Dominant Acoustic Mode Research Articles

Direct hydroacoustics analyses of pipe orifice using lattice Boltzmann method

Acoustic waves in water pipes pose a structural threat but also offer a valuable tool for non-invasive inspection. Complex pipe geometries such as bends and surface liners create complex fluid boundaries, triggering interactions between fluid flow and acoustics. The lattice Boltzmann method (LBM) excels at capturing these interactions near complex boundaries, in contrast to the finite volume method, which can result in errors. However, the application of LBM in water pipes has been limited by stability problems. This study proposes a two-step (DM-TS) collision operator based on a direct method (LBM-HA) for stable LBM simulations in water pipes. LBM-HA enables direct hydroacoustic predictions for both acoustic and dynamic flow fields in a pipe orifice. A novel acoustic-dynamic complex boundary condition was formulated from the linearized Euler's equation to account for the physical effects of the bounded domain of the LBM-HA analysis. To distinguish between dynamic and acoustic components, wavenumber and frequency decomposition techniques were applied to the pressure data obtained within the pipe. In addition, mode decomposition of the acoustic pressure was conducted to identify the dominant acoustic modes. By comparing the LBM-HA results with experimental data, we highlighted the method's potential for direct hydroacoustic analysis considering the acoustic characteristics of the water pipe.

Dominant duct azimuthal acoustic mode detection considering failed wall-installed microphones with amplitude or phase measurement biases

Reliable duct mode detection results are essential for aero-engine health condition monitoring and low-noise design. Harsh aero-engine measurement environments may cause biases in amplitude and phase measurements due to the failure of wall-mounted microphones, which will decrease the performance of advanced sparse representation algorithms for duct mode detection. In this paper, two strategies to detect the duct mode with the failed microphones are proposed. An optimization problem is constructed considering the low-rankness of theoretical array measurement and row-sparsity of the failed microphone measurements. The Strategy 1 is based on the recovery of the theoretical array measurement. The recovered array measurement can be further used to detect the duct mode via the sparsity-induced duct mode detection algorithm (generalized minimax-concave penalty in this paper). With the simulation, the Strategy 1 can not recover the theoretical array measurement completely. The Strategy 2 is based on the removal of the failed microphone measurements. With the simulation and experiment, it turns out that the Strategy 2 can detect the interested mode accurately.

Experimental mode decomposition investigation on 3-stage axial flow compressor stall phenomena using aeroacoustics measurements

Safely operating compressors is quite critical. Rotating stall is highly undesirable and affects negatively the performance and stability of aero-engine compressors. It is important to identify a precursor of such rotating stalling phenomena observed on the compressor. In this work, we conduct experimental measurements on a 3-stage axial flow compressor by using 8 acoustic pressure sensors. Emphasis is being placed on applying different mode decomposition methods such as Proper Orthogonal Decomposition (POD) and Empirical Mode Decomposition (EMD) on these acoustics data to shed lights on the stall phenomena. Comparison is then made between these methods and conventional means like continuous wavelet (CWA), and PSD (Power Spectrum Density) analyses to achieve a timely detection of stall inception and the energy distribution before and after stall occurred. It is found from POD that the dominant acoustic modes contributing to more than 90% of the total fluctuation energy are changed from 3 to 4, when the stall occurred. The EMD investigation shows that the compressor stall is associated with a few low-frequency IMFs (intrinsic mode function). Time evolutions of such IMFs are observed to grow from negligible amplitude disturbances into limit cycles. Additionally, applying CWA reveals that the stall cell is originated from the first stage and then propagates circumferentially and axially to the second and third stage. Finally, PSD analysis shows that the stall frequency related to the spike-type stall cell exists only on the first stage. This is different from the blade passing frequency as observed on all stages of the compressor. In summary, the present work offers alternative acoustic measurements to examine the dynamic instability of a compressor stall with advanced signal-processing techniques applied.

Cooling fan aerodynamic noise reduction with short inlet duct and its applicability

This paper presents experimental and numerical analyses of the noise-reduction method of applying short inlet ducts to axial-flow cooling fans. Far-field noise tests of three different-sized cooling fans with tip Mach numbers of 0.143, 0.156, and 0.174 demonstrated that tonal noise was the primary source of aerodynamic noise. Through the installation of short inlet ducts on these fans, the upstream propagation of tonal noise was reduced. The average total sound pressure levels were decreased by up to 3.5, 4.8, and 7.1 dBA, respectively. At the blade passing frequency, the dominant upstream-propagating azimuthal acoustic modes were suppressed. Full-passage unsteady computational fluid dynamics calculations revealed that the duct transforms multiple suction vortices into a single annular vortex structure and weakens the interaction with the rotor blades. In addition, the acoustic analogy was used to compute the reduction of far-field noise and the variations in the major acoustic modes. This revealed a noise-reduction trend similar to that seen in the experiments. Essentially, the duct restrains the non-uniform inlet flow to influence the various inlet–rotor–stator interaction modes. The application of this approach to various fan configurations confirmed that a duct with a parameter of L/D (duct length/duct inner diameter) equal to 0.08 can be recommended as a compact and efficient way to reduce noise.

Scattering characteristics through multiple regions of the wave-bearing trifurcated waveguide

ABSTRACT Present communication addresses the scattering of the dominant acoustic wave mode propagating from a semi-rigid waveguide. The underlying waveguide is tuned because of trifurcated inlet regions whereas outer surfaces are flexural. The matching of the scattering duct modes (orthogonal and non-orthogonal) is accomplished by employing continuity of pressure and velocity across the interface. The reflection and transmission coefficients are calculated using mode matching methods. The obtained infinite linear algebraic systems are truncated and solved numerically. The expressions for energy functionals in different duct regions are obtained and sketched subject to different parameters of interest. Through mathematical and graphical illustrations, the obtained solution is authenticated with the law of energy conservation and verification of geometric conditions.

Experimental investigation of aeroelastic instabilities in an aeroengine fan: Using acoustic measurements

Aeroelastic instabilities will occur to the aero-engine fans inevitably during transient operations, causing excessive blade vibration and threatening the structural integrity. However, the complicated fluid-solid coupling interactions in aero-engine are difficult to be described precisely using current numerical or analytical models. Instead, experimental investigations on the flow induced blade vibration can reveal the aeroelastic performance of aero-engines, which is regarded as an effective approach to provide instructive guidance in the design of aero-engine. In this paper, experiments on a realistic aero-engine fan with 3.5 stages is conducted to study the typical aeroelastic instabilities, including the blade forced response, flutter, and acoustic resonance. In addition to the conventional blade strain measurements, the duct acoustic array measurements are also implemented to characterize the distinguishing acoustic features of the aeroelastic instabilities. Particularly, the acoustic mode decomposition technique is employed to provide insight into the unsteady flow as the aeroelastic instabilities occur, by which the blade vibration and aerodynamic excitation are bridged. The run-up test is conducted firstly to investigate the blade forced response and flutter; thereafter, a transient test is designed to encounter the acoustic resonance. The results illustrate the acoustic features of aeroelastic instabilities properly, validating the feasibility of acoustic measurement for instability identification. When the blade forced response occurs in the run-up test, the amplitude of Tyler-Sofrin mode increases significantly. The flutter also occurs in the run-up test and is believed to be excited by the rotating instability, where a sloping strip in the acoustic mode spectrum can be observed. In terms of the acoustic resonance, a nonsynchronous tonal component is observed below the blade passing frequency, where the dominant acoustic mode order matches the nodal diameter number. This paper presents an alternative way to study vibrations using acoustic measurements and the acoustic mode decomposition, which is validated by experiments on a realistic aero-engine fan.

Altering Terahertz Sound Propagation in a Liquid upon Nanoparticle Immersion.

One of the grand challenges of new generation Condensed Matter physicists is the development of novel devices enabling the control of sound propagation at terahertz frequency. Indeed, phonon excitations in this frequency window are the leading conveyor of heat transfer in insulators. Their manipulation is thus critical to implementing heat management based on the structural design. To explore the possibility of controlling the damping of sound waves, we used high spectral contrast Inelastic X-ray Scattering (IXS) to comparatively study terahertz acoustic damping in a dilute suspension of 50 nm nanospheres in glycerol and on pure glycerol. Bayesian inference-based modeling of measured spectra indicates that, at sufficiently large distances, the spectral contribution of collective modes in the glycerol suspension becomes barely detectable due to the enhanced damping, the weakening, and the slight softening of the dominant acoustic mode.

Analysis of the vibro-acoustic behaviours of underwater reinforced rectangular plates

In this paper, the vibration and sound radiation of underwater stiffened plates with arbitrary boundary condition have been studied. The vibration of the structure is described by an improved Fourier series, the water-load acts on the structure is solved by Rayleigh integral, and the dynamic response is solved by an energy method. The results show that the increase of plate thickness and boundary constraint can enhance the vibration and sound transformation ability of underwater rectangular plate, and the mechanism is that the increase of vibration mode frequency increases the radiation efficiency of the dominant acoustic radiation mode. By analyzing the effect of stiffening mode and quantity on acoustic radiation, it can be seen that for the plate (1,1) mode, the longitudinal stiffening mode has the lowest vibration energy conversion rate, and the main mechanism is that the dominant acoustic radiation mode has the lowest radiation efficiency, while the transverse stiffening mode has the highest radiation efficiency. In addition, when the reinforcement is distributed, the natural frequency and the radiation efficiency of the corresponding dominant acoustic radiation modes of the stiffeners are reduced, the vibration- acoustic conversion of the plate showed a decreasing trend.

Comparison of strongly and weakly nonlinear flame models applied to thermoacoustic instability

Weakly nonlinear flame (or heater) dynamic models, only accounting for heat release rate disturbances from the flame (or heater) at forcing frequencies and omitting harmonic terms due to nonlinear mechanisms, are widely used in low-order tools for the analysis and prediction of thermoacoustic instabilities, because they have a numerical cost much cheaper than tools based on Navier–Stokes equations, and are easier to develop and validate. However, these models may lead to errors under certain conditions. The present work considers a Rijke tube model combustor, in which a classical third-order model is used to describe the flame dynamic response to the oncoming flow disturbance. We call this model the strongly nonlinear flame model. The weakly nonlinear flame model is then introduced. The wave-based approach is adopted as a low-order tool. The weakly and strongly nonlinear flame models are embedded in the low-order tool to reproduce the thermoacoustic instability of the model combustor. The natural frequency and growth rate of thermoacoustic instability are then determined by mode extracted methods. The differences between the results predicted by using the weakly and strongly nonlinear flame models are compared for a set of operating conditions, in order to find the conditions under which the weakly nonlinear flame model works. Short-time Fourier transform is adopted to analyze the extracted frequencies and growth rates of four selected cases. When the dominant acoustic mode strength is much stronger than the remaining modes, the weakly nonlinear models perform well. However, these models fail to capture the mode frequency and growth rate when multiple unstable modes are present.

Evaluation of the high-frequency oscillation parameters of a liquid-propellant rocket engine with an annular combustion chamber

The high-frequency instability (HF instability) of a liquid-propellant rocket engine (LPRE) during static firing tests is often accompanied by a significant increase in dynamic loads on the combustion chamber structure, often leading to the chamber destruction. This dynamic phenomenon can also be extremely dangerous for the dynamic strength of a liquid-propellant rocket engine with an annular combustion chamber. Computation of the parameters of acoustic combustion product oscillations is important in the design and static firing tests of such rocket engines. The main aim of this paper is to develop a numerical approach to determining the parameters of acoustic oscillations of combustion products in annular combustion chambers of liquid-propellant rocket engines taking into account the features of the configuration of the combustion space and the variability of the physical properties of the gaseous medium depending on the axial length of the chamber. A numerical approach is proposed. The approach is based on mathematical modeling of natural oscillations of a “shell structure of an annular chamber – gas” coupled dynamic system by using the finite element method. Based on the developed finite-element model of coupled spatial vibrations of the structure of the annular combustion chamber and the combustion product oscillations, the oscillation parameters of the system under consideration (frequencies, modes, and effective masses) for its dominant acoustic modes, the vibration amplitudes of the combustion chamber casing, and the amplitudes of its vibration accelerations can be determined. The operating parameters of the liquid-propellant rocket engine potentially dangerous for the development of thermoacoustic instability of the working process in the annular combustion chamber can be identified. For the numerical computation of the dynamic gains (in pressure) of the combustion chamber, a source of harmonic pressure excitation is introduced to the finite element model of the dynamic system “shell structure of an annular configuration – gas” (to the elements at the start of the chamber fire space). The developed approach testing and further analysis of the results were carried out for an engine with an annular combustion chamber (with a ratio of the outer and inner diameters of 1.5) using liquid oxygen – methane as a propellant pair. The system shapes and frequencies of longitudinal, tangential and radial modes are determined. It is shown that the frequency of the first acoustic mode in the case of a relatively low stiffness of the combustion chamber casing walls can be reduced by 40 percent in comparison with the frequency determined for a casing with rigid walls.

Experimental investigation on axial compressor stall phenomena using aeroacoustics measurements via empirical mode and proper orthogonal decomposition methods

In this work, we conduct experimental investigations on a single-stage axial compressor to shed lights on the dynamic stall phenomena via aeroacoustic measurements. For this, 8 acoustic pressure sensors are installed equally around the circumference of the compressor intake. The acoustic pressure data are simultaneously logged in real-time. Classical and conventional Fourier-transform based methods reveal that the compressor stall will occur beyond a critical pressure ratio or a flow coefficient as illustrated on a compressor map. However, there is no warning precursor obtained from the conventional signal analysis methods. Further investigations are conducted using a number of advanced signal processing techniques, such as empirical mode decomposition (EMD) and proper orthogonal decomposition (POD) and continuous wavelet (CW) methods. EMD analysis reveals that the compressor stall is corresponding to a low-frequency intrinsic mode function (IMF). It is growing rapidly from negligible amplitude random disturbances to limit cycle oscillations. POD shows that the number of the dominant acoustic modes contributing to more than 98.5% of the total fluctuation energy is changed from 3 to 7, when the stall occurs. This is insightful for low-order modeling of the compressor stall phenomena. Finally, applying CW transform could provide a warning of 0.05 s precursor on the tested axial compressor. In general, the present work opens up an alternative approach to study the dynamic physics of a compressor stall by applying an array of acoustic sensors with proper advanced data-processing methods implemented.

The Effects of Turbulent Burning Velocity Models in a Swirl-Stabilized Lean Premixed Combustor

Abstract The effects of turbulent burning velocities in a turbulent premixed combustion simulation with a G-equation are investigated using the 3D LES technique. Two turbulent burning velocity models – Kobayashi model, which takes into account the burning velocity pressure effect, and the Pitsch model, which considers the flame regions on the premixed flame structure – are implemented. An LM6000 combustor is employed to validate the turbulent premixed combustion model. The results show that the flame structures in front of the injector have different shapes in each model because of the different turbulent burning velocities. These different flame structures induce changes in the entire combustor flow field, including in the recirculation zone. The dynamic mode decomposition (DMD) method and linear acoustic analysis provide the dominant acoustic mode.

Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels by means of a slender wedge probe and direct numerical simulation

Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels were conducted by means of a slender wedge probe and direct numerical simulation. The study comprises comparative tunnel noise measurements at Mach 3, 6 and 7.4 in two Ludwieg tube facilities and a shock tunnel. Surface pressure fluctuations were measured over a wide range of frequencies and test conditions including harsh test environments not accessible to measurement techniques such as Pitot probes and hot-wire anemometry. A good agreement was found between normalized Pitot pressure fluctuations converted into normalized static pressure fluctuations and the wedge probe readings. Quantitative results of the tunnel noise are provided in frequency ranges relevant for hypersonic boundary-layer transition. Complementary numerical simulations of the leading-edge receptivity to fast and slow acoustic waves were performed for the applied wedge probe at conditions corresponding to the experimental free-stream conditions. The receptivity to fast acoustic waves was found to be characterized by an early amplification of the induced fast mode. For slow acoustic waves an initial decay was found close to the leading edge. At all Mach numbers, and for all considered frequencies, the leading-edge receptivity to fast acoustic waves was found to be higher than the receptivity to slow acoustic waves. Further, the effect of inclination angles of the acoustic wave with respect to the flow direction was investigated. An inclination angle was found to increase the response on the wave-facing surface of the probe and decrease the response on the opposite surface for fast acoustic waves. A frequency-dependent response was found for slow acoustic waves. The combined numerical and experimental approach in the present study confirmed the previous suggestion that the slow acoustic wave is the dominant acoustic mode in noisy hypersonic wind tunnels.

Mesoscale electric fluctuations interacting with zonal flows, magnetic fluctuations and turbulence

New mesoscale electric fluctuations (MSEFs) are identified in the edge plasmas of the HL-2A tokamak using multiple Langmuir probe arrays. The MSEFs, resulting from the synchronization and having components of dominant geodesic acoustic mode (GAM) and m/n = 6/2 potential fluctuations, are found at the same frequency as that of the magnetic fluctuations of m/n = 6/2 (m and n are poloidal and toroidal mode numbers, respectively). The temporal evolutions of the MSEFs and the magnetic fluctuations clearly show the frequency entrainment and the phase lock between the GAM and the m/n = 6/2 magnetic fluctuations. The results indicate that GAMs and magnetic fluctuations can transfer energy through nonlinear synchronization. The nonlinear coupling analyses show that the MSEFs couple to turbulence and low frequency zonal flows (LFZFs). This suggests that the MSEFs may contribute to the LFZF formation, reduction of turbulence level, and thus confinement regime transitions. The analysis of the envelope modulation demonstrates that both the MSEFs and the LFZFs modulate the turbulence while the MSEFs are modulated by the LFZFs.

Evaluation of scale-adaptive simulation for transonic cavity flows

Scale-adaptive simulations of transonic cavities with and without doors are presented in this paper. Results were compared with detached-eddy simulations for cavities with length-to-depth ratios of 5 and 7. The Mach and Reynolds numbers (based on the cavity length) were 0.85 and 6.5 × 106, respectively, and the grid sizes were 5.0 million for the clean cavity with doors-off and 5.5 million for the clean cavity with doors-on. Instantaneous Mach number contours showed that the shear layer broke down for both the doors on and doors off cases and that the flows had a high level of unsteadiness inside them. The two L/D ratios of cavities were seen to have similar acoustic signatures reaching maximum sound levels of 170 dB. Spectral analyses for the cavities without doors revealed that by changing the length-to-depth ratio from five to seven, the dominant acoustic modes at the front and rear of the cavities were shifted from the second and third modes to the first and second modes respectively. Proper orthogonal decomposition was used to reduce the data storage using modes constructed from flowfield snapshots taken at regular intervals.

Investigation of Diametral Acoustic Modes in a Model of a Steam Control Gate Valve

The objective of the present study is to provide an insight into mechanism of coupling between turbulent pipe flow and partially trapped diametral acoustic modes associated with a shallow cavity formed by the seat of a steam control gate valve. First, the effects of the internal pipe geometry immediately upstream and downstream of the shallow cavity on the characteristics of partially trapped diametral acoustic modes were investigated. The mode shapes were calculated numerically by solving a Helmholtz equation in a three-dimensional domain corresponding to the internal geometry of the pipe and the cavity. Second, the set of experiments were performed using a scaled model of a gate valve mounted in a pipeline that contained converging–diverging sections in the vicinity of the valve. Acoustic pressure measurements at three azimuthal locations at the floor of the cavity were performed for a range of geometries of the converging–diverging section and inflow velocities. The experimentally obtained pressure data were then used to scale the amplitude of the pressure in the numerical simulations. The present results are in good agreement with the results reported in earlier studies for an axisymmetric cavity mounted in a pipe with a uniform cross-section. The resonant response of the system corresponded to the second diametral mode of the cavity. Excitation of the dominant acoustic mode was accompanied by pressure oscillations corresponding to other acoustic modes. As the angle of the converging–diverging section of the main pipeline in the vicinity of the cavity increased, the trapped behavior of the acoustic diametral modes diminished, and additional antinodes of the acoustic pressure wave were observed in the main pipeline.

Experimental Study of Supersonic Entrainment Using a Cavity

The effect of wall-mounted three-dimensional cavities on the entrainment of gaseous injectant into a supersonic stream of Mach number 1.7 is studied. Entrainment induced by flow unsteadiness, turbulence, and exchange of fluid between the cavity and mainstream flow has been investigated. To clarify the influence of the ratio of cavity length to width on the fluid dynamic behavior, cavities of ratios between two and four were applied at a constant length-to-depth ratio . Acoustic oscillations generated from the cavity were observed to have profound impact on mass exchange between the cavity and the freestream flow. These acoustic oscillations were in turn found to be dependent on the ratio of the cavity. Shift in dominant acoustic mode was observed as the ratio was changed from three to four. Spillage of the shear layer over the cavity walls and turbulence induced in the cavity were probed by measuring velocity using laser Doppler velocimetry. Entrainment of secondary gaseous injection into the main flow from the cavity was also observed to depend on the cavity ratio.

Effects of cooling flow on the flow structure and acoustic oscillation in a swirl-stabilized combustor. Part II: Acoustic analysis

The proper orthogonal decomposition (POD) analysis was implemented to extract the acoustic modes from the flow field inside a swirl-stabilized combustor obtained from non-reacting large eddy simulation. The major frequencies and the spatial distribution of fluctuating pressure of dominant POD modes were presented. The first four dominant modes occupied more than 70 % of the total energy of the oscillatory flow fields. Cooling flow injection suppressed the amplitude of pressure fluctuation and tended to dissipate the high-order longitudinal acoustic modes. The dominant acoustic modes observed in the fluctuating pressure field in the case with no cooling flow injection were dissipated or shifted by the injection of cooling flow. .

THE SURFACE NITROGEN ABUNDANCE OF A MASSIVE STAR IN RELATION TO ITS OSCILLATIONS, ROTATION, AND MAGNETIC FIELD

We have composed a sample of 68 massive stars in our galaxy whose projected rotational velocity, effective temperature and gravity are available from high-precision spectroscopic measurements. The additional seven observed variables considered here are their surface nitrogen abundance, rotational frequency, magnetic field strength, and the amplitude and frequency of their dominant acoustic and gravity mode of oscillation. Multiple linear regression to estimate the nitrogen abundance combined with principal components analysis, after addressing the incomplete and truncated nature of the data, reveals that the effective temperature and the frequency of the dominant acoustic oscillation mode are the only two significant predictors for the nitrogen abundance, while the projected rotational velocity and the rotational frequency have no predictive power. The dominant gravity mode and the magnetic field strength are correlated with the effective temperature but have no predictive power for the nitrogen abundance. Our findings are completely based on observations and their proper statistical treatment and call for a new strategy in evaluating the outcome of stellar evolution computations.

Experimental Method Extracting Dominant Acoustic Mode Shapes for Automotive Interior Acoustic Field Coupled with the Body Structure

Experimental Method Extracting Dominant Acoustic Mode Shapes for Automotive Interior Acoustic Field Coupled with the Body Structure