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    R. King (Ed.): Active Flow Control, NNFM 95, pp. 137151, 2007.

    springerlink.com Springer-Verlag Berlin Heidelberg 2007

    Control of Wing Vortices

    I. Gursul, E. Vardaki, P. Margaris, and Z. Wang

    Department of Mechanical Engineering

    University of Bath

    Bath, BA2 7AY, United Kingdom

    Summary

    Vortex control concepts employed for slender, nonslender and high aspect ratio wings

    were reviewed. For slender delta wings, control of vortex breakdown has been the

    most important objective, which is achieved by modifications to swirl level and

    pressure gradient. Delay of vortex breakdown with the use of control surfaces,

    blowing, suction, high-frequency and low-frequency excitation, and feedback control

    was reviewed. For nonslender delta wings, flow reattachment is the most important

    aspect for flow control methods. For high aspect ratio wings, vortex control concepts

    are diverse, ranging from drag reduction to attenuation of wake hazard and noise,

    which can be achieved by modifications to the vortex location, strength, and structure,

    and generation of multiple vortices.

    1 Introduction

    Controlling vortical flows over wings may have various benefits, such as

    enhancement of lift force, generation of forces and moments for flight control,

    attenuation of buffeting, reduction of drag, and attenuation of noise due to

    vortex/blade interaction. Control methods include manipulation of one or more of the

    following flow phenomena: flow separation from the wing, separated shear layer,

    vortex formation, flow reattachment on the wing surface, and vortex breakdown. The

    occurrence and relative importance of these phenomena strongly depend on the wing

    sweep angle.For delta wings, flow reattachment and vortex breakdown are two important

    phenomena, which determine the effective flow control strategies. The reattachment

    location on the wing surface moves inboard with increasing angle of attack, and

    reaches the wing centreline at a particular incidence. Beyond this limiting angle of

    attack FR, flow reattachment to the wing surface is not possible. The variation of

    predictions [1] ofFR with wing sweep angle is shown in Figure 1. This prediction is

    only valid for slender wings, and the only experimental observation [2] for a

    nonslender wing (by visualization of the reattachment line for a sweep angle of

    =50) is also given in Figure 1. For slender wings, it is seen that the reattachmentincidence decreases with increasing wing sweep angle. Therefore, on highly swept

    wings, reattachment does not occur beyond small angles of attack.

    Vortex breakdown appears on the wing with increasing angle of attack and crosses

    the trailing-edge at a particular incidence BD. The variation of this angle of attack

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    138 I. Gursul et al.

    [3] with wing sweep angle is also shown in Figure 1. It is seen that this angle of

    attack increases with increasing wing sweep angle. On nonslender wings, vortex

    breakdown appears at very small incidences.

    As the vortex control concepts become increasingly diverse, new actuators and

    closed-loop control strategies are being developed. It is useful to consider the flowphysics and dominant mechanisms as these determine which flow control methods are

    effective. The main objective of this paper is to review the vortex control concepts,

    which mainly depend on the wing sweep.

    WING SWEEP [deg]

    ANGLEOFATTACK

    [deg

    ]

    45 50 55 60 65 70 75 80 850

    10

    20

    30

    40

    VORTEX

    BREA

    KDOW

    N

    NOBREAKD

    OWN

    NOREATTACHMENT

    REATTACHMENT

    Fig. 1. Boundaries of vortex breakdown and flow reattachment on the wing surface as a

    function of sweep angle

    2 Slender Delta Wings

    2.1 Overview of Flow Physics

    The flow over a delta wing is characterised by a pair of counter-rotating leading-edge

    vortices that are formed by the roll-up of vortex sheets. The time-averaged axial

    velocity is jet-like at low and moderate incidences. The large axial velocities in the

    vortex core are due to very low pressures, which also generate additional suction andlift force, known as vortex lift, on the delta wings. Vortex lift contribution increases

    with wing sweep angle [4].

    At a sufficiently high angle of attack, the vortices undergo a sudden expansion

    known as vortex breakdown. The axial flow downstream becomes wake-like with

    very low velocities. For slender wings (defined as 65 in this paper), vortex

    breakdown is the dominant flow mechanism that is responsible for decreased lift. It is

    also the dominant source of unsteadiness that causes wing and fin buffeting [5].

    Hence control of vortex breakdown has been the subject of many investigations [6].

    There are two important parameters affecting the occurrence and movement ofvortex breakdown: swirl level and pressure gradientaffecting the vortex core. An

    increase in the magnitude of either parameter promotes the earlier occurrence of

    breakdown. Very early experiments [7] demonstrated that vortex breakdown moves

    upstream over delta wings when the magnitude of either parameter is increased.

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    Control of Wing Vortices 139

    More recently, it was shown [8] that the minimum swirl level required for breakdown

    decrease with increasing magnitude of adverse pressure gradient. Naturally, flow

    control methods for the delay of vortex breakdown rely on modification of these two

    parameters.

    Active flow control methods generally rely on unsteady excitation in flow controlapplications. Unsteady forcing has been used for the control of vortex and breakdown

    in some cases. There are various sources of unsteadiness [5]: shear layer instabilities,

    vortex wandering, helical mode instability of vortex breakdown, oscillations of

    breakdown, vortex interaction, and vortex shedding. The frequency spectrum of the

    unsteady flow phenomena that exist over stationary wings is very wide, which is one

    of the challenges in numerical simulations of these flows. Vortex breakdown, vortex

    interactions, and vortex shedding, either alone or in combination, play an important

    role in wing and fin buffeting, although vortex breakdown is the main source of

    buffeting over slender wings. Flow control approaches for vortex breakdown arereviewed next.

    2.2 Control Surfaces

    Various control surfaces [6] have been investigated to control the formation, location,

    strength, and breakdown of the leading-edge vortices: canards, strakes, leading-edge

    flaps, apex flap, and variable-sweep wings. It is well known that canards can provide

    substantial delays [9] in vortex breakdown by affecting the external pressure gradient

    acting on the vortex core.

    Since all of the vorticity of leading-edge vortices originates from the separationpoint along the leading-edge, leading-edge flaps are particularly attractive tools that

    can be used to influence the strength and structure of these vortices. Leading-edge

    flaps that are deflected downward have been found to reduce drag and improve the

    lift-to-drag ratio [10]. On the other hand, the flap deflection in the upward direction

    causes an increase in lift as well as drag. This type of vortex management can be

    used for landing or aerodynamic manoeuvres [10]. It has been found that the flaps

    deflected upward generate a stronger vortex lift at low and moderate angles of attack.

    However, flaps may also induce vortex breakdown [11]. Leading-edge flaps modify

    the strength and location of the vortices, thereby affecting the parameters that controlvortex breakdown. It was shown [11] that breakdown location and its sensitivity

    strongly depend on incidence and flap angle. For large angles of attack, the variation

    of breakdown location is not monotonic, and therefore, not suitable for control

    purposes.

    A variable leading-edge extension [12] that effectively varies the sweep angle has

    been used to control leading-edge vortices and breakdown. The advantage of this

    method is that the variation of breakdown with sweep angle is monotonic, hence

    suitable for active control purposes. Because most of the vorticity within the vortex

    core originates from a small region near the apex of the wing, an apex flap can be aneffective control surface. It was shown [13] that a drooping apex flap could delay

    vortex breakdown.

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    140 I. Gursul et al.

    2.3 Blowing and Suction

    Blowing and suction applied at various locations are commonly used as flow control

    methods for leading-edge vortices and breakdown. The most widely used versions

    include: (a) leading-edge suction/blowing, (b) trailing-edge blowing, (c) along-the-core blowing. These are discussed in detail below.

    Since the vorticity of the leading-edge vortices originates from the separation line

    along the leading-edge, control of separation characteristics or shear layer can be used

    to influence the strength and location of the vortices as well as the location of vortex

    breakdown. Steady blowing [14, 15] and suction [15, 16] at the leading-edgehas

    been employed, but these methods differ in terms of their effect on swirl level. While

    blowing, in particular tangential blowing, may increase the swirl level, suction

    reduces the strength and swirl level due to removal of some of the vorticity shed from

    the leading-edge. Figure 2(a) shows how the location of shear layer and vortex is

    modified upstream of breakdown when suction is applied. Figure 2(b) shows theaxial velocity contours at the trailing-edge, which show the change from wake-like to

    jet-like velocity as a result of the delay of vortex breakdown. Detailed measurements

    [16] show that maximum swirl angle in the core and circulation decrease with suction,

    which causes the vortex breakdown location to move downstream. It is also worth

    noting that the leading-edge suction technique does not require thick rounded leading

    edges. Control of vortices can be achieved without the use of the Coanda jet effect.

    Fig. 2. Variation of (a) rms axial velocity upstream of breakdown location; (b) time-averaged

    axial velocity at x/c = 1.0

    The effect of trailing-edge jets on wing vortices and vortex breakdown has been

    investigated in several studies [17-20]. Blowing at the trailing-edge also modifies the

    external pressure gradient and causes delay of vortex breakdown. Favourable effect

    of a trailing-edge jet [17] could be observed even in the presence of a fin, which

    produces a strong adverse pressure gradient for a leading-edge vortex. It was shown

    that fin-induced vortex breakdown can be delayed even for the head-on collision of

    the leading-edge vortex with the fin [19]. Hence the adverse pressure gradient caused

    by the presence of the fin could be overcome with a trailing-edge jet. The

    (b)(a)

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    Control of Wing Vortices 141

    effectiveness of a trailing-edge depends on the wing sweep angle [20]. It appears that

    it becomes more difficult to delay vortex breakdown with decreasing sweep angle.

    Along-the-core blowing [21, 22] accelerates the axial flow in the core, and

    modifies the pressure gradient favourably. Figure 3 shows the effectiveness of

    various blowing/suction methods from various studies published in the literature.Here the effectiveness is defined as (xbd/c)/C, where xbd is the change in

    breakdown location (positive corresponding to delay), c is the chord length and C is

    the momentum coefficient. Figure 3 shows that along-the-core blowing is the most

    effective method in terms of delaying vortex breakdown. This can be attributed to the

    importance of the pressure gradient affecting the vortex core. It is also seen that the

    effectiveness of blowing and suction is nearly the same. The effectiveness of trailing-

    edge blowing is the lowest among all blowing methods considered.

    10-2

    10-1

    100

    101

    102

    Along-the-core blowing

    Leading-edge suction/blowing

    Trailing-edge blowing

    Xbd/c

    C

    Fig. 3. Effectiveness of various blowing/suction methods from several studies published in the

    literature

    2.4 Unsteady Control

    There have been various attempts to control vortices and breakdown by using

    unsteady excitation. These include small and large amplitude oscillations of leading-edge flaps, periodic variations of sweep angle, periodic suction or blowing, and

    combined use of leading-edge flaps and intermittent trailing-edge blowing, which are

    summarized in Reference [5]. These studies fall into two categories: (i) high-

    frequency excitation, St = fc/U = O(1), (where f is the frequency and U is the free

    stream velocity) and (ii) low-frequency excitation, St = O(0.1).

    For the high-frequency excitation, Gad-el-Hak and Blackwelder [23] applied

    periodic perturbations of injection and suction along the leading-edge of a delta wing

    ( = 60). They found maximum changes in the evolution of the shear layer when thefrequency of perturbations (St = 5.5) is the subharmonic of the frequency of Kelvin-

    Helmholtz instability. However, no results were reported regarding the structure ofthe main vortex and the effect on vortex breakdown. Gu et al. [24] applied periodic

    suction-blowing in the tangential direction along the leading-edge of the wing ( =

    75) and reported a delay of vortex breakdown. The most effective period of the

    alternate suction-blowing corresponded to fc/U = 1.3. For a less slender wing

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    142 I. Gursul et al.

    ( = 60), it was shown that oscillatory blowing at the leading edge can enhance thelift at high angles of attack [25], and optimum reduced frequency varied in the range

    of fc/U = 1 to 2. For small amplitude flap oscillations, the strength of the vortices

    was larger than that of the quasi-steady case [26], when the excitation frequency was

    St = 1.2. This range of effective frequencies presumably corresponds to subharmonicsof the Kelvin-Helmholtz instability due to vortex pairing.

    For the low-frequency excitation, there have been reports [5] of increased vortex

    circulation and delay of vortex breakdown for oscillating flaps, delay of vortex

    breakdown for harmonic variations of sweep angle for a variable sweep wing, and

    delay of breakdown for combined flaps and unsteady trailing-edge blowing. When

    the spectra of unsteady flow phenomena [5] are considered, it is less clear which

    unsteady phenomena are exploited for the low-frequency excitation. However, it is

    suggested [27] that the variations in the external pressure gradient generated by

    unsteady excitation plays a major role.

    2.5 Feedback Control

    The pressure fluctuations induced by the helical mode instability of vortex breakdown

    can be measured and used as a feedback signal for active control. In such a approach,

    the rms value of pressure is chosen as the control variable and a feedback control

    strategy is considered [28]. The monotonic variation of the amplitude of the pressure

    fluctuations with vortex breakdown location makes the feedback control possible.

    In identifying a suitable flow controller, several methods were considered,

    including blowing, suction, and flaps. However, the relationship between the vortexbreakdown location and control parameter is unknown or undesirable (i.e., not

    monotonic) for these methods. A desirable controller should have a monotonic

    relationship between the control parameter and breakdown location. It was shown

    [28] that it was feasible to use a variable sweep angle mechanism as a means of

    controlling the breakdown location by influencing the circulation of the leading-edge

    vortex. The system was idealized as a first-order system, and integral control was

    used. The feedback control of breakdown was demonstrated for stationary as well as

    pitching delta wings.

    3 Nonslender Delta Wings

    3.1 Overview of Flow Physics

    Vortical flow over nonslender delta wings ( 55) has recently become a topic of

    increased interest in the literature. While the flow topology over more slender wings,

    typically 65, has been extensively studied and is now reasonably wellunderstood, the flow over lower sweep wings has only recently attracted more

    attention [29]. Vortical flows develop at very low angles of attack, and form close to

    the wing surface. One of the distinct features of nonslender wings is that

    reattachment of the separated flow is possible even after breakdown reaches the apex

    of the wing. However, at large angles of attack in the post-stall region, reattachment

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    Control of Wing Vortices 143

    is not possible, and completely stalled flow occurs on the wing. Active and passive

    control of reattachment may be beneficial for lift enhancement in the post-stall region.

    According to Polhamus leading-edge suction analogy [4], the vortex lift

    contribution becomes a smaller portion of the total lift as the sweep angle decreases.

    Vortex breakdown occurs over the wing even at small incidences, and there is noobvious correlation between the onset of vortex breakdown over nonslender wings

    and the change of the lift coefficient. Hence, vortex breakdown is not a limiting

    phenomenon as far as the lift force is concerned for nonslender wings. On the

    contrary, flow reattachment is key to any flow control strategy as suggested in

    Figure 1.

    3.2 Passive Control with Wing Flexibility

    Passive lift enhancement for flexible delta wings has been demonstrated as a potential

    method for the control of vortex-dominated wing flows [30]. Force measurements

    over a range of nonslender delta wings (with sweep angles = 40 to 55) havedemonstrated the ability of a flexible wing to enhance lift and delay stall compared

    with a rigid wing of similar geometry. An example for = 40 is shown in Figure 4a.This recently discovered phenomenon appears to be a feature of nonslender wings.

    Flow visualization, PIV and LDV measurements show that flow reattachment takes

    place on the flexible wings in the post-stall region of the rigid wings. The lift increase

    in the post-stall region is accompanied with large self-excited vibrations of the wings

    as shown in Figure 4b. The dominant frequency of the vibrations of various

    nonslender delta wings is St = O(1). These vibrations promote reattachment of theshear layer, which results in the lift enhancement. Various measurements including

    wing-tip accelerations, rms rolling moment, and hot-wire measurements confirm that

    the dominant mode of vibrations occur in the second antisymmetric structural mode in

    the lift enhancement region. The self-excited vibrations are not observed for a half-

    wing model, hence passive flow control for a flexible wing occurs only in the

    anti-symmetric mode.

    Fig. 4. (a) Variation of lift coefficient as a function of angle of attack for rigid and flexible

    wings, = 40; (b) Variation of mean and amplitude of fluctuating wing-tip deflection with

    incidence, =40

    CL

    0 5 10 15 20 25 30 35 400

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    1

    1.1

    1.2

    1.3

    1.4

    1.5

    Flexible

    Rigid

    /s

    0 5 10 15 20 25 30 35 40 450

    0.02

    0.04

    0.06

    0.08

    0.1

    0.12

    0.14

    0.16

    0.18

    0.2

    Mean deflection

    Peak-to-peak amplitude

    (a) (b)

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    144 I. Gursul et al.

    Spectral analysis of velocity fluctuations [31] along the shear layer showed large

    sharp peaks, corresponding to the wing vibrations. There are also broad dominant

    peaks in the spectra of velocity fluctuations in the range of St = 1 to 5 for the post-

    stall incidences, and these correspond to the shear layer instabilities. The center

    frequency of these peaks decreases with streamwise distance as the shear layervortices shed conically. There is also a decrease in the spanwise direction due to the

    vortex pairing process. The frequency of the structural vibrations is in the same range

    as these natural frequencies [31].

    3.3 Active Control of Reattachment

    Rigid delta wings undergoing small amplitude oscillations [32] in the post-stall region

    exhibit many similarities to flexible wings, including reattachment in the post-stall

    region (see Figure 5). For simple delta wings and cropped delta wings [31] with =

    50, 40, and 30, an optimum frequency around St = 1 was identified for which thereattachment is observed. Note that these dramatic changes are observed with

    unsteady forcing in the post-stall region, whereas there is little effect in the pre-stall

    region. Hence active control methods in the form of leading-edge oscillations or

    blowing can be effective.

    An important parameter is the wing sweep angle. The effect of excitation on a

    swept wing is similar to the response of the flow over a backward-facing step [33] to

    the periodic excitation. However, for zero sweep angle, formation of a closed

    separation bubble at high angles of attack in the post-stall region is not possible. It

    seems that moderate sweep angles (around 50) help the formation of semi-openseparation bubbles, hence the wing sweep is beneficial in flow reattachment.

    However, there is a lower limit of sweep angle below which the beneficial effect of

    wing sweep will diminish. This lower limit of sweep angle is around = 20.Symmetric perturbations in the form of small amplitude pitching oscillations

    (1 amplitude) were studied for = 50 simple delta wing. The results show thatsymmetric perturbations also promote reattachment and vortex re-formation. Hence,

    Fig. 5. Effect of shear layer excitation for =25 and =50; (a) time-averaged fluorescent dyevisualization, (b) instantaneous visualization for one side only

    (a)

    (b)

    St=0 St=1.0

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    Control of Wing Vortices 145

    for active control purposes, both symmetric and anti-symmetric excitations are

    effective. However, passive control for a flexible wing occurs only in the anti-

    symmetric mode.

    3.4 Vortex Re-formation

    Within the reattachment region, axial flow may develop, resulting in re-formation of

    the leading-edge vortices. Figure 6 shows an example of vortex re-formation [32],

    which occurs when the amplitude of forcing is sufficiently large and the frequency of

    excitation is near an optimum value. The mean breakdown location becomes a

    maximum at an optimum frequency as shown in Figure 7.

    Fig. 6. Flow visualization for stationary and oscillating wings in water tunnel, = 50, =25

    fc/U

    xbd

    /c

    0 1 2 3 4 5 6 7 8 9 100

    0.1

    0.2

    0.3 = 25, = 5 = 25, = 1

    Fig. 7. Variation of time-averaged breakdown location as a function of dimensionless

    frequency for two values of oscillation amplitude, = 50, =25

    The time-averaged vorticity flux increases due to the oscillating leading edge,

    which leads to increased circulation. Although the leading edge vortices becomestronger due to the leading edge motion, vortex breakdown is delayed for the

    oscillating wing compared to the stationary wing for which breakdown is at the apex.

    This appears to be in contrast to the well-known studies of vortex breakdown, which

    indicate that increased strength of vortices should cause premature, rather than

    fc/U = 0 fc/U = 1.8

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    146 I. Gursul et al.

    delayed, breakdown. This result suggests that streamwise pressure gradient might be

    modified favourably due to the wing motion.

    4 High Aspect Ratio Wings

    4.1 Vortex Control Concepts

    Control of formation and development of tip vortices in both near-wake and far-wake

    has potential benefits in a variety of aerodynamic problems. Tip vortices form in a

    similar way to the leading-edge vortices over low aspect ratio wings and the roll-up

    process becomes complete within a few chord-lengths downstream of the trailing-

    edge.

    There appear to be three main applications of flow control approaches with regard

    to the tip vortices: 1) reduction of induced drag, 2) attenuation of vortex wake hazardon following aircraft, 3) reduction of helicopter noise due to the blade-vortex

    interaction. These will be briefly reviewed with regard to vortex control concepts.

    For reduction of induced drag, various methods were considered [34], including

    planform shape and span, tip shape, winglets, fences and tip sails. Wing tip devices

    are used to redistribute the vorticity near the wing tip. Similarly, any flow control

    method that can displace the tip vortices in the outboard direction can be used to

    reduce the drag as it is inversely proportional to the square of the spanwise distance

    between the vortices.

    In order to attenuate wake vortex hazard on following aircraft, the core size of the

    vortices can be increased by turbulence injection into the core. Various wing tip

    devices were tested, which were found to be not much effective in the far-wake.

    Those that seemed to decrease the vortex hazard (such as decelerating chutes and

    splines placed behind the trailing-edge) had an unacceptable drag penalty [35]. It was

    noted [36] that passive devices in the form of turbulence generators could increase the

    size of the vortex core and also promote cooperative instabilities as an additional

    benefit. A second and more promising method is to rely on the longwave

    instabilities occurring in a system of several vortices [37]. Compared to the

    alleviation of a single vortex by turbulent diffusion, excitation of the long-wave

    cooperative instabilities of multiple vortices has the potential for much fasterdestruction of vortex wakes. Hence deliberate creation of multiple vortices and

    excitation by oscillating surfaces or oscillatory blowing may be a promising strategy.

    One of the sources of helicopter noise is the interaction of rotor blades with the

    vortices shed from preceding blades. Noise and vibration caused by this interaction

    can be reduced by i) increasing the distance between the rotor blade and tip vortex,

    and (ii) by increasing the size of the vortex core and decreasing the maximum

    tangential velocity.

    4.2 Tip Blowing

    Most of the intended modifications to the tip region (such as wing tip devices), to the

    vortex location, strength, and structure can be achieved without the use of passive

    devices. Active flow control using wing tip blowing can achieve multiple tasks

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    Control of Wing Vortices 147

    during different flight regimes. It was shown that the strength, location, core

    structure, and number of vortices can be effectively manipulated by tip blowing [38].

    The effect of continuous blowing using high-aspect ratio jets is very sensitive to

    the blowing direction. The location and strength of the vortices generated by the jet

    and their interaction with the tip vortex lead to different flow configurations.Blowing in the upward direction produces additional co-rotating vortices in the near-

    wake as shown in Figure 8. These strong vortices forming on the wing surface can be

    used to increase the lift in certain applications such as hovering rotorcraft. Figure 9

    shows that a diffused vortex is obtained with spanwise blowing near the pressure

    surface. Also, downward blowing produces diffused vortices. For these cases,

    blowing appears to add a substantial amount of turbulence in the vortex core,

    resulting in diffused trailing vortices. For downward blowing, it was possible to

    displace the vortices in the outboard direction, which should result in an induced drag

    Fig. 8. Vorticity distribution for upward blowing

    Fig. 9. Vorticity distribution for spanwise blowing near the pressure surface

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    148 I. Gursul et al.

    (b)

    fc/U

    U/U

    0

    10-1

    100

    1010.6

    0.8

    1

    1.2

    synthetic jet - C=0.0004synthetic jet - C=0.0008continuous jet - C=0.0004

    y/c

    z/c

    -0.1 0

    0.1 0.2 0.3-0.1

    0

    0.1

    0.2

    0.3

    Fig. 10. (a) Velocity and vorticity for the synthetic jet (with no freestream); (b) variation of

    maximum tangential velocity at x/c=1.0 as a function of dimensionless frequency

    reduction. Spanwise blowing also results in a lift augmentation [39] as the downwash

    decreases with blowing. Drag reduction and lift augmentation are essentially inviscid

    phenomena due to an effective increase in span and aspect ratio of the wing.

    Synthetic jets can be useful for wing tip blowing as the air supply problem is

    avoided. Figure 10 shows that a synthetic jet is beneficial in terms of diffusing the

    trailing vortex [40]. Synthetic jets produce large turbulence and much larger vortex

    wandering. For the blowing coefficients used, the effect of excitation frequency was

    minor. While there are substantial changes in the circulation of the trailing vortex for

    continuous jets, the circulation remains virtually the same for synthetic jets. The

    intermittent and diffused jet vortices are weak and do not appear to affect the totalcirculation.

    5 Conclusions

    For slender delta wings, control of vortex breakdown has been the primary goal of

    many investigations. Delay of vortex breakdown is possible with the modifications to

    the swirl level and pressure gradient. The use of control surfaces such as leading-

    edge flaps makes it possible to control the location, strength, and structure of the

    vortices. Blowing and suction at the leading-edge, trailing-edge, or along the core

    have differences in terms of their effects on swirl level and pressure gradient affecting

    the vortex core. Along-the-core blowing is the most effective method for delaying

    vortex breakdown. Active flow control using high-frequency excitation targets the

    Kelvin-Helmholtz instability of the separated shear layers. In contrast, low-frequency

    excitation appears to modify the external pressure gradient only.

    For nonslender delta wings, flow reattachment is the most important aspect for

    flow control methods. Passive lift enhancement on flexible wings is due to the self-

    excited wing vibrations, which promote flow reattachment in the post-stall region.

    The frequency of the wing vibrations (St = O(1)) is in the same range as the naturalfrequencies of the shear layer instabilities. Rigid delta wings undergoing small

    amplitude oscillations in the post-stall region exhibit many similarities to flexible

    wings, including reattachment and vortex re-formation. Moderate sweep angles help

    the formation of semi-open separation bubbles, hence the wing sweep is beneficial.

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    Control of Wing Vortices 149

    For high aspect ratio wings, vortex control concepts are diverse, ranging from drag

    reduction to attenuation of wake hazard and noise. Modifications to the vortex

    location, strength, and structure are common ideas in these applications. Active flow

    control using wing tip blowing is shown to have potential for various objectives.

    Depending on the blowing configuration and direction, more diffused or strongervortices, and also, multiple vortices, that move inboard or outboard can be generated.

    The deliberate creation of multiple vortices and excitation of vortex instabilities may

    be a promising strategy for effective control in the far-wake.

    Acknowledgements

    The authors acknowledge the financial support of the Air Force Office of Scientific

    Research (AFOSR), Engineering and Physical Sciences Research Council (EPSRC)

    and the Ministry of Defence in the UK.

    References

    [1] Mangler, K.W. and Smith, J.H.B., A Theory of the Flow Past a Slender Delta Wing with

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