On the spread of rectangular jets

Results are presented of some characteristic features of turbulent free jets issuing from rectangular exits having different geometries and small aspect ratios. Emphasis is placed on the measurement in the flow field near the exit. The measured quantities include mean velocity, mean temperature and stream-wise turbulence intensity. It is found that the growth and decay of the spanwise saddle-backed profile for temperature is different from that for velocity, and that the streamwise turbulence intensity distribution shows a remarkable change according to aspect ratio, exit shape and exit velocity.This paper was presented at the Ninth Symposium on Turbulence, University of Missouri-Rolla, October 1-3, 1984     

Turbulence measurements in a rectangular mesoscale confined impinging jets reactor

Mesoscale chemical reactors capable of operating in the turbulent flow regime, such as confined impinging jets reactors (CIJR), offer many advantages for rapid chemical processing at the microscale. One application where these reactors are used is flash nanoprecipitation, a method for producing functional nanoparticles. Because these reactors often operate in a flow regime just beyond transition to turbulence, modeling flows in these reactors can be problematic. Moreover, validation of computational fluid dynamics models requires detailed and accurate experimental data, the availability of which has been very limited for turbulent microscale flows. In this work, microscopic particle image velocimetry (microPIV) was performed in a mesoscale CIJR at inlet jet Reynolds numbers of 200, 1,000, and 1,500. Pointwise and spacial turbulence statistics were calculated from the microPIV data. The flow was observed to be laminar and steady in the entire reactor at a Reynolds number of 200. However, at jets Reynolds numbers of 1,000 and 1,500, instabilities as a result of the jets impinging along the centerline of the reactor lead to a highly turbulent impingement region. The peak magnitude of the normalized Reynolds normal and shear stresses within this region were approximately the same for the Reynolds numbers of 1,000 and 1,500. The Reynolds shear stress was found to exhibit a butterfly shape, consistent with a flow field dominated by an oblique rocking of the impingement zone about the center of the reactor. Finally, the spatial auto- and cross-correlations velocity fluctuations were calculated and analyzed to obtain an understanding of size of the coherent structures.     

入口扰动频率对矩形射流影响的大涡模拟

为研究入口扰动对矩形射流的影响,对不同扰动频率下的三维矩形射流进行了大涡模拟,用分布投影法求解动量方程,对压力泊松方程采用FACR直接解法.计算结果预报的速度在横向的单峰分布和速度半宽沿流向的近似线性增长与实验结果吻合良好.计算结果表明扰动频率为射流的本征频率fe=0.22时射流的卷吸作用最强,而高频扰动则抑制卷吸作用;激励频率接近本征频率时射流提前出现速度在展向的马鞍型双峰分布,更早趋于圆射流形态.进一步的流场二维和三维瞬态涡结构图谱表明:上述现象与f=0.1和f=0.22时射流近场更频繁卷起、更丰富的大涡结构存在密切关系,正是这些大涡运动不断地在横向卷吸入外界流体,实现射流在横向的迅速扩展.     

The role of vorticity in the near field development of sharp-edged,rectangular, wall jets

Experimental results on the near field development of a turbulent rectangular wall jet with aspect ratio 10 that issues from a sharp-edged orifice at Re_h ∼ 23,000 are presented and discussed, in comparison with results obtained in a free jet with identical initial conditions. Hot wire X-probe measurements on cross plane grids provide information on the 3D characteristics of the flow field. This work, besides presenting the main features of the jet, focuses on the effect of vorticity on the development of specific flow field characteristics. Mean vorticity components were estimated by interpolation and derivation from the mean and turbulent velocity measurements and the symmetries of the flow field were imposed by suitable averaging. Several terms of the axial vorticity equation are presented and discussed to uncover some complex flow physics, related e.g. to axis switching and the formation of a dumbbell shape of the jet outline, in the early stages of development.     

Natural convection flow of a viscous incompressible fluid in a rectangular porous cavity heated from below with cold sidewalls

We consider the flow, which is induced by differential heating on the boundaries of a porous cavity heated from below. In particular we allow the sidewalls to have the same cold temperature as the upper surface, and thus the problem is a variant of the Darcy-Bénard convection problem, but one where there is flow at all non-zero Grashof numbers. Attention is focused on how the flow and heat transfer is affected by variations in the cavity aspect ratio, the Grashof number and the Darcy number. The flow becomes weaker as the Darcy number decreases from the pure fluid limit towards the Darcy-flow limit. In addition the number of cells which form in the cavity varies primarily with the aspect ratio and is always even due to the symmetry imposed by the cold sidewalls.     

The effect of boundary conditions on sound loudness radiated from rectangular plates

This paper describes the effect of boundary conditions on sound loudness radiated from rectangular plates. Based on the Rayleigh integral, the Zwicker’s loudness model and the dynamic response of the plates, the sound loudness radiated from vibrating plates and the effects of simple support, clamped support and free support on sound loudness are separately studied. The effects of boundary conditions on sound intensity level, sound intensity density and critical-band level are also studied, taking the frequency selectivity of human hearing into account. The transformation from sound intensity level to sound loudness radiated from rectangular plates, due to the frequency-selectivity characteristics of human hearing, is also illustrated.     

Three-dimensional effect on transonic rectangular cavity flows

Experiments are performed to study the characteristics of rectangular cavity flows. Mean and fluctuating surface pressures in a Mach 1.28 turbulent flow past rectangular cavities are obtained. The cavity length-to-depth ratio (L/D) is varied from 2.43 to 43.00, while the length-to-width ratio (L/W) is 0.5, 1.0 and 2.0. The three-dimensional effect is significant on the trailing edge vortex, which affects the peak pressure ahead of the rear face, pressure gradient and levels of pressure fluctuation near the trailing edge, particularly for closed and transitional cavity flows. L/W and Mach number are important for the definition of critical L/D for the cavity flowfield models. Received: 12 March 1999/Accepted: 30 August 2000     

Meandering jets in shallow rectangular reservoirs: POD analysis and identification of coherent structures

The effect of the shallowness on meandering jets in a shallow rectangular reservoir is investigated. Four meandering flows were investigated in an experimental shallow rectangular reservoir. Their boundary conditions were chosen to cover a large range of friction numbers (defined with the sudden expansion width). Due to the unsteady characteristics of the flows, a proper orthogonal decomposition (POD) of the fluctuating part of the surface velocity fields measured using Large-Scale Particle Image Velocity was used for discriminating the flow structures responsible for the meandering of the jet. Less than 1 % of the calculated POD modes significantly contribute to the meandering of the jet, and two types of instability are in competition in such a flow configuration. The sinuous mode is the dominant mode in the flow, and it induces the meandering of the flow, while the varicose mode is a source of local mixing and weakly participates to the flow. The fluctuating velocity fields were then reconstructed using the POD modes corresponding to 80 % of the total mean fluctuating kinetic energy, and the coherent structures were identified using the residual vorticity, their centres being localised using a topology algorithm. The trajectories of the structures centres emphasise that at high friction number the coherent structures are small and laterally paired in the near, middle and far fields of the jet, while with decreasing friction number, the structures merge into large horizontal vortices in the far field of the jet, their trajectories showing more variability in space and time. The analysis of the stability regime finally reveals that the sinuous mode is convectively unstable and may become absolutely unstable at the end of the reservoir when the friction number is small.     

Parameters of jets out of rectangular nozzles when they are free or near a screen

The results are given of an experimental determination of the parameters of jets out of rectangular nozzles. The distributions of the mean velocity and an impurity concentration were measured. The rearrangement of the jet flow associated with the three-dimensional structure of the jet and interaction of the jet and a screen was investigated. A model that describes the occurrence of a pressure difference and curvature of the jet trajectory when it interacts with a screen is proposed.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 39–48, July–August, 1979.     

Experimental investigation of jets from rectangular six-lobed and round orifices at very low Reynolds number

This article presents an experimental study conducted on a six-lobed rectangular jet at a very low Reynolds number of 800. The near-exit flow dynamics is compared to the reference counterpart circular jet with same initial conditions. Flow dynamics is analyzed using time-resolved flow-visualizations, hot-wire anemometry and laser Doppler velocimetry. In the round jet, flow motion is dominated by large primary Kelvin–Helmholtz (K–H) structures. In the six-lobed rectangular jet, the K–H vortices are very thin compared to the large secondary vortices generated by the high shear at the lobed nozzle lip. The inspection of mean-velocity profiles and streamwise evolutions of the spreading rates in the major and the minor planes of the lobed jet confirm the absence of the switching-over phenomenon not observed on flow images. The streamwise structures that develop in orifice troughs render the volumetric flow rate significantly higher than that of the reference circular jet. Comparison of the obtained results to available data of the literature of similar rectangular six-lobed jets investigated at very high Reynolds numbers reinforces the notion that the three-dimensional flowfields at very low and very high Reynolds numbers are similar if the geometry of the lobed nozzle is conserved. However, important variations in flow dynamics might occur if one or several geometric parameters of the lobed nozzle are modified.     

The effect of buoyancy on flow fluctuations for horizontal plane jets in low speed applications

Low speed jets have important applications in chemical process, power and aerospace industries. Velocity fluctuations in low speed laminar jets have been investigated experimentally and theoretically, in the present work. The effects of buoyancy on the mean and fluctuating components of velocity have been highlighted. It is observed that even for forced convection dominated flow, convective instabilities and the resulting local velocity fluctuations are significantly influenced by buoyancy. Both the dominant frequency and the amplitude of velocity fluctuations depend on the jet exit temperature and spatial location within the jet. For isothermal jets, the dominant frequency of oscillation increases almost linearly with Reynolds number, while for buoyant jets nonlinearity exists at lower Reynolds numbers. Numerical simulations of the present study are found to be reasonably successful in predicting the oscillatory behavior of both isothermal and non-isothermal laminar free jets accurately.     

The effect of support locations on the steady-state response of viscoelastically point-supported rectangular orthotropic plates

The vibration of orthotropic rectangular plates having viscoelastic point supports symmetrically located on its diagonals is analyzed. The plate is under the effect of a sinusoidally varying force at its center. The Lagrange equations are used in the solution process. To apply the Lagrange equations, the trial function denoting the deflection of the plate is expressed in polynomial form. By using the Lagrange equations, the problem is reduced to the solution of a system of algebraic equations. The influence of the mechanical properties, the damping of the supports and the locations of point supports on the steady-state response of the viscoelastically point-supported rectangular plates is investigated numerically for the concentrated load at center for various values of the mechanical properties characterizing the anisotropy of the plate material and for various values of damping and location of the supports for a certain stiffness value of the supports. The results are given for the considered frequency range of the external force. Convergence studies are made. The validity of the obtained results is demonstrated by comparing them with other solutions based on the Kirchhoff–Love plate theory.     

The nonlinear evolution of swirling jets

We investigate the mechanisms of vorticity concentration, reorientation and stretching in a swirling jet, whose dynamics is dominated by the competition of a Kelvin-Helmholtz-type vortex sheet instability and a centrifugal Rayleigh instability. To this end, we employ an inviscid Lagrangian vortex filament technique. It is found that the axial jet velocity profile breaks the symmetry of the pure swirling flow. Conversely, the swirl is seen to modify the case dominated by a Kelvin-Helmholtz instability in that it results in the formation of counterrotating vortex rings. A pinch-off mechanism is observed which leads to a dramatic decrease in the local jet diameter. Furthermore, the vortex ring circulation is seen to be time dependent.

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Sommario In questo lavoro si analizza la dinamica della vorticità in un setto rotante in cui siano presenti, ed in competizione reciproca, fenomeni di instabilità di Kelvin-Helmholtz e di Rayleigh. A tale scopo si adotta una metodologia di soluzione non viscosa, Lagrangiana a filamenti vorticosi. Viene mostrato come il profilo di velocità assiale del getto altera la simmetria del moto di pura rotazione. Viceversa, la presenza della rotazione modifica il flusso dominato dall'instabilità di Kelvin-Helmholtz attraverso la formazione di anelli vorticosi controrotanti. L'interazione di questi due campi di velocità porta sia ad una considerevole riduzione del diametro locale del getto, sia ad una variazione temporale della circolazione degli anelli vorticosi.

     

Thermal effect of strongly underexpanded jets on constructional elements of equipment of complicated form

During the preparation of the Soyuz spaceship for the joint mission with the Apollo spaceship, an investigation was made on a model of the thermal and mechanical effect of the exhaust jets of the Apollo guiding system on Soyuz [1]. The experiments on the model revealed a complicated structure of the gas flow field, in which there were shock waves and rarefaction zones. The experimental values of the heat fluxes were converted to the conditions of actual flight by a method based on dependences for calculating the thermal effect of a supersonic gas jet on bodies of a simple form. Calorimeters of a cylindrical form set up on the spacecraft made it possible to compare the heat fluxes measured during the joint mission with those calculated by this method. In the present paper, the method of converting the experimental model values of the heat fluxes to the actual mission conditions is explained, the construction of the calorimeters and the conditions under which the heat fluxes from the Apollo exhausts were measured are described, and the measured values of the heat fluxes are compared with the values calculated by this method.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 113–119, January–February, 1980.     

On the effect of gravity on the bifurcation of rectangular closed-loop thermosyphon

Closed-loop thermosyphons are systems in which heat is transferred from a source to a sink by means of a natural convective flow, i.e. without the help of mechanical pumping. In fact, the dynamics of such systems strongly depend both on the thermal boundary conditions and on the gravitational field in which they operate. While the effect of variations of the boundary conditions has been extensively analysed in the last decades, the dependence on gravity has never been explicitly studied.The aim of this paper is to examine the effect of variations of gravity as well as that of thermal boundary conditions on the dynamics of natural circulation loops. Such an analysis might point out some useful applications for the cooling of a generic source in reduced gravity conditions.To this purpose an experimental campaign was performed on a natural circulation operating under a gravity field varying in the range between 10^–2 g and 1.8 g, with g = 9.81 ms^–2. The dynamical behaviour detected during the experiment was used for the validation of a mathematical model, previously validated under terrestrial gravity conditions. Model simulations were found to satisfactorily reproduce the dynamics of the system under variable gravity. This proved the possibility to use the model for the construction of bifurcation diagrams describing the behaviours of natural circulation loops under variations of both the gravitational field and the thermal boundaries.     

长方体单腔室空腔环境内爆炸效应的实验研究

为了获得在典型空腔内发生爆炸后,结构壁面上爆炸载荷的分布规律和空腔结构的破坏形式,以国防工事和人防工事的等级设计规范为依据,设计了长方体单腔室空腔模型,并对该模型进行了药量逐渐递增直至可使结构破坏的内爆炸实验。用压力传感器和加速度传感器分别记录了单腔室壁面上爆炸载荷的压力时程曲线和结构壁面振动的加速度时程曲线,分析了壁面上爆炸载荷的分布规律以及模型结构的破坏形式,并将首个峰值的实测数据与理论计算和数值模拟结果进行了对比,探讨了3种研究方法产生误差的原因。     

The effects of fluctuating body forces on annular liquid jets

Summary The nonlinear dynamics of axisymmetric, inviscid, incompressible, thin, annular liquid jets subjected to fluctuating body forces is studied numerically by means of an adaptive finite difference method which maps the time-dependent, curvilinear geometry of the jet into a unit square. The fluctuating body forces may arise from fluctuations in the gravitational acceleration in inertial frames or from the acceleration of a non-inertial frame of reference which translates parallelly to an inertial one. It is shown that both the pressure coefficient and the axial location at which the annular jet becomes a solid one are periodic functions of time with a period equal to that of the imposed body force fluctuations, and that their magnitude increases as the amplitude of the body force fluctuations is increased. It has also been shown that, for both intermittent, sinusoidal or rectangular excitations, increases in the frequency of the excitation result in the creation of superharmonics, broad, albeit peaked, spectra, and closed phase planes with many loops.The research reported in this paper was supported by Project PB91-0767 from the D.G.I.C.Y.T. of Spain.     

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.