Visualization of the vortical structure of a circular jet excited by axial and azimuthal perturbations

The vortical structure of a circular water jet was investigated by a flow visualization technique. The jet was excited by axial and azimuthal perturbations to stabilize and enhance the large-scale axisymmetric and streamwise vortices. A laser fluorescent dye and a laser light sheet were used to visualize the vortical structure, and the whole view of the structure was captured by applying the Taylor hypothesis to the cross-sectional images and by scanning a laser light sheet in the streamwise direction. The visualized images reveal the details of the complicated structure of axisymmetric and streamwise vortices, and it is confirmed that the streamwise vortices have fundamental effect on the entrainment of ambient fluid. From the images, the length of jet boundary was calculated to estimate the mixing effect. The result suggests that the jet mixing is significantly increased by the break-down of the vortices enhanced by axial and azimuthal perturbations. We also discussed the jet diffusion effect in consideration of the jet widths obtained by velocity measurement. The result indicates that the vortical structure including streamwise vortices plays an important role to enhance diffusion.     

Active control of coaxial jet mixing with manipulation of primary vortical structures by arrayed micro flap actuators

Active mixing control of a methane/air isothermal coaxial jet was achieved using micro magnetic flap actuators arranged on the inner surface of the outer annular nozzle. The spatio-temporal evolution of vortical structures and the scalar mixing were studied through the particle image velocimetry and planar laser-induced fluorescence methods. In contrast to studies on jet control using acoustic forcing, the mechanical movement of the flap directly generated large-scale intense vortices. The mixing was enhanced significantly by the vortices formed in the inner shear layer, although the control input was given to the outer shear layer. It was found that the primary vortex rings dominated the near-field mixing, while streamwise vortices were responsible for the downstream mixing. It was also demonstrated that the radial range of the inner fuel transportation could be manipulated flexibly by adjusting the shedding interval of the vortices. Especially, the mixing was enhanced most significantly when the primary vortices were most densely populated near the nozzle exit at the control Strouhal number of unity.     

Existence of mach cones and helical vortical structures around the underexpanded circular jet in the helical oscillation mode

Existence of Mach cone and helical vortical structure in the helical oscillation mode of an underexpanded circular jet was confirmed by using schlieren instantaneous photographs and drawing of the envelopes of the Mach cones by the superposition of spherical sound waves radiated from two moving sound sources about the jet axis at a supersonic speed. Existence of such structures was conjectured in our earlier paper [Umeda and Ishii, J. Acoust. Soc. Am. 110, 1845-1858 (2001)]. The envelopes of a Mach cone are observed as a V-shaped pattern composed of a pair of clear fine lines starting from a prominent point, which rotates about the jet axis. The helical vortical structure is observed as a bright pattern of the gathering of the tiny specks around the jet. It always appears to overlap on the envelopes of the moving Mach cones.     

Numerical visualization of vortical structures in a spatially evolving turbulent compressible mixing layer

Large-eddy simulations are performed to numerically visualize the generation of streamwise vortical structures and its interaction with spanwisely rolled-up coherent vortical structure during the spatial development of a turbulent supersonic/subsonic mixing layer at convective Mach numberM _c =0.51. Time-dependent three-dimensional compressible conservation equations were solved with a subgrid-scale turbulence model. The numerical code used the finite-volume technique, which adopted alternately in temporal discretization the second-order, explicit MacCormack’s and modified Godunov’s schemes. Both transverse and spanwise perturbations were imposed initially for promoting the formation of spanwise rollers and counter-rotating streamwise vortices, respectively. Numerical visualizations are presented in terms of time-sequence isopressure surfaces and vorticity contours of spanwise and streamwise components. The results show that the spatial growth of three-dimensional vortical structures, in particular, the formation of chain-link-fence type structures, is adequately captured by the present computations. Vorticity dynamics is further analyzed, for the first time, to identify the dominant roles played by the convection effect followed by the vortex stretching effect on affecting the evolution of streamwise and spanwise vortical structures, respectively, forM _c <0.6.     

Visualization of multi-scale turbulent structure in lobed mixing jet using wavelets

The two-dimensional orthogonal wavelet transform was applied to the LIF image of lobed mixing jet for identifying the multi-scale turbulent structures. The digital imaging slice photographs atz /D=1.0 and 1.5 withRe=3000 were respectively decomposed into seven image components with different broad scales. These image components provided the visualized information on the multi-scale structures in a lobed mixing turbulent jet. The cores and edges of the vortices and the coherent structures at different resolutions or scales can be easily extracted. It was found that the scale range of the coherent structure becomes narrow along the downstream direction. The size of the intermediate- and small-scale structures does not vary significantly with downstream distance.     

Visualization of jet mixing in a fluidic oscillator

The fluidic oscillator is a device that generates an oscillating jet when supplied with fluid at pressure. The oscillator has no moving parts — the creation of the unsteady jet is based solely on fluid-dynamic interactions. Fluidic oscillators can operate at frequencies ranging up to 20 kHz, and are useful for flow control applications. The fluidic oscillator evaluated in the current study is comprised of two fluid jets that interact in an internal mixing chamber, producing the oscillating jet at the exit. Both porous pressure-sensitive paint (PSP) and dye-colored water flow are used to visualize the internal and external fluid dynamics of the oscillator. Porous PSP formulations have been shown to have frequency responses on the order of 100 kHz, which is more than adequate for visualizing the fluidic oscillations. In order to provide high-contrast PSP data in these tests, one of the internal jets of the fluidic oscillator is supplied with oxygen, and the other with nitrogen. Results indicate that two counter-rotating vortices within the mixing chamber drive the oscillations. It is also shown that the fluidic oscillator possesses excellent mixing characteristics.     

Chaotic mixing and transport in a meandering jet flow

Mixing and transport of passive particles are studied in a simple kinematic model of a meandering jet flow motivated by the problem of lateral mixing and transport in the Gulf Stream. We briefly discuss a model stream function, Hamiltonian advection equations, stationary points, and bifurcations. The phase portrait of the chosen model flow in the moving reference frame consists of a central eastward jet, chains of northern and southern circulations, and peripheral westward currents. Under a periodic perturbation of the meander's amplitude, the topology of the phase space is complicated by the presence of chaotic layers and chains of oscillatory and ballistic islands with sticky boundaries immersed into a stochastic sea. Typical chaotic trajectories of advected particles are shown to demonstrate a complicated behavior with long flights in both the directions of motion intermittent with trapping in the circulation cells being stuck to the boundaries of vortex cores and resonant islands. Transport is asymmetric in the sense that mixing between the circulations and the peripheral currents is, in general, different from mixing between the circulations and the jet. The transport properties are characterized by probability distribution functions (PDFs) of durations and lengths of flights. Both the PDFs exhibit at their tails power-law decay with different values of exponents.     

低马赫数射流混合层中的相干声源

通过二维射流混合层中相干时间尺度的测量,对声激励不稳定波增长过程及破碎点后的相干特性进行研究。实验结果表明,相干时间尺度在基波和第一亚谐波增长区较大,在破碎点后突然下降并在下游呈逐渐减小趋势。增加声激励强度可使不稳定波增长区的相干时间尺度增大,并对下游的流动结构有明显影响。     

Three-dimensional vibration analysis of a torus with circular cross section

The free vibration characteristics of a torus with a circular cross section are studied by using the three-dimensional, small-strain, elasticity theory. A set of three-dimensional orthogonal coordinates system, comprising the polar coordinate (r, theta) at each circular cross section and the circumferential coordinate phi around the ring, is developed. Each of the displacement components u(r), v(theta), and w(phi) in the r, theta, and phi directions, respectively, is taken as a product of the Chebyshev polynomials in the r direction and the trigonometric functions in the theta and phi directions. Eigenfrequencies and vibration mode shapes have been obtained via a three-dimensional displacement-based extremum energy principle. Upper bound convergence of the first seven eigenfrequencies accurate to at least six significant figures is obtained by using only a few terms of the admissible functions. The eigenfrequency responses due to variation of the ratio of the radius of the ring centroidal axis to the cross-sectional radius are investigated in detail. Very accurate eigenfrequencies and deformed mode shapes of the three-dimensional vibration are presented. All major modes such as flexural thickness-shear modes, in-plane stretching modes, and torsional modes are included in the analysis. The results may serve as a benchmark reference for validating other computational techniques for the problem.     

Analysis of a turbulent jet mixing flow by using a PIV-PLIF combined system

In the present paper, the development of a high-resolution PIV-PLIF combined system for the simultaneous measurements of velocity and passive scalar concentration fields is described. The high-resolution PIV-PLIF combined system is used to perform the simultaneous whole-field measurements of velocity and concentration in the near field of a turbulent jet mixing flow. The characteristics of the mass transfer process and momentum transfer process in the near field of the jet mixing flows are discussed in the terms of ensemble-averaged velocity and concentration, turbulent intensity, concentration standard deviation and the correlation terms between the fluctuating velocities and concentration.     

Neutrino mixing and see-saw mechanism

Models of neutrino masses are discussed capable of explaining in a natural way the maximal mixing between ν_μ and ν_τ observed by the Super-Kamiokande Collaboration. For three generations of leptons two classes of such models are found implying: a) Δm₂₃²Δm₁₂²≈Δm₁₃² and a small mixing between ν_e and the other two neutrinos, b) Δm₁₂²Δm₁₃²≈Δm₂₃² and a nearly maximal mixing for solar neutrino oscillations in vacuum.     

Femtosecond nonresonant degenerate four-wave mixing at atmospheric pressure and in a free jet

2 , N₂, and CO₂. For CO₂ additional experiments have been performed at reduced pressure and in a molecular beam. By delaying the probe pulse a periodic recovery of the DFWM signal is observed. The period of these transients can be assigned unambiguously to rotational Raman transitions of the ground state within the laser bandwidth. The decay of the transients yields the collisional dephasing of the Raman-induced polarization. At zero delay also optical-field-induced birefringence of electronic nature contributes to the signal. The different time scales of the Raman and electronic effects allow us to estimate their relative strength. Received: 3 August 1998 / Revised version: 21 October 1998 / Published online: 24 February 1999     

Targeted heat transfer augmentation in circular tubes using a corona jet

Natural convection heat transfer can be noticeably enhanced by corona wind in tubes and channels. A corona-induced secondary flow may be generated in tubes with no major changes in the geometry, or causing any noise or vibration. In this investigation, it is shown that the eccentric configuration of a wire electrode inside a tube forms a local jet along the eccentricity direction, which impinges on the tube wall and improves the local heat transfer. Since the direction of the corona jet is determined by the eccentricity direction of the electrode, the jet may be oriented properly to target the desired spots.     

Large eddy simulations of a circular orifice jet with and without a cross-sectional exit plate

The effect of a cross-sectional exit plane on the downstream mixing characteristics of a circular turbulent jet is in- vestigated using large eddy simulation （LES）. The turbulent jet is issued from an orifice-type nozzle at an exit Reynolds number of 5 ×104. Both instantaneous and statistical velocity fields of the jet are provided. Results show that the rates of the mean velocity decay and jet spread are both higher in the case with the exit plate than without it. The existence of the plate is found to increase the downstream entrainment rate by about 10% on average over the axial range of 8-30de （exit diameter）. Also, the presence of the plate enables the formation of vortex rings to occur further downstream by 0.5-1 .Ode. A physical insight into the near-field jet is provided to explain the importance of the boundary conditions in the evolution of a turbulent jet. In addition, a method of using the decay of the centreline velocity and the half-width of the jet to calculate the entrainment rate is proposed.     

Three-dimensional numerical simulations of cellular jet diffusion flames

Recent experimental investigations have demonstrated that the appearance of particular cellular states in circular non-premixed jet flames significantly depends on a number of parameters, including the initial mixture strength, reactant Lewis numbers, and proximity to the extinction limit (Damköhler number). For CO₂-diluted H₂/O₂ jet diffusion flames, these studies have shown that a variety of different cellular patterns or states can form. For given fuel and oxidizer compositions, several preferred states were found to co-exist, and the particular state realized was determined by the initial conditions. To elucidate the dynamics of cellular instabilities, circular non-premixed jet flames are modeled with a combination of three-dimensional numerical simulation and linear stability analysis (LSA). In both formulations, chemistry is described by a single-step, finite-rate reaction, and different reactant Lewis numbers and molecular weights are specified. The three-dimensional numerical simulations show that different cellular flames can be obtained close to extinction and that different states co-exist for the same parameter values. Similar to the experiments, the behavior of the cell structures is sensitive to (numerical) noise. During the transient blow-off process, the flame undergoes transitions to structures with different number of cells, while the flame edge close to the nozzle oscillates in the streamwise direction. For conditions similar to the experiments discussed, the LSA results reveal various cellular instabilities, typically with azimuthal wavenumber m = 1–6. Consistent with previous theoretical work, the propensity for the cellular instabilities is shown to increase with decreasing reactant Lewis number and Damköhler number.     

Experimental study of corona jet produced from a circular tube fitted with a nozzle

The flow characteristics of a corona jet, which is produced from a single needle electrode positioned at the centerline of a circular tube fitted with a grounded stainless-steel nozzle at one end of the tube, is experimentally evaluated. Six nozzles with two diameter ratios and three taper angles are evaluated for their effectiveness in accelerating the jet produced by corona discharge with positive polarity. To determine the maximum jet velocity and volume flow rate, experiments have been conducted at a voltage ranging from corona onset (5 kV) to sparkover (approximately 12.5 kV) at an increment of 2.5 kV. The results show that the jet velocity increases with the applied voltage. The maximum velocity occurs at the center line but its value decreases as the jet expands downstream. In addition, the results show that a nozzle with a smaller diameter ratio does not always perform the best in accelerating the flow or producing the maximum volume flow rate. The nozzle's taper angle further accentuates the result produced by the diameter ratio. The implications from the present results for actual applications are provided.     

Three-dimensional potential of a homogeneous circular torus in terms of equigravitating elements

The complete system of the simplest equigravitating elements presented for a homogeneous circular torus comprises a compound one-dimensional rod consisting of three links with a purely imaginary density, as well as two real point masses located at the ends of the rod. In terms of these elements, the three-dimensional gravitational potential of the torus at an arbitrary point is derived. The surface of the potential is constructed, and the family of equipotential curves is found.