Effect of shear-layer thickness on the Strouhal–Reynolds number relationship for bluff-body wakes

2006 ◽  
Vol 22 (8) ◽  
pp. 1133-1138 ◽  
Author(s):  
F.L. Ponta
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Layer Thickness ◽  
Bluff Body

Density ratio effects on reacting bluff-body flow field characteristics

2012 ◽  
Vol 706 ◽  
pp. 219-250 ◽  
Author(s):  
Benjamin Emerson ◽  
Jacqueline O’Connor ◽  
Matthew Juniper ◽  
Tim Lieuwen
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Shear Layer ◽  
Parametric Excitation ◽  
Bluff Body ◽  
Density Ratio ◽  
Global Mode ◽  
Measured Velocity ◽  
Number Range ◽  
Flow Field Characteristics

AbstractThe wake characteristics of bluff-body-stabilized flames are a strong function of the density ratio across the flame and the relative offset between the flame and shear layer. This paper describes systematic experimental measurements and stability calculations of the dependence of the flow field characteristics and flame sheet dynamics upon flame density ratio,${\rho }_{u} / {\rho }_{b} $, over the Reynolds number range of 1000–3300. We show that two fundamentally different flame/flow behaviours are observed at high and low${\rho }_{u} / {\rho }_{b} $values: a stable, noise-driven fixed point and limit-cycle oscillations, respectively. These results are interpreted as a transition from convective to global instability and are captured well by stability calculations that used the measured velocity and density profiles as inputs. However, in this high-Reynolds-number flow, the measurements show that no abrupt bifurcation in flow/flame behaviour occurs at a given${\rho }_{u} / {\rho }_{b} $value. Rather, the flow field is highly intermittent in a transitional${\rho }_{u} / {\rho }_{b} $range, with the relative fraction of the two different flow/flame behaviours monotonically varying with${\rho }_{u} / {\rho }_{b} $. This intermittent behaviour is a result of parametric excitation of the global mode growth rate in the vicinity of a supercritical Hopf bifurcation. It is shown that this parametric excitation is due to random fluctuations in relative locations of the flame and shear layer.

A Simplified Model Predicting the Kelvin–Helmholtz Instability Frequency for Laminar Separated Flows

2015 ◽  
Vol 138 (4) ◽  
Author(s):  
Daniele Simoni ◽  
Marina Ubaldi ◽  
Pietro Zunino
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Layer Thickness ◽  
Active Flow Control ◽  
Linear Stability Theory ◽  
Aerodynamic Loading ◽  
Gradient Parameter ◽  
Semi Empirical ◽  
Instability Frequency

A semi-empirical model for the estimation of the Kelvin–Helmholtz (KH) instability frequency, in the case of short laminar separation bubbles over airfoils, has been developed. To this end, the Thwaites's pressure gradient parameter has been adopted to account for the effects induced by the aerodynamic loading distribution as well as by the Reynolds number on the separated shear layer thickness at separation. The most amplified frequency predicted by linear stability theory (LST) for a piecewise linear profile, which can be considered as the KH instability frequency, has been related to the shear layer thickness at separation, hence to the Reynolds number and the aerodynamic loading distribution through the Thwaites's pressure gradient parameter. This procedure allows the formulation of a functional dependency between the Strouhal number of the shedding frequency based on exit conditions and the dimensionless parameters. Experimental results obtained in different test cases, characterized by different Reynolds numbers and aerodynamic loading distributions, have been used to validate the model, as well as to identify the regression curve best fitting the data. The semi-empirical correlation here derived can be useful to set the activation frequency of active flow control devices for the optimization of boundary layer separation control strategies.

Analysis and Scalings of Blowoff Limits of 2D and Axisymmetric Bluff Body Stabilized Flames

2012 ◽  
Author(s):  
Christopher W. Foley ◽  
Jerry Seitzman ◽  
Tim Lieuwen
Keyword(s):  
Shear Layer ◽  
Layer Thickness ◽  
Bluff Body ◽  
Scale Dependence ◽  
Flame Stabilization ◽  
Length Scale ◽  
Measured Velocity ◽  
Shearing Flow ◽  
Flame Stretch ◽  
Geometric Length

This paper considers shear layer flame stabilization with a particular focus on velocity scaling of blowoff limits. Analysis of the expression for hydrodynamic flame stretch, κ, in a shear layer shows that it consists of two contributions, associated with normal and shearing flow strain. These two contributors lead to flame stretch scalings of SL/δ and U/L, respectively, where δ and L denote shear layer thickness and characteristic geometric length scale. These two flame stretch terms have different velocity and length scalings (roughly U1/2 and U1, respectively) and so different blowoff trends can be expected depending upon which term dominates. These scalings are used to interpret a variety of bluff body blowoff data in the literature by analyzing the velocity and length scale dependence of extinction stretch rates calculated at the measured blowoff conditions. We also show that the measured velocity sensitivities to chemical time at blowoff range from U−0.3 to U−1.6. A key point of this study is that blowoff boundaries do not necessarily follow a U−1 scaling suggested by classical Damköhler number scalings and that more work is needed to understand the controlling extinction processes of near-blowoff flames.

The effect of confinement on the stability of viscous planar jets and wakes

2010 ◽  
Vol 656 ◽  
pp. 309-336 ◽  
Author(s):  
S. J. REES ◽  
M. P. JUNIPER
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Layer Thickness ◽  
Finite Thickness ◽  
Temporal Analysis ◽  
Velocity Profiles ◽  
Shear Layers ◽  
Channel Width ◽  
Planar Jet ◽  
Spatio Temporal

This theoretical study examines confined viscous planar jet/wake flows with continuous velocity profiles. These flows are characterized by the shear, confinement, Reynolds number and shear-layer thickness. The primary aim of this paper is to determine the effect of confinement on viscous jets and wakes and to compare these results with corresponding inviscid results. The secondary aim is to consider the effect of viscosity and shear-layer thickness. A spatio-temporal analysis is performed in order to determine absolute/convective instability criteria. This analysis is carried out numerically by solving the Orr–Sommerfeld equation using a Chebyshev collocation method. Results are produced over a large range of parameter space, including both co-flow and counter-flow domains and confinements corresponding to 0.1 < h2/h1 < 10, where the subscripts 1 and 2 refer to the inner and outer streams, respectively. The Reynolds number, which is defined using the channel width, takes values between 10 and 1000. Different velocity profiles are used so that the shear layers occupy between 1/2 and 1/24 of the channel width. Results indicate that confinement has a destabilizing effect on both inviscid and viscous flows. Viscosity is found always to be stabilizing, although its effect can safely be neglected above Re = 1000. Thick shear layers are found to have a stabilizing effect on the flow, but infinitely thin shear layers are not the most unstable; having shear layers of a small, but finite, thickness gives rise to the strongest instability.

Impingement Cooling Flow and Heat Transfer Under Acoustic Excitations

1997 ◽  
Vol 119 (4) ◽  
pp. 810-817 ◽  
Author(s):  
C. Gau ◽  
W. Y. Sheu ◽  
C. H. Shen
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Turbulence Intensity ◽  
Pressure Level ◽  
Unstable Wave ◽  
Impingement Cooling ◽  
Cooling Flow ◽  
Acoustic Excitations ◽  
Excitation Pressure

Experiments are performed to study (a) slot air jet impingement cooling flow and (b) the heat transfer under acoustic excitations. Both flow visualization and spectral energy evolution measurements along the shear layer are made. The acoustic excitation at either inherent or noninherent frequencies can make the upstream shift for both the most unstable waves and the resulting vortex formation and its subsequent pairing processes. At inherent frequencies the most unstable wave can be amplified, which increases the turbulence intensity in both the shear layer and the core and enhances the heat transfer. Both the turbulence intensity and the heat transfer increase with increasing excitation pressure levels Spl until partial breakdown of the vortex occurs. At noninherent frequencies, however, the most unstable wave can be suppressed, which reduces the turbulence intensity and decreases the heat transfer. Both the turbulence intensity and the heat transfer decreases with increasing Spl, but increases with increasing Spl when the excitation frequency becomes dominant. For excitation at high Reynolds number with either inherent or noninherent frequency, a greater excitation pressure level is needed to cause the enhancement or the reduction in heat transfer. During the experiments, the inherent frequencies selected for excitation are Fo/2 and Fo/4, the noninherent frequencies are 0.71 Fo, 0.75 Fo, and 0.8 Fo, the acoustic pressure level varies from 70 dB to 100 dB, and the Reynolds number varies from 5500 to 22,000.

scholarly journals A frictional Cosserat model for the slow shearing of granular materials

2002 ◽  
Vol 457 ◽  
pp. 377-409 ◽  
Author(s):  
L. SRINIVASA MOHAN ◽  
K. KESAVA RAO ◽  
PRABHU R. NOTT
Keyword(s):  
Angular Velocity ◽  
Granular Material ◽  
Shear Layer ◽  
Granular Materials ◽  
Layer Thickness ◽  
Couette Flow ◽  
Velocity Fields ◽  
Cosserat Continuum ◽  
Cosserat Model ◽  
Cylindrical Couette Flow

A rigid-plastic Cosserat model for slow frictional flow of granular materials, proposed by us in an earlier paper, has been used to analyse plane and cylindrical Couette flow. In this model, the hydrodynamic fields of a classical continuum are supplemented by the couple stress and the intrinsic angular velocity fields. The balance of angular momentum, which is satisfied implicitly in a classical continuum, must be enforced in a Cosserat continuum. As a result, the stress tensor could be asymmetric, and the angular velocity of a material point may differ from half the local vorticity. An important consequence of treating the granular medium as a Cosserat continuum is that it incorporates a material length scale in the model, which is absent in frictional models based on a classical continuum. Further, the Cosserat model allows determination of the velocity fields uniquely in viscometric flows, in contrast to classical frictional models. Experiments on viscometric flows of dense, slowly deforming granular materials indicate that shear is confined to a narrow region, usually a few grain diameters thick, while the remaining material is largely undeformed. This feature is captured by the present model, and the velocity profile predicted for cylindrical Couette flow is in good agreement with reported data. When the walls of the Couette cell are smoother than the granular material, the model predicts that the shear layer thickness is independent of the Couette gap H when the latter is large compared to the grain diameter dp. When the walls are of the same roughness as the granular material, the model predicts that the shear layer thickness varies as (H/dp)1/3 (in the limit H/dp [Gt ] 1) for plane shear under gravity and cylindrical Couette flow.

The structure of a highly decelerated axisymmetric turbulent boundary layer

2021 ◽  
Vol 929 ◽  
Author(s):  
N. Agastya Balantrapu ◽  
Christopher Hickling ◽  
W. Nathan Alexander ◽  
William Devenport
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Large Scale ◽  
Convection Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Experiments were performed over a body of revolution at a length-based Reynolds number of 1.9 million. While the lateral curvature parameters are moderate ( $\delta /r_s < 2, r_s^+>500$ , where $\delta$ is the boundary layer thickness and r s is the radius of curvature), the pressure gradient is increasingly adverse ( $\beta _{C} \in [5 \text {--} 18]$ where $\beta_{C}$ is Clauser’s pressure gradient parameter), representative of vehicle-relevant conditions. The mean flow in the outer regions of this fully attached boundary layer displays some properties of a free-shear layer, with the mean-velocity and turbulence intensity profiles attaining self-similarity with the ‘embedded shear layer’ scaling (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592–642). Spectral analysis of the streamwise turbulence revealed that, as the mean flow decelerates, the large-scale motions energize across the boundary layer, growing proportionally with the boundary layer thickness. When scaled with the shear layer parameters, the distribution of the energy in the low-frequency region is approximately self-similar, emphasizing the role of the embedded shear layer in the large-scale motions. The correlation structure of the boundary layer is discussed at length to supply information towards the development of turbulence and aeroacoustic models. One major finding is that the estimation of integral turbulence length scales from single-point measurements, via Taylor's hypothesis, requires significant corrections to the convection velocity in the inner 50 % of the boundary layer. The apparent convection velocity (estimated from the ratio of integral length scale to the time scale), is approximately 40 % greater than the local mean velocity, suggesting the turbulence is convected much faster than previously thought. Closer to the wall even higher corrections are required.

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