Sharif Digital Repository

Shape optimization of a submarine for minimized noise emission has been the objective of this work. Boundary element method and finite element methods have been employed to determine the acoustic field around the object. A combined neural network and genetic algorithm scheme is developed to find the optimum external geometric ratios of the submarine for a minimized emitted acoustic energy in certain reference points. The obtained optimum geometric values stay in normal range for minimum hydrodynamic forces and show a close agreement with the trend of change for models coming to operation. © 2004 IEEE  

The search for the development of a reliable mathematical model for understanding bubble dynamics behavior is an ongoing endeavor. A long list of complex phenomena underlies the physics of this problem. In the past decades, the lattice Boltzmann method has emerged as a promising tool to address such complexities. In this regard, we have applied a 121-velocity multiphase lattice Boltzmann model to an asymmetric cluster of bubbles in an acoustic field. A problem as a benchmark is studied to check the consistency and applicability of the model. The problem of interest is to study the deformation and coalescence phenomena in bubble cluster dynamics, as well as the screening effect on an acoustic... 

Thermoacoustics describes energy conversion processes, activated by interaction temperature oscillation, accompanying the pressure oscillation in a sound wave, with solid boundaries. In ordinary experience this interaction of sound and heat cannot be observed, because of the very low temperature differences, but under suitable conditions, it can be emphasized and amplified to create remarkable thermodynamic effects such as, steep thermal gradients, powerful convective heat fluxes, and strong sound fields. We designed and constructed a simple thermoacoustic refrigerator with an adjustable mechanical resonator, coupled with the acoustic resonator. Our experimental data showed about % 10... 

These days, the pace of utilizing CAE tools is increasing with an eye to the vehicle development time and cost considerations. This paper deals with the NVH virtual analyses performed for a trimmed body of a B-segment sedan car. Since the main source of annoyance in low frequency range is the booming noise, the NVH diagnoses are performed by FEM tools. Firstly, the finite element model of the car is generated and this is followed by the modal analysis simulations for the car and cavity models in order to investigate the frequency attributes of the car and identify the NVH characteristics of body structure and acoustic cavity. Afterwards, noise transfer function (NTF) analyses are performed... 

The luminescence parameters of laser-induced bubble in the presence of an acoustic field in water are studied. A comparison is made between parameters such as bubble radius, interior temperature, and pressure of the bubble induced by laser and an acoustic field influenced by different driving pressure amplitudes. It is found that the bubble volume induced by laser at the collapse instant is more than 106 times larger than the one induced by an acoustic field. It is also noticed, by increasing the driving pressure amplitude, the bubble radius decreases in both cases, however, the bubble interior pressure and temperature increase  

The mutual interaction of two oscillating gas bubbles in different concentrations of sulfuric acid is numerically investigated. A nonlinear oscillation for spherical symmetric bubbles with equilibrium radii smaller than 10 μm at a frequency of 37 kHz in a strong driving acoustical field Pa =1.8 bar is assumed. The calculations are based on the investigation of the secondary Bjerknes force with regard to adiabatic model for the bubble interior which appears as repulsion or attraction interaction force. In this work the influence of the various concentrations of sulfuric acid in uncoupled and coupled distances between bubbles has been investigated. It is found that the sign and value of the... 

This paper investigates the secondary Bjerknes force for two oscillating bubbles in various pressure amplitudes in a concentration of 95% sulfuric acid. The equilibrium radii of the bubbles are assumed to be smaller than 10 μm at a frequency of 37 kHz in various strong driving acoustical fields around 2.0 bars (1 bar=10 5 Pa). The secondary Bjerknes force is investigated in uncoupled and coupled states between the bubbles, with regard to the quasi-adiabatic model for the bubble interior. It finds that the value of the secondary Bjerknes force depends on the driven pressure of sulfuric acid and its amount would be increased by liquid pressure amplitude enhancement. The results show that the... 

The problem of estimating a sparse channel, i.e. a channel with a few non-zero taps, appears in many fields of communication including acoustic underwater or wireless transmissions. In this paper, we have developed an algorithm based on Iterative Alternating Minimization technique which iteratively detects the location and the value of the channel taps. In fact, at each iteration we use an approximate Maximum A posteriori Probability (MAP) scheme for detection of the taps, while a least square method is used for estimating the values of the taps at each iteration. For approximate MAP detection, we have proposed three different methods leading to three variants for our algorithm. Finally, we... 

A body insonified by a sound field is known to experience a steady force that is called the acoustic radiation force. In this paper, the method of wave function expansion is adopted to study the scattering and the radiation force function caused by a plane normal harmonic acoustic wave incident upon an arbitrarily thick-walled functionally graded cylindrical shell submerged in and filled with compressible ideal fluids. A laminate approximate model and the so-called state space formulation in conjunction with the classical transfer matrix (T-matrix) approach are employed to present an analytical solution based on the two-dimensional exact equations of elasticity. Two typical models,... 

This paper describes methods used for determining the characteristic acoustic modes and frequencies of a liquid-propellant rocket-motor combustion chamber and effects of radial baffles on the chamber's acoustic field. A multi-point sensing experimental setup, including stationary and moving sensors, was used to measure characteristic frequencies and mode shapes of a combustion chamber. A new technique based on the comparison of signal phase angles from stationary sensors to that of a moving sensor was used to map complex characteristic mode shapes of a combustor. A three-dimensional Helmholtz acoustic solver was also developed using an efficient finite volume approach for complex geometries... 

A body insonified by a constant (time-varying) intensity sound field is known to experience a steady (oscillatory) force that is called the steady-state (dynamic) acoustic radiation force. Using the classical resonance scattering theorem (RST) which suggests the scattered field as a superposition of a resonance field and a background (non-resonance) component, we show that the radiation force acting on a cylindrical shell may be synthesized as a composition of three components: background part, resonance part and their interaction. The background component reveals the pure geometrical reflection effects and illustrates a regular behavior with respect to frequency, while the others... 

Compartment noise has gained significant importance to meet customer expectation. One of the dominant sources of noise from whole engine as a system is the one which is induced from fresh air intake. Geometrical features in air induction systems (AIS) airflow path are often responsible for unusual noise due to the complex air flow structure and its interaction with the internal acoustic field. To reduce air intake noise basically resonator and expansion chamber are used on automotive vehicles. Resonators are widely used for noise reduction of air induction systems. Although, airflow bench tests are faster to evaluate various alternate design geometries, understanding the mechanism of such... 

The scattering of an oblique plane progressive monochromatic acoustic field upon a laminated composite cylindrical shell is studied based on the three-dimensional exact equations of anisotropic elasticity. Each layer of laminated structure is made of helically filament wound (fiber reinforced) homogeneous material whose degree of anisotropy is considered as monoclinic type. An approximate laminate model along with the local transfer matrix solution is used to solve the state space governing formulation within each layer. Considering the perfect bonding between the adjacent layers, the global transfer matrix is constructed as the product of the local transfer matrices employed to solve for... 

In this study, the numerical simulation of the fluid flow and acoustic field of a supersonic jet is performed by using high-order discretization and the vorticity confinement (VC) method on coarse grids. The three-dimensional Navier-Stokes equations are considered in the generalized curvilinear coordinate system and the high-order compact finite-difference scheme is applied for the space discretization, and the time integration is performed by the fourth-order Runge-Kutta scheme. A low-pass high-order filter is applied to stabilize the numerical solution. The non-reflecting boundary conditions are adopted for all the free boundaries, and the Kirchhoff surface integration is utilized to...