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.

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.

Flow Excited Resonance of a Confined Shallow Cavity in Low Mach Number Flow and Its Control

2002 ◽  
Author(s):  
S. Ziada ◽  
H. Ng ◽  
C. Blake
Keyword(s):  
Mach Number ◽  
Shear Layer ◽  
Control Method ◽  
Acoustic Resonance ◽  
Synthetic Jet ◽  
Low Mach Number ◽  
Acoustic Modes ◽  
Shallow Cavities ◽  
Number Flow ◽  
Low Mach Number Flow

Shallow cavities exposed to unbounded, low Mach number flow are generally weak aeroacoustic sources because their acoustic modes are heavily damped. This paper focuses on a cavity mounted on the wall of a duct to investigate the effect of “confinement”, i.e. solid boundaries close to the cavity, on the aeroacoustic response of shallow cavities in low Mach number flow (M < 0.3). It is found that the transverse acoustic modes of the duct-cavity combination are excited by the higher order modes of the cavity shear layer oscillations. The nature of the excitation mechanism as well as the effects of the cavity and duct dimensions are investigated by means of measurements of the amplitude and phase distributions of the acoustic pressure, complemented with flow visualization of the cavity shear layer oscillation. A method to predict the onset of resonance is also suggested. It is also shown that the acoustic resonance is effectively suppressed by a feedback control method, which generates a synthetic jet acting at the cavity upstream corner. The effect of the phase and gain of the controller transfer function is studied in some detail.

Numerical Study of Methane and Air Combustion Inside Heat-Recirculating Combustors

2013 ◽  
Author(s):  
Yanxia Li ◽  
Zhongliang Liu ◽  
Yan Wang ◽  
Jiaming Liu
Keyword(s):  
Gas Flow ◽  
Numerical Study ◽  
Convection Heat Transfer ◽  
Central Area ◽  
Small Scale ◽  
Inlet Velocity ◽  
Strong Convection ◽  
Simulation Results ◽  
Lean Fuel ◽  
Set Up

A numerical model on methane/air combustion inside a small Swiss-roll combustor was set up to investigate the flame position of small-scale combustion. The simulation results show that the combustion flame could be maintained in the central area of the combustor only when the speed and equivalence ratio are all within a narrow and specific range. For high inlet velocity, the combustion could be sustained stably even with a very lean fuel and the flame always stayed at the first corner of reactant channel because of the strong convection heat transfer and preheating. For low inlet velocity, small amounts of fuel could combust stably in the central area of the combustor, because heat was appropriately transferred from the gas to the inlet mixture. Whereas, for the low premixed gas flow, only in certain conditions (Φ = 0.8 ～ 1.2 when ν0 = 1.0m/s, Φ = 1.0 when ν0 = 0.5m/s) the small-scale combustion could be maintained.

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.

Stress Distribution Analysis of Different Types of Blade for a Fermentation System

2013 ◽  
Vol 479-480 ◽  
pp. 319-323
Author(s):  
Cheng Chi Wang ◽  
Po Jen Cheng ◽  
Kuo Chi Liu
Keyword(s):  
Stress Distribution ◽  
Maximum Stress ◽  
Effective Means ◽  
Materials Processing ◽  
Distribution Analysis ◽  
Stress Distributions ◽  
Fermentation System ◽  
Different Types ◽  
Inclined Angle ◽  
Key Parameter

Fermentation system is widely used for food manufacturing, materials processing and chemical reaction etc. Different types of blade in the tank for fermentation cause distinct stress distributions on the surface between fluid and blade, and appear various flow fields in the tank. So, this paper is mainly focused on analyzing the stress field of blades under different scales of blade with fixing rotational speed. The results show that the ratio of blade length to width influences stress distribution on the blades. At the same time, the inclined angle of blade is also the key parameter for the consideration of design and appropriate design will decrease the maximum stress. The results provide an effective means of gaining insights into the stress distribution of fermentation system.

Exploration of the Early Childhood Care and Education Provided in Kindergartens in Japan

2019 ◽  
Vol 8 (2) ◽  
pp. 24-29
Author(s):  
Manjinder Kaur
Keyword(s):  
Early Childhood ◽  
World War Ii ◽  
World War ◽  
High Quality ◽  
Early Childhood Care ◽  
Childhood Care ◽  
Different Types ◽  
Education And Care ◽  
Set Up ◽  
Shed Light

This study tends to shed light on early childhood care and education (ECCE) institutions with special reference to kindergartens in Fukuoka, Japan. The choice of the topic for study was derived by the importance of ECCE in children’s life and huge economic growth of Japan after worst effects of world war-II, which are thought to be linked with the education that children receives in Japan. The study is limited to four kindergartens in Fukuoka City and observations made for the study refers to 2018. Herein, different types of institutions providing ECCE, their infrastructural set-up, activities, along with curriculum are discussed. At the end, issues and challenges of ECCE system in Japan are discussed. It has been observed that the infrastructural facility and nature of activities are of high quality. Each and every care is being taken to inculcate habits, as well as to maintain physical and intellectual growth of children. The children seem to be highly happy and enjoy learning via various activities in these schools. It is clear that the devised policies on education and care of children are implemented in full spirit.

scholarly journals Determination of Aerodynamic Damping at High Reduced Frequencies

2018 ◽  
Author(s):  
Minghao Pan ◽  
Paul Petrie-Repar ◽  
Hans Mårtensson ◽  
Tianrui Sun ◽  
Tobias Gezork
Keyword(s):  
Acoustic Resonance ◽  
Forced Response ◽  
Economic Consequences ◽  
Navier Stokes ◽  
Flow Solver ◽  
Aerodynamic Damping ◽  
Acoustic Modes ◽  
Reduced Frequency ◽  
Standard Configuration

In turbomachines, forced response of blades is blade vibrations due to external aerodynamic excitations and it can lead to blade failures which can have fatal or severe economic consequences. The estimation of the level of vibration due to forced response is dependent on the determination of aerodynamic damping. The most critical cases for forced response occur at high reduced frequencies. This paper investigates the determination of aerodynamic damping at high reduced frequencies. The aerodynamic damping was calculated by a linearized Navier-Stokes flow solver with exact 3D non-reflecting boundary conditions. The method was validated using Standard Configuration 8, a two-dimensional flat plate. Good agreement with the reference data at reduced frequency 2.0 was achieved and grid converged solutions with reduced frequency up to 16.0 were obtained. It was concluded that at least 20 cells per wavelength is required. A 3D profile was also investigated: an aeroelastic turbine rig (AETR) which is a subsonic turbine case. In the AETR case, the first bending mode with reduced frequency 2.0 was studied. The 3D acoustic modes were calculated at the far-fields and the propagating amplitude was plotted as a function of circumferential mode index and radial order. This plot identified six acoustic resonance points which included two points corresponding to the first radial modes. The aerodynamic damping as a function of nodal diameter was also calculated and plotted. There were six distinct peaks which occurred in the damping curve and these peaks correspond to the six resonance points. This demonstrates for the first time that acoustic resonances due to higher order radial acoustic modes can affect the aerodynamic damping at high reduced frequencies.

Trapped acoustic modes in aeroengine intakes with swirling flow

2000 ◽  
Vol 419 ◽  
pp. 151-175 ◽  
Author(s):  
A. J. COOPER ◽  
N. PEAKE
Keyword(s):  
Swirling Flow ◽  
Aircraft Engine ◽  
Acoustic Resonance ◽  
Axial Fan ◽  
System Parameters ◽  
Acoustic Modes ◽  
Resonant States ◽  
Blade Row ◽  
Actuator Disc ◽  
Axial Variation

A theoretical model of an aeroengine intake–fan system is developed in order to show the existence of acoustic resonance in the intake. In general this phenomenon can be linked to instabilities in aircraft engine inlets.The model incorporates a slowly varying duct intake and accounts for the swirling flow downstream of the fan. The slow axial variation in cross-section gives rise to turning points where upstream-propagating acoustic modes are totally reflected into downstream-propagating modes. The effect of the swirling flow downstream can be to cut off a mode which is cut on upstream of the fan. It is shown that these two aspects of the flow, coupled with the effects of the fan (represented by an actuator disc), can lead to acoustic modes becoming trapped in the intake, thus giving rise to pure acoustic resonance.A whole range of system parameters, such as axial, fan and swirl Mach numbers, which satisfy the conditions for resonance are identified. The effects of a stationary blade row behind the fan are also considered leading to a second family of resonant states. In addition we find resonance due to reflection of acoustic modes at the open (inlet) end of the duct.

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