Primary and secondary vortical structures contribution in the entrainment of low Reynolds number jet flows

Particle image velocimetry measurements and time-resolved visualization are used for the reconstruction of the Kelvin–Helmholtz vortex passing in the near field of a round jet and of a lobed jet. For the round jet, the entrainment is produced in the braid region, where streamwise structures develop. In the Kelvin–Helmholtz ring, entrainment is dramatically affected by the attenuation of the streamwise structures. As for the lobed jet, the special geometry introduces a transverse shear leading to a breakdown of the Kelvin–Helmholtz structures into “ring segments.” Streamwise structures continuously develop at the resulting discontinuity regions and control the lobed jet self-induction. In this case, the entrainment rate is less affected by the primary structures dynamics.     

Organizational Modes of Large-Scale Vortices in an Axisymmetric Turbulent Jet

Large vortices occurring in the axial plane of a self-similar axisymmetric turbulent jet are educed by spatially filtering PIV data. First, the instantaneous PIV frame is convolved with a Gaussian kernel to obtain a smooth (low-pass) field. Next, the low-pass field is Galilean transformed to expose the large vortices residing near the edges of the jet. Large vortices tend to organize themselves in preferred modes; evidence of ring and helical modes is revealed by Galilean transformation of the low-pass filtered field. Both modes seem to occur prominently in jets, with the helical mode being the more frequent. The overall diameter of both ring and helical modes is comparable with the local jet width. The low-pass field occasionally exhibits arrowhead shaped structures with large entrainment at their downstream tips. Stochastic estimates computed from the Galilean-transformed low-pass filtered field indicate that jet meander and a sweep-in of ambient fluid are sufficient to reconstruct large vortices. The frequency of occurrence of modes agrees with previously quoted results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Combination interaction of Taylor–Goertler vortices in a curved shear layer of a supersonic jet

Problems of origination and evolution of streamwise vortex structures in an initial region of the shear layer of a supersonic jet are discussed. Streamwise vortices are generated with the use of artificial microroughnesses on the internal surface of polished nozzles. Results of Pitot pressure distributions measured in a supersonic nonisobaric jet both in the radial and azimuthal directions are presented. Streamline curvature in the initial region of supersonic nonisobaric jets has a significant effect on evolution of streamwise vortex structures. Azimuthal heterogeneity corresponding to streamwise vortices in the shear layer is analyzed with the use of both the Fourier analysis and wavelet analysis. PACS 47.40.Ki, 47.20.Ft, 02.30.Nw     

Combustion and stabilization characteristics of a branched flame under Helmholtz-type excitations

 The combustion and pollution characteristics of the newly rediscovered “branched flame” are experimentally investigated using a Helmholtz-type excitation. Under specific excitation conditions, high-amplitude Helmholtz excitation induces side jet ejection, which leads to a branched flame. Intense combustion and enhanced heat transfer due to strong oscillation of the flame and hot gases of the branched flame increase the heating effectiveness and fuel saving. Strong velocity oscillation results in accumulation of jet fluid ahead of the ring structure for generation of the side jet. In the side-jet evolution, the strong entrainment of the ring vortex in the initial stages followed by the early growth of the streamwise vortical structures greatly shortens the route to mixing transition of fuel and air in the upstream region of the flame. This enhanced premixing process of the side jet leading to high F probability, which is defined as the probability of the presence of a premixture of fuel and air with concentration within the flammability limits, and low strain rate has significant implications for the stabilization of the branched flame. NOx emission indices for the branched flames can be 30% higher and CO emission indices 50% lower than the unexcited case. Received: 5 June 2000 / Accepted: 21 March 2001     

不定原点喷嘴射流中流体物理的实验研究(英文)

为了能够更好地了解不定源喷嘴(indeterminate origin nozzle)射流中的物理过程,本文应用平面激光诱导荧光技术对一个大尺度的水射流进行了实验研究。流场显示的实验结果表明不定源喷嘴在射流的剪切层引入了蘑菇形反向旋转的涡对。这些涡的矢量方向与射流方向相同或相反,被称为流向涡(streamwise vortex)。由于射流中存在开尔文-亥姆霍兹不稳定,每当一个横向涡(spanwise vortex,即涡的矢量方向与射流方向垂直)从喷嘴脱流时会产生瞬时的低压,该瞬时低压促使向内发展的流向涡对在喷嘴的凹槽处生成。这些涡对在向下游流动的过程中会重组并在喷嘴的尖峰面生成向外发展的涡对。这些流向涡极大地影响了射流的发展。流向涡与横向涡的相互作用促使射流更早地发展成为湍流。由于流向涡同时也在射流中引入了径向的剪切流动,因此导致了更多的湍流生成从而增强了射流与周围流体的混合。     

不定原点喷嘴射流中流体物理的实验研究

为了能够更好地了解不定源喷嘴（indeterminate origin nozzle）射流中的物理过程，本文应用平面激光诱导荧光技术对一个大尺度的水射流进行了实验研究。流场显示的实验结果表明不定源喷嘴在射流的剪切层引入了蘑菇形反向旋转的涡对。这些涡的矢量方向与射流方向相同或相反，被称为流向涡（streamwise vortex）。由于射流中存在开尔文一亥姆霍兹不稳定，每当一个横向涡（spanwisevortex，即涡的矢量方向与射流方向垂直）从喷嘴脱流时会产生瞬时的低压，该瞬时低压促使向内发展的流向涡对在喷嘴的凹槽处生成。这些涡对在向下游流动的过程中会重组并在喷嘴的尖峰面生成向外发展的涡对。这些流向涡极大地影响了射流的发展。流向涡与横向涡的相互作用促使射流更早地发展成为湍流。由于流向涡同时也在射流中引入了径向的剪切流动，因此导致了更多的湍流生成从而增强了射流与周围流体的混合。     

Vortex dynamics and mass entrainment in turbulent lobed jets with and without lobe deflection angles

Passive control of jet flows in order to enhance mixing and entrainment is of wide applicative interest. Our purpose is to develop new air diffusers for HVAC systems, by using lobed geometry nozzles, in order to ameliorate users the thermal comfort. Two turbulent 6-lobed air jets with and without lobe deflection angles were studied experimentally and compared with a reference circular jet having the same initial Reynolds number. The main objective was to analyze the modifications occurring in the vortex dynamics of the flow, firstly by replacing a circular tube with a straight lobed tube, and secondly by a lobed tube having a double inclination of the lobes. Rapid visualizations of the flows and hot-wire measurements of the streamwise velocity spectra allow understanding the vortex roll-up mechanisms. Unlike the circular jet, where the primary rings are continuous, the Kelvin–Helmholtz vortices in the lobed jet flows were found to be discontinuous. The resulting “ring segments” detach at different frequencies whether they are shed in the lobe troughs or at the lobe sides. One explanation relies on the strong variation of the exit plane curvature. Additionally, a speculative scenario of the vortical dynamics is advanced by the authors. The discontinuous nature of the K–H vortices enables the development of secondary streamwise structures, non-influenced by the passage of the primary structures as in the case of the circular jet. Thus, the momentum flux transport role played by the streamwise structures is rendered more efficient and leads to a spectacular increase in the entrainment rate in the initial region. The amount of fluid being entrained in the lobed jet by the streamwise structures is drastically amplified by the double inclination of the nozzle exit boundary.     

Measurements of the stabilization zone of a lifted jet flame under acoustic excitation

The turbulence and concentration characteristics in the stabilization zone of a lifted jet flame with and without acoustic excitation are measured by a time-resolved Rayleigh scattering, a LDV, and a hot vire anemometry system together with other probes. Both amplification and suppression of the flow can be achieved by acoustic excitation. By careful comparison of the turbulence and concentration characteristics in the stabilization zone for the natural, amplification and suppression cases, it is found that the key parameters of lifted flame stabilization in the stabilization zone are the integral length scale, the F probability of the presence of a flammable premixture, and the G probability of the presence of a fluid with a temperature reaching the ignition temperature. Amplification excitation enhances the large-scale coherent vortices and the vortical entrainment, thus enhancing the length scale, theF probability, and the G probability of having a combustibltoe premixture in the stabilization zone. In this case, the flame shifts upstream to a higher gas speed location and restabilizes there. Suppression excitation shows the opposite results. Practical need calls for a new model capable of predicting the stabilization zone structure of excited lifted flames where the large length scale, theF andG probabilities are suggested to be important parameters.This research was kindly supported by the National Science Council, R.O.C., through contract NSC-82-0401-E006-193. This financial support is sincerely appreciated.     

Manipulation of Large-Scale Vortical Structures and Mixing in a Coaxial Jet with Miniature Jet Actuators Toward Active Combustion Control

Manipulation of large-scale vortical structures and associated mixing in a methane-air coaxial jet is carried out by using miniature jet actuators installed on the inner surface of the annular nozzle. The periodic radial miniature jet injections are achieved with a rapid-response servo-valve. The spatio-temporal primary jet structures are investigated through phase-locked 2C-PIV (2 Component Particle Image Velocimetry) and stereoscopic-PIV. It is found that intense ring-like vortices are produced perfectly in phase with the periodic miniature jet injections regardless of the valve-driven frequency f_v examined. When the Strouhal number St_v, which is defined with f_v, is larger than unity, the ring-like vortices are densely formed and thus methane/air mixing is prompted with low periodic fluctuation. The diameter of the vortices becomes small as St_v is increased, so that the transport range of the inner methane and outer air fluids can be controlled by changing St_v. In addition, the evolution of counter-rotating vortex pair is also observed in the cross-sectional plane. These streamwise vortices are directly formed as a result of the radial miniature jet injection, which leads to entrainment of the ambient fluid near the primary jet shear layer, and they also contribute to the mixing enhancement. Moreover, it is demonstrated that coaxial jet flame characteristics such as carbon monoxide (CO) emission and flame holding can be drastically improved under different equivalence ratios by the present jet control scheme.     

Large Eddy Simulation of Compressible Subsonic Turbulent Jet Starting From a Smooth Contraction Nozzle

Compressible subsonic turbulent starting jet with a relatively large Reynolds number of significant practical importance is investigated using large eddy simulation (LES), starting from a smooth contraction nozzle. The computational domain of truncated conical shape is determined through the comparison of the time-averaged numerical solution with the particle imaging velocimetry measurements for the steady jet. It is shown that the starting jet consists of a leading vortex ring followed by a quasi-steady jet, and the instantaneous velocity field exhibits contraction and expansion zones, corresponding to the high pressure (HP) and low pressure (LP) regions formed by the convecting vortex rings, and are related to the Kelvin-Helmholtz instability. The thin boundary layer inside the smooth contraction nozzle evolves into a shear layer at the nozzle exit and develops with the downstream penetration of the jet. Using λ ₂ criterion, the formation and evolution of the vortical structures are temporally visualized, illustrating distortion of vortex rings into lobed shapes prior to break-down. Rib-shape streamwise vortex filaments exist in the braid region between a pair of consecutive vortex rings due to secondary instabilities. Finally, formation and dynamics of hairpin vortices in the shear layer is identified.     

Noise Reduction in Non-Premixed Lifted-Jet Flames

The characteristic changes in non-premixed lifted flames when excited by hole tones from a cavity, placed in the flow path of the fuel gas, were studied. A significant reduction of the sound pressure level was observed in the low-frequency noise at the flame base of the lifted flame when the hole tones were induced in the jet. The liftoff height and the mean diameter of the flame base decreased for a given jet Reynolds number. The blow-off velocities also increased suggesting improved flame stability in the presence of the hole tones induced by the cavity. Incorporation of the cavity upstream of a burner nozzle is demonstrated to give a quieter lifted flame with improved stability characteristics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental characterization of the instability of the vortex rings. Part II: Non-linear phase

The first stage of the instability of a vortex ring is linear and characterized by the growth of an azimuthal stationary wave which develops around the ring. Theoretical works predict its origin, shape, number of waves and growth rate. Apart for the growth rate, experimental and numerical results in viscous fluids fit well with the predictions based on an ideal fluid hypothesis. On the other hand, the next stages of the development of the instability (which are non-linear) are not well known. Only few phenomena are described, in an isolated way, in various partial contributions. The aim of this paper is to report on a complete experimental investigation of the non-linear phase of the instability of the vortex ring. The vortices were produced in water and their Reynolds number Re _p was varied from 2,650 to 6,100. Visualizations were performed using planar laser induced fluorescence and measurements with 2D2C and 2D3C particle image velocimetry. Based on a Fourier analysis of the results, it appears that the non-linear phase begins with the development of harmonics of the linear modes (first unstable modes). But the growth of those harmonics is rapidly stopped by the development of low order modes. Then appears an m=0 mode, which corresponds to a mean azimuthal velocity around the vortex. Simultaneously, secondary vortical structures develop all around the vortex in its peripheral zone. These vortical structures are linked with the ejection of vorticity in the wake of the ring and they appear just before the transition towards turbulence. A tentative is made here to place all these phenomena chronologically, in order to propose a scenario for the transition from the linear phase to turbulence.     

微型射流涡流器对湍流边界层控制的数值研究

运用数值方法,模拟出展向分布的同向倾斜微型射流列与平板湍流边界层相互作用形成流向涡列的流场结构,验证了利用其来对湍流边界层进行控制的可能性.随射流间距减小,流向涡列控制作用流向渗透能力增强,但作用区域减小;随射流速度提高,流向涡列控制作用增强,但过大的射流速度反而会导致流向涡列在局部区域内控制作用的下降;随射流俯仰角减小、倾斜角增大,流向涡列初始控制作用增强,但过小的俯仰角、过大的倾斜角会导致流向涡列流向控制区域明显缩小.要保证流向涡列具有较强的湍流边界层控制作用,必须通过合理配置射流列各主要参数,在保证各流向涡具有一定强度的同时,还要确保各流向涡在形成时部分嵌入边界层内部.     

Experimental investigation of axisymmetric coaxial synthetic jets

Measurements were made in the near field of piston driven axisymmetric coaxial synthetic jets emanating from an orifice and a surrounding annulus of equal exit areas and cavity volumes. Piston velocity, amplitude, radial spacing between the orifice and the annulus, and exit angles had a strong influence on the dominant features of the flow. Flow visualization revealed three distinct topologies of the jet consisting of expanding, contracting and recirculating regions and doubling of the number of foci inside of the cavity compared to jet from the orifice alone. The direction of the swirl/rotation imposed on the mean flow was also dependent on the direction of the rotation of dominant foci. Interaction between flow from the orifice and the annulus amplified the azimuthal instability of ring vortices due to the periodic axial stretching and compression of the streamwise vortex filaments. Bifurcation of ring vortices into elliptical lobes reported earlier [S.V. Gaimella, V.P. Schroeder, Local heat transfer distributions in confined multiple air impingement, ASME Journal of Electronic Packaging 123 (3) (2001) 165–172] for single cavity jet was also observed in the coaxial jet. The number of cellular structures however was considerably larger than the single jet case. Large excursions of the jets from the plane of symmetry were observed. Power spectra exhibited sub-harmonic distribution of energy due to coalescence of the vortices. Growth of jet width and decay of centerline velocity were strongly influenced by the spacing and forcing frequency.     

The spatial modulation of a forced triangular jet

An investigation of the possibility of controlling the evolution of jets into the far field is presented. Driven by practical concerns the study examined a highly turbulent jet flow. To enhance controllability of the flow evolution the virtues of non-circular nozzles and active flow excitation were combined. The study examined an air jet, Re _de =8000, average turbulence intensity 1.8%, issuing into stagnant room air out of a triangular nozzle, which had piezoceramic actuators mounted on the flat sides. The evolution of the jet flow field was examined over the range of 0X/D30.Small amplitude, single mode, excitation with frequency as the varying parameter was found to be ineffective for controlling the far field evolution. In contrast, excitation of the jet with non-integer and counter propagating azimuthal modes yielded marked changes in the jet streamwise evolution. The most notable changes in the far field were the transition of the cross section from round to elliptical, increased jet cross sectional area based on half centerline velocity contour, asymmetric threedimensional flow, and an increase in the entrainment rate. The entrainment of ambient air by the jet increased slightly more than twofold for non-integer and counter propagating azimuthal modes, compared with the unexcited jet, and only a 50% increase in the entrainment for single, integer, mode excitation.While excitation of the jet with modes m=0 and 1 resulted in symmetric evolution of the jet in the x-y-t space, excitation at non-integer and counter propagating modes resulted in time-dependent asymmetric motion. The near field induced jet column motion is controlling the far-field evolution of the examined jet.The authors would like to thank Dr. S. Ragab for useful discussions, and Dr. R. Kriz for access to the Scientific Visualization Laboratories, both from the ESM department at Va. Tech. The opportunities to discuss the research with Dr. M. G. Mungal from Stanford University are highly appreciated     

The stabilization mechanism of the lifted jet diffusion flame in the hysteresis region

Recent experimental efforts focused on near-field coherent vortex dynamics, and their impact on stabilization of a lifted jet diffusion flame in the hysteresis region are reported. Simultaneous jet flow and flame visualizations are conducted first to obtain a global feature of flow/flame interaction. The statistical liftoff heights are calculated by a DIP (digital image processing) method. The gas concentration and velocity distributions induced by the vortex evolution as well as the corresponding flame front motion are deduced from phase-averaged measurements of planar Mie-scattering gas concentration images, LDV and ion-signals, respectively. The planar gas concentration technique employed here extends our previous work (Chao et al. 1990, 1991 a) to include phase-averaging. Results of the experiments show that the most probable flame base locations in the hysteresis region are at the coherent vortex roll-up and pairing locations. The deeply entrained air lump caused by large-scale vortices during roll-up and pairing is the main obstruction to flame propagation back to the nozzle exit and causes the hysteresis phenomenon.     

Flow Structure of Swirling Turbulent Propane Flames

Flow structure of premixed propane–air swirling jet flames at various combustion regimes was studied experimentally by stereo PIV, CH* chemiluminescence imaging, and pressure probe. For the non-swirling conditions, a nonlinear feedback mechanism of the flame front interaction with ring-like vortices, developing in the jet shear layer, was found to play important role in the stabilisation of the premixed lifted flame. For the studied swirl rates (S = 0.41, 0.7, and 1.0) the determined domain of stable combustion can be divided into three main groups of flame types: attached flames, quasi-tubular flames, and lifted flames. These regimes were studied in details for the case of S = 1.0, and the difference in the flow structure of the vortex breakdown is described. For the quasi-tubular flames an increase of flow precessing above the recirculation zone was observed when increased the stoichiometric coefficient from 0.7 to 1.4. This precessing motion was supposed to be responsible for the observed increase of acoustic noise generation and could drive the transition from the quasi-tubular to the lifted flame regime.     

轴对称射流场涡结构的离散涡段方法研究

本文用三维离散涡方法,模拟了轴对称圆射流涡结构的发展。     

Direct Numerical Simulation of a Square Jet Ejected Transversely into an Accelerating, Laminar Main Flow

A numerical simulation of a square jet ejected transversely into a laminar boundary-layer flow was performed at a jet-to-main-flow velocity ratio of 9.78 and jet Reynolds number of 6330. The jet consisted of a single pulse with a duration equal to the time required for the jet fluid to travel 173 jet widths. A strongly-favourable streamwise pressure gradient was applied to the boundary layer and produced a freestream acceleration that is above the typical threshold required for relaminarization. The results of the simulation illustrate the effect of the favourable streamwise pressure gradient on the flowfield created by the transverse jet. Notably, the horseshoe vortex system created upwind of the jet remains steady in time and does not induce noticeable fluctuations in the jet flow. The upwind and downwind shear layers of the jet roll-up through a Kelvin–Helmholtz-like instability into discrete shear-layer vortices. Jet vorticity in the upwind and downwind shear layers accumulates near the corners of the jet and produces two sets of vortex pairs, the former of which couple with the shear-layer vortices to produce large, counter-rotating vortices in the freestream, while the latter are unstable and periodically produce hairpin vortices in the main-flow boundary layer and elongated vortices in the freestream behind the jet. The departure of the jet flowfield from the vortical structures typically observed in transverse jets illustrates the substantive effect of the favourable streamwise pressure gradient on the flowfield created by the jet.     

近距离下射流冲击平板PIV实验研究

运用时间分辨粒子成像测速系统(time-resolved particle image velocimetry, TR-PIV)对近距离下射流冲击平板时的流场进行了直接测量, 通过对两个正交的平面流场开展测量, 揭示了冲击距离和雷诺数对射流间隙内三维流动特征及涡系结构演化规律的影响. 结果表明: 射流间隙存在三种典型的涡系结构, 分别为双涡环模式、单涡环模式和卷吸模式, 但在大流量湍流状态下, 射流可能会冲破涡环, 形成随机的高速出流, 各流动模式的出现主要与射流流态及壁面约束作用有关. 运用涡量分析对三种典型涡系结构的能量传递和损失特性进行了比较. 结果表明: 近距离冲击时, 射流的能量通过涡环模式向外传递. 在双涡环模式下, 两个涡环的旋向相反, 端面的约束作用使得两个涡环都被严格约束在射流棒端面之内, 且一次涡环强度显著大于二次涡环强度. 最后, 运用本征正交分解方法对射流间隙内的流动模态及其能量分布进行了分析. 单涡和双涡模式前十阶模态分析结果表明: 能量脉动在较低阶时即以配对的模式出现, 这表明一次涡环与二次涡环均具有良好的对称性, 同时在双涡模式中, 一次涡环是占主导作用的大尺度流动结构. 卷吸模式的前三阶模态分析表明: 射流的能量集中在射流上游, 能量随紊动扩散急剧衰减.