Far Field Acoustic Radiation From an Expansion Chamber

1989 ◽  
Vol 111 (2) ◽  
pp. 208-213
Author(s):  
W. Eversman ◽  
J. M. Park
Keyword(s):  
Acoustic Radiation ◽  
Three Dimensional ◽  
Normal Modes ◽  
Side Wall ◽  
Coupled System ◽  
Internal Resonances ◽  
Expansion Chamber ◽  
Pipe Length ◽  
Wall Stiffness ◽  
Acoustic Output

A method is presented for the assessment of the effect of several system parameters on the radiated and transmitted acoustic fields of an expansion chamber with a flexible side wall. The approach is based on a previously developed model of a three dimensional cavity in an otherwise one dimensional acoustic transmission line. The acoustic field within the cavity is represented in terms of the cavity acoustic normal modes. The flexible side wall is represented in terms of its natural modes of free vibration. Computations are made and results presented to assess the effect on the acoustic output of parameters such as side wall thickness, side wall stiffness, inlet pipe length, and inlet and outlet pipe placement. It is concluded that radiation from the side wall can occur at significant levels at certain plate resonances and at cavity internal resonances. Reduction of radiation is achieved by thickening of the side wall and by adding appropriate stiffeners. Success in reduction of radiated noise requires an understanding of the entire coupled system.

Acoustic and vibrational characteristics of a propeller–shaft–hull coupled system based on sono-elasticity theory

2016 ◽  
Vol 24 (9) ◽  
pp. 1707-1715 ◽  
Author(s):  
LB Qi ◽  
YS Wu ◽  
MS Zou ◽  
Yong Duan ◽  
MX Shen
Keyword(s):  
Acoustic Radiation ◽  
Numerical Models ◽  
Three Dimensional ◽  
Coupled System ◽  
Vibration Modes ◽  
Propeller Shaft ◽  
Floating Bodies ◽  
Vibrational Characteristics ◽  
Interface Boundary Conditions ◽  
The Ideal

The three-dimensional sono-elasticity method recently developed by Zou ((2014) Three-dimensional sono-elasticity of ships. PhD Dissertation, China Ship Scientific Research Center, China) and Wu ((1984) Hydroelasticity of floating bodies. PhD Dissertation, Brunel University, UK) is employed to explore the acoustic and vibrational characteristics of a propeller–shaft–hull coupled system. The acoustic field can be solved by introducing Green’s function for the ideal compressible fluid together with the Price–Wu generalized fluid–structure interface boundary conditions. The vibration of a ship structure is governed by the generalized equations, including added mass, damping and restoring coefficients. In order to discover the mechanisms underlying the acoustic and vibrational characteristics of the propeller–shaft–hull coupled system, numerical models for hull structures with a shaft and without a shaft are designated. Through modal analysis, the correlations of the line spectra of acoustic radiation and the corresponding vibration modes of the hull are clearly identified. Through further numerical analysis, the appropriate location of the base for the thrust bearing and installation schemes are recommended.

Investigation of Curved Polymeric Piezoelectric Active Diaphragms

2003 ◽  
Vol 125 (2) ◽  
pp. 145-154 ◽  
Author(s):  
Kelly C. Bailo ◽  
Diann E. Brei ◽  
Karl Grosh
Keyword(s):  
Polyvinylidene Fluoride ◽  
Acoustic Radiation ◽  
Numerical Models ◽  
Three Dimensional ◽  
Analytical Models ◽  
Acoustic Response ◽  
Structural Dynamic ◽  
Geometric Configurations ◽  
Parametric Studies ◽  
Acoustic Output

Piezoelectric active diaphragms hold promise as an alternative to using passive diaphragms driven by voice coils for sound generation and noise cancellation applications. This paper presents an in-depth investigation of the acoustic response for curved polymeric piezoelectric active diaphragms. Simple analytical models were derived and experimentally validated to predict the structural dynamic and acoustic responses for generic polyvinylidene fluoride (PVdF) constant curvature active diaphragms with variable geometric parameters (width, radius, subtended angle, thickness). These models are useful for design purposes and for capturing the overall behavioral trends. To analyze the acoustic response mechanisms further, three-dimensional numerical models were also developed and experimentally validated. Parametric studies based upon these models reveal the potential of high acoustic outputs (over 100 dB in the far field) from optimized geometric configurations with subtended angles differing from the conventionally utilized flat, semicircular or circular configurations. These studies, corroborated by experiments on a variety of active diaphragm prototypes, conclude that the acoustic output is governed by the structural ring resonance, which can be designed such that the most efficient acoustic radiation is within the particular frequency range of operation.

Wake Flow of Single and Multiple Yawed Cylinders

2004 ◽  
Vol 126 (5) ◽  
pp. 861-870 ◽  
Author(s):  
A. Thakur ◽  
X. Liu ◽  
J. S. Marshall
Keyword(s):  
Computational Study ◽  
Three Dimensional ◽  
Side Wall ◽  
Wake Flow ◽  
Upstream Region ◽  
Two Dimensional ◽  
Cylinder Wake ◽  
Dimensional Approximation ◽  
The Face ◽  
Drag And Lift

An experimental and computational study is performed of the wake flow behind a single yawed cylinder and a pair of parallel yawed cylinders placed in tandem. The experiments are performed for a yawed cylinder and a pair of yawed cylinders towed in a tank. Laser-induced fluorescence is used for flow visualization and particle-image velocimetry is used for quantitative velocity and vorticity measurement. Computations are performed using a second-order accurate block-structured finite-volume method with periodic boundary conditions along the cylinder axis. Results are applied to assess the applicability of a quasi-two-dimensional approximation, which assumes that the flow field is the same for any slice of the flow over the cylinder cross section. For a single cylinder, it is found that the cylinder wake vortices approach a quasi-two-dimensional state away from the cylinder upstream end for all cases examined (in which the cylinder yaw angle covers the range 0⩽ϕ⩽60°). Within the upstream region, the vortex orientation is found to be influenced by the tank side-wall boundary condition relative to the cylinder. For the case of two parallel yawed cylinders, vortices shed from the upstream cylinder are found to remain nearly quasi-two-dimensional as they are advected back and reach within about a cylinder diameter from the face of the downstream cylinder. As the vortices advect closer to the cylinder, the vortex cores become highly deformed and wrap around the downstream cylinder face. Three-dimensional perturbations of the upstream vortices are amplified as the vortices impact upon the downstream cylinder, such that during the final stages of vortex impact the quasi-two-dimensional nature of the flow breaks down and the vorticity field for the impacting vortices acquire significant three-dimensional perturbations. Quasi-two-dimensional and fully three-dimensional computational results are compared to assess the accuracy of the quasi-two-dimensional approximation in prediction of drag and lift coefficients of the cylinders.

Aerothermal Investigation of a Rib-Roughened Trailing Edge Channel With Crossing-Jets—Part I: Flow Field Analysis

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
Alessandro Armellini ◽  
Filippo Coletti ◽  
Tony Arts ◽  
Christophe Scholtes
Keyword(s):  
Flow Field ◽  
Three Dimensional ◽  
Side Wall ◽  
Field Analysis ◽  
Navier Stokes ◽  
Trailing Edge ◽  
Present Contribution ◽  
Cross Sectional ◽  
Numerical Predictions ◽  
Rib Roughened

The present contribution addresses the aerothermal, experimental, and computational studies of a trapezoidal cross-sectional model simulating a trailing edge cooling cavity with one rib-roughened wall. The flow is fed through tilted slots on one side wall and exits through straight slots on the opposite side wall. The flow field aerodynamics is investigated in Part I of the paper. The reference Reynolds number is defined at the entrance of the test section and set at 67,500 for all the experiments. A qualitative flow model is deduced from surface-streamline flow visualizations. Two-dimensional particle image velocimetry measurements are performed in several planes around midspan of the channel and recombined to visualize and quantify three-dimensional flow features. The crossing-jets issued from the tilted slots are characterized and the jet-rib interaction is analyzed. Attention is drawn to the motion of the flow deflected by the rib-roughened wall and impinging on the opposite smooth wall. The experimental results are compared with the numerical predictions obtained from the finite volume Reynolds-averaged Navier–Stokes solver, CEDRE.

FEM/BEM Simulation of Vibroacoustics of a Fluid-Loaded, Stiffened Plate Under Combined Force-Moment Excitation

2000 ◽  
Author(s):  
H. Zheng ◽  
C. Cai ◽  
G. R. Liu ◽  
K. Y. Lam
Keyword(s):  
Numerical Simulation ◽  
Acoustic Radiation ◽  
Normal Modes ◽  
Vibration Response ◽  
Structural Vibration ◽  
Stiffened Plate ◽  
Coupled Modes ◽  
Force Moment ◽  
Surrounding Fluid ◽  
Element Method

Abstract A numerical simulation of structural vibration and acoustic radiation is presented for a finite, fluid-loaded plate reinforced with two sets of orthotropic stiffeners. The attempt is to achieve a physical understanding of the dynamic behaviour and especially the acoustic radiation of the stiffened plate under combined force and moment excitations. Finite element method (FEM) is employed for calculation of the in-vacuo normal modes of the stiffened plate. The coupled modes with a heavy fluid (water), vibration response and acoustic radiation of the plate under given force and/or moment excitation are calculated using boundary element method (BEM). Numerical simulation results are detailed to address the significance of moment in combined force-moment excitations and, more importantly, the cancelling of the combined excitation in both structural vibration response and the associated acoustic radiation into the surrounding fluid.

Experimental Periodic Localized Motions in Coupled Beams With Active Nonlinearities

1995 ◽  
Author(s):  
Melvin E. King ◽  
Johannes Aubrecht ◽  
Alexander F. Vakakis
Keyword(s):  
Steady State ◽  
Vibrational Energy ◽  
Normal Modes ◽  
Coupled System ◽  
Nonlinear Normal Modes ◽  
Nonlinear Frequency ◽  
Nonlinear Phenomenon ◽  
Response Curves ◽  
Nonlinear Motion ◽  
Spatially Extended

Abstract Steady-state nonlinear motion confinement is experimentally studied in a system of weakly coupled cantilever beams with active stiffness nonlinearities. Quasi-static swept-sine tests are performed by periodically forcing one of the beams at frequencies close to the first two closely-spaced modes of the coupled system, and experimental nonlinear frequency response curves for certain nonlinearity levels are generated. Of particular interest is the detection of strongly localized steady-state motions, wherein vibrational energy becomes spatially confined mainly to the directly excited beam. Such motions exist in neighborhoods of strongly localized anti-phase nonlinear normal modes (NNMs) which bifurcate from a spatially extended NNMs of the system. Steady-state nonlinear motion confinement is an essentially nonlinear phenomenon with no counterpart in linear theory, and can be implemented in vibration and shock isolation designs of mechanical systems.

scholarly journals Effects of Central Tube on Shape of Modal Duct of a Helicoid in a Simple Expansion Chamber

2018 ◽  
Vol 2018 ◽  
pp. 1-6
Author(s):  
Daniel Omondi Onyango ◽  
Robert Kinyua ◽  
Abel Nyakundi Mayaka
Keyword(s):  
Wave Propagation ◽  
Acoustic Wave ◽  
Plane Wave ◽  
Acoustic Pressure ◽  
Three Dimensional ◽  
The Other ◽  
Expansion Chamber ◽  
Central Tube ◽  
Simple Expansion ◽  
Plane Wave Propagation

The shape of the modal duct of an acoustic wave propagating in a muffling system varies with the internal geometry. This shape can be either as a result of plane wave propagation or three-dimensional wave propagation. These shapes depict the distribution of acoustic pressure that may be used in the design or modification of mufflers to create resonance at cut-off frequencies and hence achieve noise attenuation or special effects on the output of the noise. This research compares the shapes of acoustic duct modes of two sets of four pitch configurations of a helicoid in a simple expansion chamber with and without a central tube. Models are generated using Autodesk Inventor modeling software and imported into ANSYS 18.2, where a fluid volume from the complex computer-aided-design (CAD) geometry is extracted for three-dimensional (3D) analysis. Mesh is generated to capture the details of the fluid cavity for frequency range between 0 and 2000Hz. After defining acoustic properties, acoustic boundary conditions and loads were defined at inlet and outlet ports before computation. Postprocessed acoustic results of the modal shapes and transmission loss (TL) characteristics of the two configurations were obtained and compared for geometries of the same helical pitch. It was established that whereas plane wave propagation in a simple expansion chamber (SEC) resulted in a clearly defined acoustic pressure pattern across the propagation path, the distribution in the configurations with and without the central tube depicted three-dimensional acoustic wave propagation characteristics, with patterns scattering or consolidating to regions of either very low or very high acoustic pressure differentials. A difference of about 80 decibels between the highest and lowest acoustic pressure levels was observed for the modal duct of the geometry with four turns and with a central tube. On the other hand, the shape of the TL curve shifts from a sinusoidal-shaped profile with well-defined peaks and valleys in definite multiples of π for the simple expansion chamber, while that of the other two configurations depended on the variation in wavelength that affects the location of occurrence of cut-on or cut-off frequency. The geometry with four turns and a central tube had a maximum value of TL of about 90 decibels at approximately 1900Hz.

Aero-Thermal Investigation of a Rib-Roughened Trailing Edge Channel With Crossing-Jets: Part I—Flow Field Analysis

2008 ◽  
Author(s):  
Alessandro Armellini ◽  
Filippo Coletti ◽  
Tony Arts ◽  
Christophe Scholtes
Keyword(s):  
Flow Field ◽  
Computational Study ◽  
Three Dimensional ◽  
Side Wall ◽  
Field Analysis ◽  
Trailing Edge ◽  
Present Contribution ◽  
Numerical Predictions ◽  
Flow Features ◽  
Rib Roughened

The present contribution addresses the aero-thermal experimental and computational study of a trapezoidal cross-section model simulating a trailing edge cooling cavity with one rib-roughened wall. The flow is fed through tilted slots on one side wall and exits through straight slots on the opposite side wall. The flow field aerodynamics is investigated in part I of the paper. The reference Reynolds number is defined at the entrance of the test section and set at 67500 for all the experiments. A qualitative flow model is deduced from surface-streamline flow visualizations. Two-dimensional Particle Image Velocimetry measurements are performed in several planes around mid-span of the channel and recombined to visualize and quantify three-dimensional flow features. The jets issued from the tilted slots are characterized and the jet-rib interaction is analyzed. Attention is drawn to the motion of the flow deflected by the rib-roughened wall and impinging on the opposite smooth wall. The experimental results are compared with the numerical predictions obtained from the finite volume, RANS solver CEDRE.

Fanno Design of Blow-Off Lines in Heavy Duty Gas Turbine

2013 ◽  
Author(s):  
Marco Cioffi ◽  
Enrico Puppo ◽  
Andrea Silingardi
Keyword(s):  
Gas Turbines ◽  
Gas Flow ◽  
Initial Conditions ◽  
Three Dimensional ◽  
Design Procedure ◽  
Flow Equation ◽  
Heavy Duty ◽  
Cross Sectional ◽  
Pipe Length ◽  
Choked Flow

In typical heavy duty gas turbines the multistage axial compressor is provided with anti-surge pipelines equipped with on-off valves (blow-off lines), to avoid dangerous flow instabilities during start-ups and shut-downs. Blow-off lines show some very peculiar phenomena and somewhat challenging fluid dynamics, which require a deeper regard. In this paper the blow-off lines in axial gas turbines are analyzed by adopting an adiabatic quasi-unidimensional model of the gas flow through a pipe with a constant cross-sectional area and involving geometrical singularities (Fanno flow). The determination of the Fanno limit, on the basis of the flow equation and the second principle of thermodynamics, shows the existence of a critical pipe length which is a function of the pipe parameters and the initial conditions: for a length greater than this maximum one, the model requires a mass-flow reduction. In addition, in the presence of a regulating valve, so-called multi-choked flow can arise. The semi-analytical model has been implemented and the results have been compared with a three-dimensional CFD analysis and cross-checked with available field data, showing a good agreement. The Fanno model has been applied for the analysis of some of the actual machines in the Ansaldo Energia fleet under different working conditions. The Fanno tool will be part of the design procedure of new machines. In addition it will define related experimental activities.

Nonlinear Dynamics of a System of Coupled Oscillators With Essential Stiffness Nonlinearities

2003 ◽  
Author(s):  
Alexander F. Vakakis ◽  
Richard H. Rand
Keyword(s):  
Normal Modes ◽  
Nonlinear Normal Modes ◽  
Resonance Capture ◽  
Periodic Motions ◽  
Internal Resonances ◽  
Dynamical Mechanism ◽  
Elliptic Orbits ◽  
Weakly Coupled ◽  
Resonant Dynamics ◽  
Nonlinear Normal

We study the resonant dynamics of a two-degree-of-freedom system composed a linear oscillator weakly coupled to a strongly nonlinear one, with an essential (nonlinearizable) cubic stiffness nonlinearity. For the undamped system this leads to a series of internal resonances, depending on the level of (conserved) total energy of oscillation. We study in detail the 1:1 internal resonance, and show that the undamped system possesses stable and unstable synchronous periodic motions (nonlinear normal modes — NNMs), as well as, asynchronous periodic motions (elliptic orbits — EOs). Furthermore, we show that when damping is introduced certain NNMs produce resonance capture phenomena, where a trajectory of the damped dynamics gets ‘captured’ in the neighborhood of a damped NNM before ‘escaping’ and becoming an oscillation with exponentially decaying amplitude. In turn, these resonance captures may lead to passive nonlinear energy pumping phenomena from the linear to the nonlinear oscillator. Thus, sustained resonance capture appears to provide a dynamical mechanism for passively transferring energy from one part of the system to another, in a one-way, irreversible fashion. Numerical integrations confirm the analytical predictions.

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