scholarly journals Vortex suppression and drag reduction in the wake of counter-rotating cylinders

2011 ◽  
Vol 679 ◽  
pp. 343-382 ◽  
Author(s):  
ANDRE S. CHAN ◽  
PETER A. DEWEY ◽  
ANTONY JAMESON ◽  
CHUNLEI LIANG ◽  
ALEXANDER J. SMITS
Keyword(s):  
Drag Reduction ◽  
Vortex Shedding ◽  
Rotation Rate ◽  
Reynolds Numbers ◽  
Instability Region ◽  
Complete Suppression ◽  
Rotating Cylinders ◽  
Critical Rotation ◽  
Unsteady Wake ◽  
Wake Interaction

The flow over a pair of counter-rotating cylinders is investigated numerically and experimentally. It is demonstrated that it is possible to suppress unsteady vortex shedding for gap sizes from one to five cylinder diameters, at Reynolds numbers from 100 to 200, expanding on the more limited work by Chan & Jameson (Intl J. Numer. Meth. Fluids, vol. 63, 2010, p. 22). The degree of unsteady wake suppression is proportional to the speed and the direction of rotation, and there is a critical rotation rate where a complete suppression of flow unsteadiness can be achieved. In the doublet-like configuration at higher rotational speeds, a virtual elliptic body that resembles a potential doublet is formed, and the drag is reduced to zero. The shape of the elliptic body primarily depends on the gap between the two cylinders and the speed of rotation. Prior to the formation of the elliptic body, a second instability region is observed, similar to that seen in studies of single rotating cylinders. It is also shown that the unsteady wake suppression can be achieved by rotating each cylinder in the opposite direction, that is, in a reverse doublet-like configuration. This tends to minimize the wake interaction of the cylinder pair and the second instability does not make an appearance over the range of speeds investigated here.

Numerical investigation of wake flow regimes behind a high-speed rotating circular cylinder in steady flow

2019 ◽  
Vol 878 ◽  
pp. 875-906
Author(s):  
Adnan Munir ◽  
Ming Zhao ◽  
Helen Wu ◽  
Lin Lu
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Vortex Shedding ◽  
Rotation Rate ◽  
High Speed ◽  
Three Dimensional ◽  
Flow Regimes ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Rotating Circular Cylinder

Flow around a high-speed rotating circular cylinder for $Re\leqslant 500$ is investigated numerically. The Reynolds number is defined as $UD/\unicode[STIX]{x1D708}$ with $U$, $D$ and $\unicode[STIX]{x1D708}$ being the free-stream flow velocity, the diameter of the cylinder and the kinematic viscosity of the fluid, respectively. The aim of this study is to investigate the effect of a high rotation rate on the wake flow for a range of Reynolds numbers. Simulations are performed for Reynolds numbers of 100, 150, 200, 250 and 500 and a wide range of rotation rates from 1.6 to 6 with an increment of 0.2. Rotation rate is the ratio of the rotational speed of the cylinder surface to the incoming fluid velocity. A systematic study is performed to investigate the effect of rotation rate on the flow transition to different flow regimes. It is found that there is a transition from a two-dimensional vortex shedding mode to no vortex shedding mode when the rotation rate is increased beyond a critical value for Reynolds numbers between 100 and 200. Further increase in rotation rate results in a transition to three-dimensional flow which is characterized by the presence of finger-shaped (FV) vortices that elongate in the wake of the cylinder and very weak ring-shaped vortices (RV) that wrap the surface of the cylinder. The no vortex shedding mode is not observed at Reynolds numbers greater than or equal to 250 since the flow remains three-dimensional. As the rotation rate is increased further, the occurrence frequency and size of the ring-shaped vortices increases and the flow is dominated by RVs. The RVs become bigger in size and the flow becomes chaotic with increasing rotation rate. A detailed analysis of the flow structures shows that the vortices always exist in pairs and the strength of separated shear layers increases with the increase of rotation rate. A map of flow regimes on a plane of Reynolds number and rotation rate is presented.

Interference Between Two Stationary or Elastically Supported Rigid Circular Cylinders of Unequal Diameters in Tandem and Staggered Arrangements

2013 ◽  
Vol 135 (2) ◽  
Author(s):  
Shan Huang ◽  
Andy Sworn
Keyword(s):  
Drag Reduction ◽  
Vortex Shedding ◽  
Damping Ratio ◽  
Cross Flow ◽  
Reynolds Numbers ◽  
Circular Cylinders ◽  
Test Results ◽  
Damping Parameter ◽  
Frequency Components ◽  
Elastically Supported

Analysis of model test results was carried out to investigate the hydrodynamic interaction between pairs of fixed or elastically supported rigid cylinders of dissimilar diameters in a water flume. The two cylinders are placed with one situated in the wake of the other. The spacing between the cylinders ranges from 1 to 15 times the larger cylinder diameter. The Reynolds numbers are within the subcritical range. For the vibrating cylinders which are free to oscillate in both the in-line and the cross-flow directions, the reduced velocity ranges from 1 to 13 and the low damping ratio of the test setup at 0.006 gives a combined mass-damping parameter of 0.02. For the fixed cylinders, the downstream cylinder experiences a drag reduction and it was found that this drag reduction also depends upon the diameter ratio. The lift on the fixed downstream cylinder has the frequency components derived from the upstream cylinder's vortex shedding as well as from its own vortex shedding, and the relative importance of the two sources is influenced by the spacing between the two cylinders. This is reflected in the downstream cylinder's vortex induced vibration (VIV) response which appears to be dependent upon the actual reduced velocities of both the cylinders.

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.

Suppression of vortex shedding behind a circular cylinder by another control cylinder at low Reynolds numbers

2007 ◽  
Vol 573 ◽  
pp. 171-190 ◽  
Author(s):  
A. DIPANKAR ◽  
T. K. SENGUPTA ◽  
S. B. TALLA
Keyword(s):  
Vortex Shedding ◽  
Physical Mechanism ◽  
Grid Method ◽  
Reynolds Numbers ◽  
Accurate Solution ◽  
Navier Stokes ◽  
Navier Stokes Equation ◽  
Low Reynolds Numbers ◽  
Low Reynolds ◽  
Control Cylinder

Vortex shedding behind a cylinder can be controlled by placing another small cylinder behind it, at low Reynolds numbers. This has been demonstrated experimentally by Strykowski & Sreenivasan (J. Fluid Mech. vol. 218, 1990, p. 74). These authors also provided preliminary numerical results, modelling the control cylinder by the innovative application of boundary conditions on some selective nodes. There are no other computational and theoretical studies that have explored the physical mechanism. In the present work, using an over-set grid method, we report and verify numerically the experimental results for flow past a pair of cylinders. Apart from providing an accurate solution of the Navier–Stokes equation, we also employ an energy-based receptivity analysis method to discuss some aspects of the physical mechanism behind vortex shedding and its control. These results are compared with the flow picture developed using a dynamical system approach based on the proper orthogonal decomposition (POD) technique.

scholarly journals Dynamics of viscoelastic pipe flow at low Reynolds numbers in the maximum drag reduction limit

2019 ◽  
Vol 874 ◽  
pp. 699-719 ◽  
Author(s):  
Jose M. Lopez ◽  
George H. Choueiri ◽  
Björn Hof
Keyword(s):  
Drag Reduction ◽  
Turbulent Flows ◽  
Pipe Flow ◽  
Domain Size ◽  
Reynolds Numbers ◽  
Weissenberg Number ◽  
Pipe Length ◽  
Low Reynolds Numbers ◽  
Maximum Drag Reduction ◽  
Low Reynolds

Polymer additives can substantially reduce the drag of turbulent flows and the upper limit, the so-called state of ‘maximum drag reduction’ (MDR), is to a good approximation independent of the type of polymer and solvent used. Until recently, the consensus was that, in this limit, flows are in a marginal state where only a minimal level of turbulence activity persists. Observations in direct numerical simulations at low Reynolds numbers ($Re$) using minimal sized channels appeared to support this view and reported long ‘hibernation’ periods where turbulence is marginalized. In simulations of pipe flow at $Re$ near transition we find that, indeed, with increasing Weissenberg number ($Wi$), turbulence expresses long periods of hibernation if the domain size is small. However, with increasing pipe length, the temporal hibernation continuously alters to spatio-temporal intermittency and here the flow consists of turbulent puffs surrounded by laminar flow. Moreover, upon an increase in $Wi$, the flow fully relaminarizes, in agreement with recent experiments. At even larger $Wi$, a different instability is encountered causing a drag increase towards MDR. Our findings hence link earlier minimal flow unit simulations with recent experiments and confirm that the addition of polymers initially suppresses Newtonian turbulence and leads to a reverse transition. The MDR state on the other hand results at these low$Re$ from a separate instability and the underlying dynamics corresponds to the recently proposed state of elasto-inertial turbulence.

scholarly journals A Study of Drag Reduction on Cylinders with Different V-Groove Depths on the Surface

Water ◽  
2021 ◽  
Vol 14 (1) ◽  
pp. 36
Author(s):  
Jiyang Qi ◽  
Yue Qi ◽  
Qunyan Chen ◽  
Fei Yan
Keyword(s):  
Drag Reduction ◽  
Drag Coefficient ◽  
Vortex Structure ◽  
Lift Coefficient ◽  
Pressure Coefficient ◽  
Reynolds Numbers ◽  
Groove Surface ◽  
Smooth Cylinder ◽  
Groove Structure ◽  
Reduction Effect

In this study, the drag reduction effect is studied for a cylinder with different V-groove depths on its surface using a k-ω/SST (Shear Stress Transport) turbulence model of computational fluid dynamics (CFD), while a particle image velocimetry (PIV) system is employed to analyze the wake characteristics for a smooth cylinder and a cylinder with different V-groove depths on its surface at different Reynolds numbers. The study focuses on the characteristics of the different V-groove depths on lift coefficient, drag coefficient, the velocity distribution of flow field, pressure coefficient, vortex shedding, and vortex structure. In comparison with a smooth cylinder, the lift coefficient and drag coefficient can be reduced for a cylinder with different V-groove depths on its surface, and the maximum reduction rates of lift coefficient and drag coefficient are about 34.4% and 16%, respectively. Otherwise, the vortex structure presents a complete symmetry for the smooth cylinder, however, the symmetry of the vortex structure becomes insignificant for the V-shaped groove structure with different depths. This is also an important reason for the drag reduction effect of a cylinder with a V-groove surface.

Vortex Shedding From a Bluff Body Adjacent to a Plane Sliding Wall

1991 ◽  
Vol 113 (3) ◽  
pp. 384-398 ◽  
Author(s):  
M. P. Arnal ◽  
D. J. Goering ◽  
J. A. C. Humphrey
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Solid Wall ◽  
Reynolds Numbers ◽  
Careful Examination ◽  
Recirculation Region ◽  
Stream Velocity ◽  
Flow Past ◽  
Lower Reynolds Number

The characteristics of the flow around a bluff body of square cross-section in contact with a solid-wall boundary are investigated numerically using a finite difference procedure. Previous studies (Taneda, 1965; Kamemoto et al., 1984) have shown qualitatively the strong influence of solid-wall boundaries on the vortex-shedding process and the formation of the vortex street downstream. In the present study three cases are investigated which correspond to flow past a square rib in a freestream, flow past a rib on a fixed wall and flow past a rib on a sliding wall. Values of the Reynolds number studied ranged from 100 to 2000, where the Reynolds number is based on the rib height, H, and bulk stream velocity, Ub. Comparisons between the sliding-wall and fixed-wall cases show that the sliding wall has a significant destabilizing effect on the recirculation region behind the rib. Results show the onset of unsteadiness at a lower Reynolds number for the sliding-wall case (50 ≤ Recrit ≤100) than for the fixed-wall case (Recrit≥100). A careful examination of the vortex-shedding process reveals similarities between the sliding-wall case and both the freestream and fixed-wall cases. At moderate Reynolds numbers (Re≥250) the sliding-wall results show that the rib periodically sheds vortices of alternating circulation in much the same manner as the rib in a freestream; as in, for example, Davis and Moore [1982]. The vortices are distributed asymmetrically downstream of the rib and are not of equal strength as in the freestream case. However, the sliding-wall case shows no tendency to develop cycle-to-cycle variations at higher Reynolds numbers, as observed in the freestream and fixed-wall cases. Thus, while the moving wall causes the flow past the rib to become unsteady at a lower Reynolds number than in the fixed-wall case, it also acts to stabilize or “lock-in” the vortex-shedding frequency. This is attributed to the additional source of positive vorticity immediately downstream of the rib on the sliding wall.

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