Dynamics of swirling jet flows

Experimental investigations of near-field structure of coaxial flows are presented for four different configurations: coaxial jets without rotation (reference case), outer flow rotating only (OFRO), inner-jet rotating only (IJRO) and corotating jets (CRJ). The investigations are performed in a cylindrical water tunnel, with an independent rotation of two coaxial flows. Laser tomography is used to document the flow field, and photographs are shown for different configurations. Time mean velocity profiles obtained by PIV, with and without swirl, are also presented. The dynamics of the swirling jets in the initial region (i.e. near the exit of the jets) is described. The effects of azimuthal velocity and axial velocity ratio variations on flow dynamics are examined. The appearance and growth of the first instabilities are presented and compared with some theoretical results, as is the influence of the rotation (inner or outer) on the dominating structures.     

Flow field characterization of coaxial conical and serrated (chevron) nozzles

PIV measurements were performed to provide insight into the effect of serrated (chevron) nozzles on the flow field of a coaxial circular jet. The serrations were tested on the primary nozzle. Mean flow results showed that the chevron effectively redistributes momentum from the high velocity center stream outward to the lower velocity secondary stream by creating lateral jets. This leads to a more rapid decay of the peak jet velocity and a consequent reduction in the length of the jet potential core. Local increases of up to 65% in the outer stream velocity were measured. The interaction of the secondary jets with the lower velocity outer stream produces increases in turbulent kinetic energy (TKE) near the center nozzle lip. These flow field effects correlate with the jet’s acoustic emissions: Reduction of low-frequency noise due to large scale mixing and potential core shortening, and increased high-frequency noise due to increased near-field turbulence.     

同轴交叉射流的轴线速度

研究了射流交叉角度和密度比对同轴交叉射流流场的影响。基于动量守恒原理,提出了考虑径向流动和密度差别的同轴交叉射流的当量直径和当量速度。当量速度与Dual PDA的实验结果较为吻合。该法可较好地描述射流交叉角度、密度比对轴线上轴向速度的影响。     

A study of the flow characteristics in micro-abrasive jets

An experimental study of particle velocities in micro-abrasive jets by using the particle image velocimetry (PIV) technique is presented. It has been found that the particle jet flow has a nearly linear expansion downstream. The particle velocities increase with air pressure, and the increasing rate increases with nozzle diameter within the range considered. The instantaneous velocity profile of the particle flow field in terms of the particle velocity distribution along the axial and radial directions of the jets is discussed. For the axial profile in the jet centerline downstream, there exists an extended acceleration stage, a transition stage, and a deceleration stage. For the radial velocity profiles, a relatively flat shape is observed at a jet cross-section near the nozzle exit. Mathematical models for the particle velocities in the air jet are then developed. It is shown that the results from the models agree well with experimental data in both the variation trend and magnitude.     

Analysis of the nozzle exit flow parameter effect on the turbulence characteristics and the noise level in jets issuing from nozzles of different types

The flow, turbulence, and noise parameters of cold and hot jets flowing out of nozzles of different types at subsonic and supersonic velocities are calculated using the high-resolution RANS/ILES method. The effect of the Mach number and the temperature at the nozzle exit on the flow features, the turbulent fluctuations of the velocity, the static pressure, and the temperature, together with the overall noise level is analyzed for all the jets considered. The accuracy of the calculations is confirmed by means of comparing with the available experimental data concerning certain parameters.     

Simulations of high‐aspect‐ratio jets

There are many practical situations when jets are emanating from non‐axis‐symmetric apertures, yet numerical simulations of such three‐dimensional jets are scarce and most of them have failed to reproduce some of the unique flow features. Examples of this type of jets are gas leaks from flanges. These can be treated as jets issuing from high aspect ratio rectangular orifices. The present work consists of a series of large eddy simulations typifying such jets using different inflow boundary conditions. Good agreement with available experiments was observed provided appropriate boundary conditions were present. A discrete method for formulating turbulence data with a known energy spectrum for an inflow condition is outlined and evaluated with three other inflow conditions–a steady uniform profile, a steady parabolic profile and pseudo‐random noise. The implementation of the new inlet condition results in a more realistic centreline velocity decay where the division between the end of the potential core region and the start of the characteristic decay region is clearly visible. Large velocity oscillations are also observed in the final quarter of the domain (15–20 slot widths downstream). Similar oscillations have been observed in real jets. Off‐centre mean velocity peaks are present along the major axis 10 slot widths downstream of the exit in all the simulations. The peaks are approximately 3% of the centreline velocity. The presence of the off‐centre peaks are proved to be independent of jet inflow boundary conditions and an explanation for the mechanism causing the off‐centre peaks is given. Copyright © 2002 John Wiley & Sons, Ltd.     

Investigation of the Flow Structure on a Miniature Gas-Dynamic Setup: Identification of the Secondary Flow in a Clustered Supersonic Jet Escaping Into a Rarefied Space

A possibility of using small-scale vacuum setups for experimental investigations of supersonic jets escaping from supersonic nozzles into vacuum or rarefied space is considered. Results of studying the structure of the secondary supersonic flow formed in supersonic jets with developed condensation, which is detected for the first time, are reported. The present investigations are carried out with the use of photometry and spectrometry of jets with the use of radiation excited by an electron beam; flow visualization is also performed. The results obtained in the study are analyzed; capabilities and specific features of various methods of flow registration are considered. An empirical model, which establishes the dependence between the detected secondary flow and the process of formation of large clusters in the flow, is developed and justified.     

Influence of the initial conditions on axisymmetric jets in a parallel flow

An experimental investigation was made of the initial-section flow of axisymmetric helium, air, and freon-12 jets in a parallel air flow for two different velocity profiles at the nozzle exit near the boundary of the jet. In one case, the velocity profile was determined by boundary layers on the nozzle walls; in the other case, it was produced artificially by means of a honeycomb of tubes of variable length. Measurements were made of the profiles of the mean and the pulsation velocity and the temperature. The flow was also photographed. The investigations showed that, depending on the initial conditions, the intensity of mixing of the jets in the initial section at Reynolds numbers Re 10⁴ (calculated using the jet diameter) can change from the level determined by molecular diffusion to the level characteristic of developed turbulent flow. The flow structure in the annular mixing layer also depends strongly on the initial conditions. The observed ordered structures in the mixing layer are related to a section of development of perturbations near the nozzle. The ordered structures are strongly influenced by the effect on the jet of acoustic vibrations from an external source. When the initial velocity profile is produced by the honeycomb, the transition to developed turbulence may be due to the development of long-wavelength perturbations or to the development of small-scale turbulence generated by the flow over the end of the honeycomb.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 18–24, July–August, 1980.We thank V. M. levlev and K. I. Artamonov for assistance and for discussing the work.     

On the analysis of turbulent transport processes in nonreacting multijet burner flows

Measurements of time-resolved velocity characteristics have been obtained with a laser-Doppler velocimeter in the vicinity of a model of an industrial oxy-fuel burner. The burner consists of a central axisymmetric jet surrounded by 16 circular jets, simulating the injection of oxygen in practical burners. The experiments were carried out for isothermal flows and quantify the effect of swirl for 0 ≤ S ≤ 0.9 on the mixing efficiency of the burner assembly. The results show that the present flow develops faster than related coaxial free jets with similar velocity ratios between central and peripheral air streams and, for example, for the nonswirling flow the rate of decay of the centreline velocity increases by a factor of 2. Swirl attenuates the three-dimensional structure typical of multijet flows although the peripheral jets limit the radial spreading of the swirling flow and give rise to increased values of mean shear strain and, therefore, to turbulent production. The existence of zones characterized by large turbulence anisotropy indicate the need to take account of the individual normal stresses in any proposed mathematical model to simulate the flow characteristics. Inspection of the terms in the conservation equation for the turbulent stresses quantify the extent to which interaction of normal stresses with normal strains influences the flow and suggests the likely combined magnitude of turbulent diffusion and dissipation.     

Coaxial turbulent jets with strong central blowing

The results of solving the problem of the initial section of isothermal coaxial jets with strong central blowing, when the transverse pressure gradient has only a slight effect and there is no circulation zone in the central jet are given. The problem is solved by the integral relation method with allowance for jet interference and the presence of a cocurrent flow. The results of an experimental investigation of these jets over a wide range of the geometric and regime parameters are also given. The results of the calculations made using the formulas obtained are compared with the experimental data.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 52–59, May–June, 1996.     

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 investigation of turbulence modulation in particle-laden coaxial jets by Phase Doppler Anemometry

The effect of solid particles on the flow characteristics of axisymmetric turbulent coaxial jets for two flow conditions was studied. Simultaneous measurements of size and velocity distributions of continuous and dispersed phases in a two-phase flow are presented using a Phase Doppler Anemometry (PDA) technique. Spherical glass particles with a particle diameter range from 102 to 212 μm were used in this two-phase flow, the experimental results indicate a significant influence of the solid particles and the Re on the flow characteristics. The data show that the gas phase has lower mean velocity in the near-injector region and a higher mean velocity at the developed region. Near the injector at low Reynolds number (Re = 2839) the presence of the particles dampens the gas-phase turbulence, while at higher Reynolds number (Re = 11 893) the gas-phase turbulence and the velocity fluctuation of particle-laden jets are increased. The particle velocity at higher Reynolds number (Re = 11 893) and is lower at lower Reynolds number (Re = 2839). The slip velocity between particles and gas phase existed over the flow domain was examined. More importantly, the present experiment results suggest that, consideration of the gas characteristic length scales is insufficient to predict gas-phase turbulence modulation in gas-particle flows.     

An experimental study of two-phase turbulent coaxial jets

The effect of solid particles on the flow structure of axisymmetric turbulent coaxial jets has been studied. A laser-Doppler anemometer was used to measure the mean and fluctuation velocities of both phases, and a Malvern laser diffraction instrument was applied to measure particle size and concentration. A series of velocity ratios and particle loading ratios were investigated, and the results were analysed for the effects of these ratios on the mixing characteristic and the similarity behavior of the jet. The effects of particle diameter and its distribution were also studied as well as their influence on the coaxial jet behavior.     

Experiments on the formation of a recirculation zone in swirling coaxial jets

 The near flow field of coaxial air jets, with swirl in the outer one, was studied using flow visualization and hot-wire anemometry. The flow is sensitive to both the swirl number and the mass flow ratio between the outer and inner jets. A necessary condition for the formation of an internal recirculation zone (IRZ) is that the swirl number must exceed a minimum value which depends on the mass flow ratio. Spectral analysis of the velocity fluctuations indicates that the formation of an IRZ in the present flow does not appear to be related to the growth of convective flow instabilities. Analysis of the flow visualization and X-wire data indicates that the vorticity dynamics model for vortex breakdown proposed by Brown and Lopez [J Fluid Mech (1990) 222: 553] provides a plausible mechanism for the formation of an IRZ in this flow. Received: 14 June 1999/Accepted: 7 December 1999     

Interaction of plane turbulent jets in a confined space: mean flow characteristics

The present study deals with the experimental investigations of static pressure and mean velocity fields obtained as a result of the interaction of two plane turbulent jets at impingement angles of α equal to 30° and 45°, with an additional central jet in a confined space. The investigation is carried out for the velocity ratios of U _c/U _o=1.0, 2.0 and 3.0, where U _c and U _o are the velocities in the central plane at the exit of the central jet and the outer jets, respectively. The introduction of the central jet alters the various recirculation zones present in the flow field for all the cases considered above. Also, the change in the velocity ratio U _c/U _o has a significant effect on the pressure and mean velocity flow fields. Flow visualisation results are presented which give a better physical insight into the flow field considered. Received: 26 July 1999/Accepted: 14 February 2000     

Velocity measurements in a precessing jet flow using a three dimensional LDA system

 The turbulent, three dimensional and time dependent flow field of a precessing jet is investigated. In the present case the jet precession is generated by mechanically rotating a round jet inclined relative to the axis of rotation. A conditional flow visualisation technique is used to complement three dimensional laser Doppler velocity data, time-averaged and phase-averaged at the frequency of precession. The conditional phase-averaging technique enables phase-averaged velocity contours and vectors to be obtained which reveal flow patterns and structures within the flow field. Time-averaging of the velocity data shows that these structures are significant in that they generate a reverse flow (recirculation) region between the jet and its spinning axis. They are found also to cause a rapid decay of the mean velocity. The characteristics of the precessing jet found here are compared with previous investigations of the same flow and with investigations of other turbulent jets. Received: 17 March 1995/Accepted: 7 December 1995     

Reynolds number effects on the behavior of a non-buoyant round jet

The behavior of a non-buoyant circular water jet discharged from a contraction nozzle was experimentally investigated. In this experiment, the Reynolds number of the jet, based on the mean velocity results obtained by particle image velocimetry (PIV), ranged from 177 to 5,142. From the experimental results, we found that the cross-sectional profile of the axial velocity for a laminar flow near the nozzle did not show a top-hat distribution, whereas the profiles with Reynolds number higher than 437 were almost top-hat. The length of the zone of flow establishment (ZFE) was found to decrease with increasing Reynolds number. The measured centerline velocity decayed more rapidly and, consequently, approached the theoretical equation earlier near the nozzle as the Reynolds number increased. The decay constant for the centerline velocity of the turbulent cases was relatively lower than that discovered in theory. It is assumed that this probably resulted from the use of the contraction nozzle. Verifying the similarity of the lateral velocity profiles demonstrated that the Gaussian curve was properly approximated only for the turbulent jets and not for the laminar or transitional flows. The jet half width seldom grew for the laminar or transitional flows, whereas it grew with increasing axial distance for the turbulent flows. The spreading rates for the turbulent flows gradually decreased with increasing Reynolds number. The normalized turbulence intensity along the jet centerline increased more rapidly with the axial distance as the Reynolds number increased, and tended to the constant values proposed by previous investigators. The Reynolds shear stress levels were also found to increase as the Reynolds number increased for the turbulent jets.     

On the mixing of axi-symmetric ducted jets

Because of practical application to jet pumps, ejectors, furnaces and similar devices, the turbulent discharge of a round jet into a coaxial duct and the mixing patterns in the various regions into which the flow may be divided, are of considerable interest. In this paper the mixing of an incompressible jet with a similar fluid in a cylindrical tube is considered up to the plane which marks the disappearance of potential flow. Under the assumption of similarity of velocity profile and with neglect of the wall boundary layer and nozzle wake, the continuity and momentum equations, in integral form, are solved for the velocities and mixing region radii at any given section. Prandtl's momentum transfer hypothesis may be used to determine the dependence of these on distance downstream. By examining the various flow regimes in detail this analysis is formally able to cover ratios of primary to secondary flow velocities of from one to infinity and, similarly, all ratios of duct to nozzle diameters, thereby extending earlier investigations. It also corrects work on similar basis in which inappropriate linearisations were made. The ‘exact’ results constitute a basis from which extension to include additional effects may be made.     

Calculation of the transition region of a turbulent jet

The presently known methods for calculating plane and axisymmetric turbulent jets in a wake flow are based on dividing the flow region into two segments, initial and basic [1–3], Here the matching of the parameters of the initial and basic segments is of an artificial nature, since it permits the existence of a physically impossible discontinuity of the curves of the velocity distribution and the jet width along the axis.The aerodynamic characteristics of the transition segment, extending from the point of convergence of the boundary layers at the end of the initial segment to the section corresponding to the point of inflection of the curve u_m(x), differ significantly from the characteristics of the initial and basic segments. This difference is due not only to the sharp increase of the velocity pulsations, but also the marked deformation of the average longitudinal velocity component profile. Consequently, the calculation of the transition segment, in contrast to the initial and basic segments, cannot be based on the single-parameter method.Generally speaking, the flow development in the transition segment may be calculated with the aid of the method [4], which reduces the solution of the problem to an equation of the heat conduction type and assumes the use of an experimental curve of the velocity distribution along the jet axis. Abramovich has carried out the calculation of the transition segment of a plane submerged jet on the basis of certain assumptions which are based on the results of experimental studies [1].Below is presented an approximate method of calculating the transition segment of plane and axisymmetric turbulent jets in a wake flow in which the velocity profiles obtained for the extreme sections of this segment are used for calculating the flow parameters in the initial and basic segments.