Computational modeling of turbulent mixing in a jet in crossflow

The jet in crossflow is a configuration of highest theoretical and practical importance, in which the turbulent mixing plays a major role. High-resolution measurements using Particle Image Velocimetry combined with Laser Induced Fluorescence have been conducted and used to validate simulations ranging from simple steady-state Reynolds-averaged Navier Stokes to sophisticated large-eddy simulation. The reasons for the erratic behavior of steady-state simulations in the given case, in which large-scale structures dominate the turbulent mixing, have been discussed. The analysis of intermittency proved to be an appropriate framework to account for the influence of these flow structures on the jet in crossflow, contributing to the explanation of the poor performance of the steady-state simulations.     

Turbulent fuel-air mixing study of jet in crossflow at different velocity ratios using LES

In order to study the mixing mechanism of fuel and air in gas turbine, large eddy simulation has been used to investigate the methane jet-in-crossflow with the velocity ratio (R) of 1.5 and 4. This study aims to explore the formation mechanism of vortices such as the hairpin vortices, hovering vortices and horseshoe vortices, the relationship between the fuel–air mixing and flow characteristics at different velocity ratios. The numerical methods in the present work are firstly validated with the experimental data in terms of mean and root mean square values of velocity. For R = 4, the shear layer vortices, horseshoe vortices, counter-rotating vortices pairs (CVP) and wake vortices can be observed, while the jet shear layer cannot be observed for R = 1.5. The hairpin vortices originating from the vortice-ring are lifted and shed from the downstream of the jet-outlet due to Kutta-Joukowski lift. The hairpin vortices are similar to CVP. The horseshoe vortices in R = 1.5 and 4 are formed due to the blockage of the jet (CH₄) and the crossflow (air) respectively, and its evolution is associated with the hovering vortices which only exist for R = 1.5. The uniform index and pr-obability density function are used for quantitative analysis of the mixing performance. The uniform index at X/D = 0 (fuel-inlet) and at X/D = 25 (outlet) are 0.033 and 0.335 for R = 1.5 and 0.130 and 0.047 for R = 4. For R = 4, the jet penetration is higher and the deflection angle of jet is smaller than that in case of R = 1.5. Higher R will provide more region for mixing, therefore uniform index is higher and the mixing is more uniform in the downstream.     

Self-sustained global oscillations in a jet in crossflow

A jet in crossflow with an inflow ratio of 3, based on the maximum velocity of the parabolic jet profile, is studied numerically. The jet is modeled as an inhomogeneous boundary condition at the crossflow wall. We find two fundamental frequencies, pertaining to self-sustained oscillations in the flow, using full nonlinear direct numerical simulation (DNS) as well as a modal decomposition into global linear eigenmodes and proper orthogonal decomposition (POD) modes; a high frequency which is characteristic for the shear-layer vortices and the upright vortices in the jet wake, and a low frequency which is dominant in the region downstream of the jet orifice. Both frequencies can be related to a region of reversed flow downstream of the jet orifice. This region is observed to oscillate predominantly in the wall-normal direction with the high frequency, and in the spanwise direction with the low frequency. Moreover, the steady-state solution of the governing Navier?CStokes equations clearly shows the horseshoe vortices and the corresponding wall vortices further downstream, and the emergence of a distinct counter-rotating vortex pair high in the free stream. It is thus found that neither the inclusion of the jet pipe nor unsteadiness is necessary to generate the characteristic counter-rotating vortex pair.     

Effects of dual jets distance on mixing characteristics and flow path within a cavity in supersonic crossflow

A rectangular open cavity with upstream dual injectors at a freestream Mach number of 1.9 was investigated experimentally. To evaluate the effect of the distance between the jets, the flow characteristics were investigated using the high-speed schlieren photography, particle image velocimetry, and surface oil flow techniques. The dual jet distances of 18 and 54 mm were used. Unstable flow occurs over the cavity in all cases and is not improved by changing the distance between the dual jets. Although the distance between the dual jets does not influence the flow stability, the flow field varies decidedly depending on the dual jets distance. The enhancement of air mixing depends on the distance between the jets. A long dual jets distance was found to yield better mixing characteristics within the cavity than a short one. When the jets are further apart, the mainstream between two counter-rotating vortex pairs behind the jets flows strongly into the cavity because of the increased blow-down occurring between the vortex pairs. Additionally, a counterflow with a low velocity magnitude occurs behind the jets. Hence, mixing is enhanced within the cavity by effects of the opposed flow. When the jet pairs are closer to each other, the counter-rotating vortex pairs are in contact; as a result, the blow-down effect does not occur between them. The flow drawn into the cavity from the mainstream is supplied from the sides of the test section into the cavity.     

Some observations of a large, burning jet in crossflow

Some results are presented on the temporal evolution of a large jet in crossflow consisting of the burning plume from an unabated oil well discharge. The volume rendering approach is used whereby several sequential x-y images of the jet are stacked in time, t, to produce an object in x-y-t space which can be used to conveniently examine the jet dynamics. Similar to earlier findings for free jets, the burning jet in crossflow is seen to consist of a series of large-scale organized structures which convect downstream leading to a quasiperiodic flame tip burnout. The wake region however is seen to be much less organized. Most surprising is the constant speed of the burning structures as they progress from the jet to crossflow direction. It therefore appears that an underlying organization exists in the jet in crossflow, in spite of its relatively complex, three-dimensional structure.     

Flow characteristics of slot jet impingement on a wedge

An experimental investigation is made to study the flow characteristics of slot jet impingement on a wedge whose included angle is 90 degrees. The aim of this investigation is to study the characteristics of the flow field near the wedge surface for various parameters. The different parameters like, jet velocity, slot width, distance of wedge vertex from the jet exit and the inclination of the wedge to jet axis are systematically varied to see their effect on the flow field. The flow field near the wedge vertex is similar to stagnation point flow. Far away from the vertex, the flow field is like that of wall jet. Near the vertex, very large variations of static pressure are observed in streamwise and transverse directions. This is due to large streamwise curvature and stagnation of flow. The transverse pressure gradient slowly decays in the streamwise direction, as a result, the velocity profiles are different from the similarity profiles of stagnation point flow and wall jet in the respective regions. Experiments are conducted for slot widths of 10 mm, 15 mm and 30 mm each for the distance between slot and wedge vertex of 80 mm, 120 mm and 240 mm. The static pressure and velocity profiles are measured by calibrated disk type static pressure probe and pitot tube respectively at various streamwise locations.Es wurde eine experimentelle Studie über die Strömmungscharakteristiken eines auf einem rechtwinkligen Keil auftretenden Spaltstrahls durchgeführt. Das Ziel dieser Untersuchungen ist die Studie des Strömungsverhaltens in Keilnähe, in Abhängigkeit verschiedener Parameter wie Strahlgeschwindigkeit, Spaltbreite, Entfernung der Keilspitze und Winkel zwischen Strahlachse und Keil. Diese Parameter werden systematisch variiert um deren Einfluß auf das Strömungsverhalten zu bestimmen. Die Strömung an der Keilspitze ist ähnlich der Staupunktströmung. Weit hinter der Spitze gleicht das Strömungsfeld dem eines auf einer Wand auftreffenden Strahls. In Nähe der Keilspitze sind die Änderungen des statischen Drucks in Strömungsrichtung und quer zur Strömung groß. Grund dafür sind die starke Krümmung der Strömung und die Stagnation der Strömung. Der Druckgradient in Querrichtung nimmt langsam in Strömungsrichtung ab, daher unterscheiden sich die Geschwindigkeitsprofile von den Ähnlichkeitsprofilen der Staupunktströmung und des Wandstrahls in den jeweiligen Bereichen. Die Versuche wurden für Spaltbreiten von 10, 15 und 30 mm und Keilentfernungen von 80, 120 und 240 mm durchgeführt. Der statische Druck und die Geschwindigkeitsprofile wurden mit kalibrierten scheibenförmigen Drucksonden bzw. einer Pitot-Sonde an verschiedenen Orten gemessen.     

On coherent structures and mixing characteristics in the near field of a rotating-pipe jet

Mixing characteristics and coherent structures populating the near-nozzle area of a rotating-pipe jet at the Reynolds number of 5300 were studied by Large-eddy simulation (LES). The swirl rate, defined as the ratio of the tangential velocity of the inner pipe wall to the bulk axial velocity, varied from 0 to 1, corresponding to a weak-to-moderate swirl intensity, insufficient to induce reverse flow near the nozzle. The visualization shows that for the non-swirling jet the near-wall streaky structures generated in the pipe interact with the shear layer, evolving into hairpin-like structures that become tilted at low rotation rates. For higher swirl, they cannot be recognized as they are destroyed at the nozzle exit. No large-scale coherent structures akin to Kelvin–Helmholtz vortical rings in the ‘top-hat’ jets are identifiable close to the nozzle. Using the single and joint probability density functions of velocity and passive scalar (temperature) fields we quantify the events responsible for the intensive entrainment at various swirl numbers. The isosurface of the temperature field indicates the meandering and precessing motion of the rotating jet core at the axial distance of 6D downstream, where D is the diameter of the pipe. The Fourier analysis with respect to the azimuthal angle and time reveals an interplay between the co- and counter-rotating modes. These findings explain the previously detected but not fully clarified phenomenon of the weakly counter-rotating jet core at low swirl rates.     

Analysis and reconstruction of a pulsed jet in crossflow by multi-plane snapshot POD

In this work, snapshot proper orthogonal decomposition (POD) is used to study a pulsed jet in crossflow where the velocity fields are extracted from stereoscopic particle image velocimetry (SPIV) results. The studied pulsed jet is characterized by a frequency f = 1 Hz, a Reynolds number Re _j  = 500 (based on the mean jet velocity ${\overline{U}_{j}}$  = 1.67 cm/s and a mean velocity ratio of R = 1). Pulsed jet and continuous jet are compared via mean velocity field trajectory and Q criterion. POD results of instantaneous, phase-averaged and fluctuating velocity fields are presented and compared in this paper. Snapshot POD applied on one plane allows us to distinguish an organization of the first spatial eigenmodes. A distinction between “natural modes” and “pulsed modes” is achieved with the results obtained by the pulsed and unforced jet. Secondly, the correlation tensor is established with four parallel planes (multi-plane snapshot POD) for the evaluation of volume spatial modes. These resulting modes are interpolated and the volume velocity field is reconstructed with a minimal number of modes for all the times of the pulsation period. These reconstructions are compared to orthogonal measurements to the transverse jet in order to validate the obtained three-dimensional velocity fields. Finally, this POD approach for the 3D flow field reconstruction from experimental data issued from planes parallel to the flow seems capable to extract relevant information from a complex three-dimensional flow and can be an alternative to tomo-PIV for large volume of measurement.     

Turbulent transport in an inclined jet in crossflow

The present study experimentally investigates a turbulent jet in crossflow relevant to film cooling applications. The jet is inclined at 30°, and its mean velocity is the same as the crossflow. Magnetic resonance imaging is used to obtain the full three-dimensional velocity and concentration fields, whereas Reynolds stresses are obtained along selected planes by Particle Image Velocimetry. The critical role of the counter-rotating vortex pair in the mixing process is apparent from both velocity and concentration fields. The jet entrainment is not significantly higher than in an axisymmetric jet without crossflow, because the proximity of the wall inhibits the turbulent transport. Reynolds shear stresses correlate with velocity and concentration gradients, consistent with the fundamental assumptions of simple turbulence models. However the eddy viscosity is strongly anisotropic and non-homogeneous, being especially low along the leeward side of the jet close to injection. Turbulent diffusion acts to decouple mean velocity and concentration fields, as demonstrated by the drop in concentration flux within the streamtube issued from the hole. Volume-averaged turbulent diffusivity is calculated using a mass–flux balance across the streamtube emanating from the jet hole, and it is found to vary slowly in the streamwise direction. The data are compared with Reynolds-Averaged Navier–Stokes simulations with standard k − ε closure and an optimal turbulent Schmidt number. The computations underestimate the strength of the counter-rotating vortex pair, due to an overestimated eddy viscosity. On the other hand the entrainment is increasingly underpredicted downstream of injection. To capture the correct macroscopic trends, eddy viscosity and eddy diffusivity should vary spatially in different ways. Therefore a constant turbulent Schmidt number formulation is inadequate for this flow.     

Vortical structures behind a transverse jet in a supersonic flow at high jet to crossflow pressure ratios

A three-dimensional supersonic turbulent flow with symmetric normal injection of circular jets from the channel walls is numerically simulated. The initial Favre-averaged Navier–Stokes equations closed by the k–ω turbulence model are solved by an algorithm based on an ENO scheme. The mechanism of the formation of vortical structures due to the interaction of the jet with the free stream is studied for jet to crossflow total pressure ratios ranging from 3 to 50. It is known from experiments reported in the literature that, for n ? 10, mixing of the jet with the high-velocity flow leads to the formation of a pair of vortices and of an additional separation zone near the wall behind the jet. It is demonstrated that the present numerical results are consistent with such findings and that the pressure distribution on the wall ahead of the jet in the plane of symmetry is also in reasonable agreement with available experimental data.     

Influence of mixing layer thickness on the stability characteristics of a supersonic jet

The stability of a two-dimensional supersonic isothermal flow with a mixing layer is analyzed. Asymptotic expressions describing the stability characteristics of a thin mixing layer are derived. It is demonstrated that when the finite thickness of the layer is taken into account, the waves, otherwise neutrally stable, can show evidence of instability. The situations in which the natural wavenumbers merge with one another and with branch points of the dispersion relation are separately considered. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 20–28, March–April, 1998. The study was carried out with the support of the International Scientific and Technical Center (grant No. 200-95).     

Flow characteristics of gas-liquid-particle mixing in a gas-stirred ladle system with throughflow

An experimental study is performed on air-liquid-particle mixing, resulting from an air-particle mixture injected into a liquid flowing through a slender ladle. Flow visualization combined with image processing is employed to investigate the bubble and particle behavior at the nozzle outlet. Effort is directed to particle discrimination in both the liquid and the bubbles to determine particle distribution, which affects the mixing performance of gas bubbles, solid particles and liquid. A real-time movement of bubble and particle behavior can be visualized by means of image processing with the use of a slow-motion video recording. It is disclosed that the particles injected through the nozzle may stick on the inner surface of the gas bubble, break through the bubble surface, or mingle with the gas stream to form a two-phase jet, depending on the particle-to-gas mass flow rate ratio. It is observed that when a solid-gas two-phase jet penetrates deeper in the horizontal direction, the particles and bubbles rise along the vertical sidewall and simultaneously spread in the transverse direction, thus promoting a better liquid-particle mixing. The application of the slow-motion video recording results in quantitative evaluations of both the penetration depth of particles or of gas-particles from the injection nozzle and the velocity distribution along the sidewall.List of symbols B Width of water vessel, m - B _n Nozzle location on bottom surface of water vessel, m - d _o Diameter of a gas-particle injection nozzle, m - H Height of water vessel, m - H _n Nozzle location on vertical surface of water vessel, m - L Penetration length of particles or of particles and gas from the nozzle, m - Q _g Volumetric flow rate of gas, m³/s - Q _l Volumetric flow rate of water, m³/s - Q _s Volumetric flow rate of particle, m³/s - Re _g Gas Reynolds number based on inner diameter of the air-particle injection nozzle - t Time, sec. - W Thickness of water vessel, m - x Transverse coordinate, m - y Longitudinal coordinate, m - Mass flow rate ratio of particles to gas Visiting scholar on leave from the Mechanical Engineering Department, Kagoshima University, Kagoshima, JapanThe work reported was supported by the National Science Foundation under the Grant No. CTS-8921584     

Consistency issues of Lagrangian particle tracking applied to a spray jet in crossflow

Numerical simulations are performed for multiphase jets in crossflow. The flow solver uses an Eulerian/Lagrangian approach. Turbulence in the gas phase is modeled in the framework of large eddy simulation. The dispersed phase is handled using Lagrangian particle tracking. The model assumptions of solvers for Lagrangian particle tracking are critically assessed for typical flow conditions of spray jets in crossflow. The droplets are assumed to be spherical and isolated. It is shown that several model assumptions are apparently inconsistent in larger portions of the flow field. Firstly, average Weber numbers can be so large that the model assumption to regard droplets as spherical is questionable, not only near the nozzle, but also in the far-field. Secondly, the average droplet spacing can be so low that droplets directly interact with each other, again also in the far-field. Thirdly, the average Stokes numbers in the jet region can be so large that the phase coupling between the dispersed and continuous phase is weak. Some remedies to these deficiencies are proposed.     

Experimental study on turbulent mixing of spray droplets in crossflow

Centrifugal spray injected at various angles in gas crossflow has been studied experimentally using PIV visualization system and image-processing techniques. Experiments were carried out inside a rectangular duct (95 mm × 95 mm in cross-section) at ambient temperature and pressure, with different gas Reynolds numbers (vary from 12,900 to 45,000) and three injection angles (60°, 90° and 120°). The spray angle of the centrifugal nozzle is 80°, with D₃₂ of 80 μm. The instantaneous images of droplets distribution and the values of the line-averaged D₃₂ at different positions on the cross-sections along the flow field for each condition were obtained, and their flow field configurations were achieved. Quantitative assessments of mixing degree between two phases for different injection angles were determined using a spatial unmixedness parameter. It is found that the addition of droplets into the gas crossflow enhanced the turbulence intensity of the gas crossflow and caused different-scale vortices. The flow field structure, to a great extent, is dependent on the injection angle. The entrainment and centrifugal force of large vortex lead to uneven droplet distribution and moreover influence the mixing of droplets and gas crossflow. A better mixing result can be obtained with the injection angle of nozzles of 60°.     

Turbulent mixing and diffusion combustion of a jet in a channel

An attempt is made to study in some detail the turbulent mixing of reacting (propane) and inert jets (air and carbon dioxide) in a channel. The results are given of an experimental investigation into diffusion combustion in a channel, and these are compared with calculated data obtained using a semiempirical theory of turbulence. Such a comparison makes it possible to estimate the applicability of this theory for calculating the characteristics of diffusion combustion in a channel.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 25–33, July–August, 1980.We thank V. R. Kuznsatsov and A. N. Sekundov for great interest in the work and for discussing the results and V. I. Rasshchupkin for assisting in the experiments.     

Flow patterns and mixing characteristics of gaseous fuel multiple injections in a non-reacting supersonic combustor

Using the particle-based laser scattering imaging technique, schlieren system and surface oil-flow visualization technique, the flow patterns and mixing characteristics of multiple injections with tandem multi-orifices and parallel multi-orifices in a supersonic vitiated air flow were investigated in this paper. All injectors have a declined angle of 30 degree to the freestream direction. The distance between the tandem orifices and that between the parallel orifices was varied. The experimental results showed that decreasing the distance between the tandem orifices will reduce the pressure and velocity of the stream upstream of the second jet, which results in the increase of the penetration height of the second injection and quick mixing of the whole field. For the small distance between the parallel multi-orifices, the bow shock waves upstream of the injected jets connect with each other and the air stream entered into the gap between the jets is not enough, resulting in the decrease of the mixing effect. Large distance between the parallel multi-orifices decreases the interaction between the injection jets. For the mixing enhancement, there should be a proper optimized distance between the parallel injection orifices.     

Effects of the jet-to-crossflow momentum ratio on a sonic jet into a supersonic crossflow

Numerical investigation of a transverse sonic jet injected into a supersonic crossflow was carried out using large-eddy simulation for a free-stream Mach number M = 1.6 and a Reynolds number Re = 1.38 × 10⁵ based on the jet diameter. Effects of the jet-to-crossflow momentum ratio on various fundamental mechanisms dictating the intricate flow phenomena, including flow structures, turbulent characters and frequency behaviors, have been studied. The complex flow structures and the relevant flow features are discussed to exhibit the evolution of shock structures, vortical structures and jet shear layers. The strength of the bow shock increases and the sizes of the barrel shock and Mach disk also increase with increasing momentum ratio. Turbulent characters are clarified to be closely related to the flow structures. The jet penetration increases with the increase of the momentum ratio. Moreover, the dominant frequencies of the flow structures are obtained using spectral analysis. The results obtained in this letter provide physical insight in understanding the mechanisms relevant to this complex flow.