Near-field development of large-scale vortical structures in a controlled confined coaxial jet (original) (raw)
Related papers
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.
Control of Coaxial Jets by an Azimuthal Excitation: Vortex Dynamic and Mixing Properties
Proceeding of Fifth International Symposium on Turbulence and Shear Flow Phenomena
The goal of this work is to improve the mixing properties of a coaxial jet with moderate Reynolds number by active control. Two direct numerical simulations of coaxial jets are performed. First, studying a "natural" (without deterministic control) coaxial jet, we show that the appearance of counter-rotating pairs of streamwise vortices allows ejections from the seeding regions. This initiates the turbulent mixing. However spots of unmixed fluids persist at the end of the computational domain. We use then deterministic perturbation to allow an improvement of the mixing properties of the jet. The deterministic perturbation has an azimuthal part which forces the appearance of pairs of streamwise vortices. Finally, we found a real improvement of the mixing properties with a good homogeneity at the end of the computational domain due to a quicker appearance of small scales.
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.
Coaxial Circular Jets—A Review
Fluids, 2021
This review article focuses on the near-field flow characteristics of coaxial circular jets that, despite their common usage in combustion processes, are still not well understood. In particular, changes in outer to inner jet velocity ratios, ru, absolute jet exit velocities and the nozzle dimensions and geometry have a profound effect on the near-field flow that is characterized by shear as well as wake instabilities. This review starts by presenting the set of equations governing the flow field and, in particular, the importance of the Reynolds stress distributions on the static pressure distribution is emphasized. Next, the literature that has led to the current stage of knowledge on coaxial jet flows is presented. Based on this literature review, several regions in the near-field (based on ru) are identified in which the inner mixing layer is either governed by shear or wake instabilities. The latter become dominant when ru≈1. For coaxial jets issued into a quiescent surrounding...
Flow regimes of large-velocity-ratio coaxial jets
Journal of Fluid Mechanics, 1997
An investigation of the near-field flow structure of coaxial jets with large outer to inner velocity ratio r u has been conducted. Since in all cases r u > 1, the outer jet dominates the near-field flow structure. Two flow regimes are identified depending on whether r u is larger or smaller than a critical value r uc . When r u < r uc , the fast annular jet periodically pinches the central, slow jet near the end of the inner potential cone. The pinching frequency corresponds to the outer-jet mode. The length of the inner potential cone is strongly dependent on r u and behaves like A/r u , where A depends weakly on the initial conditions. When r u > r uc , the inner potential cone is truncated and is followed by an unsteady recirculation bubble with low-frequency oscillation.
The role of streamwise vorticity in the near-field entrainment of round jets
Journal of Fluid Mechanics, 1992
The role of streamwise vortex structures in the near-field (xjd < 10) evolution of a round jet is examined. In free shear layers the streamwise vorticity develops into Bernal-Roshko structures which are streamwise vortex pairs. Similar structures are shown to exist in round jets. These structures, which evolve and amplify in the braid region between primary vortical structures, are shown to drastically alter the entrainment process in the near field and to increase the rate at which fluid is entrained into the jet. As the flow evolves downstream, the efficiency of the streamwise vorticity in entraining fluid increases relative to that of the azimuthal vorticity. Beyond the end of the potential core regime, the entrainment process is mainly controlled by streamwise vorticity. These processes are identified via flow visualization and confirmed by detailed global entrainment measurements.
The Vortex Dominated Flow Field Associated with a Confined Jet in a Crossflow
2006
This study focuses on the near field mixing of a confined circular jet that is being injected normally from a wall into a subsonic crossflow in a confined area. The streamwise, transverse and lateral mean velocities were obtained using a two-component Laser Doppler Anemometer. From these quantities the time average vorticity in three dimensions were estimated for locations between 1.5 and 4.5 jet diameters downstream of the jet exit. The jet to crossflow velocity ratio was kept constant at four. The results highlight a complex system of vortices within this flowfield that is different to that of a free jet in a crossflow. In addition the present study was able to quantify changes in the vortical structures as a function of downstream distance as well as provides evidence of what effect freestream turbulence has on the evolving vorticity.
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.
Large-eddy simulation of turbulent confined coannular jets
Journal of Fluid Mechanics, 1996
Large-eddy simulation (LES) was used to study mixing of turbulent, coannular jets discharging into a sudden expansion. This geometry resembles that of a coaxial jet-combustor, and the goal of the calculation was to gain some insight into the phenomena leading to lean blow-out (LBO) in such combustion devices. This is a first step in a series of calculations, where the focus is on the fluid dynamical aspects of the mixing process in the combustion chamber. The effects of swirl, chemical reactions and heat release were not taken into account. Mixing of fuel and oxidizer was studied by tracking a passive scalar introduced in the central jet. The dynamic subgrid-scale (DM) model was used to model both the subgrid-scale stresses and the subgrid-scale scalar flux. The Reynolds number was 38000, based on the bulk velocity and diameter of the combustion chamber. Mean velocities and Reynolds stresses are in good agreement with experimental data. Animated results clearly show that intermitten...