Geometrical investigation of bluff bodies array subjected to forced convective flows for different aspect ratios of frontal body

This paper reports an experimental study conducted to investigate the effects of aspect ratio on the reattachment length and statistical properties in turbulent flow over three-dimensional surface-mounted bluff bodies. This study focuses on a surface-mounted body whose height is significantly smaller than the thickness of the approaching turbulent boundary layer. The studied aspect ratios of the step range from w/h = 0.5 to 20, where w and h denote the spanwise width and height of the step, respectively. All experiments were carried out in an open water channel, and the velocity measurements were performed using a time-resolved particle image velocimetry (TR-PIV) system. The Reynolds number, based on the freestream of the approach boundary layer and step height, is 12540, while the ratio of the boundary layer to step height is 4.83. Two distinct regions of separation are observed on top of the step and downstream of the step. In both separation regions, the reattachment length increases monotonically as aspect ratio increases from w/h = 0.5 to 8, and the reattachment length reaches an asymptotic value and does not vary significantly with aspect ratio larger than 8. The effects of aspect ratios on the mean velocities and Reynolds stresses were also examined.

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Quasi-Periodic Structure of Vortical Flows Produced in the Wake of Finite Bluff Bodies Partially Immersed in a Boundary Layer

ASME 2010 3rd Joint US-European Fluids Engineering Summer Meeting: Volume 1, Symposia – Parts A, B, and C ◽

10.1115/fedsm-icnmm2010-30804 ◽

2010 ◽

Cited By ~ 3

Author(s):

Jason A. Bourgeois ◽

Pooria Sattari ◽

Robert J. Martinuzzi

Keyword(s):

Cross Section ◽

Wavelet Transforms ◽

Dominant Role ◽

Three Dimensional ◽

Circular Cross Section ◽

Vortex Sheet ◽

Bluff Bodies ◽

Continuous Wavelet Transforms ◽

Aspect Ratios ◽

Image Velocimetry

The quasi-periodic vortex shedding structure in the wake of finite surface-mounted square- and circular-cross-section cylinders is investigated for several aspect ratios. Complex continuous wavelet transforms (CWT’s) are used to obtain a phase function φ(t) from hot-wire measurements. Mean relative phases and phase averaged particle image velocimetry (PIV) measurements indicate an upstream bending of the initially vertical shed vortex structures for all obstacles investigated. This upstream bending mechanism reorients vorticity streamwise and is described in terms of Biot-Savart induction that occurs at the junction of the tip and side shear layers. This mechanism of vorticity concentration/reorientation is inherently three-dimensional and interacts with the nominally two-dimensional mechanism of alternate vortex sheet roll-up from the opposing obstacle side faces. This mechanism typically acts higher along the height for square-as opposed to circular-cross section cylinders and plays a more dominant role for smaller aspect ratios.

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On Oscillatory Instability of Convective Flows at Low Prandtl Number

Journal of Fluids Engineering ◽

10.1115/1.2819504 ◽

1997 ◽

Vol 119 (4) ◽

pp. 823-830 ◽

Cited By ~ 18

Author(s):

A. Yu. Gelfgat ◽

P. Z. Bar-Yoseph ◽

A. L. Yarin

Keyword(s):

Aspect Ratio ◽

Prandtl Number ◽

Lower Boundary ◽

Oscillatory Instability ◽

Weakly Nonlinear ◽

Convective Flows ◽

Stability Diagrams ◽

Aspect Ratios ◽

The Stability ◽

Upper Boundary

Numerical investigation of the oscillatory instability of convective flows in laterally heated rectangular cavities is presented. Cavities with no-slip isothermal vertical boundaries, no-slip adiabatic lower boundary, and stress-free adiabatic upper boundary are considered. Dependence of the critical Grashof number and the critical frequency of oscillations on the aspect ratio (A = length/height) of the cavity are investigated. The stability diagrams were obtained for the whole interval of the aspect ratio 1 ≤ A ≤ 10. The study was carried out for two values of the Prandtl number, Pr = 0 and 0.015. It was shown that the oscillatory instability sets in as a result of the Hopf bifurcation. It was found that at two different values of the Prandtl number considered the instability is caused by different infinitely small dominant perturbations, which means that the convective heat transfer strongly affects stability of the flow even for cases having small Prandtl number. No asymptotic behavior for large aspect ratios was found up to A = 10. Slightly supercritical oscillatory flows were approximated asymptotically by means of the weakly nonlinear analysis of the calculated bifurcation.

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Streamwise Aspect Ratio Effects on Turbulent Flow Separations Induced by Forward–Backward-Facing Steps

Journal of Fluids Engineering ◽

10.1115/1.4048686 ◽

2020 ◽

Vol 143 (2) ◽

Cited By ~ 1

Author(s):

Heath Chalmers ◽

Xingjun Fang ◽

Mark F. Tachie

Keyword(s):

Boundary Layer ◽

Aspect Ratio ◽

Turbulent Boundary Layer ◽

Step Height ◽

Bluff Bodies ◽

Reynolds Stresses ◽

Mean Flow ◽

Freestream Velocity ◽

Aspect Ratios ◽

The Mean

Abstract Separated and reattached turbulent flows induced by two-dimensional forward–backward-facing steps (FBFS) with different streamwise lengths submerged in a thick turbulent boundary layer (TBL) are investigated using time-resolved particle image velocimetry (TR-PIV). The aspect ratios (AR) of the step range from 1 to 8, and the Reynolds number based on the freestream velocity and step height is 13 200. The thickness of the incoming turbulent boundary layer is 6.5 times the step height. The effects of varying aspect ratio of the steps on the mean flow, principal stretching, Reynolds stresses, triple velocity correlation, two-point autocorrelation, and unsteadiness of turbulent separation bubbles are studied. The results indicate that the mean flow reattaches over the step for FBFS with aspect ratios of 2 and higher. Two local peaks of Reynolds stresses occur irrespective of AR, and for a sufficiently large AR, a third local peak of Reynolds stresses appear in the shear layer emanating from the trailing edge of the bluff bodies. The streamwise decay of Reynolds stresses is slower for smaller AR cases. Incoming coherent structure is strongly disturbed near an inclined edge where principal stretching switches orientation abruptly. The temporal variation of the first proper orthogonal decomposition (POD) mode and reverse flow area over the bluff bodies shows remarkable correlation, which signifies the flapping motion of separation bubble.

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Characteristics of Bluff Body Stabilized Turbulent Premixed Flames

Volume 2: Combustion, Fuels and Emissions, Parts A and B ◽

10.1115/gt2011-45089 ◽

2011 ◽

Cited By ~ 1

Author(s):

Alejandro M. Briones ◽

Balu Sekar ◽

Hugh Thornburg

Keyword(s):

Bluff Body ◽

Three Dimensional ◽

Leading Edge ◽

Reacting Flows ◽

Chemical Mechanism ◽

Reacting Flow ◽

Bluff Bodies ◽

Two Dimensional ◽

Aspect Ratios ◽

Trailing Edges

Non-reacting and reacting flows past typical flameholders are modeled with URANS and LES. The continuity, momentum, energy, species, and turbulence governing equations are solved using two- and three-dimensional configurations. Either 2-step global or 44-step reduced chemical mechanism for C3H8-air combustion, accounting for turbulence-chemistry interaction, and with temperature- and species-dependent thermodynamic and transport properties is utilized. For square and rectangular bluff bodies the flow separates at the leading edges, whereas for triangular bluff body separation occurs only at the trailing edges. These bluff bodies exhibit two shear layers at the trailing edges that shed asymmetric vortices. For rectangular bluff bodies with aspect ratios (AR) less than 2.3 there is backflow from the wake. With increasing AR from unity, backflow is gradually diminished, and the von Ka´rma´n Strouhal number (StvK) decreases. For 2.0<AR<2.3, StvK jumps to a higher value and separation again occurs at the trailing edges for AR = 2.3. Further increase in AR decreases StvK again. The simulations with URANS qualitatively and quantitatively match experimental results for StvK vs. AR. Quantitative discrepancies are, however, found for AR≥2.3. In addition, two-dimensional non-reacting flows with URANS are sufficient to predict StvK. Moreover, two-dimensional simulations of reacting flow indicate that the flame promotes static and dynamic stability for AR = 1.0 and 2.3. The flame is dynamically unstable for AR = 2.0, exhibiting a von Ka´rma´n flow pattern. Stable flames anchored at the most downstream separation location (e.g., the flame anchored at AR = 1.0 is attached to the leading edge, whereas that of AR = 2.3 is attached to the trailing edge). Realizable k-ε URANS and LES simulations for the triangular cylinder closely match the experimental StvK for both non-reacting and reacting flows. Nonetheless, LES predicts a smaller recirculation length than k-ε URANS. LES predicts a flow field in which Be´rnard/von Ka´rma´n (BvK) instability is suppressed, whereas URANS predicts a competition between the Kelvin-Helmholtz (KH) instability and BvK.

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Stability analysis of natural convective flows in horizontal annuli: Effects of the axial and radial aspect ratios

Physics of Fluids ◽

10.1063/1.2364027 ◽

2006 ◽

Vol 18 (10) ◽

pp. 104107 ◽

Cited By ~ 18

Author(s):

Giuseppe Petrone ◽

Eric Chénier ◽

Guy Lauriat

Keyword(s):

Stability Analysis ◽

Convective Flows ◽

Aspect Ratios ◽

Horizontal Annuli

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Onset of vortex shedding around a short cylinder

Journal of Fluid Mechanics ◽

10.1017/jfm.2021.1034 ◽

2021 ◽

Vol 933 ◽

Author(s):

Yongliang Yang ◽

Zhe Feng ◽

Mengqi Zhang

Keyword(s):

Linear Stability ◽

Vortex Shedding ◽

Three Dimensional ◽

Flow Patterns ◽

Bluff Bodies ◽

Mean Flow ◽

Formula One ◽

Stability Analyses ◽

Cylinder Length ◽

Aspect Ratios

This paper presents results of three-dimensional direct numerical simulations (DNS) and global linear stability analyses of a viscous incompressible flow past a finite-length cylinder with two free flat ends. The cylindrical axis is normal to the streamwise direction. The work focuses on the effects of aspect ratios (in the range of $0.5\leq {\small \text{AR}} \leq 2$ , cylinder length over diameter) and Reynolds numbers ( $Re\leq 1000$ based on cylinder diameter and uniform incoming velocity) on the onset of vortex shedding in this flow. All important flow patterns have been identified and studied, especially as ${\small \text{AR}}$ changes. The appearance of a steady wake pattern when ${\small \text{AR}} \leq 1.75$ has not been discussed earlier in the literature for this flow. Linear stability analyses based on the time-mean flow has been applied to understand the Hopf bifurcation past which vortex shedding happens. The nonlinear DNS results indicate that there are two vortex shedding patterns at different $Re$ , one is transient and the other is nonlinearly saturated. The vortex-shedding frequencies of these two flow patterns correspond to the eigenfrequencies of the two global modes in the stability analysis of the time-mean flow. Wherever possible, we compare the results of our analyses to those of the flows past other short- ${\small \text{AR}}$ bluff bodies in order that our discussions bear more general meanings.

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