侧加热腔内的自然对流

开展侧加热腔内自然对流的研究具有重大的环境及工业应用背景. 总结侧加热腔内水平温差驱动的自然对流的最新研究进展, 并概述相应的流动性质、动力机制和传热特性以及对不同无量纲控制参数的依赖也有重要的科学价值. 已取得的研究结果显示突然侧加热的腔内自然对流的发展可包括初始阶段、过渡阶段和定常或准定常阶段. 不同发展阶段的流动依赖于瑞利数、普朗特数及腔体的高宽比, 且定常或准定常阶段的流态可以是定常层流流动、非定常周期性流动或者湍流流动. 此外, 回顾了对流流动失稳机制的研究成果以及湍流自然对流方面的新进展. 最后, 展望了侧加热腔内的自然对流研究的前景.      

Shadowgraph observations of the transition of the thermal boundary layer in a side-heated cavity

The transition of the thermal boundary layer from start-up to a quasi-steady state in a side-heated cavity is observed using a shadowgraph technique. Three stages of the transition, including an initial growth stage, an entrainment development stage and a quasi-steady stage, are demonstrated. A significant feature of the transition revealed from the present flow visualisation is the formation of a double-layer structure along the sidewall at the entrainment development stage. It is believed that the reverse flow in the double-layer structure is the likely cause responsible for the unstable travelling waves at the quasi-steady state.     

LES of transient flows controlled by DBD plasma actuator over a stalled airfoil

Large-eddy simulations (LES) are employed to understand the flow field over a NACA 0015 airfoil controlled by a dielectric barrier discharge (DBD) plasma actuator. The Suzen body force model is utilised to introduce the effect of the DBD plasma actuator. The Reynolds number is fixed at 63,000. Transient processes arising due to non-dimensional excitation frequencies of one and six are discussed. The time required to establish flow authority is between four and six characteristic times, independent of the excitation frequency. If the separation is suppressed, the initial flow conditions do not affect the quasi-steady state, and the lift coefficient of the higher frequency case converges very quickly. The transient states can be categorised into following three stages: (1) the lift and drag decreasing stage, (2) the lift recovery stage, and (3) the lift and drag converging stage. The development of vortices and their influence on control is delineated. The simulations show that in the initial transient state, separation of flow suppression is closely related to the development spanwise vortices while during the later, quasi-steady state, three-dimensional vortices become more important.     

ROUTE TO SURGE FOR A THROTTLED COMPRESSOR — A NUMERICAL STUDY

Aerodynamic instabilities of axial compressors are investigated numerically and compared with experimental results. The compressor flow in the interblade-row spaces is simulated by means of 2-D Euler equations while the blade rows are modelled as quasi-steady actuator disks. The coupling of different unbladed regions of the compressor by actuator disks, which is characterized by the influence of stator and rotor, is captured in terms of conservation laws and source terms by means of compressor characteristics. At inflow and outflow nonreflecting boundary conditions are used in order to avoid any nonphysical reflections at the boundary. Numerically simulating this model for selected initial and boundary conditions, we observe that for increasing values of the imposed exit pressure the compressor flow undergoes several qualitative changes. At some critical value of exit pressure a primary stable steady state losses stability to several coexisting time-periodic states with a number of rotating stall cells. While for these time-periodic states the mass flow does not depend on time, all solution branches lead finally to surge, i.e., to states with an oscillation of mass flow in time if the exit pressure is sufficiently large. In addition, the numerical results are compared with experimental measurements.     

Investigation of the annular nozzle start in actual and virtual intermittent aerodynamic setups

The results of a numerical and experimental investigation of the starting of jet-engine annular nozzle blown with the air at room temperature and high-temperature combustion products of a stoichiometric acetylene-air mixture in an intermittent aerodynamic setup are presented. The regimes and the times of the attainment of quasi-steady stAte of the flow are studied in the virtual full-scale counterpart of the experimental setup. The special features of the starting stage and the quasi-steady flow are studied for entry channels of different shapes. The values of starting times, the pressures at different points of the flow duct, and the thrusts produced by the annular nozzle measured in the physical experiment and obtained in the virtual experimental setup are compared.     

Experimental observations of the thermal flow around a square obstruction on a vertical wall in a differentially heated cavity

The transient thermal boundary layer flow around a square obstruction placed at the middle of the hot wall in a differentially heated cavity is visualized using a shadowgraph technique. The results show that the thermal boundary layer flow, which is blocked by the obstruction, firstly forms an intrusion head under the obstruction (the lower intrusion head). Subsequently, the lower intrusion head bypasses the obstruction and reattaches to the down-stream boundary. During the reattachment process, a more complicated flow is induced, and eventually both the lower intrusion head and the thermal boundary layer destabilize. After the lower intrusion head is convected away, the thermal boundary layer flow re-stabilizes. At the quasi-steady state, the thermal boundary layer forms a double-layer structure, which is split into two sections by the obstruction. It is demonstrated that both the transient processes and the quasi-steady state flow structures of the thermal boundary layer are significantly altered by the obstruction in comparison with the case without the obstruction.     

空间发展圆管转捩的直接数值模拟

利用直接数值模拟研究圆管流动中由局部壁面引入的周期性吹吸(PSB)扰动沿流向的空间发展,流动的雷诺数Re选定为3000.在临界幅值的PSB扰动下,在较短的圆管内,圆管中的扰动沿流向快速增长,在足够长的圆管内,扰动沿流向持续增长发生转捩,流动发展到湍流阶段.     

A Comparative Study of Turbulence in Ramp-Up and Ramp-Down Unsteady Flows

DNS of a turbulent channel flow subjected to a step change in pressure gradient are performed to facilitate a direct comparison between ramp-up and ramp-down flows. Strong differences are found between behaviours of turbulence in the two flows. The wall shear stress in the ramp-up flow first overshoots, and then strongly undershoots the quasi-steady value in the initial stage of the excursion, before approaching the quasi-steady value. In a strongly decelerating flow, the wall shear stress tends to first undershoot but then overshoot the quasi-steady value. ??Slow?? response of turbulence as well as flow inertia is responsible for these behaviours. In the ramp-up flow, the response of turbulence is similar to that observed in uniformly accelerating flows from previous studies, exhibiting a three-stage development. However, the transition between the various stages is more gradual and the responding stage is much longer and slower in the flows considered here. It has been shown that the delay in the near wall region is longer than that in the buffer layer confirming that turbulence response first occurs at the location of peak turbulence production. In a strongly decelerating flow, the response of turbulence exhibits a two-stage development. In both ramp-up and ramp down flows, the energy distribution in the three components of turbulent kinetic energy deviates from that of the steady flow. In a ramp-up flow, more energy is in $u_1^\prime $ and less in $u_2^\prime $ and $u_3^\prime $ , whereas the trend is reversed in a ramp-down flow. This is a reflection of the redistribution of turbulence from $u_1^\prime $ to $u_2^\prime $ and $u_3^\prime $ .     

Perturbation threshold and hysteresis associated with the transition to turbulence in sudden expansion pipe flow

The complex flow resulting from the laminar-turbulent transition in a sudden expansion pipe flow, with expansion ratio of 1:2, subjected to an inlet parabolic velocity profile and a vortex perturbation, is investigated by means of direct numerical simulations. It is shown that the threshold amplitude for disordered motion is described by a power law scaling, with -3 exponent, as a function of the subcritical Reynolds number. The instability originates from a region of intense shear rate, which results on the flow symmetry breakdown. Above the threshold, several unsteady states are identified using space-time diagrams of the centreline axial velocity fluctuation and their energy. In addition, the simulations show a small hysteresis transition mode due to the reestablishment of the recirculation region in the subcritical range of Reynolds numbers, which depends on: (i) The initial and final quasi-steady states, (ii) the observation time and (iii) the number of intermediate steps taken when increasing and decreasing the Reynolds number.     

AEROACOUSTIC RESPONSE OF DIFFUSERS AND BENDS: COMPARISON OF EXPERIMENTS WITH QUASI-STEADY INCOMPRESSIBLE FLOW MODELS

Experimental results were obtained in the study of the aeroacoustic response of diffusers and abrupt expansions. An experimental study on the aeroacoustics of bends has also been carried out and shows similar results. In both cases the low-frequency aeroacoustic behaviour can be predicted by a quasi-steady flow model when the flow separation point is fixed by sharp edges in the geometry. At smooth pipe discontinuities nonlinear effects are more important and the response is essentially unsteady.     

Experimental determination of droplet size and density field in condensing flows

 We report a detailed experimental characterization of the process of homogeneous condensation in supersonic expanding flow. In our experiments, the supersaturated mixture expands in a Laval nozzle, where, depending on the initial conditions, a steady or periodically oscillating flow may evolve due to the non-linear interaction of nucleation and droplet growth rate with the flow field. Two experimental techniques are utilized: holographic interferometry for the determination of the density field and a time-resolved white-light extinction method. The latter is employed to derive the evolution in time of the droplet cloud (i.e., modal radius, number density, and relative width) and to measure the frequency of oscillations. In combination with the wide-field density data, droplet size measurements provide additional physical insights in the mechanism of interaction in condensing flows and serve as an excellent test case for the critical assessment of nucleation and droplet growth theories. To this purpose, the accuracy of the measurements is carefully reviewed due to the difficulties of characterizing dense sub-micron droplet clouds by means of light-scattering techniques. An important byproduct of this analysis is an evaluation of the applicability of single-scattering approximations, i.e., Lambert-Beer law, for a variety of experimental configurations. Received: 24 April 2001 / Accepted: 29 August 2001     

Flow between two unsteadily rotating disks

Summary This is a theoretical investigation of the unsteady laminar flow of a viscous incompressible fluid between two infinite parallel disks, which are rotating with angular velocities varying with time. The solution is obtained in the form of a series expansion about the quasi-steady state. The deviation of the actual instantaneous state of the flow from the quasi-steady state is determined.     

SURFACE EFFECT ON NANOSIZED VOID GROWTH IN A RIGID-PERFECTLY PLASTIC MATERIAL

The influence of the surface effect on the nanosized spherical void growth in a rigidperfectly plastic material is analyzed and the mechanism of the nanosized void growth with high triaxiality is given. Based on the Rice and Tracey model for a macro void growth, the present model is proposed to account for the nanosized void growth under a uniform remote strain rate field with consideration on the surface effect. It is concluded that the surface effect yields an evident resistant influence on the nanosized void growth. That is, this influence decays as the void radius increases. With high triaxiality, the nanosized void growth is divided into two stages： the initial stage and the mature stage. At the first stage, the void grows slowly and the influence of surface effect is relatively weak, whereas at the second stage, the influence is significant and the void grows drastically.     

Investigations of gas and particle dynamics in first generation needle-free drug delivery devices

Abstract. Transdermal powdered drug delivery involves the propulsion of solid drug particles into the skin by means of high-speed gas-particle flow. The fluid dynamics of this technology have been investigated in devices consisting of a convergent-divergent nozzle located downstream of a bursting membrane, which serves both to initiate gas flow (functioning as the diaphragm of a shock tube) and to retain the drug particles before actuation. Pressure surveys of flow in devices with contoured nozzles of relatively low exit-to-throat area ratio and a conical nozzle of higher area ratio have indicated a starting process of approximately 200 s typical duration, followed by a quasi-steady supersonic flow. The velocity of drug particles exiting the contoured nozzles was measured at up to 1050 m/s, indicating that particle acceleration took place primarily in the quasi-steady flow. In the conical nozzle, which had larger exit area ratio, the quasi-steady nozzle flow was found to be overexpanded, resulting in a shock system within the nozzle. Particles were typically delivered by these nozzles at 400 m/s, suggesting that the starting process and the quasi-steady shock processed flow are both responsible for acceleration of the particle payload. The larger exit area of the conical nozzle tested enables drug delivery over a larger target disc, which may be advantageous. Received 12 March 2000 / Accepted 8 June 2000     

Relaxation oscillations in the turbulent near wake

The method of [2] is used to examine the interaction of a shock or an expansion wave in the outer plane supersonic stream with a turbulent near wake. It is shown that the flow in the disturbed near wake may be two-valued, and for fixed conditions in the outer stream quasisteady transitional flows between these two steady states are possible. Under the influence of some external disturbing mechanism (believed to be turbulent pulsations of the large-vortex type) these quasi-steady flows become relaxation oscillations which lead to the experimentally observed base-pressure oscillations.The theoretical conclusions are confirmed by an experimental study of the base-pressure fluctuations which develop in a two-dimensional expansion-deflection nozzle with interaction of the off-design shock with the near wake.In conclusion the author wishes to thank G. G. Chernyi and G. N. Abramovich for helpful discussions of the study and L. V. Kapranova, T. S. Soboleva, and Z. A. Donskova for assistance in performing the calculations.     

Direct Numerical Simulation of Transitional Noncircular Buoyant Reactive Jets

The near field dynamics of transitional buoyant reactive jets established on noncircular geometries, including a rectangular nozzle with an aspect ratio of 2:1 and a square nozzle with the same cross-sectional area, are investigated by three-dimensional spatial direct numerical simulations. Without applying external perturbations at the inflow boundary, large vortical structures develop naturally in the flow field due to buoyancy effects. Simulation results and analysis describe the details and clarify mechanisms of vortex dynamics of the noncircular buoyant reactive jets. The interaction between density gradients and gravity initiates the flow vorticity. Among the major vorticity transport terms, the gravitational term mainly promotes flow vorticity in the cross-streamwise direction. For the baroclinic torque, it can either create or destroy flow vorticity depending on the local flow structure. The vortex stretching term has different effects on the streamwise and cross-streamwise vorticity. Streamwise vorticity is mainly created by vortex stretching, while this term can either create or destroy cross-streamwise vorticity. Under the coupling effects of buoyancy and noncircular nozzle geometry, three-dimensional vortex interactions lead to the transitional behavior of the reactive jets. Simulations also show that the rectangular jet is more vortical than the square jet. The rectangular jet has a stronger tendency of transition to turbulence at the downstream due to the aspect ratio effect. Mean flow property calculations show that the rectangular buoyant reactive jet has a higher entrainment rate than its square counterpart. Received 13 December 2000 and accepted 24 July 2001     

Transient growth of energy and aeroelastic stability of ground vehiclesCroissance transitoire de l'énergie et stabilité aéroélastique des véhicules terrestres

Transient growth of energy is shown to be possible in the lateral dynamics of passenger cars. This mechanism might be generated during one vehicle overtaking another. Starting from a simple linearized quasi-steady model, which couples the lateral displacement and the yaw angle of the vehicle, the transient growth appears when an initial condition in the yawing rate is applied. To cite this article: P. Hémon, C. Noger, C. R. Mecanique 332 (2004).     

海陆过渡相页岩气藏不稳定渗流数学模型

海陆过渡相页岩常与煤层和砂岩呈互层状产出, 储层连续性较差、横向变化快、非均质性强, 水力压裂技术是其获得经济产量的关键手段. 然而, 目前缺乏有效的海陆过渡相页岩气藏不稳定渗流数学模型, 对其渗流特征分析及储层参数评价不利. 针对这一问题, 首先建立海陆过渡相页岩气藏压裂直井渗流数学模型, 其次采用径向复合模型来反映强非均质性, 采用Langmuir等温吸附方程来描述气体的解吸和吸附, 分别采用双重孔隙模型和边界元模型模拟天然裂缝和水力裂缝, 建立并求解径向非均质的页岩气藏压裂直井不稳定渗流数学模型, 分析海陆过渡相页岩气藏不稳定渗流特征, 并进行数值模拟验证和模型分析应用. 分析结果表明, 海陆过渡相页岩气藏不稳定渗流特征包括流动早期阶段、双线性流、线性流、内区径向流、页岩气解吸、内外过渡段、外区径向流及边界控制阶段. 将本模型应用在海陆过渡相页岩气试井过程中, 实际资料拟合效果较好, 其研究成果可为同类页岩气藏的压裂评价提供一些理论支撑, 具有较好应用前景.      

On the numerical study of bubbly flow created by ventilated cavity in vertical pipe

The dispersion of bubbles into a down-liquid flow in a vertical pipe is investigated. At low flow rates, the intended design of a swarm of discrete bubbles is achieved. At high flow rates, a ventilated cavity is nonetheless formed, which is attached close to the gas sparger. Behind this ventilated cavity, three different flow regimes characterize the complex bubbly flow field downstream of the down-liquid flow: vortex region with high void fraction, transitional region and pipe flow region. In this study, a numerical model that solved the entire development of the gas–liquid flow including the extended single-phase liquid region upstream to the wall-jet and recirculating-vortex zones in order to allow a more realistic determination of the boundary conditions of the down-liquid flow was adopted. Coupling with the Eulerian–Eulerian two-fluid model to solve the respective gas and liquid phases, a population balance model was also applied to predict the bubble size distribution in the wake right below the cavity base as well as further downstream in the transitional and fully-developed pipe flow regions. The numerical model was evaluated by comparing the numerical results against the data derived from theoretical, numerical and experimental approaches. Prediction of the Sauter mean bubble diameter distributions by the population balance approach at different axial locations confirmed the dominance of breakage due to the high turbulent intensity below the ventilated cavity which led to the generation of small gas bubbles at high void fraction. Further downstream, the coalescence effect dominated leading to merging of the small bubbles to form bigger bubbles.