Role of shear layer instability in the transition of boundary layer on a bluff body

2004 ◽  
Vol 7 (2) ◽  
pp. 107-107
Author(s):  
S. P. Singh ◽  
S. Mittal
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Bluff Body ◽  
Shear Layer Instability

scholarly journals New Theories on Boundary Layer Transition and Turbulence Formation

2012 ◽  
Vol 2012 ◽  
pp. 1-22 ◽  
Author(s):  
Chaoqun Liu ◽  
Ping Lu ◽  
Lin Chen ◽  
Yonghua Yan
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Shear Layer ◽  
Boundary Layer Transition ◽  
Length Scale ◽  
Multiple Level ◽  
Shear Layer Instability ◽  
Small Length ◽  
Layer Transition ◽  
Turbulence Generation

This paper is a short review of our recent DNS work on physics of late boundary layer transition and turbulence. Based on our DNS observation, we propose a new theory on boundary layer transition, which has five steps, that is, receptivity, linear instability, large vortex structure formation, small length scale generation, loss of symmetry and randomization to turbulence. For turbulence generation and sustenance, the classical theory, described with Richardson's energy cascade and Kolmogorov length scale, is not observed by our DNS. We proposed a new theory on turbulence generation that all small length scales are generated by “shear layer instability” through multiple level ejections and sweeps and consequent multiple level positive and negative spikes, but not by “vortex breakdown.” We believe “shear layer instability” is the “mother of turbulence.” The energy transferring from large vortices to small vortices is carried out by multiple level sweeps, but does not follow Kolmogorov's theory that large vortices pass energy to small ones through vortex stretch and breakdown. The loss of symmetry starts from the second level ring cycle in the middle of the flow field and spreads to the bottom of the boundary layer and then the whole flow field.

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.

scholarly journals PIV Visualization of Flow Around Airfoil Controlled by Synthetic Jet Actuators at Shear-Layer and Wake Instability Frequencies

2021 ◽  
Author(s):  
Eric Yang ◽  
Pierre E. Sullivan
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Large Scale ◽  
Turbulent Wake ◽  
Synthetic Jet ◽  
Shear Layer Instability ◽  
Image Velocimetry ◽  
Wake Instability ◽  
Proper Orthogonal ◽  
Instability Frequency

Abstract The response of a separated boundary layer to synthetic jet flow control at the global wake instability (F+ ≈ 𝒪(1)) and the shear-layer instability (F+ ≈ 𝒪(10)) measured by particle image velocimetry are presented. The visualization shows that in each of the control cases, coherent vorticity develops and breaks down into a turbulent wake. When the jets are actuated by burst-modulation at the wake instability frequency, they induce regular formation and detachment of large-scale vorticity to form a wide turbulent wake. Excitation at the shear-layer instability frequency, on the other hand, produces a train of alternating velocity fluctuations in the boundary layer which dissipate to a narrower wake. Proper orthogonal decomposition of the velocity fields show that the physical extent of the jet-induced coherent structures is decreased with increasing addition of momentum for both excitation frequencies.

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 Effects of nozzle-exit boundary-layer profile on the initial shear-layer instability, flow field and noise of subsonic jets

2019 ◽  
Vol 876 ◽  
pp. 288-325 ◽  
Author(s):  
Christophe Bogey ◽  
Roberto Sabatini
Keyword(s):  
Boundary Layer ◽  
Velocity Profile ◽  
Shear Layer ◽  
Nozzle Exit ◽  
Momentum Thickness ◽  
Shear Layer Instability ◽  
Instability Waves ◽  
Exit Velocity ◽  
Exit Boundary ◽  
Boundary Layer Profile

The influence of the nozzle-exit boundary-layer profile on high-subsonic jets is investigated by performing compressible large-eddy simulations (LES) for three isothermal jets at a Mach number of 0.9 and a diameter-based Reynolds number of $5\times 10^{4}$, and by conducting linear stability analyses from the mean-flow fields. At the exit section of a pipe nozzle, the jets exhibit boundary layers of momentum thickness of approximately 2.8 % of the nozzle radius and a peak value of turbulence intensity of 6 %. The boundary-layer shape factors, however, vary and are equal to 2.29, 1.96 and 1.71. The LES flow and sound fields differ significantly between the first jet with a laminar mean exit velocity profile and the two others with transitional profiles. They are close to each other in these two cases, suggesting that similar results would also be obtained for a jet with a turbulent profile. For the two jets with non-laminar profiles, the instability waves in the near-nozzle region emerge at higher frequencies, the mixing layers spread more slowly and contain weaker low-frequency velocity fluctuations and the noise levels in the acoustic field are lower by 2–3 dB compared to the laminar case. These trends can be explained by the linear stability analyses. For the laminar boundary-layer profile, the initial shear-layer instability waves are most strongly amplified at a momentum-thickness-based Strouhal number $St_{\unicode[STIX]{x1D703}}=0.018$, which is very similar to the value obtained downstream in the mixing-layer velocity profiles. For the transitional profiles, on the contrary, they predominantly grow at higher Strouhal numbers, around $St_{\unicode[STIX]{x1D703}}=0.026$ and 0.032, respectively. As a consequence, the instability waves rapidly vanish during the boundary-layer/shear-layer transition in the latter cases, but continue to grow over a large distance from the nozzle in the former case, leading to persistent large-scale coherent structures in the mixing layers for the jet with a laminar exit velocity profile.

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.

A Statistical Study of Process Variables to Optimize a High Speed Curtain Coater: Part I

TAPPI Journal ◽  
2009 ◽  
Vol 8 (1) ◽  
pp. 20-26 ◽  
Author(s):  
PEEYUSH TRIPATHI ◽  
MARGARET JOYCE ◽  
PAUL D. FLEMING ◽  
MASAHIRO SUGIHARA
Keyword(s):  
Boundary Layer ◽  
Experimental Design ◽  
High Speed ◽  
Statistical Study ◽  
Process Variables ◽  
High Speeds ◽  
The Stability ◽  
Air Removal ◽  
Removal System

Using an experimental design approach, researchers altered process parameters and material prop-erties to stabilize the curtain of a pilot curtain coater at high speeds. Part I of this paper identifies the four significant variables that influence curtain stability. The boundary layer air removal system was critical to the stability of the curtain and base sheet roughness was found to be very important. A shear thinning coating rheology and higher curtain heights improved the curtain stability at high speeds. The sizing of the base sheet affected coverage and cur-tain stability because of its effect on base sheet wettability. The role of surfactant was inconclusive. Part II of this paper will report on further optimization of curtain stability with these four variables using a D-optimal partial-facto-rial design.

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