Jet Column Mode Research Articles

Self-excited oscillations in variable density counter-current round jets

The paper investigates the global instability phenomenon in variable-density counter-current round jets. The analysis is performed using a large eddy simulation and the computations are carried out employing a high-order numerical code. The configuration of two co-axial nozzles, where suction applied in an annular nozzle is a driving force for the counterflow, is investigated. In order to simulate conditions conducive to the emergence of global instability a wide range of velocity and density ratios is considered. Depending on these parameters two different types of modes are observed, i.e., Mode I — shear layer mode, and Mode II — jet column mode. The former is recognized by a high level of perturbations in the shear layer which vanish away from the shear layer reaching a low level at the jet centerline. The latter is characterized by strong disturbances throughout the entire jet region. The results obtained are in remarkable agreement with the theoretical basis reported in the literature. They reveal that the critical velocity ratio for which global instability emerges decreases along with the density ratio. Additionally, the transition from Mode II to Mode I is found for sufficiently strong suction. In general, the lower the density ratio, the weaker suction is required to induce Mode I. Consistently to the literature, Mode II is characterized by a lower frequency of oscillations compared to Mode I for which an increase in density ratio and/or velocity ratio shifts the peaks of the velocity spectra toward higher frequencies.

Experimental study of vortex formation in pulsating jet flow by time-resolved particle image velocimetry

We experimentally investigate the pulsating circular jet flow at moderate Reynolds numbers. By applying time-resolved particle image velocimetry in the axial-radial plane, we measure the near-field velocity fields with the jet source temporally modulated by sinusoidal pulsations. As a baseline, the steady jet flow with the same mean Reynolds number is tested. The direct comparisons of the mean and fluctuating velocity fields show that the whole potential core as well as the axisymmetric shear layer is modulated by the pulsation effect. Meanwhile, larger-scale vortices are formed in the shear layer with phase correlation of the pulsation cycle. As a result, the pulsation increases the turbulent mixing in the latter half of the potential core, and it extends the fluid entrainment further in the radial direction. The increased fluid entrainment of the ambient quiescent fluid is clearly identified by the attracting Lagrangian coherent structures as the bounds of the growing vortices within the shear layer. By analyzing the dynamic modes, we find that the low-frequency off-the-axis helical structures, which are dominant in the steady jet flow, are inhibited. The axisymmetric jet column mode and its harmonics along the axis are strengthened by the pulsation effect. Furthermore, the vortex formation mainly takes place particularly in the deceleration phase, whereas a shock-like wave front is formed during the acceleration, indicating the distinct roles of the pulsation phases in the jet instability.

On the role of thermo-transport properties in the convective/absolute transition of heated round jets

The effect of thermodynamic and transport properties on the convective-to-absolute transition in heated round jets is investigated with the spatio-temporal linear stability theory, by considering three sets of properties with increasing complexity. Present models include (i) a constant property model often used in the literature, (ii) a simplified model with variable properties, and (iii) a more accurate equilibrium air mixture model, accounting for dissociation reactions in the flow. A family of arbitrary single-stream and dual-stream jet profiles, representative of typical configurations studied in the literature, is investigated and adapted to each model. Our results show that considering a variable viscosity and thermal conductivity has a destabilizing effect on absolute instabilities in the viscous regime. Furthermore, this destabilization is stronger for the outer mode in dual-stream jets than for the inner mode or the jet-column mode in single-stream jets. With sufficient heating (S < 0.3), results obtained with the equilibrium model strongly depart from those of calorically perfect gas models and display absolute domains deformed by the chemical activity. For absolute instabilities triggered by the baroclinic torque such as jet-column and inner modes, the convective-to-absolute transition is shifted toward thinner and hotter configurations, while the opposite is observed for the outer mode. Finally, we observe a dependence of the equilibrium model stability properties on the static pressure.

Active plasma grid for on-demand airflow mixing increase

The present paper concerns the electromechanical characterization of an actuator composed of a ceramic plate perforated by 121 holes housing embedded and printed electrodes between which a high voltage is applied. The electrode arrangement is such that the holes where the gas flows are surrounded by surface discharges. Electrical measurements and iCCD images show that the discharge behaves as a typical surface dielectric barrier discharge with streamer and glow regimes during one period of the AC sine voltage. Particle image velocimetry has been used to measure the jet flow produced by the discharge. The plasma discharge is at the origin of a wall jet with mean velocity of about 2.2 m/s, oriented from the active electrode to the grounded one. The capability of this discharge for promoting mixing by reducing the length of the jet core is demonstrated for flow velocities from 20 up to 60 m/s. In all the tested cases, the actuator can improve the mixing downstream of the perforated plate, when periodic perturbations are imposed at the jet column mode (StD = 0.3).

Investigation of an impinging heated jet for a small nozzle-to-plate distance and high Reynolds number: An extensive experimental approach

The present work aims at investigating a particular impinging jet configuration throughout a comprehensive experimental approach. A preheated air jet at 130°C issues a fully developed circular pipe at Reynolds number 60,000 and discharges in the laboratory room to impinge a flat plate located 3 diameters downstream. The description of the velocity field and the complete Reynolds stress tensor is provided by stereoscopic particle image velocimetry (S-PIV) and Laser Doppler Velocimetry (LDV) measurements. For the first time, data are reported for the mean and fluctuating temperature of an impinging jet configuration with the help of cold-wire thermometry (CWT) measurements. The heat transfer distribution on the impinged plate is determined through an inverse method based on infrared thermography measurements on the rear face of the plate. The agreement between S-PIV and LDV measurements is shown excellent over the whole flow field. The measured Nusselt number distribution exhibits a secondary maximum at r/D=2, as observed in previous experiments for short impinging distances. Flow dynamics is characterized through a spectral analysis of time-resolved measurements while flow topology features are identified through coherent structure detection based on S-PIV spatially-resolved instantaneous velocity fields. This analysis shows that the jet column mode, associated with a Strouhal number of 0.4, plays a key role in the primary structure dynamics.

Manipulation of a grid-generated mixing with an active honeycomb dielectric barrier plasma discharge

This study defines and characterizes an active system for turbulent and scalar mixing enhancement. This system is made of an active grid composed of 121 holes where the gas flows. A high voltage is applied between printed electrodes and embedded ground electrodes in order to produce a non-thermal surface discharge at the hole exits. The goal is to modify the flow downstream of the active grid. First, electrical and optical characterizations of the actuator are proposed. Second, it is shown that the discharge strongly modifies the flow distribution of the multi-jet exhaust, and the flow change depends on the high voltage applied to the active plasma grid. A minimization of the potential core by 40% is reported when discharge frequency corresponds to jet column mode instability.

Absolute/convective instability of planar viscoelastic jets

Spatiotemporal linear stability analysis is used to investigate the onset of local absolute instability in planar viscoelastic jets. The influence of viscoelasticity in dilute polymer solutions is modeled with the FENE-P constitutive equation which requires the specification of a non-dimensional polymer relaxation time (the Weissenberg number, We), the maximum polymer extensibility, L, and the ratio of solvent and solution viscosities, β. A two-parameter family of velocity profiles is used as the base state with the parameter, S, controlling the amount of co- or counter-flow while N−1 sets the thickness of the jet shear layer. We examine how the variation of these fluid and flow parameters affects the minimum value of S at which the flow becomes locally absolutely unstable. Initially setting the Reynolds number to Re = 500, we find that the first varicose jet-column mode dictates the presence of absolute instability, and increasing the Weissenberg number produces important changes in the nature of the instability. The region of absolute instability shifts towards thin shear layers, and the amount of back-flow needed for absolute instability decreases (i.e., the influence of viscoelasticity is destabilizing). Additionally, when We is sufficiently large and N−1 is sufficiently small, single-stream jets become absolutely unstable. Numerical experiments with approximate equations show that both the polymer and solvent contributions to the stress become destabilizing when the scaled shear rate, η=WedU¯1dx2L (dU¯1dx2 is the base-state velocity gradient), is sufficiently large. These qualitative trends are largely unchanged when the Reynolds number is reduced; however, the relative importance of the destabilizing stresses increases tangibly. Consequently, absolute instability is substantially enhanced, and single-stream jets become absolutely unstable over a sizable portion of the parameter space.

Parametric study of a Mach 1.3 cold jet excited by the flapping mode using plasma actuators

This paper describes an investigation into numerical and physical parameters influencing an ideally-expanded Mach 1.3 jet excited by the m=±1 flapping mode. The nozzle exit properties of the perfectly expanded jet include: ρjet=1.576kg/(m3), Tjet=224K, Re=1.1×106, and T0jet=Tambient=300K. The excitation is imposed by eight Localized Arc Filament Plasma Actuators (LAFPA) placed around the periphery of the circular nozzle exit. The reference controlled case considers the most amplified (jet column mode) frequency corresponding to a Strouhal number of 0.3 based on the diameter of the nozzle and jet velocity, with an actuator-imposed temperature of 1500K and a duty cycle of 20%. Relative to this reference, the effects of changing frequency, duty cycle and actuator model temperature are explored and results are compared with experimental data where available. The results are analyzed with several different quantitative and qualitative metrics, including time-averaged centerline decay and jet half-width as well as phase-averaged coherent structures. Raising the frequency affects the dynamics in several ways; the number of features observed in the phase-averaged data increases and the rate of decay of the centerline velocity is reduced. The alternating vortex ring interactions observed in the reference case are not distinct but are rather replaced by smaller structures, trends which are also observed in experiment. Lowering the Strouhal number increases mixing until a reversal occurs at St=0.1. The higher duty cycle exhibits strengthened coherent structures and slightly higher jet growth along the flapping plane, but the overall dynamics remain the same. The response of the jet is relatively insensitive to actuator temperature model within reasonable limits. The latter two studies, with different duty cycles and actuator temperatures, are consistent with previous analyses demonstrating that instability manipulation, rather than heat deposition is the primary mechanism of control.

The preferred mode of incompressible jets: linear frequency response analysis

AbstractThe linear amplification of axisymmetric external forcing in incompressible jet flows is investigated within a fully non-parallel framework. Experimental and numerical studies have shown that isothermal jets preferably amplify external perturbations for Strouhal numbers in the range $0. 25\leq {\mathit{St}}_{D} \leq 0. 5$, depending on the operating conditions. In the present study, the optimal forcing of an incompressible jet is computed as a function of the excitation frequency. This analysis characterizes the preferred amplification as a pseudo-resonance with a dominant Strouhal number of around $0. 45$. The flow response at this frequency takes the form of a vortical wavepacket that peaks inside the potential core. Its global structure is characterized by the cooperation of local shear-layer and jet-column modes.

Reduced-order modeling of high-speed jets controlled by arc filament plasma actuators

Arc filament plasma actuators applied to high-speed and high Reynolds number jets have demonstrated significant mixing enhancement when operated near the jet column mode (JCM) frequency. A feedback-oriented reduced-order model is developed for this flow from experimental data. The existent toolkit of stochastic estimation, proper orthogonal decomposition, and Galerkin projection is adapted to yield a 35-dimensional model for the unforced jet. Explicit inclusion of a "shift mode" stabilizes the model. The short-term predictive capability of instantaneous flow fields is found to degrade beyond a single flow time step, but this horizon may be adequate for feedback control. Statistical results from long-term simulations agree well with experimental observations. The model of the unforced jet is augmented to incorporate the effects of plasma actuation. Periodic forcing is modeled as a deterministic pressure wave specified on the inflow boundary of the modeling domain. Simulations of the forced model capture the nonlinear response that leads to optimal mixing enhancement in a small range of frequencies near the JCM.

S054101 周囲と密度の異なる噴流のプリファード周波数([S05410]噴流,後流,およびはく離流れの基礎と応用:最前線(10))

This report aims at investigating the relationship between preferred frequency of jets and density. The disturbance frequency for mixing control based on the preferred frequency occur in a jet flow initial area of a jet column mode is often selected to enhance the diffusion of the jet flow. Air, CO_2, and He were used for jets and surroundings. Nozzle was used converging round nozzle (d=10mm). The laser light sheet technique was applied to the visualization of a cross section of vortex ring. The variation of velocity of a jet flow was measured by the hot wire anemometry and PIV to investigate the preferred frequency to determine the Strouhal number. The distance until rolling up to the vortex ring goes away from the nozzle as increasing the density ratio. The Strouhal number does not depend on Reynolds number and keep an almost constant value. These show the relationship between preferred frequency and diffusion by jet density with different surrounding gas.

Viscous linear stability of axisymmetric low-density jets: Parameters influencing absolute instability

Viscous linear stability calculations are presented for model low-density axisymmetric jet flows. Absolute growth transitions for the jet column mode are mapped out in a parametric space including velocity ratio, density ratio, Reynolds number, momentum thickness, and subtle differences between velocity and density profiles. Strictly speaking, the profiles used in most jet stability studies to date are only applicable to unity Prandtl numbers and zero pressure gradient flows—the present work relaxes this requirement. Results reveal how subtle differences between the velocity and density profiles generally used in jet stability theory can dramatically alter the absolute growth rate of the jet column mode in these low-density flows. The results suggest heating/cooling or mass diffusion at the outer nozzle surface can suppress absolute instability and potentially global instability in low-density jets.

Extremizing Feedback Control of a High-Speed and High Reynolds Number Jet

We present results of the development and application of extremizing feedback control to high-speed and high Reynolds number axisymmetric jets. In particular, we demonstrate control authority on the near-field pressure of a Mach 0.9 jet with a Reynolds number based on jet diameter of 7.8 ◊ 10 5 . Open-loop forcing experiments are presented wherein localized arc filament plasma actuators are shown to have two distinct effects on the near-field pressure, similar to their effect on the far-field acoustics reported earlier. At low forcing Strouhal numbers (StDF’s) near the jet column mode instability, a large amplification in the pressure fluctuations is observed. At higher StDF’s (close to the initial shear layer instability) a broad attenuation is observed in the near-field pressure fluctuations, especially in the axisymmetric mode. Previous experiments have shown that forcing the jet with these low and high frequencies result in jet mixing enhancement and far-field noise reduction, respectively. Two different gradient-free extremizing feedback control algorithms have been implemented, each of which can perform online minimum-seeking as well as maximumseeking. Both methods demonstrate fast convergence to the optimum followed by steady operation.

Numerical study of eigenmode forcing effects on jet flow development and noise generation mechanisms

The effect of nonlinear interaction of instability eigenmodes on jet flow transition and its near acoustic field for a high-subsonic round jet at a Reynolds number of Re=4.5×105 and a Mach number of Ma=0.9 is investigated using large-eddy simulations. At the inflow, helical perturbations of azimuthal wavenumbers |n|=4,…,8 determined from linear stability theory are superimposed on a laminar base flow in order to trigger transition to turbulence. The disturbance amplitude is varied parametrically in the range from 1.5% to 4.5% of the jet exit velocity Uj. Thereby we aim to characterize sources of noise generation and, in particular, underlying mode interactions. With increasing forcing amplitude, the transitional behavior of the jet changes which affects the mean flow and also the acoustic near-field, which are both analyzed in detail. As the forcing amplitude is increased, the axial root-mean-square peak levels along the jet centerline are reduced by approximately 7%. Simultaneously, pronounced dual-peak distributions are generated along the jet lip line which are related to the localization of vortex pairings of the jet column mode. For low-amplitude excitation the azimuthal turbulent kinetic energy spectra show that the unexcited, naturally least stable axisymmetric mode n=0 and the helical mode n=1 dominate the early nonlinear regimes between z≈6r0 and 9r0 where r0 is the jet radius. An analysis of the Fourier mode amplitude clarifies that this energy rise is linked to the helical mode n=1. For higher forcing amplitudes, in addition to the varicose mode n=0 interactions between the excited even mode n=4 and higher azimuthal harmonics thereof dominate the azimuthal energy spectra. These differences in the early nonlinear development of the eigenmodes are found to alter the acoustic near-field. At small angles from the downstream jet axis, the peak acoustic frequency occurs at a Strouhal number based on the angular frequency ω and the jet diameter Dj of St=ωDj/(2πUj)≈0.4. For low-amplitude forcing sound pressure levels are slightly enhanced which can be linked to the dominant low azimuthal wavenumbers identified in the transitional region. In the sideline direction, regardless of the excitation level, broadbanded spectra with maxima in the band 0.7≤St≤0.8 are found which is maintained at intermediate observer angles. For high forcing amplitude, however, a tonal component outside the initially excited frequency range is observed. This peak at St≈0.88 can be explained by weakly nonlinear interactions of initially forced eigenmodes n=4 and n=8 together with the jet column mode.

405 噴流の密度とコラムモードとの関係(OS6-1乱流現象の計測とシミュレーション1)

405 噴流の密度とコラムモードとの関係(OS6-1乱流現象の計測とシミュレーション1)

Linear impulse response in hot round jets

The linear impulse response of axisymmetric jets is examined for a family of variable-temperature profiles typical of the potential core. The influence of jet heating, shear layer thickness, and Reynolds and Mach numbers on the spatiotemporal stability of both axisymmetric and helical modes is investigated. The linear impulse response is retrieved from a numerical solution of the spatial eigenvalue problem, which is derived from the fully compressible equations of motion. Changes in the spatiotemporal stability of heated versus isothermal jets are shown to arise solely from the effect of the baroclinic torque. By considering the full linear impulse response, the competition between jet column modes and shear layer modes is characterized. Jet column modes are only found to occur for axisymmetric disturbances. In thin shear layer jets, the jet column mode is shown to prevail at low group velocities, whereas axisymmetric and helical shear layer modes dominate at high group velocities. The absolute mode of zero group velocity is found to always be of the jet column type. Although only convectively unstable, the maximum growth rates of the shear layer modes greatly exceed those of the jet column modes in thin shear layer jets. In thick shear layer jets, axisymmetric modes of mixed jet column/shear layer type arise. The weakened maximum growth rate of mixed modes accounts for the dominance of helical modes in temporal stability studies of thick shear layer jets.

Instabilités de l'écoulement produisant le bruit de fente

The nature of the instability governing the self-sustained tones produced by a low Mach number plane jet impinging on a slotted plate, known as slot-tone, is identified experimentally. For a given Reynolds number, the natural shear-layer and the jet column mode frequencies of the free jet delimit the values of the measured slot-tone operating frequencies. The oscillations at lower frequencies are the result of the amplification of the jet column mode, and those at higher frequencies correspond to the shear layer instabilities. To cite this article: A. Billon et al., C. R. Mecanique 332 (2004).

Jet column modes in both a plane jet and a passively modified plane jet subject to acoustic excitation

Use is made of acoustic excitation to enhance the coherent vortical structures that form in both a plane jet and a passively modified plane jet. The Strouhal number for the preferred mode based on the nozzle height of the jet is found to be unaffected by the passive modification. The existence of the jet column mode of stable pairing in the excited plane jet is unambiguously confirmed. Measurements in the excited and modified plane jet show that the passive modification suppresses this mode. Finally, the work shows the existence of a new mode of stable pairing. Both time- and phase-averaged measurements are presented to examine this mode.

A study of the flow-field evolution and mixing in a planar turbulent jet using direct numerical simulation

Turbulent plane jets are prototypical free shear flows of practical interest in propulsion, combustion and environmental flows. While considerable experimental research has been performed on planar jets, very few computational studies exist. To the authors' knowledge, this is the first computational study of spatially evolving three-dimensional planar turbulent jets utilizing direct numerical simulation. Jet growth rates as well as the mean velocity, mean scalar and Reynolds stress profiles compare well with experimental data. Coherency spectra, vorticity visualization and autospectra are obtained to identify inferred structures. The development of the initial shear layer instability, as well as the evolution into the jet column mode downstream is captured well.The large- and small-scale anisotropies in the jet are discussed in detail. It is shown that, while the large scales in the flow field adjust slowly to variations in the local mean velocity gradients, the small scales adjust rapidly. Near the centreline of the jet, the small scales of turbulence are more isotropic. The mixing process is studied through analysis of the probability density functions of a passive scalar. Immediately after the rollup of vortical structures in the shear layers, the mixing process is dominated by large-scale engulfing of fluid. However, small-scale mixing dominates further downstream in the turbulent core of the self-similar region of the jet and a change from non-marching to marching PDFs is observed. Near the jet edges, the effects of large-scale engulfing of coflow fluid continue to influence the PDFs and non-marching type behaviour is observed.

A103 円形ノズルからの間欠噴流に関する実験

This paper describes the classification of pulsating jets. The flow fields and the motions of vortices characterizing the pulsating jets were experimentally examined using both the smoke visualization method and the velocity measurement. The results of the velocity variation are found to agree with those of the flow visualization. The pulsating jets can be classified into three kinds of flow state; they are termed a row of puffs, a row of vortex rings in jet column mode, and a row of vortex rings in shear layer mode. In this paper, we defined the as a lump of fluid consisting of a leading vortex and a trailing jet following the vortex. The puff may be regarded as the state possessing the most fundamental features of a pulsating jet. Each run corresponds to a point on a 2-D regime diagram, where the governing parameters are chosen as the coordinate variables. The plotted result of the points indicates that the choice of the parameters is adequate to classify the flow states.