Comparative early development of wake vortices behind a short semicircular-section cylinder in two opposite arrangements

1996 ◽  
Vol 327 ◽  
pp. 73-99 ◽  
Author(s):  
Nathalie Boisaubert ◽  
Madeleine Coutanceau ◽  
Patrick Ehrmann
Keyword(s):  
Vortex Shedding ◽  
Body Shape ◽  
Reynolds Numbers ◽  
The Body ◽  
Initial Time ◽  
Geometrical Parameters ◽  
Present Observation ◽  
Wake Vortices ◽  
Flat Side ◽  
Flow Evolution

As a first step in a more general study of the influence of the body shape upon the initial time-development of wake vortices, we consider the case of a 5.20 aspect-ratio semicircular-section cylinder, fitted with two endplates, and with the rounded side and the flat side in turn facing the oncoming current. The flow structure is analysed by means of a detailed qualitative and quantitative analysis of numerous flow visualization pictures, for Reynolds numbers Re ranging between 60 and 600. Beyond the first phase of development, necessary for the vortex-shedding process to take place (t* [ges ] 6), a change in the flow evolution with Re is found for both body configurations, at a critical Reynolds number Rec whose final value is, within about 5%, 190 and 140 for the rounded and flat forebody respectively. However, this change varies with the body shape. Thus, above Re = 200, it is shown that the reversal of the body compared to the free stream (flat-forebody configuration) implies a clear difference in the wake development with a quasi-symmetrical shifting of the vortex cores to the rear of the recirculating zone and a complete annihilation of the process of vortex shedding, at least during the limited period of time corresponding to the present observation. The consequences of the modifications of the wake behaviour are quantitatively evaluated by considering the time- and Re-evolution of the wake geometrical parameters and of the axial velocity distribution; they are related to the body geometry.

scholarly journals Feedback control of unstable flows: a direct modelling approach using the Eigensystem Realisation Algorithm

2016 ◽  
Vol 793 ◽  
pp. 41-78 ◽  
Author(s):  
Thibault L. B. Flinois ◽  
Aimee S. Morgans
Keyword(s):  
Robust Control ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
Open Loop ◽  
The Body ◽  
Nonlinear Flow ◽  
Balanced Truncation ◽  
Unstable Flow ◽  
Feedback Controllers ◽  
Modelling Approach

Obtaining low-order models for unstable flows in a systematic and computationally tractable manner has been a long-standing challenge. In this study, we show that the Eigensystem Realisation Algorithm (ERA) can be applied directly to unstable flows, and that the resulting models can be used to design robust stabilising feedback controllers. We consider the unstable flow around a D-shaped body, equipped with body-mounted actuators, and sensors located either in the wake or on the base of the body. A linear model is first obtained using approximate balanced truncation. It is then shown that it is straightforward and justified to obtain models for unstable flows by directly applying the ERA to the open-loop impulse response. We show that such models can also be obtained from the response of the nonlinear flow to a small impulse. Using robust control tools, the models are used to design and implement both proportional and $\mathscr{H}_{\infty }$ loop-shaping controllers. The designed controllers were found to be robust enough to stabilise the wake, even from the nonlinear vortex shedding state and in some cases at off-design Reynolds numbers.

Wake dynamics of external flow past a curved circular cylinder with the free stream aligned with the plane of curvature

2007 ◽  
Vol 592 ◽  
pp. 89-115 ◽  
Author(s):  
A. MILIOU ◽  
A. DE VECCHI ◽  
S. J. SHERWIN ◽  
J. M. R. GRAHAM
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Free Stream ◽  
Three Dimensional ◽  
Axial Flow ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
The Body ◽  
Vertical Extension ◽  
Flow Past

Three-dimensional spectral/hp computations have been performed to study the fundamental mechanisms of vortex shedding in the wake of curved circular cylinders at Reynolds numbers of 100 and 500. The basic shape of the body is a circular cylinder whose centreline sweeps through a quarter section of a ring and the inflow direction lies on the plane of curvature of the quarter ring: the free stream is then parallel to the geometry considered and the part of the ring that is exposed to it will be referred to as the ‘leading edge’. Different configurations were investigated with respect to the leading-edge orientation. In the case of a convex-shaped geometry, the stagnation face is the outer surface of the ring: this case exhibited fully three-dimensional wake dynamics, with the vortex shedding in the upper part of the body driving the lower end at one dominant shedding frequency for the whole cylinder span. The vortex-shedding mechanism was therefore not governed by the variation of local normal Reynolds numbers dictated by the curved shape of the leading edge. A second set of simulations were conducted with the free stream directed towards the inside of the ring, in the so-called concave-shaped geometry. No vortex shedding was detected in this configuration: it is suggested that the strong axial flow due to the body's curvature and the subsequent production of streamwise vorticity plays a key role in suppressing the wake dynamics expected in the case of flow past a straight cylinder. The stabilizing mechanism stemming from the concave curved geometry was still found to govern the wake behaviour even when a vertical extension was added to the top of the concave ring, thereby displacing the numerical symmetry boundary condition at this point away from the top of the deformed cylinder. In this case, however, the axial flow from the deformed cylinder was drawn into the wake of vertical extension, weakening the shedding process expected from a straight cylinder at these Reynolds numbers. These considerations highlight the importance of investigating flow past curved cylinders using a full three-dimensional approach, which can properly take into account the role of axial velocity components without the limiting assumptions of a sectional analysis, as is commonly used in industrial practice. Finally, towing-tank flow visualizations were also conducted and found to be in qualitative agreement with the computational findings.

Three-dimensional effects in turbulent bluff-body wakes

1997 ◽  
Vol 343 ◽  
pp. 235-265 ◽  
Author(s):  
ANIL PRASAD ◽  
CHARLES H. K. WILLIAMSON
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Reynolds Numbers ◽  
The Body ◽  
Practical Significance ◽  
Transition Regime ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Wake Transition ◽  
Mode A

There has recently been a surge in activity concerning the development of three-dimensionality in the wakes of nominally two-dimensional bluff bodies, yielding the realization that end effects can influence the wake vortex shedding pattern over long spanlengths. Much of this work has been focused on low Reynolds numbers (Re), but virtually no studies have investigated to what extent it is possible to control shedding patterns at higher Reynolds numbers, through the use of end manipulation. In the present paper, we demonstrate that it is possible to induce parallel shedding, oblique shedding and vortex dislocations, by manipulation of the end conditions, over a large range of Reynolds number. Such patterns affect the frequency of primary wake instability and its amplitude of fluctuation, as they do at low Reynolds number, although distinct differences are found at the higher Reynolds numbers.We find that imposition of oblique shedding conditions at high Reynolds number leads to a spatial variation of both the oblique shedding angle and shedding frequency across the span, and to sparse dislocations which are not restricted to the spanwise end regions, as they are at low Reynolds numbers (under similar geometrical conditions). In the wake transition regime (Re=190–250), it is confirmed that the spontaneous appearance of vortex dislocations in mode-A shedding precludes the control of shedding patterns using end manipulation. However, it has proven possible to extend the regime of Reynolds number where dislocations ‘naturally’ exist to Re>250, by introducing them artificially through end control, where they would otherwise not occur. The possibility of introducing dislocations and of inducing oblique vortex shedding at higher Reynolds numbers has practical significance, if one can deliberately decorrelate the vortex shedding, and hence reduce the spanwise-integrated unsteady fluid forces on the body.We confirm the existence of a transition in the mode of shedding at Re≈5000 (originally found by Norberg 1987) under conditions where parallel shedding is attempted. This mode transition displays similarities to an inverse of the mode A→mode B transition that is found in the wake transition regime. It is clear that vortex dislocations occur beyond Re=5000, although it is not clear why the flow is unstable to such a mode. Furthermore, there appears to be some support for the suggestion that vortex dislocations may be a feature of the flow for Re at least up to 30×103, as evidenced by the work of Norberg (1994).

Updating long-held assumptions about fat stigma: For women, body shape plays a critical role

2020 ◽  
Author(s):  
Jaimie Krems ◽  
Steven L. Neuberg
Keyword(s):  
Body Shape ◽  
Critical Role ◽  
The Body ◽  
Theoretical Assumption ◽  
Obesity Stigma ◽  
Obese Female ◽  
Three Samples ◽  
Female Bodies ◽  
Different Parts ◽  
Fat Stigma

Heavier bodies—particularly female bodies—are stigmatized. Such fat stigma is pervasive, painful to experience, and may even facilitate weight gain, thereby perpetuating the obesity-stigma cycle. Leveraging research on functionally distinct forms of fat (deposited on different parts of the body), we propose that body shape plays an important but largely underappreciated role in fat stigma, above and beyond fat amount. Across three samples varying in participant ethnicity (White and Black Americans) and nation (U.S., India), patterns of fat stigma reveal that, as hypothesized, participants differently stigmatized equally-overweight or -obese female targets as a function of target shape, sometimes even more strongly stigmatizing targets with less rather than more body mass. Such findings suggest value in updating our understanding of fat stigma to include body shape and in querying a predominating, but often implicit, theoretical assumption that people simply view all fat as bad (and more fat as worse).

Chaos in body–vortex interactions

2010 ◽  
Vol 466 (2119) ◽  
pp. 1871-1891 ◽  
Author(s):  
Johan Roenby ◽  
Hassan Aref
Keyword(s):  
Body Shape ◽  
Mass Density ◽  
Relative Equilibrium ◽  
Large Body ◽  
Point Vortex ◽  
Cylindrical Body ◽  
The Body ◽  
Body Motion ◽  
Single Vortex ◽  
Vortex Interactions

The model of body–vortex interactions, where the fluid flow is planar, ideal and unbounded, and the vortex is a point vortex, is studied. The body may have a constant circulation around it. The governing equations for the general case of a freely moving body of arbitrary shape and mass density and an arbitrary number of point vortices are presented. The case of a body and a single vortex is then investigated numerically in detail. In this paper, the body is a homogeneous, elliptical cylinder. For large body–vortex separations, the system behaves much like a vortex pair regardless of body shape. The case of a circle is integrable. As the body is made slightly elliptic, a chaotic region grows from an unstable relative equilibrium of the circle-vortex case. The case of a cylindrical body of any shape moving in fluid otherwise at rest is also integrable. A second transition to chaos arises from the limit between rocking and tumbling motion of the body known in this case. In both instances, the chaos may be detected both in the body motion and in the vortex motion. The effect of increasing body mass at a fixed body shape is to damp the chaos.

scholarly journals Sea stars generate downforce to stay attached to surfaces

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Mark Hermes ◽  
Mitul Luhar
Keyword(s):  
Body Shape ◽  
Control Volume ◽  
Water Channel ◽  
Morphological Plasticity ◽  
The Body ◽  
Sea Star ◽  
Sea Stars ◽  
Hydrodynamic Loads ◽  
Spherical Dome ◽  
Star Models

AbstractIntertidal sea stars often function in environments with extreme hydrodynamic loads that can compromise their ability to remain attached to surfaces. While behavioral responses such as burrowing into sand or sheltering in rock crevices can help minimize hydrodynamic loads, previous work shows that sea stars also alter body shape in response to flow conditions. This morphological plasticity suggests that sea star body shape may play an important hydrodynamic role. In this study, we measured the fluid forces acting on surface-mounted sea star and spherical dome models in water channel tests. All sea star models created downforce, i.e., the fluid pushed the body towards the surface. In contrast, the spherical dome generated lift. We also used Particle Image Velocimetry (PIV) to measure the midplane flow field around the models. Control volume analyses based on the PIV data show that downforce arises because the sea star bodies serve as ramps that divert fluid away from the surface. These observations are further rationalized using force predictions and flow visualizations from numerical simulations. The discovery of downforce generation could explain why sea stars are shaped as they are: the pentaradial geometry aids attachment to surfaces in the presence of high hydrodynamic loads.

scholarly journals Hydromechanics of low-Reynolds-number flow. Part 2. Singularity method for Stokes flows

1975 ◽  
Vol 67 (4) ◽  
pp. 787-815 ◽  
Author(s):  
Allen T. Chwang ◽  
T. Yao-Tsu Wu
Keyword(s):  
Exact Solutions ◽  
Stokes Flow ◽  
Fundamental Solutions ◽  
The Body ◽  
Circular Cylinders ◽  
Geometrical Parameters ◽  
Flow Problems ◽  
Singularity Method ◽  
Wide Range ◽  
Body Shapes

The present study further explores the fundamental singular solutions for Stokes flow that can be useful for constructing solutions over a wide range of free-stream profiles and body shapes. The primary singularity is the Stokeslet, which is associated with a singular point force embedded in a Stokes flow. From its derivatives other fundamental singularities can be obtained, including rotlets, stresslets, potential doublets and higher-order poles derived from them. For treating interior Stokes-flow problems new fundamental solutions are introduced; they include the Stokeson and its derivatives, called the roton and stresson.These fundamental singularities are employed here to construct exact solutions to a number of exterior and interior Stokes-flow problems for several specific body shapes translating and rotating in a viscous fluid which may itself be providing a primary flow. The different primary flows considered here include the uniform stream, shear flows, parabolic profiles and extensional flows (hyper-bolic profiles), while the body shapes cover prolate spheroids, spheres and circular cylinders. The salient features of these exact solutions (all obtained in closed form) regarding the types of singularities required for the construction of a solution in each specific case, their distribution densities and the range of validity of the solution, which may depend on the characteristic Reynolds numbers and governing geometrical parameters, are discussed.

An Experimental Study on the Vertical Flow Past a Finite-Length Horizontal Cylinder at Low Reynolds Numbers

2014 ◽  
Vol 493 ◽  
pp. 68-73 ◽  
Author(s):  
Willy Stevanus ◽  
Yi Jiun Peter Lin
Keyword(s):  
Finite Length ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Reynolds Numbers ◽  
Horizontal Cylinder ◽  
Vortex Street ◽  
Vertical Flow ◽  
Low Reynolds Numbers ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

The research studies the characteristics of the vertical flow past a finite-length horizontal cylinder at low Reynolds numbers (ReD) from 250 to 1080. The experiments were performed in a vertical closed-loop water tunnel. Flow fields were observed by the particle tracer approach for flow visualization and measured by the Particle Image Velocimetry (P.I.V.) approach for velocity fields. The characteristics of vortex formation in the wake of the finite-length cylinder change at different regions from the tip to the base of it. Near the tip, a pair of vortices in the wake was observed and the size of the vortex increased as the observed section was away from the tip. Around a distance of 3 diameters of the cylinder from its tip, the vortex street in the wake was observed. The characteristics of vortex formation also change with increasing Reynolds numbers. At X/D = -3, a pair of vortices was observed in the wake for ReD = 250, but as the ReD increases the vortex street was observed at the same section. The vortex shedding frequency is analyzed by Fast Fourier Transform (FFT). Experimental results show that the downwash flow affects the vortex shedding frequency even to 5 diameters of the cylinder from its tip. The interaction between the downwash flow and the Von Kármán vortex street in the wake of the cylinder is presented in this paper.

scholarly journals Fish larvae exploit edge vortices along their dorsal and ventral fin folds to propel themselves

2016 ◽  
Vol 13 (116) ◽  
pp. 20160068 ◽  
Author(s):  
Gen Li ◽  
Ulrike K. Müller ◽  
Johan L. van Leeuwen ◽  
Hao Liu
Keyword(s):  
Larval Fish ◽  
Fish Larvae ◽  
Fluid Dynamic ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Bony Fish ◽  
The Body ◽  
Functional Explanation ◽  
Image Velocimetry ◽  
Median Fins

Larvae of bony fish swim in the intermediate Reynolds number ( Re ) regime, using body- and caudal-fin undulation to propel themselves. They share a median fin fold that transforms into separate median fins as they grow into juveniles. The fin fold was suggested to be an adaption for locomotion in the intermediate Reynolds regime, but its fluid-dynamic role is still enigmatic. Using three-dimensional fluid-dynamic computations, we quantified the swimming trajectory from body-shape changes during cyclic swimming of larval fish. We predicted unsteady vortices around the upper and lower edges of the fin fold, and identified similar vortices around real larvae with particle image velocimetry. We show that thrust contributions on the body peak adjacent to the upper and lower edges of the fin fold where large left–right pressure differences occur in concert with the periodical generation and shedding of edge vortices. The fin fold enhances effective flow separation and drag-based thrust. Along the body, net thrust is generated in multiple zones posterior to the centre of mass. Counterfactual simulations exploring the effect of having a fin fold across a range of Reynolds numbers show that the fin fold helps larvae achieve high swimming speeds, yet requires high power. We conclude that propulsion in larval fish partly relies on unsteady high-intensity vortices along the upper and lower edges of the fin fold, providing a functional explanation for the omnipresence of the fin fold in bony-fish larvae.

The development and validation of the body shape questionnaire

1987 ◽  
Vol 6 (4) ◽  
pp. 485-494 ◽  
Author(s):  
Peter J. Cooper ◽  
Melanie J. Taylor ◽  
Zafra Cooper ◽  
Christopher G. Fairbum
Keyword(s):  
Body Shape ◽  
The Body ◽  
Development And Validation
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