Flow Excitation of Diametral Acoustic Modes of Axisymmetric Cavities

2006 ◽  
Author(s):  
Kareem Awny ◽  
Samir Ziada
Keyword(s):  
Acoustic Pressure ◽  
Cavity Length ◽  
Low Flow ◽  
Cylindrical Pipe ◽  
Mode Pattern ◽  
Instability Modes ◽  
Symmetric Cavity ◽  
Shallow Cavity ◽  
Lock In ◽  
Axisymmetric Cavities

Flow-excited resonances of the acoustic diametral modes of a cylindrical pipe housing an axi-symmetric shallow cavity are investigated experimentally. The aeroacoustic response of the cavity-pipe combination is studied up to a Mach number of 0.4 and for several ratios of cavity length to its depth. Although the diametral modes do not have a preferred orientation because of the system axi-symmetry, they are found to be strongly excited by any of the first three instability modes of the cavity shear layer. Intense acoustic pressure levels, up to 170 dB, and wide lock-in resonance ranges have been observed. The acoustic pressure and its phase are measured along the cavity circumference to examine the orientation of the excited diametral modes within the un-preferential domain of the axi-symmetric cavity. Preliminary results suggest that the excited modes are stationary at low flow velocities, but they switch to spinning mode pattern at higher velocities.

Investigation of Diametral Acoustic Modes in a Model of a Steam Control Gate Valve

2014 ◽  
Vol 136 (6) ◽  
Author(s):  
Oleksandr Barannyk ◽  
Peter Oshkai
Keyword(s):  
Gate Valve ◽  
Acoustic Pressure ◽  
Three Dimensional ◽  
Turbulent Pipe Flow ◽  
Mode Shapes ◽  
Main Pipeline ◽  
Scaled Model ◽  
Axisymmetric Cavity ◽  
Acoustic Modes ◽  
Shallow Cavity

The objective of the present study is to provide an insight into mechanism of coupling between turbulent pipe flow and partially trapped diametral acoustic modes associated with a shallow cavity formed by the seat of a steam control gate valve. First, the effects of the internal pipe geometry immediately upstream and downstream of the shallow cavity on the characteristics of partially trapped diametral acoustic modes were investigated. The mode shapes were calculated numerically by solving a Helmholtz equation in a three-dimensional domain corresponding to the internal geometry of the pipe and the cavity. Second, the set of experiments were performed using a scaled model of a gate valve mounted in a pipeline that contained converging–diverging sections in the vicinity of the valve. Acoustic pressure measurements at three azimuthal locations at the floor of the cavity were performed for a range of geometries of the converging–diverging section and inflow velocities. The experimentally obtained pressure data were then used to scale the amplitude of the pressure in the numerical simulations. The present results are in good agreement with the results reported in earlier studies for an axisymmetric cavity mounted in a pipe with a uniform cross-section. The resonant response of the system corresponded to the second diametral mode of the cavity. Excitation of the dominant acoustic mode was accompanied by pressure oscillations corresponding to other acoustic modes. As the angle of the converging–diverging section of the main pipeline in the vicinity of the cavity increased, the trapped behavior of the acoustic diametral modes diminished, and additional antinodes of the acoustic pressure wave were observed in the main pipeline.

The Influence of Flow Instability on the Lock-In of Distributed Elastic Resonators

2008 ◽  
Author(s):  
Kristin Lai-Fook Cody ◽  
Stephen A. Hambric ◽  
Martin L. Pollack ◽  
Michael L. Jonson
Keyword(s):  
Shear Layer ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Flow Instability ◽  
Coupling Factor ◽  
Flow Instabilities ◽  
Analytical Models ◽  
Low Flow ◽  
Shear Layer Instability ◽  
Lock In

Lock-in occurs between many different types of flow instabilities and structural-acoustic resonators. Factors that describe the coupling between the fluid and structure have been defined for low flow Mach numbers. This paper discusses how different flow instabilities influence lock-in experimentally and analytically. A key concept to the lock-in process is the relative source generation versus dissipation. The type of fluid instability source dominates the generation component of the process, so a comparison between a cavity shear layer instability with a relatively stronger source, for example wake vortex shedding from a bluff body, will be described as a coupling factor. In the fluid-elastic cavity lock-in case, the shear layer instability produced by flow over a cavity couples to the elastic structure containing the cavity. In this study, this type of lock-in was not achieved experimentally. A stronger source, vortex shedding from a bluff body however, is shown experimentally to locks into the same resonator. This study shows that fluid-elastic cavity lock-in is unlikely to occur given the critical level of damping that exists for a submerged structure and the relatively weak source strength that a cavity produces. Also in this paper, a unified theory is presented based on describing functions, a nonlinear control theory used to predict limit cycles of oscillation, where a self-sustaining oscillation or lock-in is possible. The describing function models capture the primary characteristics of the instability mechanisms, are consistent with Strouhal frequency concepts, capture damping, and are consistent with mass-damping concepts from wake oscillator theory. This study shows a strong consistency between the analytical models and experimental results.

Acoustic Response of Multiple Shallow Cavities and Prediction of Self-Excited Acoustic Oscillations

2018 ◽  
Vol 140 (9) ◽  
Author(s):  
Ayman A. Shaaban ◽  
Samir Ziada
Keyword(s):  
Prediction Model ◽  
Industrial Applications ◽  
The Self ◽  
Acoustic Oscillations ◽  
Source Characteristics ◽  
Standing Wave Method ◽  
Wide Range ◽  
Shallow Cavities ◽  
Lock In ◽  
Axisymmetric Cavities

Self-sustaining oscillations of flow over ducted cavities and in corrugated pipes are a known source of tonal noise and excessive vibration in industrial applications. Corrugated pipes can be modeled as a series of axisymmetric cavities. In the current study, the aero-acoustic sources generated by one-, two-, and three-cavity configurations have been experimentally investigated by means of the standing wave method (SWM) for a wide range of Strouhal numbers and acoustic excitation levels. The source strength is found to increase in a nonlinear manner with increasing the number of cavities. Moreover, the self-excited acoustic resonances of the same cavity combinations are investigated. The source characteristics are compared with the observed lock-in range from the self-excited experiments. A prediction model is also developed to utilize the measured source characteristics for estimating the amplitude of the cavities self-sustained oscillations. The self-excited experimental data are used to assess the effect of acoustic absorption at the cavity edges. This absorption is found to be substantial and must be accounted for in the prediction model. When the model is supplemented with appropriate loss coefficients, it predicts fairly well the pulsation amplitude within the resonance lock-in range of the studied multiple cavity configurations.

scholarly journals Challenges of Investigating Fluid-Elastic Lock-In of a Shallow Cavity and a Cantilevered Beam at Low Mach Numbers

2005 ◽  
Author(s):  
Kristin Lai-Fook Cody ◽  
Stephen A. Hambric ◽  
Martin L. Pollack
Keyword(s):  
Shear Layer ◽  
Experimental System ◽  
Feedback Mechanism ◽  
Analytical Models ◽  
Low Flow ◽  
Critical Parameters ◽  
Shear Layer Instability ◽  
Cantilevered Beam ◽  
Mach Numbers ◽  
Lock In

At low flow Mach numbers, fluid-elastic lock-in may occur when a shear layer instability interacts with an adjoining or nearby structure and the resulting vibration of the structure reinforces the shear layer instability. Despite the significant amount of study of lock-in with acoustic resonators, fluid-elastic lock-in of a shear layer fluctuation over a cavity and a structural resonator is not well understood and has not been thoroughly studied. Design of an experimental system is described and preliminary diagnostics are addressed as a basis for a platform for developing a fundamental understanding of the feedback mechanism, analytical models for predicting and describing fluid-elastic lock-in conditions, and the roles of the fluid and structural dynamics in the process. Features of the system investigated here include design for characterization of modal excitation of a beam-like structure from the shear layer fluctuation, isolation of the predominant instability source to the shear layer fluctuation over the cavity, variation of the cavity size to identify critical parameters that govern fluid-elastic lock-in, and alteration of the inflow boundary layer momentum thickness. So far, lock-in between the cavity and the distributed elastic resonator has not been achieved. Further investigations to determine the role of the source and resonator attributes are underway.

Quantitative Visualization of Unstable, Acoustically Coupled Shear Layers in Deep Axisymmetric Cavities

2014 ◽  
Author(s):  
Peter Oshkai ◽  
Oleksandr Barannyk
Keyword(s):  
Shear Layer ◽  
Flow Structure ◽  
Large Scale ◽  
Acoustic Pressure ◽  
Noise Suppression ◽  
Internal Flow ◽  
Shear Layers ◽  
Optical Recording ◽  
Transport Systems ◽  
Axisymmetric Cavities

High-amplitude acoustic pressure fluctuations associated with locked-on, resonant flow states frequently occur in engineering systems that involve internal cavities located in pipelines, such as components of gas transport systems, steam delivery pipelines and jet engines. This paper describes the evolution of fully turbulent, acoustically coupled shear layers that form across deep, axisymmetric cavities. Effects of geometric modifications of the cavity edges on the separated flow structure were investigated using digital particle image velocimetry (PIV). The internal flow was non-intrusively accessed by means of a borescope, which allowed illumination and optical recording of flow tracers inside the cavity. Instantaneous, phase- and time-averaged patterns of velocity and vorticity provided insight into the flow physics during flow tone generation and noise suppression by the geometric modifications. In particular, the first mode of the shear layer oscillations was significantly affected by shallow chamfers located at the upstream and, to a lesser degree, the downstream edges of the cavity. Specifically, the introduction of the chamfers affected the phase and the location of formation of large-scale vortical structures in the shear layer, which is associated with a maximum of the vorticity thickness across the cavity opening. In turn, these changes in the flow structure affected the amplitude of acoustic pressure pulsations.

Numerical Analysis on the Start-Up Flow Past a Resonant Cavity

2008 ◽  
Vol 130 (10) ◽  
Author(s):  
Antonio Filippone
Keyword(s):  
Unsteady Flow ◽  
Computational Analysis ◽  
Cavity Length ◽  
Resonant Cavity ◽  
Three Dimensional ◽  
Freestream Velocity ◽  
Short Life Span ◽  
Start Up ◽  
Symmetric Cavity ◽  
Secondary Vortices

This paper presents the results of a computational analysis on a three-dimensional unsteady flow inside a resonant cavity. The cavity was fully immersed in a channel flow, had a squared cross section, and a spanwise aspect ratio equal to 3. It was partly closed to the inflow by slits upstream and downstream. The lid was 1∕4 of the cavity length. The Reynolds number was Re=8000 based on the freestream velocity. The numerical simulations were carried out for flow times up to 380 units. Results are presented for a symmetric cavity with respect to the normal to the freestream. The analysis shows complex three-dimensional vortex structures, with Taylor–Görtler-type vortices, filament vortices, and other secondary vortices, some having a relatively short life-span. It is shown that the flow is substantially symmetric, with small spanwise instabilities. It is further shown that there is an asymptotic tendency to an unsteady flow with large wavelengths. A primary vortex establishes at the center of the cavity. Most vortex regions disappear and that they depend on the type of geometry and the state of the boundary layer at the inlet.

Dynamic test of an acoustic/pressure sensor with precise cavity length control

2010 ◽  
Author(s):  
Wenhui Wang ◽  
Nan Wu ◽  
Ye Tian ◽  
Charles Guthy ◽  
Xingwei Wang ◽  
...  
Keyword(s):  
Pressure Sensor ◽  
Acoustic Pressure ◽  
Cavity Length ◽  
Dynamic Test
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