Romanian Journal of Acoustics and Vibration

Infiltration Air Flow Variance Analysis in Relation to The Reverberation Time and Weighted Global Noise Level Difference

2023-03-08T12:34:14+02:00

The acoustic method for the experimental determination of the air permeability for buildings manages to overcome the disadvantages of other experimental methods or phenomenological modelling. However, the current degree of development of this method is applicable only to a small number of building types. In this research we adapt this method in order to extend its applicability for different types of buildings. To achieve this, we use an experimental approach, completed by numerical simulations for the formation of a database. More concrete, the purpose of this paper is to analyze the variance of the air infiltration flow (Q) considering the different Window Types (WT) for different Reverberation Time (RT_1000Hz) of the receiving room and Global Weighted Noise Level Difference (ΔLA). The statistical model ‘Analysis of Variance (ANOVA)’, offers the possibility to follow the evolution of a phenomenon under different conditions. In this paper, the research objective is to test the hypothesis that Q varies, depending on the four RT_1000Hz values for the eight ΔLA values. The article highlights the influence of the two parameters on the infiltrated air flow, as well as the way these parameters must be introduced in a more general model, with wider applicability for different building types.

Modification of Noise Reduction by SiO2 Nanocomposite: The Case of Earplugs

2023-03-08T12:52:16+02:00

When applying different control measures fail to reduce noise exposure to a safe level, using hearing protection devices (HPDs) becomes necessary. The study aim was to investigate the effect of SiO2 nanoparticles on the noise reduction efficiency of PVC earplugs. PVC polymer of type S-60 as the main matrix and SiO2 with a size of 30 nm were used. The PVC / SiO2 nano-composite was mixed at a temperature of 160 ° C and 40 rpm and samples with 0, 0.2 and 0.5 weight percent of SiO2 nanoparticles were prepared. Samples of earplugs with PVC / SiO2 nanoparticles (0.2; 0.5% by weight) when compared to raw earplugs showed the noise attenuation at low frequencies (125-500 Hz) almost the same as that of the raw earplugs. At high frequencies (2-8 KHz), however, the noise attenuation performance of earplugs with SiO2 nanoparticles increased significantly (P <0.05). The results of the present study showed that samples with SiO2 nanoparticles show a stronger noise attenuation at higher frequencies compared to samples without nanoparticles.

Interior Ballistic Simulation for 9 mm Gun for Bullet and Blank Shot Applying Spalart-Allmars Turbulence Model

2023-03-08T13:02:37+02:00

Internal ballistic for a bullet shot and a blank shot in a 9 mm gun was simulated. The length and the thickness of the barrel of the gun were 125 mm and 3 mm, respectively. In this simulation viscous Spalart-Allmars turbulence model was applied and the movement of the bullet was modeled with six degrees of freedom. Four different initial conditions for the bullet were considered that involved the initial position of the bullet in the barrel, pressure of gas behind the bullet, and speed of the bullet in that position. The simulated muzzle speed for each initial condition was compared to experimental data and the error (in %) for each initial condition was calculated. Also, Mach number, gas pressure, flow field, and sound pressure level were simulated in the area outside of the gun barrel. Sound level reduced in both axial and radial directions but the reduction of sound in the radial direction was more than axial direction. In the case of the blank shot, also, it was observed that the reduction of sound in the radial direction was more than the axial direction. The sound pressure level for the bullet shot at the muzzle was 239.7 dB and for the blank shot, it was 220.5 dB at the muzzle. In both bullet and blank shots, it is observed from simulations that at the muzzle of the gun pressure of gas dropped and speed increased significantly. Mach number outside of the barrel was more than 1, which means the flow of gas was supersonic, and in some regions, Mach number reached 6. The shock wave produced from the supersonic flow of gas is detectable in the simulations.

Aluminum Nanocomposite based on Ensemble of Carbon Materials for Enhanced Vibration Damping

2023-03-08T13:10:21+02:00

Many aluminum nanocomposites consisting of a single particulate phase have been synthesized and tested in the past. However, vibration damping characteristics of aluminum nanocomposite based on ensemble or combination of two morphologically different carbon-based materials have not been reported. Therefore, in this work the focus is to synthesize an aluminum nanocomposite based on the combination of two different carbon-based particulate phases namely graphite flake and carbon fiber in order to identify any improvement in the damping ability of the as-synthesized aluminum nanocomposite material. Aluminum nanocomposite was prepared using stir casting method. The particulate phases of carbon were used in varying weight compositions but were always in an equal ratio relative to each other. The material was casted in form of thin rectangular bars using metallic dies and tested in a cantilever beam configuration. MEMS based accelerometer via Arduino microcontroller was used to collect the time dependent response at the free end of the beams. The damping constant ζ was estimated from the response curves. Experiments indicate that compared to the pristine aluminum bars, the damping ability of the as-synthesized aluminum nanocomposite increases by almost 117 % at 3 wt% of the particulate phase. Aluminum nanocomposite is successfully prepared using combination of two morphologically different carbon-based particulate phases. The as-synthesized material shows favorable damping performance that can be exploited in structural, automotive, marine and aerospace applications.

Strain-Based Mindlin Finite Element for Vibration Analysis of Rectangular Plates Coupled with a Fluid

2023-03-08T13:16:26+02:00

This paper presents an extension of the recently published quadrilateral finite element to analyze the free vibration of plates vibrating in air or in interaction with the fluid. The previously proposed four-node element is based on the strain approach and the Reissner-Mindlin plate theory. The developed finite element, which contains the three essential external degrees of freedom at each of the four corner nodes, is based on assumed functions of the strain field that satisfy the compatibility equations. A comparison is presented with experimental and numerical data available in the literature to demonstrate the performance of the current element. The dynamic characteristics (natural frequencies and associated modes) of a rectangular plate simply supported or clamped totally submerged in water, a cantilever square plate, and a vertical plate partially submerged are numerically computed by the proposed element. The frequencies obtained by this element are of excellent accuracy compared to numerical, analytical, or experimental solutions from the literature.

On the Reduction of a System of Planar Cylindrically Jointed Bars with Small Deformations

2023-03-08T13:29:08+02:00

In this paper we present a general method to obtain the equivalent system for an arbitrary planar system of cylindrically jointed bar. The equivalent system is one for which the displacement of the origin (the point at which all bars are jointed) is equal to the displacement of the same point for the original system. It is proved that, in general, any system of bars fulfilling some additional conditions is equivalent to a system of two bars. This equivalence is valid only for small deformations of the system. An example highlights the theory.

Dynamic Modelling of Vibrating Equipment for Fine Grinding of Granular Material

2023-03-08T13:53:08+02:00

The process of fine grinding of the granular materials from crushed stone, merble or brittle solid oxide materials is carried out with vibratinf equipment with two or more grinding chambers. The granular material is fed o the fine grinding chamber together with the spherical or cylindrical grinding bodies.

The vibration regime the whole assembly is subjected causes the material to be brought to a powder state with a very high degree of fineness in the range of (5÷10) μm.

The shredded material is used to coat the welding electrodes, the insulation of electrical conductor, the anti-corrosion coatings for special equipment as well as in the chemical and /or pharmaceutical industry.

The study include the dynamic computational model and the foundation of the dynamic vibration regimes so that the technological grinding process may be realized.

Thus, the calculation formulas for the forced vibration amplitudes and working pulsations are given to achieve the optimal regime for achieving the fine grinding degree at particle size class level required by the industrial process technologies.

Thea results of the dynamic modelling, of the optimization of the calculation of the vibration parameters and of the correlation with the grinding fineness were applied for several equipment at the following companies: Ductil Buzău - Bucharest, Medicines Factory Iași, Cement Factory Bicaz-Tașca all in Romania.

Modal Analysis of the Inertial Platform of the Laser ELI-NP Facility in Magurele-Bucharest

2023-03-08T15:32:52+02:00

Starting from the need to determine the deformability of the inertial platform from the Research Facility of ELI-NP in Magurele-Bucharest in case of changing the position of some very heavy partition walls, a vibration study of the entire inertial platform was made. The research team studied various aspects of this issue and some of the results were published. The paper presented the obtained results of the dynamic analysis, in a unitary approach and with the conclusions that emerge from these researches. The results may be useful in the study of problems of anti-vibration isolation of very large masses, generally used in research infrastructure.

Crack Detection in A Cantilever Beam Using Correlation Model and Machine Learning Approach

2023-03-08T16:14:32+02:00

Crack in a structural member alters local stiffness that affects the dynamic response, such as natural frequency and mode shapes. The purpose of structural health monitoring is to diagnose and predict structural health. In this paper, a correlation model is developed to detect crack parameters, i.e., crack location and crack depth, in the beam. To evaluate the authenticity of the developed correlation model, the Artificial Intelligence-based approach is used to predict the crack parameters. Twenty-three Artificial Intelligence algorithms were used to predict the locations and depths of the crack in a cantilever beam. The developed correlation model used the first two normalized natural frequencies to predict the crack parameters. On the other hand, the first three normalized natural frequencies were used to input the machine learning models to predict the crack parameters. In this research study, V-shaped and U-shaped open edges cracks were considered on the cantilever beam. FEA software, ANSYS, is used to do the modal vibration analysis of various cracked cases of beams. The data set of V-shaped and a U-shaped cracked case obtained from finite element analysis (FEA) were used to develop the correlation model and machine learning models. The results for crack locations and crack depth obtained from the correlation model and machine learning models agree with the actual results. In the future, the proposed correlation model of crack detection can be used to detect cracks in more complicated structures.

Acoustic Analysis of Musical Timbre of Wooden Aerophones

2023-03-08T16:36:06+02:00

The characterization of the musical timbre, which allows the quantitative evaluation of audios, is still an open-ended research topic. This paper evaluates a set of dimensionless descriptors for studying musical timbre in monophonic recordings of woodwind instruments from the TinySOl audio library, considering the region of frequencies common to all instruments in their three dynamic levels (pianissimo, mezzo-forte, and fortissimo). These descriptors are calculated using the spectra obtained from the Fast Fourier Transform (FFT) using the Python programming language. From the analysis of the distribution of the coefficients, it was possible to verify that the Affinity coefficient (A) allows discrimination in all octaves of musical sounds. The analysis of the data through the empirical distribution of the coefficients shows that the timbral variations due to the dynamics are reflected through the coefficients Sharpness (S) and Mean Affinity (MA). The coefficients are examined using the Principal Component Analysis (PCA), and it was observed that the variability in the distribution of the first principal component is mainly due to the Sharpness (S) and Mean Contrast (MC) coefficients (~55%), and in ~43% by the Affinity coefficient. Similarly, the variation in the second principal component is due to 62 % of the MC coefficient and 49% due to MA. It is concluded that the proposed descriptors are sufficient to differentiate the aerophones studied by octaves and musical dynamics.

Experimental Investigations and Numerical Simulations of Vibratory Compaction of Weakly Cohesive Soils

2023-03-09T10:53:08+02:00

The compaction of weakly cohesive soils presents particularities in terms of improving the geotechnical properties under the loads transmitted into the terrain. In this case, the evaluation of the compaction process requires the development of a roller-terrain interaction model whose constitutive parameters are directly correlated with information available from laboratory and in-situ experimental tests. A simulation model is implemented in Matlab on the basis of the mathematical equation of a Kelvin-Voigt lumped vibratory system. The simulation results are presented and compared to the experimental investigations results obtained for a road structure modernization project (Romania, DN 2, km 39+200, Movilița site).

Analysis of Vibration Suppression in Multi-Degrees of Freedom Systems

2023-03-13T13:45:15+02:00

In the paper, we propose to show that a complex mechanical system, with elastic elements, subject to periodic excitations, can function as a dynamic absorber for some frequencies. A study of this problem allows determining the conditions under which this occurs. The results are illustrated with an example. The advantage of this approach is the fact that the attachment of an additional dynamic absorber in the system is avoided, which, without changing the response of the already designed system, involves additional costs.

Improved Mathematical Relation of The Modal Shapes of Thin Rectangular Plates

2023-03-13T15:09:38+02:00

In this paper a new analytical solution for solving the issues that arise in determining the correct form of the eigenmodes, for the case of simply supported on two opposite edges and clamped on the other two of a rectangular plate, is described. The case of a homogenous plate, isotropic, with constant thickness and uniformly distributed weight, and dynamically driven in the direction normal to the plane of the plate with a median surface in the xOy plane is considered. The deformed shape of the middle area Z(x, y) is analyzed. The theory of elasticity states that this phenomenon can be described by a bi-harmonic differential equation of smooth plates having constant thickness. However, the eigenfunctions obtained for this scenario, lead to eigenmodes representations which are accurate only for modes n=1 and m=1. In this paper it is shown that the proposed analytical solution accurately describes the eigenmodes for the above mentioned plate, with the given limit constraints, for every m and n modes. Moreover, this new solution stands out for its simplicity. MatLab environment was used for simulations. The results obtained using the proposed solution were compared with the results obtained by means of classical method. From the analysis we carried out, we observed a symmetrical distribution of the eigenmodes, on both sides of the equilibrium position.

Free Vibration Analysis of Anti-Symmetric FGM Sandwich Circular Beams Using a Fifth-Order Circular Beam Theory

2023-03-20T16:43:45+02:00

This paper focused on the fundamental frequency analysis of anti-symmetric FGM sandwich circular beams. A fifth-order circular beam theory considering the influence of transverse shear and normal strains is developed in this study. The theory assumes fifth-order variation of thickness coordinate in axial (tangential) displacement and fourth-order variations of thickness coordinate in transverse (radial) displacements. Hamilton’s principle is applied to derive equations of motion. An exact solution for the frequency analysis of a simply-supported circular beam is obtained using the Navier technique. Anti-symmetric FGM sandwich circular beams are considered for numerical studies. The material properties of face sheets are graded in the thickness direction of the beam according to the power-law whereas the core of the beam is made up of isotropic material. The fundamental frequencies are obtained for different values of radius of curvature (R), the power-law index (p), and lamination schemes of FGM sandwich circular beams. For the validation purpose, fundamental frequencies for straight beams obtained using the present theory are compared with the previously published papers and found in good agreement with those. This study also presents benchmark results of the FGM sandwich circular beams. Based on the comparison of the numerical results and discussion it is concluded that for the same length and thickness, the value of non-dimensional fundamental frequency increases as the radius of curvature is decreased. Also, the non-dimensional frequency decreases as the power-law index increases.