Transverse Acoustic Excitation Research Articles

Impact of Transverse Acoustic Excitation and Air Swirl on a Confined Hollow Cone Sheet

Abstract This study investigates the effect of imposing symmetric transverse acoustic excitation and counter-rotating air swirl flow on a confined hollow cone spray sheet. We study the impact of these forcing conditions on two unstable modes—sheet flapping and sinuous wave growth modes—and their phasic relationship. We used the shadowgraphy technique, and only the sheet edges were analyzed using the method of snapshot proper orthogonal decomposition (POD). In this study, we used postprocessing techniques to estimate the nondimensional breakup length, phase averaged cone angle, and continuous wavelet transform (CWT) frequency. Results show that imposing acoustic excitation alone causes the sheet edges to flap, axisymmetrically, at the pressure antinode, while at the pressure node and base flow condition, the asymmetric sinuous wave mode is dominant. At the pressure intermediary, we find the possible dominance of both modes. While the dominant effect of air swirl alone was difficult to ascertain, results from CWT analysis show that POD mode 1 and mode 3 have very similar spectral content, with mode 3 showing a flapping mode. Finally, air swirl flow has a more dominant effect than acoustics in causing sheet breakup.

Investigation of dynamic mixedness characteristics of a transverse acoustically excited turbulent jet by high-repetition-rate acetone tracer planar laser-induced fluorescence technique

The dynamic mixedness characteristics of a bluff-body stabilized turbulent jet under transverse acoustic excitations were investigated using high-repetition-rate acetone planar laser-induced fluorescence (PLIF) at 7 kHz and multipoint scanning hot-wire measurements. Acetone mixedness imaging was applied for the turbulent jet to assess the interaction between the turbulent jet and the imposed transverse acoustic excitations at a driving frequency of 50 Hz. The high-repetition-rate acetone PLIF images showed that the acetone mixedness distribution swung left and right frequently under the transverse acoustic excitation, and the deflection angle could reach about 6°. The mixedness area of a turbulent jet flow could also be increased by 13.3% when excited by a transverse acoustic wave. Meanwhile, the sequence of instantaneous acetone PLIF images illustrated how the wrinkled edges were generated when acoustic excitations were imposed. The curvature of the acetone PLIF interface showed that the portion of large curvatures increased by 60% after applying an acoustic wave of 123 dB. Multipoint hot-wire measurements further stressed that the turbulent intensity at the transverse acoustic excitation of 123 dB increased to be about 1.3 times the natural turbulence. The proper orthogonal decomposition results showed that the large and small scales of the jet wrinkles both increased with the sound pressure level. RANS transient simulation also implied that distorted velocity streamlines could be achieved inside the turbulent jet due to the transverse acoustic excitation. They could further lead to inhomogeneous convection and increased mixing between the turbulent jet and the surrounding air.

Structure and Dynamics in Liquid Iron at High Pressure and Temperature. A First Principles Study

AbstractWe have studied the evolution of structural and dynamic properties of liquid Fe as a function of pressure for 11 thermodynamic states close to the melting line. The pressure range considered goes from ambient pressure to 323 GPa, and the study has been carried out by using the ab‐initio molecular dynamics technique. The agreement between the calculated static structure and the available experimental data is very good, including details like an asymmetric second peak, which remains over most of the whole pressure range and suggests a significant local icosahedral short‐range order in the liquid. The dynamical structure is studied through the characteristics of the propagating density fluctuations and the associated longitudinal and transverse particle currents. The transverse dispersion relations expose two branches of modes for all pressures, whose range of appearance is analyzed and put in connection with the double‐peak structure of the Fourier spectra of velocity autocorrelation functions. We have also investigated the existence of fingerprints of transverse acoustic excitation modes in the dynamic structure factor for the high pressure states similar to those observed in the inelastic X‐ray scattering intensity data of liquid Fe at ambient pressure. The calculated electronic density of states shows that with increasing pressure there is a widening of the conduction band along with a decreasing significance of spin polarization. Finally, we also report results for transport coefficients like self‐diffusion, shear viscosity and adiabatic sound velocity, which are compared with the available experimental data.

The dynamics of non-premixed flames subjected to a transverse acoustic mode

A non-premixed methane/air flame under a transverse acoustic mode is studied by combining theoretical analysis and numerical simulations to understand the acoustic and flow mechanisms affecting the flame response. The experimental configuration under investigation consists of a combustion chamber, a coaxial injector with coflow, and speakers inducing stationary transverse acoustic waves. Numerical simulations reveal a strong response of the flow in the injector to transverse acoustic excitation, resulting in large fluctuations in the mass flow rates of the fuel and oxidizer streams. An analytical model is developed to describe the acoustic response of the flow in the injector to transverse excitation. This model is employed to describe unsteady inflow conditions in large-eddy simulations for different excitation amplitudes. The simulations show that the injector acoustic coupling must be considered to accurately predict the flame lift-off dynamics. Comparisons of numerical results with experimental measurements reveal similar flame structures arising from the longitudinal forcing and good agreement in the lift-off behavior. Further analyses of the flow field, mixing, and flame structure show that the coflow longitudinal forcing induces vortices that corrugate the stoichiometric surface. In addition, the jet forcing enhances the mixing at greater radial locations. The combination of these two mechanisms results in a periodically increased mixing at the stoichiometric surface, leading to significant heat release rate oscillations near the inflow. This work supports the hypothesis that strong acoustic/flame coupling can be present in non-premixed systems when the injectors are longitudinally excited by transverse acoustic waves.

Hysteresis and Bi-Stability in Transversely Excited Swirling Flows

Abstract We investigate the effect of transverse acoustic excitation on non-reacting swirling jets. The work is motivated by the azimuthal instabilities in annular gas turbine combustor which are one of the major challenges in aero-engines. We have designed and fabricated a multi-nozzle linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features a dual co-rotating radial swirler configuration. Two compression drivers placed on either side of the combustion chamber are used to generate acoustic fields in the direction transverse to the flow. Simultaneous 2D PIV and high-frequency pressure measurements are conducted to measure the time-averaged velocity field and the chamber acoustics, respectively. It is observed that once the swirling jet is excited with a transverse acoustic forcing, it instantaneously transitions to a wall-jet state. In wall-jet state, the flow moves radially outwards and remains attached to the walls on either side of the nozzle, and is characterised by a recirculation zone with a strong negative axial velocity. In our experiments we demonstrate that transverse acoustic excitation can lead to a bistable state in swirling flows. We investigated the acoustic response to low-amplitude forcing on the combustion chamber by performing a forced acoustic response analysis using COMSOL. It is observed that acoustic forcing leads to a peak response of the radial and azimuthal velocities at the flare of the swirler, which could induce an axisymmetric deflection of the jet shear layer pushing it into a wall-jet state.

Low energy excitation in liquid Sb and liquid Bi observed in inelastic x-ray scattering spectra

The dynamic structure factor S(Q, E), where Q and E are momentum and energy transfer, respectively, has been measured for liquid Sb, using inelastic x-ray scattering. A modified damped harmonic oscillator model function was applied to analyse S(Q, E) of liquid Sb and also to that of liquid Bi by Inui et al (2015 Phys. Rev. B 92, 054206). The obtained excitation energy was in fairly good agreement with that predicted by ab initio molecular dynamics simulations on these liquid semi-metals. The excitation energy of the longitudinal acoustic mode in liquid Sb and liquid Bi exhibits flat-topped Q dependence whereas the lower excitation energy below the longitudinal acoustic excitation shows Q-gap behaviour. From the viscosity estimated from the Q-gap experimentally obtained, it is inferred that the lower energy excitation arises from the transverse acoustic excitation in the liquids.

Forced flow response analysis of a turbulent swirling annular jet flame

This study of an externally forced, amplifier-type turbulent reacting swirling annular jet presents a low-order model for the flow response to transverse acoustic excitation and compares the model's predictions with experimental measurements. The model is formulated based on linear stability calculations about the turbulent mean flow and eddy viscosity fields obtained from separate measurements of the unforced flow. The stability calculations yield weakly global spatial modes associated with the forcing frequency, which serve as a basis upon which to project the forcing input. Thus, the model constitutes a hydrodynamic transfer function connecting the input forcing to the output coherent flow response through the linearized low Mach number compressible Navier–Stokes equations. Following a detailed presentation of the stability analysis underlying the model, the response predictions are evaluated against previously reported experiments where the jet was transversely excited at both an acoustic pressure node and an antinode. The results reveal excellent agreement between the predicted response and the measured fluctuating fields, suggesting that the low-order linear model based on the turbulent mean flow field captures the essential physics of the mode selection process in this forced configuration. This work provides further evidence that linear hydrodynamics govern the growth and decay of spatiotemporally coherent vortical structures in the swirling, turbulent jet flame, and, in particular, explains the dominance of co-rotating spiral structures.

Flow dynamics of shear-coaxial cryogenic nitrogen jets under supercritical conditions with and without acoustic excitations

The three-dimensional flow structures and dynamics of shear-coaxial nitrogen jets under supercritical conditions are comprehensively investigated using the large-eddy simulation technique. The theoretical framework is based on the full conservation laws and accommodates real-fluid thermodynamics and transport theories over the entire range of fluid states. Cryogenic liquid nitrogen (132 K) is delivered through the inner tube of a shear-coaxial nozzle, and gaseous nitrogen (191 K) is injected through the outer annulus, into a high-pressure environment at 233 K. Particular attention is paid to the influence of operating parameters on the flow evolution of the coaxial jets. Two supercritical pressure conditions, 4.94 and 10.0 MPa, and three outer-to-inner velocity ratios in the range of 2.03–3.75, are considered. Results reveal that the flowfield downstream of the coaxial jets is characterized by two potential cores and three shear layers. Two counter-rotating recirculation zones are formed next to the central post-tip. The characteristic frequencies of flow recirculation and shear-layer instability are analyzed using power spectral analysis. Increasing the pressure and the velocity ratio enhances turbulent mixing, promotes the entrainment of the outer stream into the inner region, and advances the pinching point of vortical motion to the centerline, thereby leading to a decreased length of the inner dark core. The predicted dark core length and outer spreading angle show good agreement with experimental results. The introduction of transverse acoustic excitation induces large sinusoidal oscillations of the inner dense-fluid stream along the direction of acoustic motion and promotes the mixing of the inner and outer streams.

Experimental and numerical study of transcritical oxygen-hydrogen rocket flame response to transverse acoustic excitation

The response of a transcritical oxygen-hydrogen flame to transverse acoustic velocity was investigated using a combination of experimental analyses and numerical modelling. The experiment was conducted on a rectangular rocket combustor with shear coaxial injectors and continuously forced transverse acoustic field. Simultaneous high-speed shadowgraph and filtered OH* radiation images were collected and reduced using dynamic mode decomposition in order to characterise the flame response to the acoustic disturbance. CFD modelling of a representative single injector under forcing conditions was carried out to gain insights into the three-dimensional features of the reacting flow field. Invisible in the 2D projection, the model reveals that the excited LOX jet develops into a flattened and widened structure normal to the imposed acoustic velocity. The comparison of co-located structures allowed features in the imaging to be attributed to the deformation and transverse displacement of lower density oxygen surrounding the denser liquid oxygen core by the transverse acoustic velocity.

Laminar Microjet Diffusion Flame Response to Transverse Acoustic Excitation

ABSTRACT The present experiments focused on the response of burning gaseous fuel jets to prescribed transverse acoustic excitation as a means of exploring the coupling of reactive, acoustic, and flow processes relevant to combustion instabilities. Single reactive methane microjets were exposed to transverse resonant standing wave disturbances within an acoustic waveguide for which a range of amplitudes of excitation were applied. Temporal flame response to acoustic excitation was studied via simultaneous phase-locked OH* chemiluminescence and visible imaging, and additionally via high speed, temporally resolved visible imaging. Quantification of flame dynamics was achieved for phase-locked data via the Rayleigh and temporal flame distortion measurements, and for time-resolved data via proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). Characteristic signatures associated with various types of flame response were identified, including weakly oscillatory combustion, full-scale flame coupling and lock-in to excitation, and multi-mode flame dynamics preceding flame extinction. It was found that modal decomposition of flame dynamics via POD and DMD analysis revealed important additional dynamical features and signatures in metrics characteristic of transitions in the flame-acoustic coupling.

Comparison of Single- and Multinozzle Reacting Swirl Flow Dynamics

This paper compares flow structures in a single- and triple-nozzle swirl combustor. Realistic combustor hardware usually contains several nozzles; however, test facilities for studying flame dynamics commonly use one nozzle. As such, it is important to determine how nozzle–nozzle interactions influence the dynamics of the system. Simultaneous 5 kHz OH planar laser-induced fluorescence and stereoscopic particle image velocimetry measurements were obtained from a premixed, swirling methane–air flame. Time-averaged flowfields of the single- and triple-nozzle configurations are compared, showing agreement in the axial velocities and flow spreading angles. However, the comparison reveals nonnegligible differences in reverse flow velocities of the recirculation zone and in flame spreading angles. The paper then compares the response of the unsteady flowfield to transverse acoustic excitation, for two configurations that excite strong axisymmetric and helical shear-layer disturbances. These disturbances exhibit similar spatial growth and decay trends as well as nearly identical convection speeds upstream of the jet merging region. These results corroborate previous findings, which show that, despite differences in time-averaged quantities, comparable flow dynamics occur in single- and multinozzle swirl flames.

Combustion of hydrogen in round and plane microjets in transverse acoustic field at small Reynolds numbers as compared to propane combustion in the same conditions (Part I)

In the first part of this study, experimental (hot–wire and visualization) results on the effect of transverse acoustic oscillations on characteristics of round and plane air microjets at low Reynolds numbers and on their propane and hydrogen combustion are presented. In both cases, the transverse acoustic excitation results in the flame bifurcation. A phenomenon observed at the nozzle exit is the so-called «bottleneck flame». This is found both at acoustic excitation and without it at combustion of hydrogen. More details on this phenomenon are given in the second part of the study titled as: “Features of diffusion combustion of hydrogen in round and plane high-speed microjets (Part II)”.

Experimental study of propagation of instability waves in a submerged jet under transverse acoustic excitation

An experimental study was conducted on the specific features of instability wave propagation in the mixing layer of a turbulent jet when the jet is excited by an external acoustic wave. We used the technique of conditional phase averaging of data obtained by particle image velocimetry using the reference signal of a microphone placed near the jet. The influence of the excitation frequency on the characteristics of large-scale structures in the mixing layer was investigated. It is shown that the propagation patterns of the instability waves agree well with previously obtained data on the localization of acoustic sources in turbulent jets.

Transverse excitations in liquid Fe, Cu and Zn

Transverse acoustic (TA) excitation modes were observed in inelastic x-ray scattering spectra of liquid Fe, Cu and Zn. From the analysis of current correlation functions, we concluded that TA excitation modes can experimentally be detected through the quasi-TA branches in the longitudinal current correlation spectra in these liquid metals. The microscopic elastic constants are estimated and a characteristic difference from macroscopic polycrystalline value was found in Poisson's ratio of liquid Fe, which shows an extremely softer value of ∼0.38 compared with the macroscopic value of ∼0.275. The lifetime of the TA modes were determined to be ∼0.45 ps for liquid Fe and Cu and ∼0.55 ps for liquid Zn, reflecting different interatomic correlations between liquid transition metals and non-transition metals. The propagation length of the TA modes are ∼0.85 nm in all of liquid metals, corresponding to the size of icosahedral or similar size of cages formed instantaneously in these liquid metals.

Visualization of Shear Layer Dynamics in a Transversely Forced Flow and Flame

This study addresses the response of a swirling annular jet flow and flame to transverse acoustic excitation in order to better describe key velocity-coupled processes during transverse combustion instabilities in lean, premixed flames. In particular, visualization and velocimetry techniques provide information about the effects of acoustic excitation on unsteady vortex development in the shear layers. Without acoustic forcing, the shear layers roll up into small vortices, driven by the Kelvin–Helmholtz instability, that convect downstream and grow. In the presence of high-amplitude acoustic forcing, as would be present during a combustion instability, the acoustic oscillations drive shear layer vortices to undergo a strong rollup event. Smoke visualization provides visual evidence of the rollup, while particle image velocimetry measurements show the development and trajectory of this large structure. Finally, planar laser-induced fluorescence of OH shows how the large coherent structure causes flame wrinkling at the base of the flame. The vortex rollup, responding at the frequency of acoustic forcing, creates large-scale wrinkles on the flame and can be the main driver of flame response during combustion instability.

Effect of Density Gradient on Flame–Acoustic Interaction

Dynamic interactions between shear coaxial injector flames and periodic compression waves are investigated under controlled density ratio variations. Turbulent diffusion flames are established between an oxygen slot jet in the middle and two hydrogen slot jets on the sides, representing a sectional view of a shear coaxial injector flowfield. Transverse acoustic excitation is applied using a compression driver mounted on one side of the combustor to force possible flame–acoustic interactions. While maintaining a stoichiometric fuel–oxidizer ratio, the density of each stream is controlled by diluting the propellant gases with inert helium and argon by appropriate amounts. Results show strong dependence of flame–acoustic interaction on the density gradient between the propellants. The observed trend is consistent with the vorticity production mechanism created by the baroclinic source term in the flowfield. It is observed that the flames become more resilient to acoustic coupling when the fuel–oxidizer density difference is reduced. The results open up the possibility of using density gradient manipulations to extend the overall stability margin of shear coaxial injector flames.

Transverse excitations in liquid Sn

Transverse acoustic (TA) excitation modes were observed in inelastic x-ray scattering (IXS) spectra of liquid Sn. The excitation energies and widths of the TA modes are in good agreement with results of an ab initio molecular dynamics simulation. By comparing current correlation spectra between the experimental and theoretical results quantitatively, we have concluded that the TA modes can be detected experimentally through the quasi-TA branches in the longitudinal current correlation spectra. The lifetime and propagation length of the TA modes were determined to be ∼0.7 ps and 0.8–1.0 nm, respectively, corresponding to the size of cages formed instantaneously in liquid Sn.

1121 SWIRL FLAME INTERACTION WITH TRANSVERSE AND AXIAL ACOUSTIC FORCING

The present study is an experimental investigation on the effect of axial and transverse acoustic excitation on a swirl-stabilized flame. A single burner test-rig with transverse extensions to facilitate transverse acoustic excitation is employed in this experimental study. The swirl flow is established using a generic radial swirl generator. Results presented here include phase-averaged crosswise flow field oscillations at isothermal flow conditions and partial orthogonal decomposition (POD) analysis of flow field acquired in the streamwise plane at isothermal and reacting conditions. Through analysis of radial velocity profile oscillations obtained from crosswise measurements and modal decomposition of the velocity field in the streamwise direction, conclusions have been derived for flow and flame response to transverse forcing. It is found that while out-of-phase transverse forcing can interact with natural helical oscillations in the swirl flow, the effect of in-phase forcing is dominantly observed in the form of flow response to induced axial oscillations. The study complements previous investigations on swirl flow and flame interaction with transverse acoustics, and provides information for future efforts on the issue of thermoacoustic instability in annular combustors.

Recirculation zone dynamics of a transversely excited swirl flow and flame

This work investigates the response of the vortex breakdown region of a swirling, annular jet to transverse acoustic excitation for both non-reacting and reacting flows. This swirling flow field consists of a central vortex breakdown region, two shear layers, and an annular fluid jet. The vortex breakdown bubble, a region of highly turbulent recirculating flow in the center of the flowfield, is the result of a global instability of the swirling jet. Additionally, the two shear layers originating from the inner and outer edge of the annular nozzle are convectively unstable and rollup due to the Kelvin-Helmholtz instability. Unlike the convectively unstable shear layers that respond in a monotonic manner to acoustic forcing, the recirculation zone exhibits a range of response characteristics, ranging from minimal response to exhibiting abrupt bifurcations at large forcing amplitudes. In this study, the response of the time-average and fluctuating recirculation zone is measured as a function of forcing frequency, amplitude, and symmetry. The time-average flow field is shown to exhibit both monotonically varying and abrupt bifurcation features as acoustic forcing amplitude is increased. The unsteady motion in the recirculation zone is dominated by the low frequency precession of the vortex breakdown bubble. In the unforced flow, the azimuthal m = −2 and m = −1 modes (i.e., disturbances rotating in the same direction as the swirl flow) dominate the velocity disturbance field. These modes correspond to large scale deformation of the jet column and two small-scale precessing vortical structures in the recirculation zone, respectively. The presence of high amplitude acoustic forcing changes the relative amplitude of these two modes, as well as the character of the self-excited motion. For the reacting flow problem, we argue that the direct effect of these recirculation zone fluctuations on the flame response to flow forcing is not significant. Rather, flame wrinkling in response to flow forcing is dominated by shear layer disturbances. Recirculation zone dynamics primarily influence the time-average flame features (such as spreading angle). These influences on the flame response are indirect, as they control the transfer function relating shear layer fluctuations and the resulting flame response.

Swirl effects on harmonically excited, premixed flame kinematics

This paper describes the response of a swirling premixed flame with constant burning velocity to non-axisymmetric harmonic excitation. This work extends prior studies of axisymmetric forcing, which have shown that wrinkles are excited on the flame that propagate downstream along the mean flame surface at a speed given by U o cos ψ, where U o is the mean flow velocity and ψ is the flame angle. The swirl component in the flow field introduces an azimuthal transport mechanism for disturbances on the flame. As such, the flame response at any given position is a superposition of flame wrinkles excited at earlier times, upstream axial locations, and different azimuthal positions. These swirl transport effects do not arise in problems where axisymmetric flames are subjected to axisymmetric excitation, but enter quite prominently in the presence of non-axisymmetries, such as when the flame is subjected to transverse excitation. The solution characteristics are strongly dependent upon the ratio of angular rotation rate to excitation frequency, denoted by σ = Ω/ ω, which describes the fraction of azimuthal rotation a disturbance makes in one acoustic period. When σ ≪ 1 and σ ≫ 1, the axial wavelength of flame wrinkles scales with the convective wavelength, λ c , but becomes much longer for σ ∼ O(1). The spatial variation in phase of flame wrinkling is also strongly dependent upon σ. Regardless of swirl number, flame wrinkles propagate in helical spirals along the solution characteristics at a phase speed equal to the local tangential velocity. The axial phase characteristics of flame wrinkling at a fixed azimuthal location, as would be measured by laser sheet imaging, are much more complex. For σ < 1, the wrinkles exhibit the familiar negative roll-off character for the phase with axial downstream distance, indicative of an axially convecting disturbance. The slope of this phase roll-off decreases with increasing σ, however, and becomes zero at σ = 1 for a compact flame. For σ > 1, the wrinkles actually have a positive roll-off character for the phase with axial downstream distance, indicating a flame wrinkle with a negative trace velocity, but whose actual propagation velocity is positive. Finally, these results show that while the flame response to transverse acoustic excitation is quite strong locally, its spatially integrated effect is much smaller for acoustically compact flames. This suggests that the dominant mechanism through which the flame responds globally to transverse excitation is the induced vortical and longitudinal acoustic fluctuations.