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

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.

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

ASME 2008 Noise Control and Acoustics Division Conference ◽

10.1115/ncad2008-73016 ◽

2008 ◽

Cited By ~ 1

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.

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Fluid-Elastic Lock-in of a Cavity Shear Layer Instability With the Modes of a Submerged Cantilevered Beam

Journal of Vibration and Acoustics ◽

10.1115/1.4044302 ◽

2019 ◽

Vol 141 (6) ◽

Author(s):

Kristin L. Cody ◽

Michael L. Jonson ◽

Martin L. Pollack ◽

Stephen A. Hambric

Keyword(s):

Shear Layer ◽

Bluff Body ◽

Flow Instability ◽

Separated Flow ◽

Shear Layer Instability ◽

Describing Function ◽

Function Model ◽

Cantilevered Beam ◽

Modal Mass ◽

Lock In

AbstractLock-in flow tones can occur for many different types of flow instabilities and structural-acoustic resonators at low Mach number. This paper examines the interaction between a shear layer instability generated by flow over a shallow cavity and the modes of an elastic cantilevered beam containing the cavity. A describing function model indicates that a cavity shear layer instability capable of producing lock-in with acoustic pipe resonances cannot achieve lock-in with equivalent structural beam resonances, particularly resonances of submerged structures. Fluid-elastic cavity lock-in is unlikely to occur due to the high level of damping that exists for a submerged structure, the high fluid-loaded modal mass, and the relatively weak source strength a cavity generates. Limited experimentation using pressure, acceleration, and particle image velocimetry (PIV) measurements has been performed which are consistent with the describing function model. A stronger source produced by a larger scale flow instability—separated flow over a bluff body—was able to lock-in with modes of the same submerged structure, further demonstrating that the concern for lock-in from a cavity shear layer instability is isolated to systems capable of stronger coupling or those dominated by fluid-acoustic resonances.

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On the nature of the amplitude modulation of jet shear layer instability waves

The Physics of Fluids ◽

10.1063/1.864089 ◽

1983 ◽

Vol 26 (11) ◽

pp. 3180 ◽

Cited By ~ 8

Author(s):

Peter A. Monkewitz

Keyword(s):

Shear Layer ◽

Amplitude Modulation ◽

Shear Layer Instability ◽

Instability Waves

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The effect of shear layer instability on jet exhaust noise

Structure and Mechanisms of Turbulence II - Lecture Notes in Physics ◽

10.1007/bfb0012628 ◽

2005 ◽

pp. 254-264 ◽

Cited By ~ 5

Author(s):

C. J. Moore

Keyword(s):

Shear Layer ◽

Shear Layer Instability ◽

Exhaust Noise

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Instability patterns between counter-rotating disks

Nonlinear Processes in Geophysics ◽

10.5194/npg-10-281-2003 ◽

2003 ◽

Vol 10 (3) ◽

pp. 281-288 ◽

Cited By ~ 3

Author(s):

F. Moisy ◽

T. Pasutto ◽

M. Rabaud

Keyword(s):

Aspect Ratio ◽

Shear Layer ◽

Particle Image ◽

Shear Layer Instability ◽

Free Shear Layer ◽

Rotating Disks ◽

Height Ratio ◽

Aspect Ratios ◽

Image Velocimetry ◽

Local Reynolds Number

Abstract. The instability patterns in the flow between counter-rotating disks (radius to height ratio R/h from 3.8 to 20.9) are investigated experimentally by means of visualization and Particle Image Velocimetry. We restrict ourselves to the situation where the boundary layers remain stable, focusing on the shear layer instability that occurs only in the counter-rotating regime. The associated pattern is a combination of a circular chain of vortices, as observed by Lopez et al. (2002) at low aspect ratio, surrounded by a set of spiral arms, first described by Gauthier et al. (2002) in the case of high aspect ratio. Stability curve and critical modes are measured for the whole range of aspect ratios. From the measurement of a local Reynolds number based on the shear layer thickness, evidence is given that a free shear layer instability, with only weak curvature effect, is responsible for the observed patterns. Accordingly, the number of vortices is shown to scale as the shear layer radius, which results from the competition between the centrifugal effects of each disk.

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CFD Simulation of Flow Tones From Grazing Flow Past a Deep Cavity

Noise Control and Acoustics ◽

10.1115/imece2006-15633 ◽

2006 ◽

Cited By ~ 5

Author(s):

Ted G. Bagwell

Keyword(s):

Shear Layer ◽

Cfd Simulation ◽

Stokes Equations ◽

Weak Interactions ◽

Cavity Resonator ◽

Navier Stokes ◽

Flow State ◽

Deep Cavity ◽

Eddy Simulation ◽

Lock In

Locked-in flow tones due to shear flow over a deep cavity are investigated using Large Eddy Simulation (LES). An isentropic from of the compressible Navier-Stokes equations (pseudo-compressibility) is used to couple the vertical flow over the cavity mouth with the deep cavity resonances (1). Comparisons to published experimental data (2) show that the pseudo-compressible LES formulation is capable of predicting the feedforward excitation of the deep cavity resonator, as well as the feedback process from the resonator to the flow source. By systematically increasing the resonator damping level, it is shown that strong lock-in results in a more organized shear layer than is observed for the locked-out flow state. By comparison, weak interactions (non-locked-in) produce no change in the shear layer characteristics. This supports the 40 dB definition of lock-in defined in the experiment.

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Mitigating Blockage Effects on Flow Over a Circular Cylinder in an Adaptive-Wall Wind Tunnel

Journal of Fluids Engineering ◽

10.1115/1.4004421 ◽

2011 ◽

Vol 133 (8) ◽

Cited By ~ 2

Author(s):

Michael Bishop ◽

Serhiy Yarusevych

Keyword(s):

Reynolds Number ◽

Wind Tunnel ◽

Circular Cylinder ◽

Shear Layer ◽

Cylinder Surface ◽

Blockage Ratio ◽

Shear Layer Instability ◽

Frequency Increase ◽

Pressure Distributions ◽

Instability Frequency

The effect of wall streamlining on flow development over a circular cylinder was investigated experimentally in an adaptive-wall wind tunnel. Experiments were carried out for a Reynolds number of 57,000 and three blockage ratios of 5%, 8%, and 17%. Three test section wall configurations were investigated, namely, geometrically straight walls (GSW), aerodynamically straight walls (ASW), and streamlined walls (SLW). The results show that solid blockage effects are evident in cylinder surface pressure distributions for the GSW and ASW configurations, manifested by an increased peak suction and base suction. Upon streamlining the walls, pressure distributions for each blockage ratio investigated closely match distributions expected for low blockage ratios. Wake blockage limits wake growth in the GSW configuration at 7.75 and 15 diameters downstream of the cylinder for blockages of 17% and 8%, respectively. This adverse effect can be rectified by streamlining the walls, with the resulting wake width development matching that expected for low blockage ratios. Wake vortex shedding frequency and shear layer instability frequency increase in the GSW and ASW configurations with increasing blockage ratio. The observed invariance of the near wake width with wall configuration suggests that the frequency increase is caused by the increased velocity due to solid blockage effects. For all the blockage ratios investigated, this increase is rectified in the SLW configuration, with the resulting Strouhal numbers of about 0.19 matching that expected for low blockage ratios at the corresponding Reynolds number. Blockage effects on the shear layer instability frequency are also successfully mitigated by streamlining the walls.

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Flow Excitation of Diametral Acoustic Modes of Axisymmetric Cavities

Volume 9: 6th FSI, AE and FIV and N Symposium ◽

10.1115/pvp2006-icpvt-11-93133 ◽

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.

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Numerical parametric study on three-dimensional rectangular counter-flow thrust vectoring control

Proceedings of the Institution of Mechanical Engineers Part G Journal of Aerospace Engineering ◽

10.1177/0954410020925602 ◽

2020 ◽

Vol 234 (16) ◽

pp. 2221-2247 ◽

Cited By ~ 1

Author(s):

Kexin Wu ◽

Guang Zhang ◽

Tae Ho Kim ◽

Heuy Dong Kim

Keyword(s):

Mach Number ◽

Shear Layer ◽

Static Pressure ◽

Three Dimensional ◽

Critical Parameters ◽

Mechanical Equipment ◽

Counter Flow ◽

Thrust Coefficient ◽

Thrust Vectoring ◽

Gap Height

Recently, fluidic thrust vectoring control is popular for micro space launcher propulsion systems due to its several advantages, such as fast dynamic responsiveness, better control effectiveness, and no moving mechanical equipment. Counter-flow thrust vectoring control is an especially effective technique by utilizing less suction flow to control the mainstream deflection flexibly. In the current work, theoretical and numerical analyses are performed together to elaborate on the performance of the three-dimensional rectangular counter-flow thrust vectoring control system. A new propulsion nozzle of Mach 2.5 is devised by method of characteristics. To testify the feasibility and accuracy of the present research methodology, numerical results were validated against experimental data from the open literature. The computational result using the standard k-epsilon turbulence model reveals a good match with experimentally measured static pressure values along the centerline of the upper suction collar. The influence of several key parameters on vectoring performance is investigated herein, including the mainstream temperature, collar radius, horizontal collar length, and gap height. Critical parameters have been quantitatively analyzed, such as static pressure distribution along the centerline of the upper suction collar, pitching angle, suction mass flow ratio, and thrust coefficient. Furthermore, the flow-field features are qualitatively expounded based on the static pressure contour, streamline, iso-turbulent kinetic energy contour, and iso-Mach number contour. Some important conclusions are offered for further studies. The mainstream temperature mainly affects different dynamic characteristics of the mixing shear layer, including the convective Mach number of the shear layer, density ratio, and flow velocity ratio. The collar radius influences the pressure gradient near the suction collar surface significantly. The pitching angle increases rapidly with the increasing collar radius. As the horizontal collar length increases, the systematic deflection angle initially increases then decreases. The highest pitching angle is obtained for L/ H = 3.53. With regard to the gap height, a larger gap height can achieve a higher pitching angle.

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