A study of three-dimensional aspects of vortex shedding from a bluff body with a mild geometric disturbance

1997 ◽  
Vol 330 ◽  
pp. 85-112 ◽  
Author(s):  
N. TOMBAZIS ◽  
P. W. BEARMAN
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Vortex Formation ◽  
Three Dimensional ◽  
Base Pressure ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Trailing Edges ◽  
Mode Transitions

Experiments have been carried out to study the three-dimensional characteristics of vortex shedding from a half-ellipse shape with a blunt trailing edge. In order to control the occurrence of vortex dislocations, the trailing edges of the models used were constructed with a series of periodic waves across their spans. Flow visualization was carried out in a water tunnel at a Reynolds number of 2500, based on trailing-edge thickness. A number of shedding modes were observed and the sequence of mode transitions recorded. Quantitative data were obtained from wind tunnel measurements performed at a Reynolds number of 40000. Two shedding frequencies were recorded with the higher frequency occurring at spanwise positions coinciding with minima in the chord. At these same positions the base pressure was lowest and the vortex formation length longest. Arguments are put forward to explain these observations. It is shown that the concept of a universal Strouhal number holds, even when the flow is three-dimensional. The spanwise variation in time-average base pressure is predicted using the estimated amount of time the flow spends at the two shedding frequencies.

The effect of base bleed on vortex shedding and base pressure in compressible flow

1981 ◽  
Vol 110 ◽  
pp. 273-292 ◽  
Author(s):  
F. Motallebi ◽  
J. F. Norbury
Keyword(s):  
Supersonic Flow ◽  
Compressible Flow ◽  
Vortex Shedding ◽  
Transonic Flow ◽  
Vortex Formation ◽  
Base Pressure ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Base Bleed ◽  
The Relationship

Experiments have been carried out to investigate the phenomenon of vortex shedding from the blunt trailing edge of an aerodynamic body in transonic and supersonic flow. The effect of a discharge of bleed air from a slot in the trailing edge has been included and the relationship between the vortex formation and base pressure has been considered.In transonic flow a small amount of bleed air was found to produce a rearward shift in the point of origin of the vortices with a consequent substantial increase in base pressure. The effect was less marked in supersonic flow. At higher rates of bleed two different regimes of vortex shedding were identified and increase in bleed rate caused a reduction in base pressure. For bleed rates giving near-maximum base pressure no vortex shedding occurred.

Cellular vortex shedding in the wake of a tapered plate

2008 ◽  
Vol 617 ◽  
pp. 355-379 ◽  
Author(s):  
VAGESH D. NARASIMHAMURTHY ◽  
HELGE I. ANDERSSON ◽  
BJØRNAR PETTERSEN
Keyword(s):  
Experimental Data ◽  
Reynolds Number ◽  
Vortex Shedding ◽  
High Reynolds Number ◽  
Three Dimensional ◽  
Base Pressure ◽  
Recirculation Bubble ◽  
Apparent Lack ◽  
Secondary Motion ◽  
High Reynolds

Direct numerical simulation (DNS) of vortex shedding behind a tapered plate with the taper ratio 20 placed normal to the inflow has been performed. The Reynolds numbers based on the uniform inflow velocity and the width of the plate at the wide and narrow ends were 1000 and 250, respectively. For the first time ever cellular vortex shedding was observed behind a tapered plate in a numerical experiment (DNS). Multiple cells of constant shedding frequency were found along the span of the plate. This is in contrast to apparent lack of cellular vortex shedding found in the high-Reynolds-number experiments by Gaster & Ponsford (Aero. J., vol. 88, 1984, p. 206). However, the present DNS data is in good qualitative agreement with similar high-Reynolds-number experimental data produced by Castro & Watson (Exp. Fluids, vol. 37, 2004, p. 159). It was observed that a tapered plate creates longer formation length coupled with higher base pressure as compared to non-tapered (i.e. uniform) plates. The three-dimensional recirculation bubble was nearly conical in shape. A significant base pressure reduction towards the narrow end of the plate, which results in a corresponding increase in Strouhal number, was noticed. This observation is consistent with the experimental data of Castro & Rogers (Exp. Fluids, vol. 33, 2002, p. 66). Pressure-driven spanwise secondary motion was observed, both in the front stagnation zone and also in the wake, thereby reflecting the three-dimensionality induced by the tapering.

scholarly journals Numerical Investigation of the Cavitation Effects on the Vortex Shedding from a Hydrofoil with Blunt Trailing Edge

Fluids ◽  
2020 ◽  
Vol 5 (4) ◽  
pp. 218
Author(s):  
Jian Chen ◽  
Linlin Geng ◽  
Xavier Escaler
Keyword(s):  
Numerical Investigation ◽  
Vortex Shedding ◽  
Reference Frames ◽  
Vortex Formation ◽  
Wake Flow ◽  
Trailing Edge ◽  
Hydraulic Machinery ◽  
Laminar To Turbulent Transition ◽  
Blunt Trailing Edge ◽  
Shedding Frequency

Vortex cavitation can appear in the wake flow of hydrofoils, inducing unwanted consequences such as vibrations or unstable behaviors in hydraulic machinery and systems. To investigate the cavitation effects on hydrofoil vortex shedding, a numerical investigation of the flow around a 2D NACA0009 with a blunt trailing edge at free caviation conditions and at two degrees of cavitation developments has been carried out by means of the Zwart cavitation model and the LES WALE turbulence model which permits predicting the laminar to turbulent transition of the boundary layers. To analyze the dynamic behavior of the vortex shedding process and the coherent structures, two identification methods based on the Eulerian and Lagrangian reference frames have been applied to the simulated unsteady flow field. It is found that the cavitation occurrence in the wake significantly changes the main vortex shedding characteristics including the morphology of the vortices, the vortex formation length, the effective height of the near wake flow and the shedding frequency. The numerical results predict that the circular shape of the vortices changes to an elliptical one and that the vortex shedding frequency is significantly increased under cavitation conditions. The main reason for the frequency increase seems to be the reduction in the transverse separation between the upper and lower rows of vortices induced by the increase in the vortex formation length.

The Effect of Base Bleed on the Flow behind a Two-Dimensional Model with a Blunt Trailing Edge

1967 ◽  
Vol 18 (3) ◽  
pp. 207-224 ◽  
Author(s):  
P. W. Bearman
Keyword(s):  
Vortex Formation ◽  
Reynolds Numbers ◽  
Base Pressure ◽  
Two Dimensional ◽  
Dimensional Model ◽  
Trailing Edge ◽  
Model Base ◽  
Blunt Trailing Edge ◽  
Two Dimensional Model ◽  
Base Bleed

SummaryThe effects of base bleed on the flow about a two-dimensional model with a blunt trailing edge were examined at Reynolds numbers, based on model base height, between 1·3×104 and 4·1×104. The ratio of boundary layer thickness at the trailing edge to half the model base height was approximately 0·4. Measurements were made of base pressure, vortex shedding frequency and the distance to vortex formation. With a sufficiently large bleed quantity the regular vortex street pattern disappeared and the base drag of the section was reduced to about a third of its value without bleed. The base pressure was found to vary linearly with the inverse of the vortex formation distance. Results of a previous splitter plate investigation were found to agree closely with those of the present experiments.

scholarly journals Vortex shedding from a blunt trailing edge with equal and unequal external mean velocities

1976 ◽  
Vol 75 (4) ◽  
pp. 721-735 ◽  
Author(s):  
D. R. Boldman ◽  
P. F. Brinich ◽  
M. E. Goldstein
Keyword(s):  
Steady State ◽  
Flow Visualization ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Free Stream ◽  
Theoretical Calculations ◽  
Vortex Formation ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Good Agreement

A flow-visualization study has shown that strong Kármán vortices develop behind the blunt trailing edge of a plate when the free-stream velocities over both surfaces are equal and that the vortices tend to disappear when the surface velocities are unequal. This observation provides an explanation for the occurrence and disappearance of certain discrete tones often found to be present in the noise spectra of coaxial jets. Both the vortex formation and the tones occur at a Strouhal number based on the lip thickness and the average of the external steady-state velocities of about 0.2.Results from theoretical calculations of the vortex formation, based on an inviscid incompressible analysis of the motion of point vortices, were in good agreement with the experimental observations.

Interaction and acoustics of separated flows from a D-shaped bluff body

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Guangyuan Huang ◽  
Ka Him Seid ◽  
Zhigang Yang ◽  
Randolph Chi Kin Leung
Keyword(s):  
Vortex Shedding ◽  
Bluff Body ◽  
Leading Edge ◽  
Separated Flows ◽  
Pressure Waves ◽  
Bluff Bodies ◽  
Trailing Edge ◽  
Content Type ◽  
Trailing Edges ◽  
Edge Vortex

Purpose For flow around elongated bluff bodies, flow separations would occur over both leading and trailing edges. Interactions between these two separations can be established through acoustic perturbation. In this paper, the flow and the acoustic fields of a D-shaped bluff body (length-to-height ratio L/H = 3.64) are investigated at height-based Reynolds number Re = 23,000 by experimental and numerical methods. The purpose of this paper is to study the acoustic feedback in the interaction of these two separated flows. Design/methodology/approach The flow field is measured by particle image velocimetry, hotwire velocimetry and surface oil flow visualization. The acoustic field is modeled in two dimensions by direct aeroacoustic simulation, which solves the compressible Navier–Stokes equations. The simulation is validated against the experimental results. Findings Separations occur at both the leading and the trailing edges. The leading-edge separation point and the reattaching flow oscillate in accordance with the trailing-edge vortex shedding. Significant pressure waves are generated at the trailing edge by the vortex shedding rather than the leading-edge vortices. Pressure-based cross-correlation analysis is conducted to clarify the effect of the pressure waves on the leading-edge flow structures. Practical implications The understanding of interactions of separated flows over elongated bluff bodies helps to predict aerodynamic drag, structural vibration and noise in engineering applications, such as the aerodynamics of buildings, bridges and road vehicles. Originality/value This paper clarifies the influence of acoustic perturbations in the interaction of separated flows over a D-shaped bluff body. The contribution of the leading- and the trailing-edge vortex in generating acoustic perturbations is investigated as well.

Experimental and Numerical Study of the Three Dimensional Instabilities in the Wake of a Blunt Trailing Edge Profiled Body

2010 ◽  
Author(s):  
Arash Naghib Lahouti ◽  
Lakshmana Sampat Doddipatla ◽  
Horia Hangan ◽  
Kamran Siddiqui
Keyword(s):  
Reynolds Number ◽  
Numerical Study ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
The Body ◽  
Bluff Bodies ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Image Velocimetry

The wake of nominally two dimensional bluff bodies is dominated by von Ka´rma´n vortices, which are accompanied by three dimensional instabilities beyond a threshold Reynolds number. These three dimensional instabilities initiate as dislocations in the von Ka´rma´n vortices near the trailing edge, which evolve into pairs of counter-rotating vortices further downstream. The wavelength of the three dimensional instabilities depends on profile geometry and Reynolds number. In the present study, the three dimensional wake instabilities for a blunt trailing edge profiled body, composed of an elliptical leading edge and a rectangular trailing edge, have been studied in Reynolds numbers ranging from 500 to 1200, based on the thickness of the body. Numerical simulations, Laser Induced Fluorescence (LIF) flow visualization, and Particle Image Velocimetry (PIV) methods have been used to identify the instabilities. Proper Orthogonal Decomposition (POD) has been used to analyze the velocity field data measured using PIV. The results confirm the existence of three dimensional instabilities with an average wavelength of 2.0 to 2.5 times thickness of the body, in the near wake. The findings are in agreements with the values reported previously for different Reynolds numbers, and extend the range of Reynolds numbers in which the three dimensional instabilities are characterized.

Turbulent wake behind a curved circular cylinder

2014 ◽  
Vol 742 ◽  
pp. 192-229 ◽  
Author(s):  
José P. Gallardo ◽  
Helge I. Andersson ◽  
Bjørnar Pettersen
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Vortex Formation ◽  
Axial Flow ◽  
Turbulent Wake ◽  
Outer Face ◽  
Wake Region ◽  
Axial Curvature

AbstractThis paper reports results from a direct numerical simulation of the flow past a circular cylinder with axial curvature. The main objective is to explore the effects of spanwise curvature on the stability of the shear layers and the turbulent wake at the subcritical Reynolds number of 3900. The bluff-body geometry is adapted from a previous study conducted at lower Reynolds numbers, in which a quarter segment of a ring represented the deformed cylinder. A convex configuration in which the free-stream direction is towards the outer face of the ring is adopted here. The present results show a striking distinction between the upper and lower wake regions. Despite the turbulent character of the wake, the upper wake region is more coherent due to the periodic vortex shedding of primary vortical structures, which are in close alignment with the axial curvature. A mild axial flow develops upwards along the lee face of the curved cylinder, displacing the vortex formation region further downstream from the location expected for a straight cylinder at the same Reynolds number. In the lower wake region the vortex shedding strength is drastically reduced due to larger local inclination, resulting in higher three-dimensionality and loss of coherence. A strong downdraft with a swirling pattern is the dominating feature in the lower base region. This is associated with a substantial decrease of the base suction, and the suppression of the characteristic recirculating backflow.

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