Elimination of Vortex Streets in Bluff-Body Flows

S. Dong,1,* G. S. Triantafyllou,2 and G. E. Karniadakis3

1Department of Mathematics, Purdue University, West Lafayette, Indiana 47907, USA2National Technical University of Athens, Greece

3Division of Applied Mathematics, Brown University, Providence, Rhode Island 02912, USA(Received 29 August 2007; published 21 May 2008)

We present an effective technique for suppressing the vortex-induced vibrations of bluff bodies byeliminating the von Karman street formed in their wake. Specifically, we find that small amounts ofcombined windward suction and leeward blowing around the body modify the wake instability and lead tosuppression of the fluctuating lift force. Three-dimensional simulations and stability analysis areemployed to quantify our findings for the flow past fixed and flexibly mounted circular cylinders.

DOI: 10.1103/PhysRevLett.100.204501 PACS numbers: 47.32.ck

Vortex shedding from a bluff body is omnipresent innature and man-made structures. By bluff-body flow wemean flows past blunt objects, such as the wind blowingaround a high-rise building or a long bridge, which areopposed to those past a streamlined object such as anaircraft wing. The phenomenon has been studied by severalpioneers in fluid dynamics, e.g., Strouhal, Rayleigh,Benard, and von Karman, and has remained the focus ofmany modern theoretical and experimental studies [1]. Ofinterest is the unique Strouhal-Reynolds relationship [2]and also the type of instability that sustains the vortexstreet, irrespective of any external excitations [3,4]. Theasymmetric vortices shed in the bluff-body wake inducelarge unsteady side forces that, in turn, can lead to largestructural vibrations if the body is flexibly mounted, espe-cially in the cross-flow direction [5].

Suppression of the vortex street could reduce suchvortex-induced vibrations (VIV) and the wake turbulence,with a direct impact on many engineering applications. Tothis end, many flow-control techniques have been pro-posed, e.g., splitter plate, base bleeding, small controlcylinder, to name but a few; see [6] for several representa-tive techniques, and [7] for a review of passive controlmethods. Blowing or suction as a means for cylinder dragreduction or vortex manipulation has also been the subjectof several previous studies [8–10]. The ability of blowingor suction to modify the wake has long been recognized[9,11]. For example, by experimentally ejecting or suckingfluid through two rows of small holes on the cylindersurface, Williams et al. [9] observed that the produceddisturbances significantly modified the pattern and fre-quencies of vortex shedding and the mean flow. Simi-larly, in their two-dimensional numerical simulations of asteady suction or blowing applied at the rear stagnationpoint around the Reynolds number [12] Re � 47 (at whichthe steady flow transitions to an unsteady state with vortexshedding), Delaunay and Kaiksis [10] observed that forRe> 47 slight blowing or sufficiently high suction stabi-lized the wake while for Re< 47 suction destabilized thewake and blowing had no detectable effect. More recently,

Kim and Choi [13] studied a forcing scheme numericallyfor cylinder drag reduction by blowing or suction of fluidthrough two slits located on the surface at an angle �90�

from the front stagnation line. It was observed that the in-phase forcing from the two slits reduced the drag and couldalso attenuate or even annihilate the vortex shedding.

There are two fundamental limitations with the flow-control techniques used so far in suppressing vortex-induced vibrations. First, they may be effective only forstationary cylinders (not for flexibly mounted structures).Indeed, it has been shown for some techniques (e.g., wavycylinders and cylinders with bumps) that even thoughvortex shedding may be suppressed when the cylinder isfixed, significant oscillations still develop if the cylinder isallowed to freely vibrate [14]. Second, almost all activecontrol schemes proposed require an excessive energyinput that makes them impractical at high Reynoldsnumbers.

The control scheme we propose in the current workaddresses both issues. It is very effective in suppressingthe vortex street and the vortex-induced vibrations forflexibly mounted structures, and it can readily lead topractical passive control schemes that require no energyinput. Specifically, we consider the flow past a long rigidcircular cylinder under two situations: the cylinder is(1) fixed (stationary case), and (2) allowed to freely vi-brate, but only in the cross-flow direction (VIV case). Weinvestigate the effect of a combined suction and blowingscheme, in which a steady suction is applied on the wind-ward half of the cylinder surface while a steady blowing isapplied on the leeward half of the surface. It will be calledthe WSLB control (WSLB standing for windward suctionleeward blowing) hereafter in this Letter. We compareWSLB to two similar schemes: suction-only andblowing-only, with which a steady suction (or blowing) isapplied on the entire surface.

We have simulated two Reynolds numbers, Re � 500and 1000. We solve the three-dimensional incompressibleNavier-Stokes equations employing a Fourier expansionalong the cylinder axis (or the spanwise direction, in which

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the flow is assumed to be homogeneous) and a spectralelement discretization in the streamwise–cross-flowplanes. For the VIV case, the cylinder is elasticallymounted and is modeled as a spring-mass oscillation sys-tem. The coupled fluid-structure equations are solved in acoordinate system attached to the cylinder axis underwhich the cylinder becomes stationary. Details of the nu-merical techniques for the stationary cylinder simulationsand for VIV simulations are documented in [15].

The WSLB, suction-only, and blowing-only controlsstudied here are characterized by a blowing or suctionvelocity normal to the cylinder surface with a uniformmagnitude (hereafter called the control velocity, Vcontrol).In the implementation, the Dirichlet boundary condition isapplied on the cylinder surface in accordance with thecontrols; for the case without control, the no-slip conditionis imposed. The computational domain extends from�20D at the inlet to 40D at the outlet, and from �20Dto 20D in the cross-flow direction; the spanwise dimensionis 3�D. We have performed extensive grid refinement testswith different resolutions, and validation against experi-mental results; see [16]. Global physical parameters com-puted from the simulations without control are in goodagreement with the experimental data [17]. In the simula-tions, we have employed a mesh with 1860 quadrilateralelements in the streamwise–cross-flow planes, with theelement order and the number of Fourier planes in thespanwise direction 6 and 192, respectively.

The effects of WSLB, suction-only, and blowing-onlycontrols for the stationary case at Re � 500 are comparedin Fig. 1(a), in which we plot the root-mean-square (rms)lift coefficient versus Vcontrol=U0. We observe that thefluctuating lift can be significantly reduced, and even com-pletely suppressed at high control velocities. However, thethree schemes exhibit quite different characteristics.Suction-only is effective for lift reduction only at highsuction velocities. Low suction velocity, on the otherhand, appears to have the opposite effect; withVcontrol=U0 � 0:05 or below, the rms lift coefficient isactually increased. For WSLB the lift coefficient decreaseslinearly as Vcontrol increases and is below a certain value(0:15U0). Beyond this point, the lift coefficient becomes

essentially negligible. Both WSLB and suction-only cancompletely suppress the fluctuating lift at the highestVcontrol considered here (0:2U0). For identical Vcontrol

WSLB appears more effective than suction-only in termsof lift reduction. With blowing-only, the lift coeffi-cient decreases consistently as Vcontrol increases. AtVcontrol=U0 � 0:05 blowing-only appears more effectivethan the other two schemes; However, as Vcontrol increasesits rate of lift reduction decreases and it becomes the leasteffective among the three. At Vcontrol=U0 � 0:2, the fluc-tuating lift still remains quite significant with this control.Overall, WSLB appears the most effective among the threeschemes. At low control velocities it avoids the lift increasewith suction-only, while at high control velocities it retainsa high rate of reduction, unlike blowing-only. In Fig. 1(b)we demonstrate the effect of Reynolds number with WSLBby plotting the rms lift coefficient versus Vcontrol at Re �500 and 1000 for the stationary case. The lift reductioncurve at Re � 1000 shows characteristics similar to that atRe � 500. WSLB control appears equally effective asReynolds number increases.

To assess the effect of control on free vibrations, wehave simulated the flow past a freely oscillating cylinder(in cross-flow direction only) at Re � 500 for two cases:without control and with WSLB control. We have consid-ered only WSLB for the VIV case since results from thestationary case show that it is the most effective among thethree. Figure 2(a) shows the rms cylinder displacement(normalized by cylinder diameter) versus Vcontrol for two(structural) damping coefficients (0.0046 and 0.046). Thecylinder mass ratio (with respect to the fluid) is 5.09. Thenatural frequency (fN) of the oscillation is set to be equal tothe Strouhal frequency of the flow past a stationary cylin-der at the same Reynolds number. By natural frequency wemean the oscillation frequency in the absence of any fluideffects. Evidently, the oscillation amplitude decreases withincreasing Vcontrol. At high Vcontrol values the oscillation iscompletely suppressed. To further demonstrate the effec-tiveness of WSLB in VIV reduction, we vary the naturalfrequency over a range of values. In Fig. 2(b) we plot therms cylinder displacement versus the reduced velocity,U0=�fND�, with and without control, for a fixed

Vcontrol /U0

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FIG. 1. Vortex street suppression of astationary cylinder: rms lift coefficientCL versus normalized control velocityVcontrol=U0 for different schemes at Re �500 (a) and for WSLB control at twoReynolds numbers (b).

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Vcontrol=U0 � 0:1 at Re � 500. This is for a damping co-efficient 0.0046 and the same mass ratio as before. Thecontrol has significantly reduced the oscillation over theentire range of reduced velocity values.

The effects of flow control on wake structures are dem-onstrated by Fig. 3, in which we plot isosurfaces of theinstantaneous intermediate eigenvalue (denoted by �2)following the vortex identification method of [18]. Theseare for the stationary case at Re � 500, without control[Fig. 3(a)] and with WSLB control [Fig. 3(b)]. The wakestructures have been significantly modified. Compared tothe no-control case, the wake with control is in generaldepleted of structures. Most notably, the streamwise braidsin the near wake have diminished substantially and seem tobe clustered at a few spanwise locations. As Vcontrol in-creases, the wake structures become more severely weak-ened, and at 0:15U0 and above the structures vanish fromthe wake.

To explore the underlying reasons for lift or VIV sup-pression with flow controls, we have performed a stabilityanalysis [3] of the stationary case. By solving the Orr-Sommerfield equation based on the mean streamwise ve-locity profiles at various downstream (i.e., x) locations, wecan determine the coordinates of the ‘‘critical point’’ [3] inthe complex frequency! plane,!0. Its imaginary part,!I,is related to the growth rate of perturbations, and the sign of!I determines the nature of the instability. A positive !Iindicates an absolute instability, and a negative value in-dicates a convective instability. Figure 4(a) shows the

mapped curves in the ! plane from several lines in thecomplex wave number plane (!R denoting the real part of!). This is for the downstream location x=D � 1:0, with-out control, at Re � 500. We can clearly observe thecritical point (see curve 4).

We have computed !0 at several downstream x loca-tions. In Fig. 4(b) we plot !I of the critical points versus xfor several cases. For the case without control, we observea region of absolute instability near the cylinder and aregion of convective instability further downstream. WithWSLB the region of absolute instability shrinks and isdisplaced downstream; the very near wake also changesfrom an absolute instability (no control) to a convectiveinstability (see the case Vcontrol=U0 � 0:1). As Vcontrol in-creases, the highest !I value in the absolutely unstableregion decreases. At Vcontrol=U0 � 0:2 all !I have becomenegative, suggesting a convective instability in the entirewake. As a result, the vortex shedding and the fluctuatinglift are completely suppressed. In Fig. 4(c) we plot the lociin the complex ! plane of the critical point !0�x� foundfrom local stability analysis as x is varied. The shapes ofthe curves suggest the presence of a saddle point in thecomplex x domain that determines the frequency of theglobal mode, in agreement with the stability criterion of[4].

In summary, we observe that WSLB is effective toeliminate the vortex street and the VIV in flexibly mountedstructures. It is an active control scheme requiring externalenergy input. Regarding how expensive it is, we first note

FIG. 3 (color online). Effect of controlon wake structures: Isosurfaces of instan-taneous �2 � �2 (yellow) and �0:8(cyan) for flow past a stationary cylinderwithout control (a) and with the WSLBcontrol (Vcontrol=U0 � 0:1) (b). �2 is theintermediate eigenvalue in the method of[18].

Vcontrol /U0

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FIG. 2. VIV suppression with WSLBcontrol (freely oscillating cylinder): rmscylinder displacement in cross-flow di-rection versus the control velocity (a),and versus the reduced velocity (for afixed control velocity) (b).

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that the required control velocity decreases as the structuraldamping increases and most surprisingly if periodic on-offcontrol strategies are set along the cylinder axis (results notshown here). With respect to the Reynolds number, weobserve that the required normalized control velocities forelimination of vortex street are about the same for Re �500 and 1000. More importantly, the technique can beeasily implemented as a passive control scheme by usingporous surfaces or by forming communicating channelsbetween the forward and aftward stagnation points ofappropriate width. In preliminary simulations, we havetested several such designs, verifying that indeed this isfeasible with the required channel width decreasing as Reincreases. Results of two recent experiments [19] withdesigns along this line have supported the findings of thisLetter, one with the ‘‘guided porosity’’ (flow coming inthrough holes positioned along the front stagnation lineand going out through a row of streamwise-oriented slitslocated at the top and bottom sides of the cylinder), and theother with a row of holes connecting the front and rearstagnation lines. Regarding the effect on the Strouhalnumber, we observe in the stationary case that theStrouhal number initially increases slightly with increasingVcontrol, reaching a peak around Vcontrol=U0 � 0:1, and thendecreases as Vcontrol further increases.

We gratefully acknowledge the support from NSF, ONR,and TeraGrid (computer time), and the useful discussionswith Professor M. S. Triantafyllou (MIT).

*Corresponding author.sdong@math.purdue.edu

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ωR

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FIG. 4. Stability analysis: (a) Map of lines kI � constant in the ! plane (x=D � 1:0), k being the complex wave number and kI itsimaginary part. The critical point lies at the cusp of curve 4. (b) Imaginary part of the critical point versus streamwise location x.(c) Loci in the complex ! plane of the critical point !0�x� found from local stability analysis as x is varied.

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