Acoustic Pressure Fluctuations Research Articles

A simple method for coupled acoustic-mechanical analysis with application to gradient-based topology optimization

A simple computational framework for analysis of acoustic-mechanical coupling is proposed. The method is based on extended finite element models for structural vibrations and acoustic pressure fluctuations using artificial mechanical and acoustic parameters in the non-structural and non-acoustic domains, respectively. The acoustic-mechanical interaction is created using a self-coupling matrix assembled in the entire computational domain, effectively generating coupling at acoustic-mechanical interface boundaries. The simple analysis tool circumvents the need for explicit interface tracking with accuracy controlled explicitly using a contrast parameter between the physical and artificial material parameters. Furthermore, the method’s direct applicability to gradient-based topology optimization, where elements can turn from mechanical to acoustic and vice versa, is demonstrated and illustrated using a simple example.

Extensions of the meshless finite volume particle method (FVPM) for static and dynamic free-surface flows

The Finite Volume Particle Method (FVPM) is a meshless method that incorporates features of both Smoothed Particle Hydrodynamics and the Finite Volume Method. Here, two new formulations are presented which enhance its performance in simulation of free surface flows. One is a method for determining the velocity of the free surface, making use of a partial Riemann problem to analyse the flow between the particle barycentre and the geometric free surface. The second is a well-balanced formulation for gravity forces that enables hydrostatic equilibrium to be preserved exactly. Results are presented for hydrostatic, 1D impact, dambreak and deep-water standing wave test cases. Computations display convergence and good agreement with experimental data. The new methods recover exact hydrostatic equilibrium, improve robustness and reduce acoustic pressure fluctuations in dambreak flows, and reduce dissipation.

Numerical study of potential application of active suction and blowing through blade tip perforation to reduction of helicopter rotor thickness noise

The article explains a new concept (called here ATNC – active thickness noise control) of the application of surface transpiration to the reduction of helicopter main rotor harmonic noise. The ATNC method is based on an introduction of four cavities covered by perforated plates (connected to pressure reservoirs) at the leading and trailing edges of the outer 20% of blade span. For an exemplary, two-bladed (NACA 0012 airfoil) model helicopter rotor of Purcell in low-lifting, hover conditions (tip Mach number MaT = 0.66 and collective θ = 1.5° ) the results of numerical simulations, based on the SPARC code (Spalart-Allmaras turbulence and Bohning-Doerffer transpiration models), prove that the acoustic pressure fluctuations are significantly reduced by ATNC in the near-field of blade tip. The peak amplitude is attenuated by more than 45%, with a reduction of the Overall Sound Pressure Level OASPL by 3.4 dB. It is shown also that the venting has not only a large impact on the acoustic radiation, but also on the aerodynamic performance of the rotor, expressed in terms of the torque penalty of 38%. The ATNC method proves to be a promising candidate as a mean of helicopter rotor thickness noise control, but not in the current arrangement.

Premixed Flame Dynamics in Response to Two-Dimensional Acoustic Forcing

ABSTRACTThermoacoustic instability in annular combustors involves feedback interaction between individual (acoustically compact) flames and – predominantly – azimuthal acoustic modes of the annular combustion chamber. During such an interaction, the flames experience acoustic pressure fluctuations in the combustor and transverse acoustic velocity fluctuations simultaneously. To investigate the dynamics of premixed swirl flames under the influence of transverse acoustics, experiments were performed on a single-burner atmospheric combustion rig with controlled simultaneous axial and transverse acoustic forcing, where the axial acoustics simulates the effect of acoustic pressure fluctuations in the combustor. In this paper, we present results on the descriptive analysis of spatially resolved flame surface fluctuations, acquired via OH-PLIF (OH* planar laser-induced florescence) diagnostics of the forced flame and establish the role of the interaction among coherent structures generated by axial and transverse velocity components in generating global flame response changes with respect to the phase difference between axial and transverse acoustic forcing as reported recently (Saurabh and Paschereit, Combustion Flame 182, 2017).

Analysis of Acoustic Pressure Fluctuation around Wind Farms

Analysis of Acoustic Pressure Fluctuation around Wind Farms

Noise investigation of a cavitating orifice: use of cfd simulation and ffowcs williams–hawkings (fw-h) formulation to gain insight into acoustics condition

In piping systems, hydrodynamic cavitation is well known for its destructive capabilities such as generation of intense noise. The investigation of the effect of the cavitation on acoustic pressure fluctuations and resonance frequencies provides much information about the geometry of the cavitation area, the propagation and the damping characteristics of the cavitation medium. The acoustical effects of cavitation caused by a water flow through a single-hole orifice have been investigated by using Computational Fluid Dynamics (CFD) and Ffowcs Williams–Hawkings (FW-H) formulation. The fluid zone downstream of the orifice where the cavitation happens considered as acoustic source. Time-accurate solutions of the flow-field variables are obtained from large eddy simulation (LES). Two cavitation regimes are investigated and for each regime, the sound pressure signals far downstream of the orifices are computed by applying the Ffowcs Williams–Hawkings (FW-H) formulation. From the results, it can be concluded that the spectral characteristics of the sound pressure fluctuations are influences by the cavitation condition. The super cavitation condition generates significantly more noise than the developed cavitation condition. Moreover, the results demonstrate that a combination of LES and FW-H formulation is a promising tool for acoustical study of cavitating orifices.

Numerical analysis of supersonic flows over an aft-ramped open-mode cavity

The characteristics of supersonic cavity flows were investigated using large eddy simulation together with the acoustic analogy method. A fifth-order hybrid compact-weighted essentially non-oscillatory scheme was applied to calculate the convective flux, and a sixth-order compact scheme was used for the viscous flux. Farassat's Formula 1A was used to solve the Ffowcs William–Hawkings equations to obtain the far-field acoustic pressure fluctuations. The effects of cavity configuration and flow Mach number on the pressure waves generated by the interaction of shear vortices and cavity were compared. The ramped rear-step of the cavity can increase the more-organized level of flow coherent structures, and decrease the static pressure distribution on the cavity bottom-wall. The attenuation of the interactions between the shear layer vortex and the aft-ramped wall of the cavity can reduce the feedback of pressure waves upstream. The energy of the main frequency significantly decreases for the far-field acoustics in the upstream flow over the ramped rear-step cavity. Thus, the effectiveness of noise reduction by the rear-ramp is considerable for the area upstream of the cavity for the present conditions, but it is not the case opposite to and downstream of the cavity. The present conclusions are valuable for evaluating the performance of noise suppression by cavities embedded in supersonic inflows in engineering applications.

Direct and Indirect Noise Generated by Entropic and Compositional Inhomogeneities

Flow disturbances are generated inside a duct via pulsed injection of helium into a flow of air. This leads to the generation of an acoustic pulse (direct noise), as well as the production of entropic and compositional inhomogeneities, which are convected with the mean flow. As these inhomogeneities are convected through a choked nozzle, they generate indirect noise. The resulting acoustic pressure fluctuations are measured experimentally using pressure transducers upstream of the nozzle. Insight obtained from theoretical models and a time-delay analysis can be used to isolate and extract the contributions of direct and indirect noise in the experimental signal. These results are directly compared to existing one-dimensional (1D) direct and indirect noise models. The experimental measurement of indirect noise is found to be in good agreement with the theoretical models for entropy noise and compositional noise for a compact 1D isentropic nozzle.

Flame Dynamics Intermittency in the Bistable Region Near a Subcritical Hopf Bifurcation

We report experimental evidence of thermoacoustic bistability in a lab-scale turbulent combustor over a well-defined range of fuel–air equivalence ratios. Pressure oscillations are characterized by an intermittent behavior with “bursts,” i.e., sudden jumps between low and high amplitudes occurring at random time instants. The corresponding probability density functions (PDFs) of the acoustic pressure signal show clearly separated maxima when the burner is operated in the bistable region. The gain and phase between acoustic pressure and heat release rate fluctuations are evaluated at the modal frequency from simultaneously recorded flame chemiluminescence and acoustic pressure. The representation of the corresponding statistics is new and particularly informative. It shows that the system is characterized, in average, by a nearly constant gain and by a drift of the phase as function of the oscillation amplitude. This finding may suggest that the bistability does not result from an amplitude-dependent balance between flame gain and acoustic damping, but rather from the nonconstant phase difference between the acoustic pressure and the coherent fluctuations of heat release rate.

Decomposition of Hydro and Acoustic Pressure Fluctuations in Turbulent Boundary Layer by using Wavenumber Frequency Spectrum Measurement

Decomposition of Hydro and Acoustic Pressure Fluctuations in Turbulent Boundary Layer by using Wavenumber Frequency Spectrum Measurement

Stability switching behavior of thermoacoustic oscillation in Rijke tube

Large-amplitude self-excited thermoacoustic oscillations arising due to the interaction between unsteady heat release and acoustic pressure fluctuations have been encountered in many thermal devices. These oscillations may lead to unwanted structural vibrations and efficiency reduction while emitting loud noises, and thus the predicting of such oscillations is very important. Physically, oscillation is a kind of instability, so stability analysis can be applied to understanding such a phenomenon. The present work focuses on the role of time delay between unsteady heat release and flow perturbation in the stability of thermoacoustic system. To this end, one-dimensional Rijke tube model with both open ends is numerically investigated. In the Rijke tube model, an electric heater is located at the first quarter of the Rijke tube and its unsteady heat release rate is modeled by an empirical model proposed by Heckl. Non-dimensional momentum equation and energy equation of the acoustic perturbation are derived and solved in time domain by using the Galerkin technique. The time evolution of the thermoacoustic oscillations with continuous increase in the time delay is calculated in two different acoustic damping cases, namely the heavily damped case and the weakly damped case, while other parameters are fixed. It is found that in both the heavily damped case and the weakly damped case, the system stability switches between stability and instability as the time delay increases, which is called stability switching and is a typical nonlinear phenomenon in a delay-dependent system. However, compared with in the heavily damped case, in the weakly damped case, the stability region is enlarged and the amplitude of the limit cycle oscillation is increased. Besides, in the weakly damped system, the dominating mode of system shifts in the first three modes instead of keeping in the first mode during increasing the time delay, which suggests that for the weakly damped system, the higher modes cannot be neglected and the system cannot be analyzed with a single-mode model either. Further, the bifurcation plots for the variation of the time delay for these two cases show that the system stability changes with time delay for a period of two, which is equal to the period of the first acoustic mode. As a conclusion, the results of present work indicate that the time delay between unsteady heat release and flow perturbations plays a critical role in generating thermoacoustic oscillations, and the findings of stability switching can help to understand the nonlinear phenomena in thermoacoustic systems.

Wavenumber-Frequency Analysis of Internal Aerodynamic Noise in Constriction-Expansion Pipe

High-pressure gas is produced during the oil production process at offshore plants, and pressure relief devices, such as valves, are widely used to protect related systems from it. The high-pressure gas in the pipes connected to the flare head is burned at the flare stack, or, if it is nontoxic, is vented to the atmosphere. During this process, excessive noise is generated by the pressure relief valves that are used to quickly discharge the high-pressure gas to the atmosphere. This noise sometimes causes severe acoustic-induced vibration in the pipe wall. This study estimated the internal aerodynamic noise due to valve flow in a simple constriction-expansion pipe, by combining the large eddy simulation technique with a wavenumber-frequency analysis, which made it possible to decompose the fluctuating pressure into the incompressible hydrodynamic pressure and compressible acoustic pressure. First, the steady-state flow was numerically simulated, and the result was compared with a quasi-one-dimensional theoretical solution, which confirmed the validity of the current numerical method. Then, an unsteady simulation analysis was performed to predict the fluctuating pressure inside a pipe. Finally, the acoustic pressure modes in a pipe were extracted by applying the wavenumber-frequency transform to the total pressure field. The results showed that the acoustic pressure fluctuations in a pipe could be separated from the incompressible ones. This made it possible to obtain accurate information about the acoustic power, which could be used to assess the likelihood of a piping system failure due to acoustic-induced vibration, along with information about the acoustic power spectrum of each acoustic mode, which could be used to facilitate the systematic mitigation of the potential acoustic-induced vibration in piping systems.

Experimental observation of pulsating instability under acoustic field in downward-propagating flames at large Lewis number

According to previous theory, pulsating propagation in a premixed flame only appears when the reduced Lewis number, β(Le-1), is larger than a critical value (Sivashinsky criterion: 4(1 +3) ≈ 11), where β represents the Zel'dovich number (for general premixed flames, β ≈ 10), which requires Lewis number Le > 2.1. However, few experimental observation have been reported because the critical reduced Lewis number for the onset of pulsating instability is beyond what can be reached in experiments. Furthermore, the coupling with the unavoidable hydrodynamic instability limits the observation of pure pulsating instabilities in flames. Here, we describe a novel method to observe the pulsating instability. We utilize a thermoacoustic field caused by interaction between heat release and acoustic pressure fluctuations of the downward-propagating premixed flames in a tube to enhance conductive heat loss at the tube wall and radiative heat loss at the open end of the tube due to extended flame residence time by diminished flame surface area, i.e., flat flame. The thermoacoustic field allowed pure observation of the pulsating motion since the primary acoustic force suppressed the intrinsic hydrodynamic instability resulting from thermal expansion. By employing this method, we have provided new experimental observations of the pulsating instability for premixed flames. The Lewis number (i.e., Le ≈ 1.86) was less than the critical value suggested previously.

Oxy-fuel combustion of methane in a swirl tubular flame burner under various oxygen contents: Operation limits and combustion instability

Oxy-fuel combustion of methane was conducted in a rapidly mixed tubular flame burner, as oxygen content (mole fraction) was varied from 0.25 to 1.0. The fuel and oxidizer were injected from individual slits, and the slit widths were varied to promote mixing. Temporal variation of flame structure and acoustic pressure fluctuations were simultaneously measured by using a high speed video camera and a microphone respectively to examine unsteady combustion. Detailed results show that when the oxygen mole fraction was below 0.5, a steady flame can be established in a wide range from lean to rich; above 0.6, combustion-driven oscillation occurred by increasing the equivalence ratio to around 1.0. By reducing fuel slit width to half of the oxidizer slit and assigning CO2 to both fuel and oxidizer slits to maintain the same, high injection velocities, steady tubular combustion range was much expanded. However, when the oxygen mole fraction exceeded 0.7, steady combustion evolved into periodic oscillations by gradually increasing equivalence ratio; around stoichiometry the flame became steady, and thereafter evolved into unsteady state in the rich condition. Based on pressure fluctuation measurements and frequency spectral analyses, high frequency (around 2000Hz) combustion-driven oscillation was observed. This study shed lights on the operation limits of steady oxy-methane tubular combustion under various oxygen contents. For combustion under ultrahigh oxygen contents (0.7–1.0), the inlet width should be much thinner, and the fuel and oxidizer streams be maintained at the same, high injection velocities to achieve steady tubular flame around the stoichiometry.

Thermoacoustic instability as mutual synchronization between the acoustic field of the confinement and turbulent reactive flow

Thermoacoustic instability is the result of a positive coupling between the acoustic field in the duct and the heat release rate fluctuations from the flame. Recently, in several turbulent combustors, it has been observed that the onset of thermoacoustic instability is preceded by intermittent oscillations, which consist of bursts of periodic oscillations amidst regions of aperiodic oscillations. Quantitative analysis of the intermittency route to thermoacoustic instability has been performed hitherto using the pressure oscillations alone. We perform experiments on a laboratory-scale bluff-body-stabilized turbulent combustor with a backward-facing step at the inlet to obtain simultaneous data of acoustic pressure and heat release rate fluctuations. With this, we show that the onset of thermoacoustic instability is a phenomenon of mutual synchronization between the acoustic pressure and the heat release rate signals, thus emphasizing the importance of the coupling between these non-identical oscillators. We demonstrate that the stable operation corresponds to desynchronized aperiodic oscillations, which, with an increase in the mean velocity of the flow, transition to synchronized periodic oscillations. In between these states, there exists a state of intermittent phase synchronized oscillations, wherein the two oscillators are synchronized during the periodic epochs and desynchronized during the aperiodic epochs of their oscillations. Furthermore, we discover two different types of limit cycle oscillations in our system. We notice a significant increase in the linear correlation between the acoustic pressure and the heat release rate oscillations during the transition from a lower-amplitude limit cycle to a higher-amplitude limit cycle. Further, we present a phenomenological model that qualitatively captures all of the dynamical states of synchronization observed in the experiment. Our analysis shows that the times at which vortices that are shed from the inlet step reach the bluff body play a dominant role in determining the behaviour of the limit cycle oscillations.

Investigation of air-assisted sprays submitted to high frequency transverse acoustic fields: Droplet clustering

An experimental investigation of the effects of a high amplitude transverse acoustic field on coaxial jets is presented in this paper. Water and air are used as working fluids at ambient pressure. The coaxial injectors are placed on the top of a semi-open resonant cavity where the acoustic pressure fluctuations of the standing wave can reach a maximum peak-to-peak amplitude of 12 kPa at the forcing frequency of 1 kHz. Several test conditions are considered in order to quantify the influence of injection conditions, acoustic field amplitude, and injector position with respect to the standing wave acoustic field. A high speed back-light visualization technique is used to characterize the jet response. Image processing is used to obtain valuable information about the jet behavior. It is shown that the acoustic field drastically affects the atomization process for all atomization regimes. The position of the injector in the acoustic field determines the jet response, and a droplet-clustering phenomenon is highlighted in multi-point injection conditions and quantified by determining discrete droplet location distributions. A theoretical model based on nonlinear acoustics related to the spatial distribution of the radiation pressure exerted on an object explains the behavior observed.

Dynamics of premixed swirl flames under the influence of transverse acoustic fluctuations

Flame dynamics in the presence of transverse acoustic fluctuations is an aspect of relevance to modern annular combustors. In particular, it is a key element underlying the occurrence and characteristics of thermoacoustic instability in annular combustors, which primarily involves azimuthal acoustic modes. In this paper, we present our experimental investigation of the response of a generic swirl-stabilized premixed flame to various transverse acoustic forcing configurations. Experiments were performed on a single burner rig featuring a combustor with transverse extensions. The flame was subjected to transverse acoustic fluctuations by generating controlled standing acoustic modes in this combustor. Two main results constitute this study: First, the obtained results reveal that flame response to transverse acoustic pressure fluctuations is quantitatively different when compared to axial forcing for low frequencies. Second, transverse acoustic velocity affects flame response to axial forcing such that the effects are dependent on the amplitude of transverse velocity fluctuations, the phase difference between axial and transverse forcing, and the inherent asymmetry of the flame.

Research on effecting mechanism of environmental parameters on human ear

Sound and air pressure both have an important effect on the ear of human beings. In this paper, we established the finite element model of human middle ear to study the sound transmission mechanism and the stress of human ear under the three conditions of acoustic environment, low pressure acoustic environment and pressure fluctuation environment. Experiment and theoretical calculation were also done in the analysis process. The results show that the surface state of the tympanic membrane can reflect the human auditory sensation to a certain extent. Low pressure can reduce the acoustic energy into human ear, and pressure fluctuation leads the ear to bear great stress and temporary hearing loss. In this paper, the quantification of some objective factors is of great significance to the optimization of some standards and the decoupling of multi environmental effects.

Criterion for vortex acoustic lock-on in combustors with backward facing step

In thermoacoustic systems vortex separation and breakdown are an important source of heat release rate fluctuations. The coupling between the acoustic field and the energy released by vortex breakdown can cause instability in these systems under certain operating conditions. Non-linear simulations show that combustion instability occurs after vortex acoustic lock-on. The objective of this work is to determine the criterion for vortex acoustic lock-on and thus the criterion for combustion instability as well. The 0-to-peak amplitude of the velocity fluctuations in the system are used as a criterion to determine the transition of Helmholtz number from vortex shedding mode to acoustic mode. Helmholtz number and acoustic pressure fluctuations computed using the model are compared with experimental investigations. The phenomenological model shows a good agreement with the experiments and the criterion determined for vortex lock-on provides a better understanding of the phenomenon.

Unsteady flow behavior of a steam turbine control valve in the choked condition: Field measurement, detached eddy simulation and acoustic modal analysis

Abstract The unsteady flow behavior of a steam turbine control valve in the choked condition, which occurs during the warming-up process of a steam turbine, was studied through complementary techniques, including field measurements of the valve spindle’s vibration behavior, detached eddy simulation (DES) of the unsteady flow field, and acoustic modal analysis of the valve chamber. Three peak frequencies at St = 0.044, St = 0.17, and St = 0.59 were identified from field measurements of the vibration behavior. Subsequently, the unsteady flow fields in the control valve were determined from DES and then analyzed using the state-of-the-art data-driven proper orthogonal decomposition (POD) method and cross-correlation analysis, which extracted the dominant unsteady flow behavior in relation to the valve spindle’s vibrations. The findings demonstrated that the valve spindle’s lateral force fluctuations at St = 0.019 occurred due to the alternating oscillations of the annular wall-attached jet, which resulted from the first two POD modes occupying 25% of the turbulent fluctuation energy. The valve spindle’s axial force fluctuations at St = 0.043 were attributed to the synchronous oscillations of the annular wall-attached jet, which resulted from the third, fourth, and fifth POD modes occupying 15% of the turbulent fluctuation energy. Finally, through acoustic modal analysis that compared the pressure fluctuations extracted from the DES results, the axial acoustic mode in the valve’s cavity at St = 0.19 was found to be associated with the valve spindle’s axial force fluctuations at St = 0.174, while the first circumferential acoustic mode of the valve diffuser at St = 0.61 was found to be associated with the valve spindle’s lateral force fluctuations at St = 0.62. This confirmed the potential coupling between the acoustic mode pattern and pressure fluctuation pattern. These complementary techniques were demonstrated to be effective methodologies for the flow-induced vibration behavior and intensive acoustics in valves.