Acoustic Damping Research Articles

Modeling nonlinear acoustic damping due to flow separation

Nonlinear acoustic damping has been observed in many high-amplitude acoustic systems as a result of flow separation and shear layer vortical motion, eventually transforming some of the acoustical energy into heat. The amount of nonlinear acoustic damping helps determine the nonlinear limit cycle amplitude, e.g., damping caused by baffle blades in a liquid rocket engine to reduce combustion instabilities. The damping mechanism is dependent on both the location and phase of flow separation. Identifying the flow separation is a function of both the boundary layer growth and the acoustically imposed pressure gradient. When the acoustic pressure gradient is adverse, the boundary layer is more prone to separation. Using this as a basis, a model can be created that is applicable to general geometry, which will then be used to approximate the nonlinear acoustic damping in various situations. The constructed model will be compared to established cases, such as an orifice in a duct, to validate the model.

Meta-interface textures for increased frictional damping in elastic structures

The unique behavior of acoustic and elastic metamaterials results from engineered subwavelength material composition and geometry. Though there has been a profusion of metamaterial research over the last two decades, relatively little focus has been on the dissipation of vibro-acoustic energy. Most research on metamaterials for acoustic damping have focused on viscous or viscoelastic effects [1]. However, the design of interfaces within materials or at connecting surfaces between elastic structures to amplify micro-friction events due to the relative motion of contacting surfaces is of interest for the reduction of vibro-acoustic energy. Dissipation due to friction is best represented as a nonlinear constitutive response. Here, we report on the use of friction via meta-interface textures to create interfaces with elevated damping, leading to increased macroscopic dissipation. We employ the five-parameter Bouc-Wen model to describe the complex behavior at these interfaces and study the influence of subwavelength interface texture on the parameters of the friction model. This increased dissipation from this approach can be used to reduce structural vibrations and the transmission of acoustical energy in structural components. [1] C. L. Bacquet et al., “Metadamping: Dissipation emergence in elastic metamaterials.” Adv. App. Mech. 51, 115i–164, (2018).

Effective Grüneisen constant in sodium fluoride crystals

In this paper, we propose a new approach to determining the components of the Grüneisen tensor and establishing unambiguous relationships between the effective Grüneisen constant, which characterizes the nonlinearity of interatomic interaction forces, and the attenuation anisotropy of acoustic waves. The cubic crystals of sodium fluoride were taken as an object of study. Measurements were carried out by the method of Bragg diffraction of light by sound in the frequency range 0.4–1.6 GHz.The obtained values of velocity and attenuation of acoustic waves were used to determine the real and imaginary components of the elasticity tensor. Next, the acoustic damping coefficients and the independent components of the Grüneisen tensor in sodium fluoride crystals were determined. It is shown that the calculations performed trough the attenuation coefficients make it possible to determine the effective Grüneisen constants in two ways, and these results are in good agreement with each other. Thus, the components of the acoustical Grüneisen tensor for sodium fluoride crystals are determined for the first time and the dependence of the effective Grüneisen constant on the direction of propagation and polarization of acoustic waves is calculated.

Investigation for acoustic performance analysis of stainless-steel, aluminium-alloy and epoxy-glass oil-pan

The research-paper compares the acoustic properties of oil pan made of stainless steel, aluminium alloy and epoxy glass fiber to identify best material to reduce the radiated noise of oil pan. This paper contains pictorial representation of the acoustic pressure and sound pressure level in forms such as isosurface view SPL plots and polar SPL plots to get spatial response and radiation pattern. In entire study target frequency of 1500 Hz is used. Acoustic analysis is performed by plotting sound pressure level plots for all three materials to identify material having lowest sound pressure level using ANSYS software. Epoxy glass fiber oil pan had higher acoustic damping properties as observed by sound pressure level plots and polar SPL plots. While stainless steel oil pan had very high acoustic contribution towards sound pressure level of engine.

Experimental investigation of geometric shape effect of coupled Helmholtz resonators on aeroacoustics damping performances in presence of low grazing flow

Helmholtz resonators are widely used as acoustic dampers to mitigate noise in gas turbines and automotive exhaust systems. In order to increase the damping performance and broaden the damping frequency range, multiple Helmholtz resonators are typically implemented. In this work, two connected Helmholtz resonators by sharing the same flexible or rigid sidewall, i.e. parallel-coupled are designed and tested on a cold-flow pipe system with a mean flow present. Two different geometric shapes of the coupled Helmholtz resonators are considered. One is cylindrically shaped and the other is rectangular. Both experimental and 3D numerical investigations are conducted to study the effects of the geometric shape and the mean grazing flow on the noise damping performance of the resonators. The numerical model is built in frequency-domain and solving linearized Navier-Stokes equations. To characterize and quantify the resonators' noise damping performance, transmission loss in dB is determined. It is experimentally found that the cylindrical shaped resonators are associated with much larger transmission loss. Compared with the rectangular-shaped resonators, approximately 12 dB more sound pressure level reduction is achieved by the cylindrical resonators, which is independent on the rigidness of the shared sidewall. The rigidity of the shared sidewall is shown to shift the frequencies corresponding to the maximum damping peaks, depending on the sidewall's flexural rigidity. Numerical results confirm that the geometric shape does play an important role in determining the resonator's damping. Further investigation of the mean grazing flow effect is conducted. It is shown experimentally and numerically that when the Mach number of the mean grazing flow is lower than 0.01, the noise damping performance of the coupled Helmholtz resonators is little changed over the frequency range from 150 to 600 Hz. However, as the mean flow Mach number is increased, the damping performance becomes deteriorated as observed numerically. The present work reveals the critical roles played by the geometric shape and the mean grazing flow on the aeroacoustics damping performance of the coupled-resonators.

Dust charge fluctuation and ion acoustic wave propagation in dusty plasma with q-nonextensive hot and Maxwellian cold electrons

We have employed the self-consistent kinetic theory to study the linear dispersion relation of ion acoustic waves in a four-component plasma consisting of nonextensive hot electrons, Maxwellian cold electrons, positive ions, and dust particles. The dust charging process with the modified ion acoustic wave damping, as well as its unstable mode, has been graphically illustrated. It is found that the dust charging mechanism depends on the density of hot electrons, the degree of nonextensive electron distribution, and the temperature ratio of hot to cold electrons. It is shown that the damping and instability rates of ion acoustic waves due to dust charge fluctuations explicitly depend on the choice of electron distribution and the magnitude of dusty plasma parameters. In addition, we have studied the ion acoustic Landau damping in the absence of dust particles. It is found that the weak damping region broadens, while the strong damping region shrinks and is shifted toward the short wavelength region for the increase in the temperature ratio of hot to cold electrons.

Investigation of the Effect of Electrolytes on the Breakaway of Air Bubbles at an Underwater Capillary Using High-Speed Cinematography and Passive Acoustic Techniques

Saline water froth flotation has received increasing attention in recent years due to sustainability-related concerns. Although the presence of electrolytes in these flotation systems is known to produce the desired bubble swarms, i.e., a macroscopic observation, the fundamental mechanism through which the solutes produce such an effect at the microscopic level remains obscure. For example, there is no agreed mechanism (i.e., break-up or coalescence—two major bubble formation mechanisms) of how the effect is achieved. Not only is understanding the impact of electrolytes on the bubble formation mechanisms a fundamental question, but it can also provide insight into the design of more efficient air dispersing mechanisms for saline flotation systems. Previous studies have demonstrated that electrolytes can inhibit coalescence, but their potential impact on break-up remains vague, which is the focus of this study. It is hypothesized that electrolytes have an impact on break-up, and by isolating break-up from coalescence, the effects of electrolytes on break-up can be revealed. A break-up-only bubble formation system was built. Under this condition, any impact from the electrolytes on the produced bubble can be attributed to an impact on break-up. High-speed cinematography and a passive acoustic technique were employed to capture the bubble size, acoustic frequency, and damping ratio during the break-up process. Under the quasi-static condition, an increase in the electrolyte concentration increased the bubble size produced via break-up, contradicting the common observations made for bubble swarms. The break-up imparted an initial capillary wave to the bubble surface, which is correlated with the bending modulus of the air/water interface affected by the electrolytes. No direct correlation was observed between the acoustic damping ratio and that of the capillary wave, suggesting that the electrolytes affect the break-up via a different mechanism from that by surfactants.

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.

Beam Spray Thresholds in ICF-Relevant Plasmas.

Beam spray measurements suggest thresholds that are a factor of ≈2 to 15× less than expected based on the filamentation figure of merit often quoted in the literature. In this moderate-intensity regime, the relevant mechanism is forward stimulated Brillouin scattering. Both weak ion acoustic wave damping and thermal enhancement of ion acoustic waves contribute to the low thresholds. Forward stimulated Brillouin scattering imparts a redshift to the transmitted beam. Regarding the specific possibility of beam spray occurring outside the laser entrance holes of an indirectly driven hohlraum, this shift may be the most concerning feature owing to the high sensitivity of crossed-beam energy transfer to the interacting beam wavelengths in the subsequent overlap region.

Acoustic and structural damping properties of fibrogranular composites made of jute and crumb rubber

Synthetic materials are mostly used in sound absorption and structural damping that cause harm to the environment and living beings. Previous studies have shown that jute and jute-derived composites are excellent sound-absorbing fibrous materials. The recent study also shows that the inclusion of granular particles like crumb rubber in the fiber enhances sound absorption. Fibrogranular material of natural and recycled material is an alternative to synthetic material. This study aims to characterize the effect of granular material, i.e., crumb rubber with the jute fiber, to understand the effect of granular particles on the jute fiber for sound absorption and the structural damping properties. Further, the size of crumb rubber is varied to understand its effect on the sound-absorbing and structural damping properties. The impedance tube transfer function method is used to measure the acoustic properties for normal incidence of sound. Then the measured acoustic properties are used to inverse characterize using the classical porous material model to evaluate the non-acoustic parameters, which is further used to characterize the structural damping properties based on the existing methodologies. The study discusses the sound absorption and structural damping properties of jute and crumb rubber fibrogranular composite

Acoustic Anomalies and the Critical Slowing-Down Behavior of MAPbCl3 Single Crystals Studied by Brillouin Light Scattering

Inelastic light scattering spectra of organic–inorganic halide perovskite MAPbCl3 single crystals were investigated by using Brillouin spectroscopy. Sound velocities and acoustic absorption coefficients of longitudinal and transverse acoustic modes propagating along the cubic [100] direction were determined in a wide temperature range. The sound velocities exhibited softening upon cooling in the cubic phase, which was accompanied by the increasing acoustic damping. The obtained relaxation time showed a critical slowing-down behavior, revealing the order–disorder nature of the phase transition, which is consistent with the growth of strong central peaks upon cooling toward the phase transition point. The temperature dependences of the two elastic constants C11 and C44 were obtained in the cubic phase for the first time. The comparison of C11 and C44 with those of other halide perovskites showed that C11 of MAPbCl3 is larger and C44 is slightly smaller compared to the values of MAPbBr3 and MAPbI3. It suggests that MAPbCl3 has a more compact structure (smaller lattice constant) along with stronger binding forces, causing larger C11 and bulk modulus in this compound, and that the shear rigidity is exceedingly small similar to other halide perovskites. The reported elastic constants in this study may serve as a testbed for theoretical and calculational approaches for MAPbCl3.

Modeling nonlinear acoustic damping due to flow separation

Nonlinear acoustic damping has been observed in many high-amplitude acoustic systems as a result of flow separation and shear layer vortical motion, eventually transforming some of the acoustical energy into heat. The amount of nonlinear acoustic damping helps to determine the nonlinear limit cycle amplitude, e.g., damping caused by baffle blades in a liquid rocket engine to reduce combustion instabilities. The damping mechanism is dependent on both the location and phase of flow separation. Identifying the flow separation is a function of both the boundary layer growth and the acoustically imposed pressure gradient. When the acoustic pressure gradient is adverse, the boundary layer is more prone to separation. Using this as a basis, a model can be created that is applicable to general geometry, which will then be used to approximate the nonlinear acoustic damping in various situations. The constructed model will be compared to established cases, such as an orifice in a duct, to validate the model.

Experimental study on resonant acoustic damping for cabin noise reduction

Abstract Toward the goal of overcoming the weight-and-space redundancy caused by simple implementations of various noise-reducing structures each with a single function, it is natural one of the effective solutions that combine the forms and performances of different noise-reducing structures subtly. To this end, we ingeniously blend the dynamic absorber and the damping material into one structure, which is denoted as the resonant acoustic damping (RAD). In this letter, we conducted several preliminary experiments on an aircraft cabin model to verify the effectiveness of RAD for vibration suppression and noise reduction on complex engineering structures. The experimental results show that RAD possesses a dual-effect feature, that is, the narrowband vibration absorption like dynamic absorbers and the broadband vibration suppression like damping materials.

Theory of sound attenuation in amorphous solids from nonaffine motions

We present a theoretical derivation of acoustic phonon damping in amorphous solids based on the nonaffine response formalism for the viscoelasticity of amorphous solids. The analytical theory takes into account the nonaffine displacements in transverse waves and is able to predict both the ubiquitous low-energy diffusive damping ∼k 2, as well as a novel contribution to the Rayleigh damping ∼k 4 at higher wavevectors and the crossover between the two regimes observed experimentally. The coefficient of the diffusive term is proportional to the microscopic viscous (Langevin-type) damping in particle motion (which arises from anharmonicity), and to the nonaffine correction to the static shear modulus, whereas the Rayleigh damping emerges in the limit of low anharmonicity, consistent with previous observations and macroscopic models. Importantly, the k 4 Rayleigh contribution derived here does not arise from harmonic disorder or elastic heterogeneity effects and it is the dominant mechanism for sound attenuation in amorphous solids as recently suggested by molecular simulations.

Dynamics of Thermoacoustic Oscillations in Swirl Stabilized Combustor without and with Porous Inert Media

Lean premixed (LPM) combustion processes are of increased interest to the gas turbine industry due to their reduction in harmful emissions. These processes are susceptible to thermoacoustic instabilities, which are produced when energy added by an in-phase relationship between unsteady heat release and acoustic pressure is greater than energy dissipated by loss mechanisms. To better study these instabilities, quantitative experimental resolution of heat release is necessary, but it presents a significant challenge. Most combustion systems are partially premixed and therefore will have spatially varying equivalence ratios, resulting in spatially variant heat release rates. For laminar premixed flames, optical diagnostics, such as OH chemiluminescence, are proportionally related to heat release. This is not true for turbulent and partially premixed flames, which are common in commercial combustors. Turbulent eddies effect the strain on flame sheets which alter light emission, such that there is no longer a proportional relationship. In this study, phased, averaged, and spatially varying heat release measurements are performed during a self-excited thermoacoustic instability without and with porous inert media (PIM). Previous studies have shown that PIM can passively mitigate thermoacoustic instabilities, and to the best of the authors’ knowledge, this is the first-time that heat release rates have been quantified for investigating the mechanisms responsible for mitigating instabilities using PIM. Heat release is determined from high-speed PIV and Abel inverted chemiluminescence emission. OH ∗ chemiluminescence is used with a correction factor, computed from a chemical kinetics solver, to calculate heat release. The results and discussion show that along with significant acoustic damping, PIM eliminates the direct path in which heat release regions can be influenced by incoming perturbations, through disruption of the higher energy containing flow structures and improved mixing.

Hybrid assessment of acoustic radiation damping combining in-situ mobility measurements and the boundary element method

A hybrid experimental-numerical approach is proposed for assessing acoustic radiation damping – a major energy dissipating mechanism in lightweight structures. The vibrational behavior is characterized by distributed mobility measurements using laser Doppler vibrometry allowing to realistically capture the mechanical behavior of the structure under test. The experimentally obtained matrix of mobilities are coupled to a boundary element model to evaluate the radiated sound power numerically. Thereby, acoustic measurements and associated low frequency limitations are avoided, which results in two salient features of the proposed hybrid approach: modeling of diffuse incident acoustic fields and consideration of acoustic short-circuiting induced by slits and gaps. These features contribute to an accurate and excitation-dependent estimation of acoustic radiation damping in the low frequency range. The proposed hybrid approach is applied to flat and C-shaped aluminum sandwich panels mounted onto a tub-shaped foundation. The results are compared to those obtained by a previously reported numerical method.

Failure analysis of auxetic lattice structures under crush load

Lattice structures are promising materials in the terms of energy absorption, acoustic and vibrational damping, high strength-to-weight ratios and thermal management capabilities. In particular, auxetic lattice structures, among others, show high energy absorption performances due to their characteristic negative Poisson’s ratio. In this study, it is aimed to compare auxetic deformation mechanisms under quasi-static crush loads. Ti6Al4V tensile test specimens were produced with Electron Beam Melting (EBM) Additive Manufacturing (AM) technology. Moreover, a constitutive equation was derived and calibrated according to tensile results. The calibrated data were used to generate non-linear computational crush models including elastoplastic material data, damage initiation criterion, damage evaluation law and element deletion. The computational models are utilized for optimum topology design and mechanical performance prediction of different auxetic cells, including anti-tetrachiral, hexachiral, re-entrant and honeycomb lattice structures that are prone to prematurely fail under crush loading conditions. Consequently, it was found that the chiral auxetic deformation mechanism experienced better energy absorption ability over re-entrant deformation mechanism for metallic lattice structures.

Mechanical characterization of advanced carbon fibre reinforced polymers for down selection of aero-structural materials

Carbon fibre reinforced polymers are widely used in primary and secondary aircraft structures, mainly due to their excellent fatigue endurance and specific strength. Nowadays, improvements related to their manufacturing costs, impact resistance and fracture toughness are pursued through the investigation of new matrix-fibre combinations and the application of cutting-edge manufacturing technologies. This work presents the results of the coupons test campaign that has been conducted to assess the mechanical properties of different advanced carbon fibre reinforced polymers, being compared with those obtained for a composite material traditionally used in aero-structures. The effect of the introduction of innovative elements such as reinforcing-matrix Carbon Nanotubes or acoustic damping veil is evaluated. Moreover, the influence of ageing is assessed through the evaluation of the mechanical properties after specimen environmental conditioning. From the execution of this test campaign, a comprehensive knowledge base of mechanical properties has been generated, enabling the comparative analysis with reference composite and down selection of the materials that will be applied in the regional aircraft fuselage concept developed in Clean Sky 2 JU. The study shown here belongs to the scope of SHERLOC project. The purpose of the project is to perform a down selection of the most advanced composite materials, manufacturing processes and Structural Health Monitoring (SHM) systems that contribute moving towards a Condition-Based Maintenance.

Exploring the Potential to Repurpose Flexible Moulded Polyurethane Foams as Acoustic Insulators.

Polyurethane flexible foams are widely used for a variety of applications to improve comfort and durability. Their long-term frequent use inevitably leads to the generation of waste that needs to be treated. The recycling and reuse of polyurethane waste are essential to achieve an environmentally friendly economy. The present study investigates the potential to reuse and repurpose flexible polyurethane foam from automotive seat cushion waste materials. Flexible foams were prepared with different hardnesses using isocyanate–polyol ratios between 0.8 and 1.2 NCO-index. Dry heat aging tests were performed to mimic the long-term usage of the materials. The decrease in compressive strength was compared with the change in acoustic damping properties before and after the aging tests using an acoustic tube, and the change in foam cell structures was also analyzed by micro-CT. On the basis of the results obtained, although the foam systems are no longer suitable to be used as seat cushions due to aging, they can still be used as sound insulation materials within a given frequency range, as their sound absorption capacity is suitable for such purpose.

Surface texture modulation via buckling in porous inclined mechanical metamaterials

Porous materials with well-defined periodicity are commonly encountered in biological and synthetic structures and exhibit a wide range of behaviors, ranging from negative Poisson’s ratios, to high energy absorption and acoustic damping. Recently, the response of these systems has been shown to be enhanced by mechanical instabilities that lead to sudden and reversible geometric transformations. Although buckling induces planar transformations in most of 2D porous metamaterials, here we describe the emergence of 3D morphologies triggered by mechanical instabilities in an elastomeric block with tilted cylindrical holes. As a proof of concept, we demonstrate that these structures can be leveraged to tune surface properties including friction and light reflection, thus providing a new experimental platform for investigating deformation-dependent dynamics for tribological and optical applications.