experiment, it can be deduced that the change of pressure and swirling strength in the shear layer, pressure change on the impinging plate, and vortex merging in the jet shear layer are interdependent with the impinging tone. The effects of nozzle lip thickness on the impinging jet flow field have been investigated. The results show that the values of pressure fluctuation and vortex swirling strength in the shear layer near the nozzle have an extremum with the variation of the nozzle lip thickness. The results provide a theoretical foundation for the design of supersonic nozzles. Copyright © 2011 John Wiley & Sons, Ltd.     

A Multi‐Thresholding Algorithm for Sizing out of Focus Particles

A methodology is presented that enables efficient acquisition of sufficient droplet information (e.g. diameter and aspect ratio) from images of in and out of focus droplets. The newly developed multi‐threshold algorithm is successfully implemented in the automatic particle/droplet image analysis (PDIA) system. Under the same optical hardware set‐up, and compared to the dual threshold methods [1], the multi‐threshold method increases the measurable/acceptable depth of field (DoF) of particles, especially for the small particles of diameters less than 50 μm (1098 pixels in this optical set‐up). When testing the 70 μm~110 μm and 100 μm~200 μm moving glass spheres, the dual threshold method can only detect 11%~29 % of the particles found by the multi‐threshold method. The multi‐threshold method is also capable of generating the aspect ratios of particles more accurately than dual threshold methods.     

Numerical simulation of droplet coalescence behavior in gas phase under the coupling of electric field and flow field

The coalescence behavior of droplets in an electric field belongs to the important research contents of electrohydrodynamics. Based on the phase field method of the Cahn–Hilliard equation, the electric field and the flow field are coupled to establish the numerical model of twin droplet coalescence in a coupled field. The effects of flow rate, electric field strength, droplet diameter, and interfacial tension on the coalescence behavior of droplets during the coalescence process were investigated. The results show that the dynamic behavior of the droplets is divided into coalescence, after coalescence rupture, and no coalescence under the coupling of electric field and flow field. The proper increase of the electric field strength will accelerate the coalescence of the droplets, and the high electric field strength causes the droplets to burst after coalescence. Excessive flow rates make droplets less prone to coalescence. Under the coupling field, the larger the droplet interface tension, the smaller the droplet diameter, the smaller the flow rate, and the shorter the droplet coalescence time. The results provide a theoretical basis for the application of electrostatic coalescence in gas–liquid separation technology.     

Semi-analytical and computational investigation of different dust loading structures affecting the performance of a fibrous air filter

In this study, a semi-analytical model was developed to illustrate the relationship between filtration performance （filtration efficiency and pressure drop） and dust loading under two different particle deposit structures based on theoretical analysis and computational fluid dynamic （CFD） technology. Under the compact deposit structure, within the practical parameter ranges （fiber diameter, air velocity, dust loading mass）, a slight efficiency enhancement （∽10%） occurred at the most penetration particle size （MPPS） and pressure drop increased significantly （∽100%） in response to the solidity increase from 5% to 15%. However, under the dendritic particle deposit structure, both filtration efficiency （∽40%） and pressure drop （4600%） increased significantly with the same solidity increase due to the larger air velocity and swerve change between fibers.     

端铣Ti40阻燃钛合金的刀具磨损破损形态及其机理

分别采用油冷和低温喷雾射流冷却方法开展了涂层硬质合金刀具端铣Ti40阻燃钛合金试验,结合铣削力和铣削温度,对刀具磨损破损形态及其磨损机理进行了系统研究.结果表明:涂层硬质合金刀具在铣削Ti40阻燃钛合金时主要磨损形式为前刀面月牙洼磨损和后刀面边界磨损;前后刀面破损形态主要表现为涂层贝壳状剥落、裂纹和微崩刃.磨损机理主要是黏结磨损、磨粒磨损、氧化磨损及疲劳破损的综合作用.铣削高温是造成硬质合金刀具快速磨损的主要原因.低温喷雾射流冷却方式明显降低了铣削温度,减轻了刀具黏结磨损、氧化磨损,同时抑制了刀具边界磨损的发展速度,因而大幅度提高了刀具的耐用度.     

液滴撞击孔口附近壁面运动过程的模拟研究

采用光滑粒子流体动力学(SPH)方法模拟了液滴撞击带孔壁面的问题, 提出了随计算区域变化的链表搜索法. 结合实验进一步研究了不同物理条件下黏性、重力和内部压应力波动对铺展过程中液滴在 孔口运动情况的影响, 详细分析了有限时间段内孔口断面处的压强变化. 结果表明: 液滴撞击表面后快速向两端铺展, 到达孔口上方时形成射流, 在极短暂时间内重力对射流的影响很小, 但是黏性会引起射流向孔内弯曲. 在内部压应力和惯性作用下射流下部产生有规律的波动, 使得孔口上方流体反复的膨胀和吸收将附近应力较高区域流体吸入孔内发生孔吸现象. 内部压应力是导致液滴被吸入孔内并撞击另一侧孔壁形成飞溅现象的主要原因, 模拟效果和实验结果符合良好. 关键词： 液滴 带孔壁面 光滑粒子流体动力学(SPH) 链表搜索法     

基于大密度差多相流SPH方法的二维液滴碰撞数值模拟

该文结合了Ott提出的修正连续性方程和Adami改进的动量方程, 对空气中的液滴碰撞问题进行了二维数值模拟. 为有效提高计算精度, 推导了适用于大密度差多相流的人工黏性和人工应力方程. 通过表面张力作用下方形液滴自然变化和空气中两液滴互溶的算例, 验证了算法的有效性; 对不同韦伯数 (8.8, 19.8)、不同碰撞参数 (0, 0.5)下的液滴碰撞过程进行了数值模拟, 并与VOF方法对比,取得了较为一致的结果; 进一步计算多个韦伯数、多个碰撞参数下的液滴碰撞, 得到了空气中二维液滴碰撞结果分布图,与实验结果相符合. 结果表明, 该算法对于求解涉及大密度差多相流的液滴碰撞破碎问题十分有效,而且该方法容易拓展到三维, 从而为进一步模拟火箭发动机的二次雾化过程奠定了基础. 关键词： 光滑粒子流体动力学 大密度差 多相流 液滴碰撞     

T型微通道内溶胶液滴形成过程

以制备空心玻璃微球的前体溶胶和硅油为原料,采用实验观测和数值模拟的方法,对T型微通道内溶胶乳液形成过程进行研究。基于液滴的受力分析,建立了液滴形成过程的数学模型,探讨了液滴大小的变化规律。研究结果表明:对于给定的物料体系和T型微通道,通过改变两相流量可以有效地控制液滴尺寸;在相同的分散相流量条件下,增大连续相流量可以减小液滴尺寸,但连续相流量大到一定程度后,这种效果逐渐减弱;在给定的连续相流量条件下,分散相流量越大,液滴直径越大;利用数学模型计算出的液滴直径与实验值偏差在10%左右。根据模拟结果和摄像分析,液滴产生过程经历了静态长大和缩颈剥离两个主要阶段。     

Large-eddy simulation of impinging jets with embedded azimuthal vortices

We have carried out large-eddy simulations of an impinging jet with embedded azimuthal vortices, a model of the wake of a helicopter hovering in ground effect. The azimuthal vortices are generated by sinusoidal forcing of the velocity at the jet exit. They strengthen while they are advected towards the ground; when they are close to the solid surface, a layer of opposite-sign vorticity is formed at the wall, and lifted up to form a secondary vortex that interacts with the primary one. Regions of reversed flow are caused by the strong, localised, adverse pressure gradient. After this interaction, the primary vortices begin to decay, mostly due to the Reynolds shear stresses, which contribute to the turbulent diffusion of vorticity term in the budget of the phase-averaged azimuthal vorticity. This mechanism is extremely robust, and plays the most important role in the vortex decay even if no turbulence is initially present in the jet, or if the no-slip condition is removed. A three-dimensional instability also plays a role: removing it leads to slower decay. Our results also point out some challenges for turbulence models for the unsteady Reynolds-averaged Navier–Stokes equations.