Review of Flow-Excited Resonance of Acoustic Trapped Modes in Ducted Shallow Cavities

2016 ◽  
Vol 138 (4) ◽  
Author(s):  
Kareem Aly ◽  
Samir Ziada
Keyword(s):  
Interaction Mechanism ◽  
Acoustic Resonance ◽  
Feedback Mechanism ◽  
Mode Shapes ◽  
Trapped Modes ◽  
Resonance Amplitude ◽  
Sound Fields ◽  
Trapped Mode ◽  
Shallow Cavities ◽  
Cavity Flow Oscillations

Flow-excited resonances of acoustic trapped modes in ducted shallow cavities are reviewed in this paper. The main components of the feedback mechanism which sustains the acoustic resonance are discussed with particular emphasis on the complexity of the trapped mode shapes and the strong three-dimensionality of the cavity flow oscillations during the resonance. Due to these complexities of the flow and sound fields, it is still difficult to theoretically or numerically model the interaction mechanism which sustains the acoustic resonance. Strouhal number and resonance amplitude charts are therefore included to help designers avoid the occurrence of resonance in new installations, and effective countermeasures are provided which can be implemented to suppress trapped mode resonances in operating plants.

Effect of Upstream Edge Geometry on the Trapped Mode Resonance of Ducted Cavities

2013 ◽  
Author(s):  
Michael Bolduc ◽  
Manar Elsayed ◽  
Samir Ziada
Keyword(s):  
Gas Flow ◽  
Effective Means ◽  
Acoustic Resonance ◽  
Trapped Modes ◽  
Acoustic Modes ◽  
Trapped Mode ◽  
Edge Geometry ◽  
Different Types ◽  
Shallow Cavities ◽  
Set Up

Gas flow over ducted cavities can excite strong acoustic resonances within the confined volumes housing the cavities. When the wavelength of the resonant acoustic modes is comparable to, or smaller than, the cavity dimensions, these modes are referred to as trapped acoustic modes. The excitation mechanism causing the resonance of these trapped modes in axisymmetric shallow cavities has been investigated experimentally in a series of papers by Aly & Ziada [1–3]. In this paper, the same experimental set-up is used to investigate the effect of the upstream edge geometry on the acoustic resonance of trapped modes. The investigated geometries include sharp and rounded cavity corners, chamfering the upstream edge, and spoilers of different types and sizes. Rounding off the cavity edges is found to increase the pulsation amplitude substantially, but the resonance lock-on range is delayed, i.e. it is shifted to higher flow velocities. Similarly, chamfering the upstream corner delays the onset of resonance, but does not increase its intensity. Spoilers, or vortex generators, added at the upstream edge have been found to be the most effective means to suppress the resonance. However, the minimum spoiler size which is needed to suppress the resonance increases as the cavity size becomes larger.

Passive Control of Trapped Mode Resonance of Ducted Cavities

2014 ◽  
Vol 136 (5) ◽  
Author(s):  
M. Bolduc ◽  
M. Elsayed ◽  
S. Ziada
Keyword(s):  
Gas Flow ◽  
Passive Control ◽  
Effective Means ◽  
Acoustic Resonance ◽  
Trapped Modes ◽  
Mean Flow ◽  
Acoustic Modes ◽  
Trapped Mode ◽  
Shallow Cavities ◽  
Set Up

Gas flow over ducted cavities can excite strong acoustic resonances within the confined volumes housing the cavities. When the wavelength of the resonant acoustic modes is comparable with, or smaller than, the cavity dimensions, these modes are referred to as trapped acoustic modes. The flow excitation mechanism causing the resonance of these trapped modes in axisymmetric shallow cavities has been investigated experimentally in a series of papers by Aly and Ziada (2010, “Flow-Excited Resonance of Trapped Modes of Ducted Shallow Cavities,” J. Fluids Struct., 26, pp. 92–120; 2011, “Azimuthal Behaviour of Flow-Excited Diametral Modes of Internal Shallow Cavities,” J. Sound Vib., 330, pp. 3666–3683; 2012, “Effect of Mean Flow on the Trapped Modes of Internal Cavities,” J. Fluids Struct., 33, pp. 70–84). In this paper, the same experimental set-up is used to investigate the effect of the upstream edge geometry on the acoustic resonance of trapped modes. The investigated geometries include sharp and rounded cavity corners, chamfering the upstream edge, and spoilers of different types and sizes. Rounding-off the cavity edges is found to increase the pulsation amplitude substantially, but the resonance lock-on range is delayed, i.e., it is shifted to higher flow velocities. Similarly, chamfering the upstream corner delays the onset of resonance, but maintains its intensity in comparison with that of sharp edges. Spoilers, or vortex generators, added at the upstream edge have been found to be the most effective means to suppress the resonance. However, the minimum spoiler size which is needed to suppress the resonance increases as the cavity size becomes larger.

Flow-Excited Acoustic Resonance of Single Finned Cylinder in Cross-Flow

2014 ◽  
Author(s):  
Nadim Arafa ◽  
Atef Mohany
Keyword(s):  
Aspect Ratio ◽  
Acoustic Pressure ◽  
Interaction Mechanism ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Effective Diameter ◽  
Pressure Amplitude ◽  
Resonance Amplitude ◽  
Fin Thickness ◽  
Fin Spacing

The flow-excited acoustic resonance of single straight finned cylinder in cross-flow is investigated experimentally in this work. This phenomenon has been investigated in some detail for the case of bare cylinders; however, the effect of adding fins to the cylinders on the flow-sound interaction mechanism is not yet fully understood. During the experiments, the acoustic cross-modes of the duct housing the cylinder are self-excited due to the vortex shedding that emerges from the cylinder’s surface. In order to determine the effect of different fin parameters on the onset and intensity of acoustic resonance, fourteen different finned cylinders with fin thickness ranging from 0.35 to 1.5 mm and fin density ranging from 4 to 13.7 fin/inch are investigated for a Reynolds number ranging from 3.2×104 to 2.6×105. The onset and intensity of the acoustic resonance generated from each finned cylinder are compared to those generated from an equivalent bare cylinder with the same effective diameter. It is observed that the finned cylinders experience an earlier acoustic resonance and higher levels of acoustic pressure compared to their equivalent bare cylinders. This suggests that adding fins to the cylinder enhances the flow coherence along the cylinder’s span and thus makes the flow more susceptible to acoustic excitation. Moreover, it is observed that for constant fin spacing the acoustic pressure amplitude increases and the acoustic resonance occurs at earlier velocities as the fin thickness increases. On the other hand, for constant fin thickness, as the fin spacing increases the amplitude of the acoustic pressure decreases, while the onset of the resonance is delayed. Finally, the effect of the cylinder’s aspect ratio is investigated in three different test sections. It is observed that the amplitude of the excited acoustic resonance depends on the cylinder’s aspect ratio. The acoustic resonance amplitude is weaker for finned cylinders with aspect ratio less than 5 compared to their equivalent bare cylinders. However, finned cylinders with aspect ratio higher than 6 produces stronger acoustic resonance compared to their equivalent bare cylinders.

Flow-Excited Acoustic Resonance of Trapped Modes of a Ducted Rectangular Cavity

2016 ◽  
Vol 138 (3) ◽  
Author(s):  
Michael Bolduc ◽  
Samir Ziada ◽  
Philippe Lafon
Keyword(s):  
Shear Layer ◽  
Particle Velocity ◽  
Acoustic Mode ◽  
Velocity Fields ◽  
Mode Shapes ◽  
Trapped Modes ◽  
Mean Flow ◽  
Cross Sectional ◽  
Shallow Cavities ◽  
Axisymmetric Cavities

Flow over ducted cavities can lead to strong resonances of the trapped acoustic modes due to the presence of the cavity within the duct. Aly and Ziada (2010, “Flow-Excited Resonance of Trapped Modes of Ducted Shallow Cavities,” J. Fluids Struct., 26(1), pp. 92–120; 2011, “Azimuthal Behaviour of Flow-Excited Diametral Modes of Internal Shallow Cavities,” J. Sound Vib., 330(15), pp. 3666–3683; and 2012, “Effect of Mean Flow on the Trapped Modes of Internal Cavities,” J. Fluids Struct., 33, pp. 70–84) investigated the excitation mechanism of acoustic trapped modes in axisymmetric cavities. These trapped modes in axisymmetric cavities tend to spin because they do not have preferred orientation. The present paper investigates rectangular cross-sectional cavities as this cavity geometry introduces an orientation preference to the excited acoustic mode. Three cavities are investigated, one of which is square while the other two are rectangular. In each case, numerical simulations are performed to characterize the acoustic mode shapes and the associated acoustic particle velocity fields. The test results show the existence of stationary modes, being excited either consecutively or simultaneously, and a particular spinning mode for the cavity with square cross section. The computed acoustic pressure and particle velocity fields of the excited modes suggest complex oscillation patterns of the cavity shear layer because it is excited, at the upstream corner, by periodic distributions of the particle velocity along the shear layer circumference.

scholarly journals Localization of Increased Noise at Operating Speed of a Passenger Wagon

2021 ◽  
Vol 13 (2) ◽  
pp. 453
Author(s):  
Ján Ďungel ◽  
Peter Zvolenský ◽  
Juraj Grenčík ◽  
Lukáš Leštinský ◽  
Ján Krivda
Keyword(s):  
Natural Frequencies ◽  
Internal Noise ◽  
High Volume ◽  
Acoustic Properties ◽  
Normal Operation ◽  
Mode Shapes ◽  
Railway Transport ◽  
Noise Sources ◽  
Sound Fields ◽  
Real Picture

Noise generated by railway wagons in operation is produced by large numbers of noise sources. Although the railway transport is considered to be environmental friendly, especially in production of CO2 emissions, noise is one of problems that should be solved to keep the railway transport competitive and sustainable in future. In the EU, there is a strong permanent legislation pressure on interior and exterior noise reduction in railway transport. In the last years in Slovakia, besides modernization of existing passenger wagons fleet as a cheaper option of transport quality improvement, quite a number of coaches have been newly manufactured, too. The new design is usually aimed at increased speed, higher travel comfort, in which reduction of noise levels is expected. However, not always the new designs meet all expectations. Noise generation and propagation is a complex system and should be treated such from the beginning. There are possibilities to simulate the structural natural frequencies to predict vibrations and sound generated by these vibrations. However, the real picture about sound fields can be obtained only by practical measurements. Simulations of the wagon’s natural frequencies and mode shapes and measurements in real operation using a digital acoustic camera Soundcam have been done, which showed that for the calculated speeds the largest share of noise from the chassis was not radiated through the floor of the wagon, as was expected, but through the ceiling of the wagon. To improve the acoustic properties of the wagon at higher speed, it was proposed to use high-volume textile insulation in the ceiling of the wagon. The paper briefly presents modern research approaches in the search for ways to reduce internal noise in selected wagons used in normal operation on the Slovak railways.

scholarly journals Trapped modes in a waveguide with a long obstacle

2000 ◽  
Vol 403 ◽  
pp. 251-261 ◽  
Author(s):  
N. S. A. KHALLAF ◽  
L. PARNOVSKI ◽  
D. VASSILIEV
Keyword(s):  
Two Dimensional ◽  
Trapped Modes ◽  
Acoustic Waveguide ◽  
Trapped Mode ◽  
Rectangular Obstacle

Consider an infinite two-dimensional acoustic waveguide containing a long rectangular obstacle placed symmetrically with respect to the centreline. We search for trapped modes, i.e. modes of oscillation at particular frequencies which decay down the waveguide. We provide analytic estimates for trapped mode frequencies and prove that the number of trapped modes is asymptotically proportional to the length of the obstacle.

Heat Transfer Implications of Acoustic Resonances in HP Turbine Blade Internal Cooling Channels

2015 ◽  
Author(s):  
C. Selcan ◽  
B. Cukurel ◽  
J. Shashank
Keyword(s):  
Heat Transfer ◽  
Turbine Blade ◽  
Cooling System ◽  
Resonance Condition ◽  
Order Approximation ◽  
Acoustic Resonance ◽  
Mode Shapes ◽  
Internal Cooling ◽  
The Impact ◽  
Standing Sound

In an attempt to investigate the acoustic resonance effect of serpentine passages on internal convection heat transfer, the present work examines a typical high pressure turbine blade internal cooling system, based on the geometry of the NASA E3 engine. In order to identify the associated dominant acoustic characteristics, a numerical FEM simulation (two-step frequency domain analysis) is conducted to solve the Helmholtz equation with and without source terms. Mode shapes of the relevant identified eigenfrequencies (in the 0–20kHz range) are studied with respect to induced standing sound wave patterns and the local node/antinode distributions. It is observed that despite the complexity of engine geometries, as a first order approximation, the predominant resonance behavior can be modeled by a same-ended straight duct. Therefore, capturing the physics observed in a generic geometry, the heat transfer ramifications are experimentally investigated in a scaled wind tunnel facility at a representative resonance condition. Focusing on the straight cooling channel’s longitudinal eigenmode in the presence of an isolated rib element, the impact of standing sound waves on convective heat transfer and aerodynamic losses are demonstrated by liquid crystal thermometry, local static pressure and sound level measurements. The findings indicate a pronounced heat transfer influence in the rib wake separation region, without a higher pressure drop penalty. This highlights the potential of modulating the aero-thermal performance of the system via acoustic resonance mode excitations.

Applicability of The Equivalent Diameter Approach to Estimate Vortex Shedding Frequency and Acoustic Resonance Excitation From Different Finned Cylinders In Cross-Flow

2021 ◽  
Author(s):  
Mohammed Alziadeh ◽  
Atef Mohany
Keyword(s):  
Strouhal Number ◽  
Vortex Shedding ◽  
Interaction Mechanism ◽  
Acoustic Resonance ◽  
Resonance Excitation ◽  
Equivalent Diameter ◽  
Effective Diameter ◽  
Finned Tubes ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

Abstract This article explores the applicability of utilizing different equivalent diameter (Deq) equations to estimate the vortex shedding frequency and onset of self-excited acoustic resonance for various types of finned cylinders. The focus is on three finned cylinder types that are commonly used in industrial heat exchangers: straight, twist-serrated, and crimped spirally finned cylinders. Within each type of fins, at least three different finned cylinders are investigated. The results indicate that at off-resonance conditions, utilizing the appropriate equivalent diameter collapses the Strouhal number data within the typical Strouhal number variations of an equivalent diameter circular, bare cylinder. However, when acoustic resonance is initiated, the onset and the peak of resonance excitation in all of the finned cylinder cases generally occurred at a reduced flow velocity earlier than that observed from their equivalent diameter bare cylinders. This suggests that although utilizing the appropriate equivalent diameter can reasonably estimate the vortex shedding frequency away from acoustic resonance excitation, it cannot be used to predict the onset of acoustic resonance in finned tubes. The findings of this study indicate that the effective diameter approach is not sufficient to capture the intrinsic changes in the flow-sound interaction mechanism as a result of adding fins to a bare cylinder. Thus, a revision of the acoustic Strouhal number charts is required for finned tubes of different types and arrangements.

Large-Scale Disturbances and Their Mitigation Downstream of Shallow Cavities Covered by a Perforated Lid

2004 ◽  
Vol 126 (5) ◽  
pp. 851-860 ◽  
Author(s):  
Stephen A. Jordan
Keyword(s):  
Fundamental Frequency ◽  
Large Scale ◽  
Periodic Oscillation ◽  
Feedback Mechanism ◽  
Complete Disappearance ◽  
Freestream Velocity ◽  
Design Engineers ◽  
Large Scale Disturbance ◽  
Key Factor ◽  
Shallow Cavities

Flow past cavities covered by perforated lids pose a challenging problem for design engineers. Kelvin–Helmholtz waves appear early in the separated shear layers above the perforations that quickly mature into large-scale coherent structures far downstream. This evolution is sustained by a hydrodynamic feedback mechanism within the cavity even when its aft wall is far removed from the lid. Herein, the results from large-eddy simulations show analogous fundamental characteristics between open and perforated-cover cavities. Both adequately scale the fundamental frequency of the large-scale disturbance using the freestream velocity and the cavity width (or lid length). Moreover, the dimensionless frequencies jump to higher modes at equivalent length scales. Unlike the open cavity, one can invoke certain conditions that instigate the instability above the perforations but not a simultaneous long-term feedback mechanism necessary to fully sustain the periodic oscillation. The lid itself offers options for mitigating (or even eliminating) the instability. Results (for laminar separation) show the perforation spacing as the key factor. While maintaining the same fundamental frequency, one can easily dampen its spectral peak to complete disappearance by extending the perforation spacing.

Rayleigh–Bloch surface waves along periodic gratings and their connection with trapped modes in waveguides

1999 ◽  
Vol 386 ◽  
pp. 233-258 ◽  
Author(s):  
R. PORTER ◽  
D. V. EVANS
Keyword(s):  
Finite Number ◽  
Surface Waves ◽  
Cross Section ◽  
Electromagnetic Waves ◽  
Mode Shapes ◽  
Special Choice ◽  
Trapped Mode ◽  
Uniform Cross Section ◽  
New Type ◽  
Incident Waves

Rayleigh–Bloch surface waves are acoustic or electromagnetic waves which propagate parallel to a two-dimensional diffraction grating and which are exponentially damped with distance from the grating. In the water-wave context they describe a localized wave having dominant wavenumber β travelling along an infinite periodic array of identical bottom-mounted cylinders having uniform cross-section throughout the water depth. A numerical method is described which enables the frequencies of the Rayleigh–Bloch waves to be determined as a function of β for an arbitrary cylinder cross-section. For particular symmetric cylinders, it is shown how a special choice of β produces results for the trapped mode frequencies and mode shapes in the vicinity of any (finite) number of cylinders spanning a rectangular waveguide or channel. It is also shown how one particular choice of β gives rise to a new type of trapped mode near an unsymmetric cylinder contained within a parallel-sided waveguide with locally-distorted walls. The implications for large forces due to incident waves on a large but finite number of such cylinders in the ocean is discussed.

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