Effects of non-linear rheology on electrospinning process: A model study

We develop an analytical bead-spring model to investigate the role of non-linear rheology on the dynamics of electrified jets in the early stage of the electrospinning process. Qualitative arguments, parameter studies as well as numerical simulations, show that the elongation of the charged jet filament is significantly reduced in the presence of a non-zero yield stress. This may have beneficial implications for the optimal design of future electrospinning experiments.     

熔喷双槽形喷嘴气体射流流场初探

在熔喷非织造布加工中,气体射流作为工作介质使聚合物熔体实现拉伸,气体射流流场的研究对熔喷气流拉抻数学模型研究非常重要。熔喷双槽形喷嘴形成的流场可以看作两股平面射流的合成。从单个点涡的性质出发,研究了涡偶的性质和涡偶代替射流的可行性。研究表明,在喷丝孔轴线附近,涡偶和射流的速度分布趋势相同,且有比较相近的速度分布,从而说明以涡偶代替射流是可行的。在此基础上,用两个涡偶分别代替两股射流,然后进行合成,推导出两股射流合成后速度分布的理论公式,该公式的计算结果与实验结果吻合较好。将该公式引入熔喷气流拉伸数学模型,预测出的纤维直径与采用经验公式时的预测结果几乎完全相同。结果表明,应用涡偶代替射流推导出的气流速度分布公式能够较好地描述熔喷双槽形喷嘴气体射流流场,可以用于完善熔喷气流拉伸数学模型。     

The trajectories of large fire fighting jets

This article describes a computer simulation of the trajectories of large water jets which allow the effects of changes in initial velocity, elevation, nozzle diameter, and head and tail winds to be examined. The rather limited information on aerodynamic drag of large jets obtained by other workers is used. The predicted trajectories compare well with the limited data available in the literature.The results also show that for a given flow rate an optimum pressure, and hence an optimum nozzle diameter, exists for maximum throw distance which has important implications for the design of the whole system including the pumps. The optimum elevation in still air lies in the range 30–40°. Wind effects are shown to be very important.     

A micro-aerodynamic decelerator based on permeable surfaces of nanofiber mats

This work deals with nonwoven permeable light mats made of submicron-diameter nanofibers. The nanofibers were obtained through electrospinning of polymer solutions. The mats were positioned on light pyramid-shaped frames. These platforms fell freely through the air, apex down, at a constant velocity. The drag of such passive airborne platforms is of significant interest in a number of modern aerodynamics applications including, for example, dispersion of "smart dust" carrying various chemical and thermal sensors, dispersion of seeds, as well as movement of small organisms with bristle appendages. In the present work, drag is measured using the free fall method supplemented by extensive flow visualization. The effects of platform weight, average nanofiber diameter, and porosity of the nonwoven mats on the drag force are studied. The results are compared to data for the corresponding impermeable structures that are covered with plastic wrap. The data are presented in the form of standard dependencies of drag coefficient on the Reynolds number of the structure. It was found that permeable platforms with holes on the order of several microns (which is about ten times the diameter of the nanofibers) are essentially impermeable for airflow.     

Drag reduction on the 25° slant angle Ahmed reference body using pulsed jets

This paper highlights steady and unsteady measurements and flow control results obtained on an Ahmed model with slant angle of 25° in wind tunnel. On this high-drag configuration characterized by a large separation bubble along with energetic streamwise vortices, time-averaged and time-dependent results without control are first presented. The influence of rear-end periodic forcing on the drag coefficient is then investigated using electrically operated magnetic valves in an open-loop control scheme. Four distinct configurations of flow control have been tested: rectangular pulsed jets aligned with the spanwise direction or in winglets configuration on the roof end and rectangular jets or a large open slot at the top of the rear slant. For each configuration, the influence of the forcing parameters (non-dimensional frequency, injected momentum) on the drag coefficient has been studied, along with their impact on the static pressure on both the rear slant and vertical base of the model. Depending on the type and location of pulsed jets actuation, the maximum drag reduction is obtained for increasing injected momentum or well-defined optimal pulsation frequencies.     

Computer simulation of the trajectories of large water jets

A three-dimensional computer simulation of the motion of a water jet is described which includes the effects of wind from any direction. The simulation is useful in the design of fire-fighting systems, particularly those used in offshore situations. The equations of motion are presented in vector form and the problem of the fluid dynamic drag variation is discussed. Semi-empirical approximations for the drag components along and across the jet are presented which involve four unknown constants. These are reduced to three by using previous data on the efficiency of vertical jets. To fix the remaining constants, information was available from a series of large jets tests carried out to prove an offshore fire fighting system. In these tests different nozzle shapes were tried and, using the best of these shapes, a large number of trajectories were measured photographically. These were used to fix the simulation drag constants and good agreement is shown between measurements and predictions. The simulation enables the effects of flow-rate, pressure, nozzle size, elevation and wind strength to be evaluated in the system design     

Leading-Order Dynamics of Gravity-Driven Flows in?Free-Standing Soap Films

We derive the leading-order equations that govern the dynamics of the flow in a falling, free-standing soap film. Starting with the incompressible Navier?CStokes equations, we carry out an asymptotic analysis using parameters that correspond to a common experimental setup. We account for the effects of inertia, surface elasticity, pressure, viscous stresses, gravity, and air drag. We find that the dynamics of the flow is dominated by the effects of inertia, surface elasticity, gravity, and air drag. We solve the leading-order equations to compute the steady-state profiles of velocity, thickness, and pressure in an experiment in which the film is in the Marangoni elasticity regime. The computational results, which include a Marangoni shock, are in good accord with the experimental measurements.     

Numerical Investigation of Active Flow Control Around a Generic Truck A-Pillar

Large Eddy Simulations (LES) are conducted to study the actuated flow field around a bluff body. The model is a simplified section of a truck. The aim of the work is to model the separation of the flow acting at the front rounded corners, the so called A-pillars, and to minimize the separation of the flow by means of Zero Net Mass Flux synthetic jets. LES data show the interaction of the flow main structures, the separation mechanism and the effects of the actuation on the flow field. The flow is post processed using modal and frequency decompositions. Relevant results in terms of drag reduction were observed for the actuated flow. The principle flow mechanisms are discussed and an optimal actuation frequency, in terms of induced fluctuations and drag reduction, is identified.     

Analyse spatio-temporelle de jets axisymétriques d'air et d'héliumSpace–time analysis of axisymmetric jets of air and helium.

The effects of strong density variations on the dynamics of instabilities which develop in axisymmetric jets of pure air or pure helium are studied in the near field. By using LDV measurements associated with fast visualization techniques, space–time diagrams are built in order to show the evolution of the structures along the jets according to their Reynolds number and their density. In particular, the global character of the helium jet instabilities is highlighted. To cite this article: S. Boujemaa et al., C. R. Mecanique 332 (2004).     

Active control of flow past an elliptic cylinder using an artificial neural network trained by deep reinforcement learning

The active control of flow past an elliptical cylinder using the deep reinforcement learning (DRL) method is conducted. The axis ratio of the elliptical cylinder $\Gamma$ varies from 1.2 to 2.0, and four angles of attack $\alpha=0^\circ, 15^\circ, 30^\circ$, and $45^\circ$ are taken into consideration for a fixed Reynolds number $Re=100$. The mass flow rates of two synthetic jets imposed on different positions of the cylinder $\theta_1$ and $\theta_2$ are trained to control the flow. The optimal jet placement that achieves the highest drag reduction is determined for each case. For a low axis ratio ellipse, i.e., $\Gamma=1.2$, the controlled results at $\alpha=0^\circ$ are similar to those for a circular cylinder with control jets applied at $\theta_1=90^\circ$ and $\theta_2=270^\circ$. It is found that either applying the jets asymmetrically or increasing the angle of attack can achieve a higher drag reduction rate, which, however, is accompanied by increased fluctuation. The control jets elongate the vortex shedding, and reduce the pressure drop. Meanwhile, the flow topology is modified at a high angle of attack. For an ellipse with a relatively higher axis ratio, i.e., $\Gamma\ge1.6$, the drag reduction is achieved for all the angles of attack studied. The larger the angle of attack is, the higher the drag reduction ratio is. The increased fluctuation in the drag coefficient under control is encountered, regardless of the position of the control jets. The control jets modify the flow topology by inducing an external vortex near the wall, causing the drag reduction. The results suggest that the DRL can learn an active control strategy for the present configuration.     

Robust design and numerical simulation on drag reduction by a mixture film for liquid turbulence in a channel

An important way of increasing the speed and lowering the fuel consumption of ships is by decreasing the frictional drag. One of the most promising techniques for reducing drag is the use of air bubbles. The goal of this investigation is to establish a set of optimum robust parametric levels for drag reduction by a mixture (air–water) film in turbulent channel flow. Based on the conditions laid out by the Taguchi orthogonal array method, turbulent flows, with air bubbles injected into a channel, are simulated using commercial computational fluid dynamics software. The local shear stress on the upper wall is computed to evaluate the efficiency of drag reduction. Many factors can affect drag reduction. The factors investigated in this study are the rate of air injection, bubble size, area of air injection, flow speed, and measured position of the shear stress. These factors have been investigated through the analysis of variance, which has revealed that the rate of air injection and water flow speed dominate the efficiency of drag reduction by a mixture film. According to the results, the drag can be reduced by an average of 83.4%; and when the configuration of the parametric levels is optimum the maximum drag reduction of 88.5% is achieved. Copyright © 2008 John Wiley & Sons, Ltd.     

动力刚化问题的实验研究

应用频散可控耗散格式对环形激波在圆柱形激波管内绕射、反射和聚焦 的问题进行了数值模拟研究. 研究结果表明环形激波形成强烈聚焦的关键因素是环形激波在 圆柱形管道中向对称轴运动时，绕射激波就不断加速而不作通常情况下的衰减；不同马赫数 的环形激波绕射也产生不同马赫数及形状的准柱形激波，导致聚焦效果和位置的差异；另外， 环形激波聚焦于一个点而圆柱形激波聚焦于一条线,两者有本质不同.     

The visualization of the flowfield about a delta wing with spanwise blowing

This paper describes a nonintrusive method for the visualization of the flow about a delta wing with spanwise blowing jets, based on the schlieren technique. The effects of the jet/leading-edge vortex interference are visualized by using both air and helium for the jets. The visualization of the leading-edge vortex trajectories and their breakdown, as well as the influence of the jets on them is achieved by spanwise blowing of air. The visualization of the jets' paths and the effects of the leading-edge vortices on these paths is achieved by spanwise blowing of helium.     

电雾化、电纺丝和带电射流稳定性研究

简述了电雾化和电纺丝的基本原理及其应用,总结了电雾化和电纺丝现象中几个基本的力学 机理,并重点对电雾化和电纺丝现象中的带电射流稳定性问题及其研究进展作了介绍.     

Flexural perturbations of free jets of maxwell and Doi-Edwards liquids

Flexural perturbations of high-velocity free jets of drop liquids moving in air are reinforced by the fact that the air pressure on the concave sections of the jet surface is greater than on the convex sections. The linear and nonlinear stages of development of flexural perturbations were studied in [1–5] for viscous Newtonian fluids. The effect of elastic stresses in the fluid on the growth of flexural perturbations of jets was first examined in [6], where it was assumed in an analysis of the growth of small disturbances that surface tension was constant along the jet, i.e., the investigators actually studied a tensed string. The studies [7, 8] examined the linear stage of growth of flexural perturbations of jets of Maxwell liquids. Our goal here is to analyze the dynamics of long-wave flexural perturbations of jets of viscoelastic fluids in both the linear and nonlinear stages of development. The rheological behavior of the fluid is described by two models — the phenomenological (Maxwell) model and the physical-molecular (Doi-Edwards) model. It is shown that the disturbances are oscillatory in character in the nonlinear stage of development. Meanwhile, the results of calculations performed with the Maxwell (M) and Doi-Edwards (DE) rheological models in the given problem agree with each other quantitatively as well as qualitatively.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 43–53, November–December, 1986.     

Fluid dynamics of slender,thin, annular liquid jets

Perturbation methods are used to obtain the one-dimensional, asymptotic equations that govern the fluid dynamics of slender, thin, inviscid, incompressible, axisymmetric, irrotational, annular liquid jets from the Euler equations. It is shown that, depending on the magnitude of the Weber number, two flow regimes are possible: an inertia-dominated one corresponding to large Weber numbers, and a capillary regime for Weber numbers of the order of unity. The steady equations governing these two regimes have analytical solutions for the liquid's axial velocity component and require a numerical integration to determine the jet's mean radius for inertia-dominated jets. The one-dimensional equations derived in this paper are shown to be particular cases of a hydraulic model for annular liquid jets, and this model is used to determine the effects of gravity modulation on the unsteady fluid dynamics of annular liquid jets in the absence of mass injection into the volume enclosed by the jet and mass absorption. It is shown that both the convergence length and the pressure coefficient are periodic functions of time which have the same period as that of the gravity modulation, but undergo large variations as the amplitude, frequency and width of gravitational pulses is varied.     

气流作用下同轴带电射流的不稳定性研究

通过对气体驱动同轴电流动聚焦的实验模型进行简化,开展了电场力和惯性力共同作用下同轴带电射流的不稳定性理论研究.在流动为无黏、不可压缩、无旋的假设下,建立了三层流体带电射流物理模型并得到了扰动在时间域内发展演化的解析形式色散关系,利用正则模方法求解色散方程发现了流动的不稳定模态,进而分析了主要控制参数对不稳定模态的影响.结果表明,在参考状态下轴对称模态的最不稳定增长率最大,因此轴对称扰动控制整个流场.外层气流速度越高,气体惯性力越大,射流的界面越容易失稳.内外层液-液同轴射流之间的速度差越大,射流越不稳定.表面张力对射流不稳定性起到促进作用.轴向电场对射流不稳定性具有双重影响:当加载电场强度较小时,射流不稳定性被抑制;当施加电压大于某一临界值时,轴向电场会促进射流失稳.临界电压的大小与界面上自由电荷密度和射流表面扰动发展关系密切.这些结果与已有的实验现象吻合,能够对实验的过程控制提供理论指导.     

Experimental simulation of heat transfer in an HF-plasmatron with lengthened segmented discharge channel

Subsonic and supersonic air induction plasma flows in a VGU-4 100 kW plasmatron with segmented water-cooled cylindrical nozzle with the outlet cross-section 40 mm in diameter are investigated experimentally. The enthalpy on the axis of flow is measured in subsonic air jets. The heat fluxes are measured at the stagnation flow point on a cylindrical water-cooled model 50 mm in diameter located in subsonic air and nitrogen jets. The effect of the generator power, nozzle length, and pressure in the plasmatron pressure chamber on the distributions of the heat flux and the pressure at the stagnation point on the surface of cylindrical models 20 mm in diameter with a plane and hemispheric nose is investigated along the axis of underexpanded dissociated air jets.     

A new drag model for TFM simulation of gas–solid bubbling fluidized beds with Geldart-B particles

In this work, a new drag model for TFM simulation in gas–solid bubbling fluidized beds was proposed, and a set of equations was derived to determine the meso-scale structural parameters to calculate the drag characteristics of Geldart-B particles under low gas velocities. In the new model, the meso-scale structure was characterized while accounting for the bubble and meso-scale structure effects on the drag coefficient. The Fluent software, incorporating the new drag model, was used to simulate the fluidization behavior. Experiments were performed in a Plexiglas cylindrical fluidized bed consisting of quartz sand as the solid phase and ambient air as the gas phase. Comparisons based on the solids hold-up inside the fluidized bed at different superficial gas velocities, were made between the 2D Cartesian simulations, and the experimental data, showing that the results of the new drag model reached much better agreement with experimental data than those of the Gidaspow drag model did.