Simulations of Asymmetric Flow Structures around a Moving Sphere at Moderate Reynolds Number

2015 ◽  
Vol 31 (6) ◽  
pp. 757-769
Author(s):  
D.-L. Young ◽  
C.-S. Wu ◽  
C. Wu ◽  
Y.-C. Lin
Keyword(s):  
Reynolds Number ◽  
Laboratory Experiment ◽  
Vortex Ring ◽  
Ring System ◽  
Wake Flow ◽  
Flow Structures ◽  
Leeward Side ◽  
Moderate Reynolds Number ◽  
Side Flow ◽  
Physical Instability

ABSTRACTThe evolution of asymmetric leeward-side flow structures around a moving sphere in the viscous flow is investigated. Simulations are carried out to investigate the variations of vortex-ring system at the moderate Reynolds number. A parallel laboratory experiment is undertaken in this study. The sphere travels a certain distance at constant speed and then stops to collide with a wall. The motion of moving sphere in fluid is described by the hybrid Cartesian immersed boundary method. Drag forces behind the moving sphere are extremely substantial as the solid body falls through viscous fluid for comprehending the formation of wake flow. The dynamic behavior consists of growth and breakup of the vortices which depend on two specific moderate Reynolds numbers. The onset of physical instability in the wake is obviously affected at the Reynolds number of 800. The generated vortex-ring system rolls upward to compact the primary vortex ring and interact with the secondary vortex. An asymmetric generation of the pairs of vortices is developed since the physical instability effect leads to shed in the wake with the increasing Reynolds number. The results from numerical simulations are also conducted to exhibit good comparison with those from the laboratory experiment.

Numerical study of a vortex ring impacting a flat wall

2010 ◽  
Vol 660 ◽  
pp. 430-455 ◽  
Author(s):  
MING CHENG ◽  
JING LOU ◽  
LI-SHI LUO
Keyword(s):  
Reynolds Number ◽  
Vortex Ring ◽  
Three Dimensions ◽  
Angle Of Incidence ◽  
Secondary Vortex ◽  
Primary Ring ◽  
Flat Wall ◽  
Hydrodynamic Behaviour ◽  
Moderate Reynolds Number ◽  
Wall Interaction

We numerically study a vortex ring impacting a flat wall with an angle of incidence θ ≥ 0°) in three dimensions by using the lattice Boltzmann equation. The hydrodynamic behaviour of the ring–wall interacting flow is investigated by systematically varying the angle of incidence θ in the range of 0° ≤ θ ≤ 40° and the Reynolds number in the range of 100 ≤ Re ≤ 1000, where the Reynolds number Re is based on the translational speed and initial diameter of the vortex ring. We quantify the effects of θ and Re on the evolution of the vortex structure in three dimensions and other flow fields in two dimensions. We observe three distinctive flow regions in the θ–Re parameter space. First, in the low-Reynolds-number region, the ring–wall interaction dissipates the ring without generating any secondary rings. Second, with a moderate Reynolds number Re and a small angle of incidence θ, the ring–wall interaction generates a complete secondary vortex ring, and even a tertiary ring at higher Reynolds numbers. The secondary vortex ring is convected to the centre region of the primary ring and develops azimuthal instabilities, which eventually lead to the development of hairpin-like small vortices through ring–ring interaction. And finally, with a moderate Reynolds number and a sufficiently large angle of incidence θ, only a secondary vortex ring is generated. The secondary vortex wraps around the primary ring and propagates from the near end of the primary ring, which touches the wall first, to the far end, which touches the wall last. The rings develop a helical structure. Our results from the present study confirm some existing experimental observations made in the previous studies.

scholarly journals Decomposition of wake dynamics in fluid–structure interaction via low-dimensional models

2019 ◽  
Vol 867 ◽  
pp. 723-764 ◽  
Author(s):  
T. P. Miyanawala ◽  
R. K. Jaiman
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
High Amplitude ◽  
Wake Flow ◽  
High Dimensional ◽  
Near Wake ◽  
Structure Interaction ◽  
Moderate Reynolds Number ◽  
Low Dimensional ◽  
Lock In

We present a dynamic decomposition analysis of the wake flow in fluid–structure interaction (FSI) systems under both laminar and turbulent flow conditions. Of particular interest is to provide the significance of low-dimensional wake flow features and their interaction dynamics to sustain the free vibration of a square cylinder at a relatively low mass ratio. To obtain the high-dimensional data, we employ a body-conforming variational FSI solver based on the recently developed partitioned iterative scheme and the dynamic subgrid-scale turbulence model for a moderate Reynolds number ($Re$). The snapshot data from high-dimensional FSI simulations are projected to a low-dimensional subspace using the proper orthogonal decomposition (POD). We utilize each corresponding POD mode to detect features of the organized motions, namely, the vortex street, the shear layer and the near-wake bubble. We find that the vortex shedding modes contribute solely to the lift force, while the near-wake and shear layer modes play a dominant role in the drag force. We further examine the fundamental mechanism of this dynamical behaviour and propose a force decomposition technique via low-dimensional approximation. To elucidate the frequency lock-in, we systematically analyse the decomposed modes and their dynamical contributions to the force fluctuations for a range of reduced velocity at low Reynolds number laminar flow. These quantitative mode energy contributions demonstrate that the shear layer feeds the vorticity flux to the wake vortices and the near-wake bubble during the wake–body synchronization. Based on the decomposition of wake dynamics, we suggest an interaction cycle for the frequency lock-in during the wake–body interaction, which provides the interrelationship between the high-amplitude motion and the dominating wake features. Through our investigation of wake–body synchronization below critical $Re$ range, we discover that the bluff body can undergo a synchronized high-amplitude vibration due to flexibility-induced unsteadiness. Owing to the wake turbulence at a moderate Reynolds number of $Re=22\,000$, a distorted set of POD modes and the broadband energy distribution are observed, while the interaction cycle for the wake synchronization is found to be valid for the turbulent wake flow.

Freely Vibrating Two Side-by-Side Square Columns With Combined Translational Motions

2016 ◽  
Author(s):  
Meng-Zhao Guan ◽  
Rajeev K. Jaiman ◽  
Chang-Wei Kang ◽  
Teck-Bin Arthur Lim
Keyword(s):  
Experimental Data ◽  
Reynolds Number ◽  
Low Reynolds Number ◽  
Wake Flow ◽  
Stationary Condition ◽  
Arbitrary Lagrangian Eulerian ◽  
Mesh Motion ◽  
Moderate Reynolds Number ◽  
Flow Induced Vibrations ◽  
Low Reynolds

This paper presents a set of numerical simulations of flow-induced vibrations (FIV) and coupled wake flow behind two identical square columns in a side-by-side configuration. To observe the four regimes as a function of different gap ratios, the computational results of the configuration at low Reynolds number Re=ρfUDμf in stationary condition are firstly compared with existing experimental data of moderate Reynolds number. We next investigate the configuration of elastically mounted square columns, which are free to oscillate in both streamwise and transverse directions. The simulations are performed by the Petrov-Galerkin finite-element method and Arbitrary Lagrangian-Eulerian technique to account for the fluid mesh motion. The four regimes of stationary side-by-side configuration follow the same trend of the experimental data conducted at moderate Reynolds number, while the ranges of each regime differ due to the turbulent wake properties. For the freely vibrating condition, all the simulations are computed at low Reynolds number (Re = 200), mass ratio equal to 10m*=Mmf and reduced velocity in the range of Ur ∈ [1,50] where Ur=UfND and in free-damping condition ζ = C2KM = 0. The four regimes in vibrating condition are investigated as a function of gap ratios g* = g/D, which is the ratio of spacing between the inner column surfaces to the diameter of the column. The effects of reduced velocity on the force variations, the vibration amplitudes and the vorticity contours are analyzed systematically to understand the underlying FIV physics of side-by-side columns in the four regimes. Finally, we present a FIV study of the full semi-submersible model at moderate Reynolds number Re = 20,000 which can be considered as the application of side-by-side configuration.

Reynolds number and aspect ratio effects on the leading-edge vortex for rotating insect wing planforms

2013 ◽  
Vol 717 ◽  
pp. 166-192 ◽  
Author(s):  
R. R. Harbig ◽  
J. Sheridan ◽  
M. C. Thompson
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Leading Edge ◽  
Fruit Fly ◽  
Flow Structures ◽  
Leeward Side ◽  
Aerodynamic Forces ◽  
Leading Edge Vortex ◽  
Wing Span ◽  
Edge Vortex

AbstractPrevious studies investigating the effect of aspect ratio ($\mathit{AR}$) for insect-like regimes have reported seemingly different trends in aerodynamic forces, however no detailed flow observations have been made. In this study, the effect of $\mathit{AR}$ and Reynolds number on the flow structures over insect-like wings is explored using a numerical model of an altered fruit fly wing revolving at a constant angular velocity. Increasing the Reynolds number for an $\mathit{AR}$ of 2.91 resulted in the development of a dual leading-edge vortex (LEV) structure, however increasing $\mathit{AR}$ at a fixed Reynolds number generated the same flow structures. This result shows that the effects of Reynolds number and $\mathit{AR}$ are linked. We present an alternative scaling using wing span as the characteristic length to decouple the effects of Reynolds number from those of $\mathit{AR}$. This results in a span-based Reynolds number, which can be used to independently describe the development of the LEV. Indeed, universal behaviour was found for various parameters using this scaling. The effect of $\mathit{AR}$ on the vortex structures and aerodynamic forces was then assessed at different span-based Reynolds numbers. Scaling the flow using the wing span was found to apply when a strong spanwise velocity is present on the leeward side of the wing and therefore may prove to be useful for similar studies involving flapping or rotating wings at high angles of attack.

The evolution of an elliptic vortex ring

1981 ◽  
Vol 109 ◽  
pp. 189-216 ◽  
Author(s):  
M. R. Dhanak ◽  
B. DE Bernardinis
Keyword(s):  
Reynolds Number ◽  
Vortex Ring ◽  
Ideal Fluid ◽  
Vortex Rings ◽  
Core Size ◽  
Vorticity Distribution ◽  
The Core ◽  
Moderate Reynolds Number ◽  
Break Up

The evolution of a vortex ring in an ideal fluid under self-induction from a flat and elliptic configuration is followed numerically using the cut-off approximation (Crow 1970) for the velocity at the vortex. Calculations are presented for four different axes ratios of the initial ellipse. A particular choice is made for the core size and vorticity distribution in the core of the vortex ring. When the initial axes ratio is close to 1, the vortex ring oscillates periodically. The periodicity is lost as more eccentric cases are considered. For initial axes ratio 0·2, the calculations suggest a break-up of the ring through the core at one portion of the ring touching that at another, initially distant, portion of the ring.Results from quantitative experiments, conducted at moderate Reynolds number with the vortex rings produced by puffing air through elliptic orifices, are compared with the calculations. The agreement is fairly good and it is found that a vortex ring produced from an orifice of axes ratio 0·2 breaks up into two smaller rings. The relevance of the results to the vortex trail of an aircraft is discussed.

scholarly journals Low Mass-Damping Vortex-Induced Vibrations of a Single Cylinder at Moderate Reynolds Number

2014 ◽  
Vol 136 (5) ◽  
Author(s):  
Y. Jus ◽  
E. Longatte ◽  
J.-C. Chassaing ◽  
P. Sagaut
Keyword(s):  
Reynolds Number ◽  
Wake Flow ◽  
Physical Analysis ◽  
Computational Domain ◽  
Moving Mesh Method ◽  
Structural Dynamic ◽  
Vortex Induced Vibrations ◽  
Moderate Reynolds Number ◽  
Wide Range ◽  
Low Mass

The feasibility and accuracy of large eddy simulation is investigated for the case of three-dimensional unsteady flows past an elastically mounted cylinder at moderate Reynolds number. Although these flow problems are unconfined, complex wake flow patterns may be observed depending on the elastic properties of the structure. An iterative procedure is used to solve the structural dynamic equation to be coupled with the Navier–Stokes system formulated in a pseudo-Eulerian way. A moving mesh method is involved to deform the computational domain according to the motion of the fluid structure interface. Numerical simulations of vortex-induced vibrations are performed for a freely vibrating cylinder at Reynolds number 3900 in the subcritical regime under two low mass-damping conditions. A detailed physical analysis is provided for a wide range of reduced velocities, and the typical three-branch response of the amplitude behavior usually reported in the experiments is exhibited and reproduced by numerical simulation.

scholarly journals Influence of pressure on the wake of a hydrophobic circular cylinder for a flow regime with Reynolds number of 2.2×105

2021 ◽  
Vol 2119 (1) ◽  
pp. 012011
Author(s):  
K G Dobroselsky
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Vortex Flow ◽  
Flow Region ◽  
Velocity Fields ◽  
Wake Flow ◽  
Flow Structures ◽  
Reynolds Stresses ◽  
Two Dimensional Image

Abstract Vortex flow structures in a turbulent wake behind a circular Teflon cylinder immersed in an incoming flow with a change in pressure for the Reynolds number Re = 2.2×105 have been experimentally studied using a two-dimensional image (2D-PIV) of particles in a closed-circuit water tunnel. The obtained results are presented in the form of time-averaged velocity fields, Reynolds stresses, and distributions of turbulent kinetic energy. The flow data showed that the size of the wake flow region, Reynolds stresses and turbulent kinetic energy change depending on the pressure in the flow. As a result of a 20% reduction in pressure, the size of the vortex zone in the wake increases by about 20%.

Initial motion of a viscous vortex ring

1994 ◽  
Vol 446 (1928) ◽  
pp. 589-599 ◽  
Keyword(s):  
Reynolds Number ◽  
Asymptotic Analysis ◽  
Vortex Ring ◽  
Kinematic Viscosity ◽  
Reynolds Numbers ◽  
Initial Radius ◽  
Dimensionless Form ◽  
Ring Radius ◽  
Initial Motion ◽  
Matched Asymptotic Analysis

The behaviour of a viscous vortex ring is examined by a matched asymptotic analysis up to three orders. This study aims at investigating how much the location of maximum vorticity deviates from the centroid of the vortex ring, defined by P. G. Saffman (1970). All the results are presented in dimensionless form, as indicated in the following context. Let Γ be the initial circulation of the vortex ring, and R denote the ring radius normalized by its initial radius R i . For the asymptotic analysis, a small parameter ∊ = ( t / Re ) ½ is introduced, where t denotes time normalized by R 2 i / Γ , and Re = Γ/v is the Reynolds number defined with Γ and the kinematic viscosity v . Our analysis shows that the trajectory of maximum vorticity moves with the velocity (normalized by Γ/R i ) U m = – 1/4π R {ln 4 R /∊ + H m } + O (∊ ln ∊), where H m = H m ( Re, t ) depends on the Reynolds number Re and may change slightly with time t for the initial motion. For the centroid of the vortex ring, we obtain the velocity U c by merely replacing H m by H c , which is a constant –0.558 for all values of the Reynolds number. Only in the limit of Re → ∞, the values of H m and H c are found to coincide with each other, while the deviation of H m from the constant H c is getting significant with decreasing the Reynolds number. Also of interest is that the radial motion is shown to exist for the trajectory of maximum vorticity at finite Reynolds numbers. Furthermore, the present analysis clarifies the earlier discrepancy between Saffman’s result and that obtained by C. Tung and L. Ting (1967).

Vortex shedding from a circular cylinder in moderate-Reynolds-number shear flow

1980 ◽  
Vol 101 (4) ◽  
pp. 721-735 ◽  
Author(s):  
Masaru Kiya ◽  
Hisataka Tamura ◽  
Mikio Arie
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Shear Flow ◽  
Vortex Shedding ◽  
Transverse Velocity ◽  
Number Range ◽  
Uniform Shear ◽  
Reynolds Number Range ◽  
Moderate Reynolds Number ◽  
Shear Parameter

The frequency of vortex shedding from a circular cylinder in a uniform shear flow and the flow patterns around it were experimentally investigated. The Reynolds number Re, which was defined in terms of the cylinder diameter and the approaching velocity at its centre, ranged from 35 to 1500. The shear parameter, which is the transverse velocity gradient of the shear flow non-dimensionalized by the above two quantities, was varied from 0 to 0·25. The critical Reynolds number beyond which vortex shedding from the cylinder occurred was found to be higher than that for a uniform stream and increased approximately linearly with increasing shear parameter when it was larger than about 0·06. In the Reynolds-number range 43 < Re < 220, the vortex shedding disappeared for sufficiently large shear parameters. Moreover, in the Reynolds-number range 100 < Re < 1000, the Strouhal number increased as the shear parameter increased beyond about 0·1.

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