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Parameters Governing Separation Control with Sweeping Jet Actuators

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Rene Woszidlo
Abstract:
Parameters governing separation control with sweeping jet actuators over a deflected flap are investigated experimentally on a generic "Multiple Flap Airfoil" (MFA). The model enables an extensive variation of geometric and aerodynamic parameters to aid the scaling of this novel flow control method to full-size applications. Sweeping jets exit from discrete, millimeter-scale nozzles distributed along the span and oscillate from side-to-side. The sweeping frequency is almost linearly dependent on the supplied flowrate per actuator. The measured thrust exerted by a row of actuators agrees well with vectored momentum calculations. Frequency and thrust measurements suggest that the jet velocity is limited to subsonic speeds and that any additional increase in flowrate causes internal choking of the flow. Neither the flowrate nor the momentum input is found to be a sole parameter governing the lift for varying distance between adjacent actuators. However, the product of the mass flow coefficient and the square root of the momentum coefficient collapses the lift onto a single curve regardless of the actuator spacing. Contrary to other actuation methods, separation control with sweeping jets does not exhibit any hysteresis with either momentum input or flap deflection. A comparison between sweeping and non-sweeping jets illustrates the superior control authority provided by sweeping jets. Surface flow visualization on the flap suggests the formation of counter-rotating pairs of streamwise vortices caused by the interaction of neighboring jets. The actuation intensity required to attach the flow increases with increasing downstream distance from the main element's trailing edge and increasing flap deflection. No obvious dependence of the ideal actuation location on actuator spacing, flap deflection, angle of attack, or actuation intensity is found within the tested range. Comparisons between experimental and numerical results reveal that the inviscid flow solution appears to be a suitable predictor for the effectively and efficiently obtainable lift of a given airfoil configuration. The flap size affects the achievable lift, the accompanying drag, and the required flap deflection and actuation intensity. By controlling separation, the range of achievable lift coefficients is doubled without significant penalty in drag even when considering a safety margin for the maximum applicable incidence.

T ABLE OF CONTENTS

L IST OF FIGURES

................................ ...............................

8

LIST OF TABLES

................................ ...............................

11

ABSTRACT

................................ ................................ ........

12

NOMENCLATURE

................................ ..............................

14

1

INTRODUCTION

................................ ............................

17

Research Objectives

................................ ............................... 21

1.1 2

EXPERIMENTAL SET - UP AND INSTRUMENTATION

23

3

PARAMETER OVERVIEW AND DISCUSSION

............

30

4

HOW SHOULD ACTUATION BE APPLIED?

...............

38

Characterization of Actuators

................................ ................ 38

4.1

Effects of Reynolds Number

................................ .................. 48

4.2

Effects of Hysteresis

................................ ............................... 49

4.3

Quantification of Actuation Intensity

................................ .... 52

4.4

Surface Flow Visualization

................................ ..................... 58

4.5

Sweeping vs. Non - Sweeping Jets

................................ ......... 64

4.6 4.6.1

Quantitative Comparison

................................ ................................ ..

64

4.6.2

Qualitative Comparison

................................ ................................ ....

71

5

WHERE SHOULD ACTUATION BE APPLIED?

...........

78

Actuation from a Single Location

................................ .......... 78

5.1

7

TABLE OF CONTENTS

Continued

Actuation from Multiple Locations

................................ ........ 87

5.2 6

ON WHAT AIRFOIL CONFIGURATION SHOULD ACTUATION BE APPLIED?

................................ ..............

90

Comparisons between Numerical and Experimental Data

. 90

6.1 6.1.1

Cambered Flap

................................ ................................ .................

91

6.1.2

Symmetric Flap

................................ ................................ .................

98

Effects of Flap Deflection

................................ ....................

104

6.2

Effects of Flap Size

................................ ..............................

105

6.3

Effects of Angle of A ttack

................................ ...................

112

6.4 7

HOW CAN ACTUATION BE SCALED?

.....................

118

8

CONCLUSIONS

................................ ...........................

124

9

FU TURE WORK

................................ ..........................

130

REFERENCES

................................ ................................ ..

134

8

L IST OF FIGURES

Figure 1 –

Conceptual actuator designs: a) discrete je t outlets [40] , b) continuous sweeping range [39] .

................................ ................................ ................................ ............................

19

Figure 2 –

Design of Multiple Flap Airfoil (MFA).

................................ .........................

23

Figure 3 –

Various flap s izes and shapes tested with MFA model.

................................ ..

24

Figure 4 –

Complete assembly of MFA in 20% flap configuration.

................................

27

Figure 5 –

Time series

(top) & spectrum (bottom) of exemplary sweeping jet actuator, f sweep =2.2kHz.

................................ ................................ ................................ ....................

39

Figure 6 –

Sweeping frequency vs. volume flowrate per actuator for various Δz .

...........

40

Figure 7 –

Ratio of measured & calculated jet momentum vs. flowrate per actuator.

.....

43

Figure 8 –

Approximated hotwire and lo ad cell time signals.

................................ ..........

44

Figure 9 –

Calculated force ratio as a function of sweeping angle.

................................ ..

47

Figure 10 –

Flap 60B, effects of Re ,  TS =40°,  =0°, ∆z =1in.

................................ ...........

48

Figure 11 –

Flap 40A, hysteresis of changing C  ,  TS =45°,  =0°, AFC @ 7% c flap , ∆z =1in.

................................ ................................ ................................ ..............................

50

Figure 12 –

F lap 40A, hysteresis of changing  ,  TS =40°, AFC @ 7% c flap , Δz =1in.

......

52

Figure 13 –

Flap 40B, required C 

& C Q

for fixed C l =1.5,  =0°, AFC @ 4% c flap .

........

53

Figure 14 –

Flap 40B, actuation intensity for various Δz ,  =0°, AFC @ 4% c flap .

..........

54

Figure 15 –

Flap 40B, actuation intensity for various Δz ,  =0°,  TS =30°.

.......................

55

Figure 16 –

Actuation intensity for various Δz

& flap sizes,  =0°.

................................ ..

56

Figure 17 –

Surface flow visualization,  =0°,  TS =30°, baseline.

................................ ....

59

Figure 18 –

Surface flow visualization,  =0°,  TS =30°, AFC @ 0% c flap , C  =3% Δz =1in., entire flap (left), close - up view at actuator exit (right).

................................ ....................

60

Figure 19 –

Surface flow visualization,  =0°,  TS =30°, AFC @ 0% c flap , C  =3%, Δz =1in., streamwise streaks down to flap TE.

................................ ................................ ................

61

Figure 20 –

Surface flow visualization,  =0°,  TS =3 0°, AFC @ 0% c flap , C  =6%, Δz =1in., entire flap (left), close - up view at actuator exit (right).

................................ ....................

63

Figure 21 –

Flap 20A, lift vs. actuation intensity,  =0°, non - sweeping jets, AFC @ 18% c flap .

................................ ................................ ................................ ................................ ....

65

Figure 22 –

Flap 20A, lift vs. flowrate coefficient,  =0°, non - sweeping jets, AFC @ 18% c flap .

................................ ................................ ................................ ................................ ....

66

Figure 23 –

Flap 20A, hystere sis of changing C  ,  TS =40°,  =0°, AFC @ ~20% c flap , ∆z =1in.,

................................ ................................ ................................ .............................

67

Figure 24 –

Flap 20A, sweeping vs. non - sweeping jets with (C dt +C  ) min , AFC @ ~20% c flap , Δz =0.5in.

................................ ................................ ................................ ...................

68

Figure 25 –

Flap 20A, sweeping vs. non - sweeping jets with (C dt +C E ) min , AFC @ ~20% c flap , Δz =0.5in.

................................ ................................ ................................ ...................

70

Figure 26 –

Flap 20A, surface flow visualization, no f ree - stream, AFC @ ~20% c flap , entire flap, left: non - sweeping jets, right: sweeping jets, A) Δz =0.5in., Q↔C  =3% @ Re =6 ⋅ 10 5 , B) Δz =1in., Q↔C  =6% @ Re =6 ⋅ 10 5 , C) Δz =1.5in., Q↔C  =4.5% @ Re =6 ⋅ 10 5 . ................................ ................................ ................................ ................................ ...........

72

9

LIST OF FIGURES

Continued

Figure 27 –

Flap 20A, surface flow visualization,  =0°,  TS =40°, AFC @ ~20% c flap , entire flap, left: non - sweeping jets, right: sweeping jets, A) Δz =0.5in., C  =3%, B) Δz =1in., C  =6%, C) Δz =1.5in., C  =4.5%.

................................ ................................ ........

74

Figure 28 –

Flap 20A, surface flow visualization,  =0°,  TS =40°, AFC with non - sweeping jets @ ~20% c flap , Δz =1in., C  =6%, close - up view of actuator outlet.

.............

75

Figure 29 –

Flap 20A, surface flow visualization,  =0°,  TS =40°, AFC with sweeping jets @ ~20% c flap , Δz =1.5in., C  =4.5%, entire flap.

................................ ................................

77

Figure 30 –

Flap 40B, lift vs. actuation lo cation,  =0°, Δz =1in.

................................ ......

78

Figure 31 –

Flap 40B, lift vs. actuation location,  =0°,  TS =40°.

................................ ....

80

Figure 32 –

All flaps, lift vs. actu ation location,  =0°,  TS =40°, Δz =1in.

........................

81

Figure 33 –

Flap 40A, lift & total drag vs. C  ,  =0°,  TS =0°, Δz =1in.

.............................

83

Figure 34 –

Fla p 40A, C p - distributions & wake profiles,  TS =0°,  =0°, C  =6%, Δz =1in.

85

Figure 35 –

Flap 40B, lift vs. C  (100 - x act /c flap ) 0.5 ,  =0°, Δz =1in.

................................ .....

86

Figure 36 –

Flap 40B, lift vs. C 

& C Q ,  =0°,  f =40°, Δz =1in.

................................ ........

88

Figure 37 –

Flap 40B, lift and moment vs.  TS ,  =0°, AFC @ 4% c flap , ∆z =1in.

.............

91

Figure 38 –

Flap 40B, drag vs.  TS ,  =0°, AFC @ 4% c flap , ∆z =1in.

...............................

92

Figure 39 –

Flap 40B, C p - distributions,  TS =20°,  =0°, AFC @ 4% c flap , Δz =1in.

..........

94

Figure 40 –

Flap 40B, C p - distributions & wake profiles,  TS =30°,  =0°, AFC @ 4% c flap , Δz =1in.

................................ ................................ ................................ ..............................

97

Figure 41 –

Flap 40B, C p - distributions,  TS =4 0°,  =0°, AFC @ 4% c flap , Δz =1in.

..........

98

Figure 42 –

Flap 40A, lift and moment vs.  TS ,  =0°, AFC @ 7% c flap , ∆z =1in.

.............

99

Figure 43 –

F lap 40A, drag vs.  TS ,  =0°, AFC @ 7% c flap , ∆z =1in.

.............................

100

Figure 44 –

Flap 40A, C p - distributions,  TS =0°,  =0°, AFC @ 7% c flap , Δz =1in.

..........

101

Figure 45 –

Flap 40A, C p - distributions,  TS =20°,  =0°, AFC @ 7% c flap , Δz =1in.

........

102

Figure 46 –

Flap 40A, C p - distributions,  TS =40°,  =0°, AFC @ 7% c flap , Δz =1in.

........

103

Figure 47 –

Flap 40A, Δ C l

vs. C  ·(100 - x act /c flap ) 0.5 /  TS

(left, Δz =1in.) & (0.5C  ) 0.5 ·C Q ·(100 - x act /c flap ) 0.5 /  TS

(right, all Δz ),  =0°.

................................ ..................

104

Figure 4 8 –

All flaps, differential lift improvements vs. C 

& C  ,flap ,  TS =40°, Δz =1in.

106

Figure 49 –

Flap 60B & 40B, best performance based on (C dt +C  ) min

vs. C l,des , best location , Δz =1in.

................................ ................................ ................................ .............

108

Figure 50 –

Flap 40A & 20A, best performance based on (C dt +C  ) min

vs. C l,des , best location , Δz =1in.

................................ ................................ ................................ .............

109

Figure 51 –

All flaps, maxim um performance coefficients vs. Δz ,  =0°, best actuation locations.

................................ ................................ ................................ .........................

111

Figure 52 –

Flap 40A, best performance based on (C dt +C  ) min

for all  - sweeps vs. C l,des , AFC @ 7% c flap , Δz =1in. ................................ ................................ ................................ ..

114

Figure 53 –

Flap 40A, best performance based on [(C dt +C  ) min ] safe

for all  - sweeps with 20% safety margin in C l,max

vs. C l,des , AFC @ 7% c flap , Δz =1in.

................................ .....

115

10

LIST OF FIGU RES

Continued

Figure 54 –

Flap 40A, best performance based on (C dt +C E ) min

for all  - sweeps vs. C l,des , AFC @ 7% c flap , Δz =1in. ................................ ................................ ................................ ..

117

11

L IST OF TABLES

Table 1 –

Tested parameters for various flap configurations.

................................ ..........

32

Table 2 –

Flap 20A, maximum efficiency comparison, sweeping vs. non - sweeping jets.

71

12

A BSTRACT

Parameters governing separation control with sweeping jet actuators over a deflected flap are investigated experimentally on a generic “Multiple Flap Airfoil” (MFA). The model enables an extensive variation of geometric

and aerodynamic paramet ers to aid the

scaling of this novel flow control method to full - size

applications.

S weeping jets exit from discrete, millimeter - scale nozzles distributed along the span and oscillate

from side - to - side.

The sweeping frequency is almost linearly dependent o n the supplied flowrate per actuator. The measured thrust exerted by a row of actuators agrees well with vectored momentum calculations. Frequency and thrust measurements suggest that the jet velocity is limited to subsonic speeds and that any additional i ncrease in flowrate cause s

internal choking of the flow.

Neither the flowrate nor the momentum input

is found to be a sole parameter governing the lift for varying distance between adjacent actuators. However, the product of the mass flow coefficient and the

square root of the momentum coefficient collapses the lift onto a single curve regardless of the actuator spacing. Contrary to other actuation methods , separation control with sweeping jets does not exhibit

any hysteresis with either momentum input or flap deflection .

A comparison between sweeping and non - sweeping jets illustrates the superior control authority provided by sweeping jets.

Surface flow visualization on the flap suggests the formation of counter - rotating pairs of streamwise vortices caused

by the interaction of neighboring jets.

13

The actuation intensity

required to attach the flow increases with increasing downstream distance from the main element’s trailing edge

and increasing flap deflection. No obvious dependence of the ideal actuation l ocation on actuator spacing, flap deflection, angle of attack, or actuation intensity is found within the tested range . Comparisons between exp erimental and numerical results reveal that the inviscid flow solution appears to be a suitable predictor for the

effectively and efficiently obtainable lift

of a given airfoil configuration .

The flap size affects the achievable lift, the accompanying dra g, and the required flap deflection

and actuation intensity. By controlling separation ,

the range of achievable li ft coefficients is doubled without significant penalty in drag even when considering a safety margin for the maximum applicable incidence.

14

N OMENCLATURE

A flap

=

total flap area

A nozzle

=

exit area of one nozzle

A ref

=

reference area

b

=

airfoil/wing span

c

=

total chord length

c flap

=

flap chord length

C dt

=

total drag coefficient

C E

=

input power coefficient

C l

=

lift coefficient

C l ,des

=

desired lift coefficient

C p

=

pressure coefficient

C Q

=

volume flowrate coefficient

C 

=

momentum coefficient based on total chord

length

C  , flap

=

momentum coefficient based on flap chord

length

F calculated

=

calculated total jet momentum

F jet

=

total theoretical jet momentum

F measured

=

measured jet momentum in x - direction

F M

=

figure of

merit

F +

=

reduced frequency

f sweep

=

sweeping frequency

J

=

total jet momentum

15

NOMENCLATURE –

Continued

L

=

lift force

L/D

=

lift - to - drag ratio

=

mass flowrate

n

=

total number of actuators per row

P

=

input power for actuation

P atm

=

atmospheric press ure

P cal.

=

calibration pressure of flow meter

P gauge

=

gauge pressure at flow meter

Q

=

pressure corrected volume flowrate

Q ind.

=

indicated volume flowrate on flow meter

Re

=

Reynolds number

t

=

airfoil thickness

U jet

=

jet velocity

U inf

=

free - stream ve locity

x

=

streamwise direction

x act

=

actuation location on the flap

x sep

=

separation location of baseline flow

y

=

direction normal to the free - stream

z

=

spanwise direction

∆z

=

spanwise spacing between adjacent actuators

=

angle of attack

16

NOMENCLATURE –

Continued

 jet

=

jets’ exit angle to local surface

 f

=

flap deflection angle

(between main element chord

and flap chord )

 TS

=

surface deflection angle ( between main eleme nt top surface and flap top surface )

 sweep

=

half - angle of the jet’s sweeping range

 C 

=

momentum

efficiency

 C E

=

power efficiency

=

free - stream kinematic viscosity

 inf

=

free - stream density

 jet

=

density of jet at exit

Subscripts

max

=

maximum val ue

min

=

minimum value

safe

=

value within a prescribed safety margin

17

1

I NTRODUCTION

The ability to passively and actively manipulate a flow field has be come one of the most researched topics in fluid mechanics since Prandtl’s first boundary layer control

experiments in 1904 [ 1 ] . The list of possible applications for

passive and active flow control is almost endless

(e.g., increase in aerodynamic performance

of lifting bodies , delay of laminar - to - turbulent transition, noise redu ction, enhanced combustion, increase in aircraft speed, weapon applications, maneuverability of aircrafts , etc.) .

Especially over the last decades the need to reduce the fuel consumption and emission of greenhouse gases from commercial and military aircraf ts has been

driven by economic challenges and growing concerns regarding climate change. A number of approaches to reduce an aircraft’s fuel consumption are being pursued including lightweight materials, design optimizations, and engine improvements, just to name a few. In particular, the control of flow separation from internal and external aerodynamic surfaces has gained signifi cant interest as summarized by the proceedings of the biennial AIAA Flow Control Conferences which started in 2002. A multitude o f passive and active me ans

to control flow separation have been proposed and investigated. Some of the most common approaches involve vortex generators [ 2 , 3 ] , miniature trailing edge devices [ 4 ] , continuous and oscillatory blowing

and

suction [ 5 - 9 ] , microjets [ 10 - 13 ] ,

and zero - net - mass - flux actuation (e.g., plas ma actuators [ 14 ]

and synthetic jets [ 15 - 23 ] ) . T hese methods and their feasibility for full - scale applications have been reviewed and evaluated by numerous authors [ 24 - 30 ] . The various methods entail different underlying mechanisms

to control flow separation and therefore, they inher i t specific advantages and disadvantages. For example, passive

18

control devices can be effective in controlling the flow but add parasitic drag when not needed . Active control methods, such as

continuous blowing and suction , can

energize the boundary layer by injecting high momentum fluid into the boundary layer or removing low - momentum flui d from the boundary layer. Although these methods are effective

due the significant momentum input , the benefits often do not outweigh the high flowrates required. A different approach is provided by synthetic jets which do not require

any source of mass f low. T he oscillatory control inten d s

to amplify natural instabilities in the shear layer of the separated flow to enhance mixing with the outer flow region and thereby energize the boundary layer. However, limitations in momentum output ,

and the complexity

and weight of the equipment leave the effectiveness of this method questionable for full - scale applications [ 7 ] . In order to overcome this limitation and maintain the advantageous effects of oscillatory control ,

oscillatory blowing (with and without suction) has been investigated and shown to be an effective mean s

to control flow separation [ 5 , 6 ] . However, the required mechan isms

and complex system integration remain disadvantages of this method. An alternative to controlling the spanwise vorticity ,

as intended by

aforementioned control methods, is offered by discretely distributed, high velocity jets [ 10 - 13 ] . These jets

reduce the required bleed air because they

not only control flow separation by the inserted momentum but mostly

by enhancing the mixing with the outer flow through streamwi se vortices [ 11 , 12 ] . Although this method has been proven to be effective in controlling separation and

reduc ing

the required flowrates, the spacing between adjacent jets remains to o

small in order to be eff icient . T herefore ,

a further reduction in flowrate is limited. Consequently , an effective ,

but simple

and practical

19

actuation method involving high velocity jets at a larger spacing and the advantageous effects of an oscillatory nat ure is desired. These requirements are met by fluid ic

oscillators or so - called sweeping jet actuators.

Figure 1

Conceptual actuator designs: a) discrete jet outlets [ 40 ] , b) conti nuous sweeping range [ 39 ] .

S weeping jet actuators emit a continuous but spatially oscillating jet when supplied with fluid under pressure. The oscillations do not involve any moving parts and are solely based on the internal fluid dynamics by incorporating a fluidic feedback loop into the actuator design ( Figure 1 ). As the fluid passes through the actuator, the bistable effect of fluid entrainment from a limited space forces the jet to at tach to either wall of the chamber (i.e., the Coanda effect) thereby delivering a signal through the feedback channel .

The interaction at the inlet of the chamber causes the jet to separate from that side and attach to the opposite side of the chamber wher e the process is reinitiat ed . The jet’s sweeping frequency can vary from the order of 10Hz [ 31 , 32 ]

to the order of 20kHz [ 33 ]

depending on the actuator’s geometry, size,

and supplied flowrate. Although discovered more than fifty years ago at the Harry Diamond Research Laboratories, these devices have mostly

been

operated

with liquids as a working fluid [ 34 , 35 ]

(e.g., windshield washers, sprinklers, shower heads, Jacuzzis, etc.). Only in recent years has

this concept

been

applied in conjunction with active flow control (e.g., separation [ 36 , 37 ] , noise [ 38 , 39 ] , and a)

b )

20

combustion control [ 40 ] ). The characteristics of v arious a ctuator types ,

conceptually similar to the designs shown in Figure 1 ,

have been investigated experimentally [ 31 - 33 , 41 - 48 ]

and numerically [ 48 - 52 ]

by numerous researchers . Several a ctive and passive concepts to provide the oscillatory character of the

emitted jet

have been proposed , including self - induced

feedback [ 43 ]

and

external control [ 42 , 44 - 47 ]

designs. Early actuat ors

for flow control used a splitter in the outlet nozzle [ 36 , 40 , 55 ] , which f orced the jet to exit through discrete outlets ( Figure 1 a) instead of having a continuous sweeping range

( Figure 1 b ) .

R emarkable results on separation control [ 37 , 53 , 54 , 56 - 60 ]

were also achieved with fluidic oscillators

providing contin uous sweep from side to side as illustrated in Figure 1 b

which conceptually represents the design of the actuators used in th is

study. They are small (order of millimeters) and driven by a high - pressure air source that enable the jet to oscillate in spanwise direction at large angles relative to the axis of the nozzle and at frequencies of the order of kHz. Therefore, reduced frequencies of O (10)

and higher are achieved which circumvent the common wisdom of F + = O (1) [ 19 ] . DeSalvo et al. [ 60 ]

argue that high frequency actuation may be beneficial because it decoupl es the actuation method from the dominant frequencies of the separated flow . Wygnanski [ 19 ]

and Nagib et al . [ 7 ]

indicated that the ideal frequency for periodic excitation is dependent on incidence and flap deflection due to the changing size of coherent

structures in the separated flow regime. Therefore, low - frequency actuation requires detailed knowledge of the natural flow patterns under various conditions which makes it a challenging task to optimize this actuation method. This difficulty may

be avoid ed by high - frequency actuation if it can provide similar effectiveness.

21

In r ecent

years , significant aerodynamic improveme nts were achieved with the type of actuators

shown in Figure 1 b by controlling flow separation over the fla p of several

two - dimensional (NACA 0015 [ 2 , 53 , 54 ] , NACA 0021 [ 56 ] , V - 22 airfoi l [ 37 ] , diffuser model [ 36 ] , DU96 airfoil [ 55 ] , and ADVINT airfoil [ 58 - 60 ] ) and three - dimensional models (finite wing

and 1/10 th

scale ,

powered V - 22 model [ 37 ] , and a  - wing configuration [ 57 ] ) . The

effe ctiveness of these devices is exemplified by separation control on various V - 22 configurations [ 37 ]

at moderate inputs of mass and m omentum . The lift - to - drag ratio of a finite wing/nacelle configuration was incre ased by 60% and the download - to - thrust ratio of a 1/10 th

scale ,

powered V - 22 model was reduced by 30% which is superior

to the

17% reduction achieved with earlier

actuation methods [ 23 ] .

Research Objectives

1.1 Alth ough the effectiveness of sweeping jet actuators has been convincingly demonstrated, most of the parameters involved were chosen intuitively or from limited experience with other actuation methods. Therefore, this work presents a parametric study to establ ish parameters governing separation control over the

flap

of a generic “Multiple Flap Airfoil”

(MFA) with sweeping jet actuators and thereby aid the scaling of these devices to full - size applications. After characterizing the properties of the particular a ctuators used ,

their performance on controlling flow separation is evaluated in conjunction with numerous parameters (e.g., flap deflection, size and geometry, incidence, actuation location, spacing between adjacent actuators, actuation intensity,

sweeping

frequency, free - stream velocity, etc.). Although no detailed investigation of the flow field is

conducted, the underlying mechanisms responsible for

the effective control

22

of

flow

separation with sweeping jet actuators are discussed based on surface flow v isualization experiments and other implicative observations. Limitations in effectively and efficiently achievable lift obtained with this actuation method

are assessed by comparisons to potential flow solutions. Due to typically

small levels of momentum t he improvements obtained with other actuation methods are

often limited by the lift expected from incidence and camber [ 7 , 19 , 27 ] . However, s ince sweeping jets are emitted at high velocity and frequency, supercirculation based on the jet flap principle [ 61 - 65 ]

Full document contains 141 pages
Abstract: Parameters governing separation control with sweeping jet actuators over a deflected flap are investigated experimentally on a generic "Multiple Flap Airfoil" (MFA). The model enables an extensive variation of geometric and aerodynamic parameters to aid the scaling of this novel flow control method to full-size applications. Sweeping jets exit from discrete, millimeter-scale nozzles distributed along the span and oscillate from side-to-side. The sweeping frequency is almost linearly dependent on the supplied flowrate per actuator. The measured thrust exerted by a row of actuators agrees well with vectored momentum calculations. Frequency and thrust measurements suggest that the jet velocity is limited to subsonic speeds and that any additional increase in flowrate causes internal choking of the flow. Neither the flowrate nor the momentum input is found to be a sole parameter governing the lift for varying distance between adjacent actuators. However, the product of the mass flow coefficient and the square root of the momentum coefficient collapses the lift onto a single curve regardless of the actuator spacing. Contrary to other actuation methods, separation control with sweeping jets does not exhibit any hysteresis with either momentum input or flap deflection. A comparison between sweeping and non-sweeping jets illustrates the superior control authority provided by sweeping jets. Surface flow visualization on the flap suggests the formation of counter-rotating pairs of streamwise vortices caused by the interaction of neighboring jets. The actuation intensity required to attach the flow increases with increasing downstream distance from the main element's trailing edge and increasing flap deflection. No obvious dependence of the ideal actuation location on actuator spacing, flap deflection, angle of attack, or actuation intensity is found within the tested range. Comparisons between experimental and numerical results reveal that the inviscid flow solution appears to be a suitable predictor for the effectively and efficiently obtainable lift of a given airfoil configuration. The flap size affects the achievable lift, the accompanying drag, and the required flap deflection and actuation intensity. By controlling separation, the range of achievable lift coefficients is doubled without significant penalty in drag even when considering a safety margin for the maximum applicable incidence.