Control of Coaxial Jets by an Azimuthal Excitation: Vortex Dynamic and Mixing Properties (original) (raw)
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Mixing enhancement in coaxial jets through inflow forcing: A numerical study
Physics of Fluids, 2007
Direct numerical simulations are performed to analyze the flow dynamics and the mixing properties of natural (unforced) and excited coaxial jets at moderate Reynolds number. First, the study of the natural coaxial jet, with species injected in the outer jet alone, allows us to understand the role of the coherent vortices on the mixing process during the transition stage. It is observed that the global flow behavior is controlled by the dynamics of the outer shear layer during the transition. The streamwise vortices are shown to play a significant role in the mixing process since they initiate intense ejections from the seeding regions. Spots of pure (unmixed) species from the outer jet are seen to persist far downstream. Two different types of inflow forcing are then considered based on the information provided by the natural coaxial jet: first, a purely axisymmetric excitation and second, combined axisymmetric and azimuthal excitations all of moderate amplitude. These excitations a...
Passive control of mixing in a coaxial jet
2008
An experimental investigation regarding interacting shear layers in a coaxial jet geometry has been performed. The present paper confirms experimentally the theoretical result by Talamelli and Gavarini (2006), who proposed that the wake behind the separation wall between the two stream of a coaxial jet creates the condition for an absolute instability. This instability, by means of the induced vortex shedding, may provide a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers has been demonstrated.
Control of shear perturbation in coaxial swirling turbulent jets
Aerospace Science and Technology, 2010
The present study explores the swirl jet characteristics and the possibility of using artificial means for control of shear layers with the application as swirl jet mixing. A subsonic open jet facility and a mechanical perturbation device with straight-lobes are uniquely designed and fabricated. Furthermore, the numerical analysis is performed to investigate the helical-lobed perturbation effects for relatively high speed jets. It is shown that the overall response of the swirling jet to excitation is strongly dependent on the number of lobes. The excitation from both straight-and helical-lobes affects the flow structures in the vortex core and the shear layer around the jet periphery simultaneously. Especially, the preferred helicity shows the maximum dispersion and the wake reduction at the vortex core. In addition, from the particle trace results, the injected flow spreads out in radial direction with wide angle than any other cases, and a relatively small amount of flow stays along the jet centerline at the preferred helicity. These reveal the mixing phenomena between the inner and outer jets as well as between the jet and the surrounding media, which are not presented in their counterparts. Also, the perturbed swirl jet experiences the fast thermal decay along the jet centerline and the wide diffusion in radial direction at the same time.
Mixing and coherent vortices in turbulent coaxial jets
Comptes Rendus Mécanique, 2005
Direct numerical simulations associated with mixing in constant-density round coaxial jets are performed. They are validated by comparison against laboratory experiments. The mixing process is studied by seeding a passive tracer first in the outer annular jet, then in the inner jet. We demonstrate the important role played by coherent vortices in the mixing mechanisms. The turbulent mixing exhibits an intermittent character as a consequence of fluid ejections caused by the counter-rotating streamwise vortices. We quantify also the domination of the outer jet and show that the fluid issuing from the central jet remains confined.
Journal of Turbulence, 2007
Direct numerical simulations (DNS) are performed to investigate mixing in free round coaxial jets. A great attention has been put on the influence of upstream conditions upon the global flow structure and the mixing process. The mixing behavior is studied through the spatial and temporal development of the mixture fraction of the annular and the inner fluids, and examined by means of flow visualization and statistics. It is shown that the turbulent mixing process and the mixture fraction field in coaxial jets depend on the upstream conditions, even though a quasi self-similar state is reached. The mixing alterations are explained by the understanding of the flow dynamics modifications implied by the different upstream conditions. These alterations are mainly due to the intense generation of streamwise vortices, favored by high inlet velocity gradients and velocity ratios, as well as low ratios between the inner and the outer jet diameters. This is associated with a high quality of mixing, as far as global mixedness is concerned. It is also shown that the annular fluid reaches the inner fluid and mixes swiftly into it. Conversely, the latter remains confined. Additionally, spots of pure unmixed species are observed at the end of the computational domain, and shown to be due to the annular jet.
Journal of Fluid Mechanics, 2000
The stirring and mixing properties of one-phase coaxial jets, with large outer (annular) to inner velocity ratio r u = u 2 /u 1 are investigated. Mixing is contemplated according to its geometrical, statistical and spectral facets with particular attention paid to determining the relevant timescales of the evolution of, for example, the interface area generation between the streams, the emergence of its scale-dependent (fractal) properties and of the mixture composition after the mixing transition. The two key quantities are the vorticity thickness of the outer, fast stream velocity profile which determines the primary shear instability wavelength and the initial size of the lamellar structures peeled-off from the slow jet, and the elongation rate γ = (u 2 − u 1 )/e constructed with the velocity difference between the streams and the gap thickness e of the annular jet. The kinetics of evolution of the interface corrugations, and the rate at which the mixture evolves from the initial segregation towards uniformity is prescribed by γ −1 . The mixing time t s , that is the time needed to bring the initial scalar lamellae down to a transverse size where molecular diffusion becomes effective, and the corresponding dissipation scale s(t s ) are
Mixing in Turbulent Coaxial Jet
Journal of Mechanical Engineering Research and Developments, 2019
This paper studies the mixing process in a compressible turbulent coaxial jet, using a Large Eddy Simulation (LES) type approach. Two cases were considered: Case 1, a passive scalar injected into the annular jet and Case 2, a passive scalar injected into the central jet. This study is based was articulated on the implementation of a modern and efficient a high-performance and modern numerical method to meet both the results accuracy and calculation cost requirements. A new solver for the linearization of the equation's characteristic of the approximate Riemann problem was used to calculate inviscid fluxes. The mean and instantaneous mixture fraction were traced and analysed. Then, using the probability density function, the mixture quality was studied. Results The results of this study provided useful information. In particular, active mixing regions were associated with high concentrations of Root Mean Square (RMS) values. Coherent vortices play a very important role in the mixing process, and the efficiency of the mixture is maintained when the passive scalar is injected into the central jet. All results obtained are in agreement with similar experimental results of the simulation jets in this study.
Recirculation Patterns in the Initial Region of Coaxial Jets
Journal of Applied Mechanics, 1972
This experimental study is concerned with the mixing in the initial region of turbulent coaxial streams. The investigation is limited to the low-speed incompressible case with the inner stream of lower velocity than the outer stream. Velocity ratios ranged from infinity to one. The inner to outer stream density ratio was either 4 or 1. Experimental measurements were made with a hot-wire anemometer system, static pressure probes, and shadowgraphy. The data show that a backflow region is formed in this particular apparatus at outer to inner stream velocity ratio of 13 for the homogeneous case and at outer to inner stream velocity ratio of 26 for the heterogeneous case. For higher velocity ratio, a circulating toroidal vortex is established which enhances mixing between the inner and outer streams. This vortex pattern exhibits similarity of velocity with velocity ratio variations.
Effects of controlled vortex generation and interactions in transverse jets
Physical Review Fluids
This experimental study examined the effects of controlled vortex generation and interactions created by axisymmetric excitation of a transverse jet, with a focus on the structural and mixing characteristics of the flow. The excitation consisted of a double-pulse forcing waveform applied to the jet, where two distinct temporal square-wave pulses were prescribed during a single forcing period. The two distinct pulses produced vortex rings of different strength and celerity, the strategic selection of which promoted vortex ring interactions or collisions in the near field to varying degrees. Jet flow conditions corresponding to a transitionally convectively and absolutely unstable upstream shear layer (USL) in the absence of forcing, at a jet-to-cross-flow momentum flux ratio of J = 10, and to an absolutely unstable USL at J = 7, were explored for a jet Reynolds number of 1800. Acetone planar laser-induced fluorescence imaging was utilized to quantify the influence of different prescribed temporal waveforms. All forcing conditions enhanced the spread, penetration, and molecular mixing of the jet as compared to the unforced jet, though to differing degrees. Interestingly, when the jet was convectively unstable, forcing which promoted vortex collisions provided the greatest enhancement in molecular mixing, whereas the absolutely unstable jet produced the greatest enhancement in mixing when the vortex rings did not interact, with important implications for optimized jet control.
2005
We carry out direct numerical simulation (DNS) of scalar transport and mixing in a coaxial jet issued into a small model combustor. Analysis is made on the detailed mechanism of mixing enhancement achieved by an active control of the nearfield large-scale vortical structures. The main interest lies in the relationship between these vortical structures and associated scalar transport processes in the near field downstream of the nozzle exit. From the phase-averaged statistics, the mechanism of mixing enhancement is revealed in the case of the best mixing achieved.