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.

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.

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

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.

Suppression of Self-Excited Flow Instability in a Radial Diffuser Using Rough Surfaces

2006 ◽  
Author(s):  
Saad A. Ahemd ◽  
Hayder Salem
Keyword(s):  
Flow Rate ◽  
Smooth Surface ◽  
Flow Instability ◽  
Rough Surfaces ◽  
Flow Coefficient ◽  
Flow Instabilities ◽  
Low Flow ◽  
Flow Rates ◽  
Flow Limit ◽  
Stable Operating

Flow instabilities in a compression system at low flow rates set the flow limit of the stable operating range. Experiments to investigate the feasibility of controlling the stall in the radial diffuser of a low speed centrifugal compressor were carried out. The technique was very simple and involved using rough surfaces (i.e., sand papers) attached to the diffuser shroud. The results showed that the flow instability in the diffuser (stall) was delayed to a lower flow coefficient (the mass flow rate could be reduced to 70% of its value with the smooth surface) when the rough surfaces were positioned on the diffuser shroud.

scholarly journals Design and Analysis of A Vortex Induced Vibration Based Oscillating Free Stream Energy Converter

2021 ◽  
pp. 112-117
Author(s):  
Ratan Kumar Das ◽  
Muhammad Taharat Galib
Keyword(s):  
Vortex Shedding ◽  
Large Scale ◽  
Bluff Body ◽  
Free Stream ◽  
Theoretical Formula ◽  
Pendulum Motion ◽  
Ansys Fluent ◽  
Vortex Induced Vibration ◽  
Karman Vortex ◽  
Lock In

The Kármán Vortex Shedding is one of the special types of vortex that is generated from asymmetric flow separation. For many years engineers tried to suppress the vortex shedding as it brings unnecessary motion to the static members inside the flow field. A converter model is designed and studied to harness the energy associated with this vortex shedding and convert it into usable form rather than suppressing it. It is a bluff body placed on the free stream incurring vortex-induced vibration and giving out a swinging pendulum motion. This motion is utilized to produce electricity. The model is analyzed on the free stream of water and conversion efficiency of 8.9% is achieved. A theoretical formula is derived regarding the force acting on the bluff body during the motion. Various parameters such as aspect ratio, flow velocity, lock-in delay, frequency of oscillation, etc. as well as their relations are studied by simulating the model in ANSYS FLUENT 18.1 for different configurations. From the simulated results it is obvious that as the lift force on the bluff body increases, more power generation is possible. Also, the experimental results paved the way for further study for practical large-scale implementation of the converter.

On the coherent structures and stability properties of a leading-edge separated aerofoil with turbulent recirculation

2011 ◽  
Vol 683 ◽  
pp. 395-416 ◽  
Author(s):  
V. Kitsios ◽  
L. Cordier ◽  
J.-P. Bonnet ◽  
A. Ooi ◽  
J. Soria
Keyword(s):  
Shear Layer ◽  
Bluff Body ◽  
Time History ◽  
Leading Edge ◽  
Numerical Data ◽  
Water Tunnel ◽  
Shear Layer Instability ◽  
Flow Configuration ◽  
Temporal Properties ◽  
Statistical Measures

AbstractThe present study is motivated by a need to produce stability modes to assist in the understanding and control of unsteady separated flows. The flow configuration is a NACA 0015 aerofoil with laminar leading-edge separation and turbulent recirculation. In previous water tunnel experiments, this flow configuration was measured in an unperturbed (uncontrolled) separated state, and a harmonically perturbed (controlled) reattached state. This study presents numerical data of the unperturbed case, and recovers stability modes to describe the evolution of perturbations in this environment. The unperturbed flow is numerically generated using large eddy simulation. Its temporal properties are quantified via a Fourier analysis of the velocity time history at selected points in space. The leading-edge shear layer instability is characterized by instantaneous vortex structures, and the bluff body shedding is illustrated by proper orthogonal decomposition modes. Statistical measures of the velocity field agree well with the water tunnel measurements. Finally a stability analysis is undertaken using a triple decomposition to distinguish between the time averaged field, the unsteady scales of motion, and a coherent wave (perturbation). This analysis identifies that perturbations in the region immediately downstream of the separated shear layer have the highest spatial growth rates. The associated frequency is of the order of the sub-harmonic of the shear layer instability.

A numerical study of vortex shedding from flat plates with square leading and trailing edges

1992 ◽  
Vol 236 ◽  
pp. 445-460 ◽  
Author(s):  
Yuji Ohya ◽  
Yasuharu Nakamura ◽  
Shigehira Ozono ◽  
Hideki Tsuruta ◽  
Ryuzo Nakayama
Keyword(s):  
Shear Layer ◽  
Vortex Shedding ◽  
Stokes Equations ◽  
Numerical Study ◽  
Vortical Flow ◽  
Navier Stokes ◽  
Shear Layer Instability ◽  
Flat Plates ◽  
Navier Stokes Equations ◽  
Trailing Edges

This paper describes a numerical study of the flow around flat plates with square leading and trailing edges on the basis of a finite-difference analysis of the two-dimensional Navier—Stokes equations. The chord-to-thickness ratio of a plate, d/h, ranges from 3 to 9 and the value of the Reynolds number based on the plate's thickness is constant and equal to 103. The numerical computation confirms the finding obtained in our previous experiments that vortex shedding from flat plates with square leading and trailing edges is caused by the impinging-shear-layer instability. In particular, the Strouhal number based on the plate's chord increases stepwise with increasing d/h in agreement with the experiment. Numerical analyses also provide some crucial information on the complicated vortical flow occurring near the trailing edge in conjunction with the vortex shedding mechanism. Finally, the mechanism of the impinging-shear-layer instability is discussed in the light of the experimental and numerical findings.

Manipulation of the Flow Around a Circular Cylinder by Utilizing the Flow Receptivity

2007 ◽  
Author(s):  
Yasuaki Kozato ◽  
Satoshi Kikuchi ◽  
Shigeki Imao
Keyword(s):  
Circular Cylinder ◽  
Shear Layer ◽  
Vortex Shedding ◽  
Velocity Fields ◽  
Shear Layer Instability ◽  
Hot Wire ◽  
Acoustic Disturbance ◽  
Wire Probe ◽  
Separated Shear Layer ◽  
Karman Vortex

An attempt to control the flow around a circular cylinder by utilizing the receptivity to the external acoustic disturbance was carried out and its mechanism was also studied. The velocity fields around the cylinder vicinity are carefully investigated with an X-type hot-wire probe. When the disturbance of a higher frequency related to the separated shear layer instability is added, the development of turbulence and the spreading of the shear layer are restrained. And, the amplification of the fluctuating velocity component of the Karman vortex shedding is delayed and its degree is reduced. Furthermore, the process of the gradual scale modification of the shear layer instability that appears prior to the transition of the flow is suppressed.

Lock-in phenomenon of vortex shedding in oscillatory flows: an analytical investigation pertaining to combustors

2019 ◽  
Vol 872 ◽  
pp. 115-146
Author(s):  
Abraham Benjamin Britto ◽  
Sathesh Mariappan
Keyword(s):  
Parameter Space ◽  
Acoustic Field ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Excitation Frequency ◽  
High Amplitude ◽  
Analytical Investigation ◽  
Domain Model ◽  
Critical Threshold ◽  
Lock In

An analytical investigation is performed to understand the lock-in phenomenon, observed in vortex shedding combustors. Several aeroengine afterburners and ramjets use a bluff body to stabilize the flame. The bluff body sheds vortices. During the occurrence of high-amplitude combustion instability, the frequency of vortex shedding locks in to the frequency of the chamber acoustic field. This phenomenon is termed vortex-acoustic lock-in. In general, there is a two-way coupling between the vortex shedding process and the acoustic field, making analytical investigation difficult. Since the frequency of the latter remains largely unaltered, performing an investigation to study the response of vortex shedding to external excitation not only allows one to gain insights, but also make the problem analytically tractable. We begin with a lower-order model available in the literature to describe the vortex shedding process in non-reacting flows, arising from sharp corners in the presence of upstream velocity excitation. The continuous time domain model is transformed to a discrete map, which connects the time instances of two successive vortex shedding events. The frequency and amplitude of excitation are varied to study the instantaneous vortex shedding time period, as the response of the system. In the absence of forcing, the iterates of the map form a period-1 solution with the frequency equalling the natural vortex shedding frequency. On increasing the amplitude of excitation, quasi-periodic behaviour of the iterates is observed, followed by a period-1 lock-in solution, where vortex shedding occurs at the excitation frequency. On further increasing the amplitude, de-lock-in occurs. From the map, an analytical solution is extracted, which represents the lock-in state. The condition and thereby the region in the frequency–amplitude parameter space where a general$p:1$lock-in occurs is then identified. Several important analytical expressions, such as for (1) critical threshold frequency above which lock-in occurs, (2) boundary of lock-in region in the parameter space, that are of direct importance to the design of quieter combustors are obtained. The study also identifies the transition of higher-order$p:1$to$1:1$lock-in state, through a series of lock-in and de-lock-in steps, whose occurrence could be verified from future experiments.

Vortex Shedding Lock-In due to Stream-Wise Oscillation of a 2-D Wind Turbine Blade Section at High Angles of Attack

2014 ◽  
Author(s):  
Alberto Pellegrino ◽  
Craig Meskell
Keyword(s):  
Wind Turbine ◽  
Turbine Blade ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Oscillation Amplitude ◽  
Wind Turbine Blade ◽  
Sst Turbulence Model ◽  
Blade Section ◽  
Lock In ◽  
High Angles Of Attack

The unsteady, incompressible flow around a translating two-dimensional wind turbine blade section (NREL S809) in the stream-wise direction has been simulated using unsteady RANS with the transition SST turbulence model. The Reynolds number is Re = 106 referred to a chord length of 1 m. A prescribed sinusoidal stream-wise motion has been applied at a fixed amplitude of 0.25 m for a range of high angles of attack [30° < α < 150°]. At these incidences, the airfoil will behave more like a bluff body and may experience periodic vortex shedding. It is well known that oscillations can lead to a synchronization (lock-in) of the vortex shedding frequency, fv, with the body’s motion frequency, fs, in bluff body flows. In order to investigate the susceptibility of the wind turbine blade section to lock-in, a parametric study has been conducted varying the frequency ratio r, (r = fs/fv0), in a range around r = 1 and r = 0.5. The lock-in region boundaries have been proposed and an analysis of the effect of the oscillation amplitude has been conducted. The synchronization map obtained suggests that, for the vibration amplitude considered, the risk of vortex-induced vibration is more significant in the regions of α = 35° and α = 145°. Furthermore, it has been found that for some stream-wise amplitudes, increasing the oscillation amplitude, lock-in appears to be unexpectedly suppressed in the vicinity of r = 1.

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