Lock-in and Sinusoidal Control of Mixing for Jets in Crossflow

2021

The jet in crossflow, or a transverse jet, eventually undergoes a linear transition from convectively to absolutely/globally unstable as the crossflow to jet velocity ratio increases. This flow field, however, has an extremely complex dynamics. Hence, determining this transition location is not a trivial task. It has long been known that this transition is associated with the upstream shear layer connected to the near field Kelvin-Helmholtz vortex ring, but it was only recently that the most unstable global mode and wave maker were located there as well. These findings led to the realization that an inviscid and planar linear stability analysis of the local velocity profile extracted from the jet in the crossflow upstream shear layer was strongly correlated with its transition to absolute/global instability. It is shown in this paper that such an analysis can be turned into an accurate predictive tool with the use of a viscous (instead of inviscid) and round (instead of planar) mixing layer. In other words, replacing an inviscid analysis of a planar mixing layer with counterflow by a viscous analysis of a round coaxial jet with outer nozzle suction leads to an accurate prediction of the jet in crossflow convective to an absolute instability transition.

Physics of Fluids

In this study, we consider the oscillatory behavior of mutually impinging jets in an enclosed, domeshaped mixing chamber. The frequency of the impinging jet oscillations is dictated by the flow rate, with the oscillatory behavior being grouped into three regimes: a low flow rate regime (Re < 1500), a transition regime (1500 < Re < 2000), and a high flow rate regime (Re > 2000). The detailed characteristics of the oscillations in the high flow rate regime (Re = 6800 in the present study) are investigated through simultaneous frequency and refractive-index-matched particle image velocimetry measurements. The oscillation mechanism in the high flow rate regime was found to be similar to that of the other two regimes, where jets collide and interact in an oscillatory manner. However, in the high flow rate regime, there is a distinct and phase-evolving process of saddle point formation and jet bifurcation that is not present at the lower flow rates. The jet bifurcation process is also distinctly related to the balance of vortical structures inside the mixing chamber, and saddle point formation plays a key role in the internal and external flow field of this configuration. The external sweep angle of the exiting jet increases with the flow rate throughout the low and transition flow rate regimes, but a constant sweep angle was found to persist in the high flow rate regime. Thus, formation and location of the internal saddle point is directly linked to the external sweep angle of the jet.

Physical Review Fluids

This experimental study explored the dynamics of lock-in phenomena associated with upstream shear layer (USL) instabilities for an equidensity gaseous jet in crossflow (JICF). Axisymmetric sinusoidal forcing of the jet fluid at different forcing amplitudes and frequencies was used to explore lock-in under flow conditions corresponding to naturally occurring absolutely/globally unstable and convectively/locally unstable shear layers, at relatively low and high jet-to-crossflow momentum flux ratios J, respectively. Dynamical phenomena were quantified via hotwire anemometry, which not only documented differences in spectral characteristics but also in Poincaré maps obtained via time-delay embedding in the temporal data. The experiments provided evidence of unconditional lock-in as well as quasiperiodicity in response to forcing, reflective of marginal lock-in phenomena; these phenomena were observed for both globally unstable and convectively/locally unstable shear layers in the absence of forcing. The free jet limit in the absence of crossflow also exhibited unconditional lock-in, with some characteristics similar to those for the JICF at applied forcing frequencies above the fundamental mode. For the globally unstable JICF, a simple van der Pol nonlinear oscillator model used to represent the dynamics of the USL showed consistency with experimental findings and thus provided additional insights into the nature of shear layer dynamics.

The development of the velocity and scalar fields in a coaxial jet mixer has been experimentally investigated applying simultaneously Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) methods. Mixing of a turbulent jet (water solution of Rhodamin 6G) issued from a long round nozzle (l/d = 60) with co-flow (water) was studied. Because of the nozzle length the boundary layers of the inner flow already merged upstream of the jet exit. The issued jet conditions were changed installing vortex generators (tabs) at the nozzle exit. The tabs of rectangular and triangular forms with heights of 13 15 % the inner nozzle diameter accelerated mixing significantly but the scalar field developed to the homogeneous state faster than the velocity one. Turbulent characteristics measured downstream of the jet exit gave evidence the mixing specific behind the rectangular and triangular tabs.

Physics of Fluids, 2018

The "Twin-Fluid Jet-in-Crossflow (TF-JICF)" is a nascent variation of the classical JICF, in which a liquid jet is co-injected with an annular sleeve of gas into a gaseous crossflow. Jet-engine designers are interested in using TF-JICF for liquid-fuel injection and atomization in the next-generation combustors because it is expected to minimize combustor-damaging auto-ignition and fuel-coking tendencies. However, experimental data of TF-JICF are sparse. Furthermore, a widely accepted TF-JICF model that correlates the spray's penetration to the combined liquid-gas momentum-flux ratio (J eff) is increasingly showing discrepancy with emerging results, suggesting a gap in the current understanding of TF-JICF. This paper describes an investigation that addressed the gap by experimentally characterizing the TF-JICF produced by a single injector across wide ranges of operating conditions (i.e., jet-A injectant, crossflow of air, crossflow Weber number = 175-1050, crossflow pressure P cf = 1.8-9.5 atm, momentum-flux ratio J = 5-40, and air-nozzle dP = 0%-150% of P cf). These covered the conditions previously used to develop the J eff model, recently reported conditions that produced J eff discrepancies, and high-pressure conditions found in jet-engines. Dye-based shadowgraph was used to acquire high-resolution (13.52 µm/pixel) images of the TF-JICF, which revealed wide-ranging characteristics such as the disrupted Rayleigh-Taylor jet instabilities, air-induced jet corrugations, spray-bifurcations, and prompt-atomization. Analyses of the data showed that contrary to the literature, the TF-JICF's penetration is not monotonically related to J eff. A new conceptual framework for TF-JICF is proposed, where the flow configuration is composed of four regimes, each having different penetration trends, spray structures, and underlying mechanisms.

The mixing performance of multiple transverse jets has been evaluated experimentally. Measurement techniques included laser Doppler velocimetry and planar laser induced fluorescence. Basic findings are consistent with results presented in literature for single jet mixing behavior. Mixing performance has been compared to literature for the single jet case and the Holdeman parameter has been re-evaluated for effectiveness at low jet numbers. A single jet in a confined crossflow was found to have a local minimum at (⁄). Results for two jets indicate monotonically decreasing unmixedness for the range of conditions tested, with no local optimum apparent. Data for three jets indicate a local optimum at (⁄) and relatively flat range of mixing performance in the range of (⁄). Six jets indicate a minimum unmixedness near (⁄) , but exhibited poorer mixing performance than all other configurations at the highest values of (⁄) tested. The most optimum configuration tested was six jets at (⁄) , resulting in an unmixedness of 0.0192. This value was 76% lower than the next lowest configuration (three jets) at the same (⁄) Total momentum was found to collapse the data well, as configurations more closely matched a historical correlation for second moment of a single confined jet more closely.

Physical Review Fluids

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

Proceeding of Sixth International Symposium on Turbulence and Shear Flow Phenomena

The present work investigates passive scalar turbulent mixing by means of experimental optical techniques capable to simultaneously measure the instantaneous velocity and scalar fields in a non-intrusive way. In particular, passive scalar mixing of a water jet ejected from a pipe duct onto the surrounding quiescent pure fluid is studied by simultaneously using Particle Image Velocimetry for the velocity field and Planar-Laser Induced Fluorescence for the scalar field. The Reynolds numbers (Re) of the jet flow, based on the jet diameter and exit maximum velocity, is 23700 so a fully turbulent jet emerges from the pipe allowing investigating the mixing mechanisms driven by velocity fluctuations. In order to avoid diffusion to be important compared to advection, a scalar substance, Fluorescein sodium salt, with a Schmidt number equal to 2050 has been employed. This ensures turbulent transport to be investigated focussing on Reynolds fluxes, <u i 'c'>, whose measure is the main aim of this work. A detailed and accurate description of these quantities is given, unveiling some new interesting features in the pipe jet near field, 0<x/D<17, where few experimental data are available for the scalar concentration field. At the end of the investigated region, the present data approach the Literature data for the self-similar region.

The paper presents a discussion of the mixing intensification caused by unstable modes in variable density and countercurrent jets and its application to the optimization of gas burners. Very high mixing intensity can be attained in flows under so called absolute instability. The idea of absolute instability proposed originally by Landau relies on the perturbation development both in space and time. If the small perturbation develops in time in a given place it finally reaches the highest possible oscillation amplitude limited only by non-linear flow interactions. Moreover because the perturbation is not convected from its onset it affects its source leading to self-exciting oscillations amplifying the effect of mixing enhancement. According to recent theoretical and experimental studies the absolute instability appears in low-density and countercurrent jets provided that certain critical density or velocity ratios are attained. The countercurrent technology seems to be very promising in application to gas burners optimization since this technology can be combined effectively with the reburning idea leading to very efficient NO x emission reduction.