1

Fundamentals and Applications of UnsteadySuction / Blowing for Boundary Layer Control

Avraham “Avi” Seifert

School of Mechanical EngineeringFaculty of Engineering, Tel-Aviv University, ISRAEL

Acknowledgment:Arwatz, Yehoshua, Stalnov, Shtendel, Dolgopyat, Shay, MaromGordon, Meadow, IMOD, US Army, EU FP7 Funds (AFLONEXT)

Oscillatory

GDR, Lyon, Nov. ‘16AeroVehicles2,Chalmers, ‘16

2

Background Motivation: Lack of design tools Modified Unsteady Integral BL Eq. Limiting cases of the MUIBL Eq. Experimental Examples SaOB actuation interaction with LBL SaOB application to thick turbulent airfoils Conclusions and Outlook

Talk Outline

3

Know your actuator Work in its most efficient point Use multiple actuators Space them optimally Utilize flow instability Create multiple vorticity modes Understand the flow physics Consider the IBL Equations Actuator comparisons Conclusions

Enhancing Efficiency

4

Motivation – AFC for BLC Active control of boundary layer

separation can lead to:– Aeronautical benefits: Clmax, L/D, D, M, Y– Efficient, short drag reducing devices– Control of bluff body wake– Reduced sensitivity to side winds– Reduced sensitivity to unsteady winds– Cab – Trailer – Gap penalties

AFC system efficiency:key enabling drag reduction factor

How can efficiency be improved? Focus on:

– Actuation systems– Actuator – Actuator, &– Actuation – BL interactions

Vibrating LE ribbon

Cl

Cµ(Seifert et al, 1996)

(Neuburger, ’89)

Modified UIBL Eq.• 2D steady Von Karman BL Momentum Integral Equation (1921):

• A modified version is proposed; unsteady with AFC Effects:

• Unsteady wall AFC BC’s:

• A is a velocity scale• is the wall transpiration direction, 0 ≤ ≤ 2 ,• Frequency is = 2f• is phase distribution.• B is the Steady offset velocity factor , -1<B<1.• Fx is an AFC body force (e.g., Plasma, de Oliveira, 2015, MHD and more)• All functions of time and space(van Rooij, 1996, Sarimurat and Dang, 2014, Seifert, 2017)

= 2 − − (2 + ) + − −

= 2 − (2 + )

= + + = +

Modified UIBL Eq. – Limiting cases (1)• BL on verge of separation• Negligible skin friction• Constant H and U• Neglecting body force AFC

• Assuming that: ≫• UIBL equation: = 1 −• Purpose: < near slot

• We note:• Suction is significantly moreeffective than blowing

• Especially using low magnitude• Where ≪• Experimental evidence?

(Seifert and Pack (2002), Gluauert “Hamp”)

Modified UIBL Eq. – Limiting cases (2)• BL on verge of separation• Negligible skin friction• Constant H and U, neglecting body force AFC

• Assuming that: ≫• UIBL equation: = 1 −• Purpose: < near excitation slot

• We note:• For blowing to become effective,its wall tangential,downstream directed component,must be larger than thefree-stream velocity, >

• Experimental evidence?

• ≡ ⁄ ⁄ =2*0.005>1% (Seifert et al, 1996, NACA 0015)

Modified UIBL Eq. – Limiting cases (3)• Large amplitude cases, with ≫ 1, and relatively high frequency

• Flow unsteadiness will cease to be important,• Skin friction greatly increases, due to wall jet,• Therefore cannot be neglected• Large Cf always reduces AFC effectiveness.• Therefore, the need for distributed actuation that will minimize Cf “overshoot”,

• We are left with the following Eq.:

• Unsteady wall AFC BC’s:

= 2 += + +

(Seifert et al, 1996, NACA 0015)

Modified UIBL Eq. – Limiting cases (4)• When using low magnitude slot suction

• UIBL equation reduces to:

• Purpose: < near slot

• Steady, wall normal, low magnitude, suction most effective• Pulsed suction has great potential• Using ZMF excitation it was found:• Strouhal ~ 1 is optimal• What about Pulsed Suction?

(Seifert et al, 1996, NACA 0015)

Pulsed Suction - Experimental Setup - Airfoil

10

• GOE-222 airfoil section• Chord length of 165mm• Max thickness 18.5% at

29.2% chord.• Actuator ports of 1mm

diameter with 10mmspacing, located at 10%chord length

•Actuator Ports

(Morgulis and Seifert, Wind Energy, 2015)

AFC – Pulsed Suction

A series of measurements atAoA=10deg, 100k<Re<400k

Oscillatory Suction enhanced theeffect of steady suction by up to 40%

Optimal frequency F+=0.611

-800

-600

-400

-200

0

200

0 0.02 0.04 0.06

Inle

t Pr

essu

re [P

a]

t[sec]

Pulsed Suction

V

cfF

*

drag

lift

(Morgulis and Seifert, Wind Energy, 2015)

12

Conclusions

Approaches to increase AFC system Efficiency suggested A modified version of the Unsteady Integral Boundary

layer equations was introduced Helps understand experimentally determined data trends Should enable 3D unsteady low order design tool Steady and Unsteady suction are very effective Actuators for pulsed suction are being developed

Steady Suction and Oscillatory BlowingInteraction with a Laminar Boundary Layer

Liad Marom and Avi SeifertMeadow Aerodynamics Laboratory

School of Mechanical Engineering, Faculty of EngineeringTel Aviv University, ISRAEL

May 2015 1

The SaOB Actuator Setup

2

U∞

PB Slot Width ~ */2

Steady suction results, Us/U=0.5m, d~*

May 2015 3

• Single suction hole generates stabilizing effect directly down streamfrom hole and de-stabilizing effects at the sides

• Two suction holes, 15mm (~4d) are too far apart• Staggered lines of suction holes are more beneficial, more uniform

Oscillatory Blowing (OB) Set-up

May 2015 4

0

5

10

15

20

25

30

-40 -20 0 20 40

Vmea

n [m

/s]

Z [mm]

side rowupper row

* 7 suction holes on the box side, outside tunnel, allow flow to enter the box

020406080

100120140160

0 1 2 3 4

FBT

freq

[Hz]

Pressure supply [Psi]

side rowupper rowWorking Point

This entire test

Y Profile of Oscillatory Blowing at Nozzle

5

X=548mmPin0.30.02 Psif=36.4±0.2 Hz

Z=0

Uinf

Measurements performed right at the blowing nozzle, no suction

Y Profile of OB at Nozzle

6

Pin0.30.02 Psif=36.4±0.2 Hz

Z Profile of OB Downstream from Nozzle

7

Pin0.30.02 Psif=36.4±0.2 Hz

U

Fingers structure emerging

Pulsed Blowing Phase-Locked U(t)

8

Tear-drop shaped jet due to curving of the jet De-stabilized region

9

Lift of the flowdue to counter

clockwiserotating vortex

SaOB Interaction with Laminar BL

10

Uinf

Y=1mm, f=36Hz

Jet is mush fuller and closer to the floor

Mean

Fluctuaion

SaOB Velocity Surfaces at Y=1mm

SaOB with 14 suctionholes in 2 staggeredrows, more uniform butIncrease of ~93% inskin friction estimate(above Baseline)

PB OnlySkin friction increase by~73% downstream fromnozzle, relative toBaseline BL

SaOB with 7 suction holesjet more tangential to the surfaceand more robustSkin friction increase by ~116%downstream from nozzle, relativeto BL with PB only

No Suction

One Suction Row

Two Suction Rows(Staggered)

SaOB surfaces at Y=4mm

Uniformstabilizing effectdue to theadded upstreamsuction holeprevents fingerseparation

Separations offinger due to strongfluctuations fromupstream suctionholes

Strong velocitygradient generatesstrong shear stressand highfluctuations

SuctionAlone(+51%)

SuctionAlone (+76%)

Added suction effect causes the Y profileto be much fuller (even away from C.L.)

May 2015 13

Suction Effect

One row of 7 suction holesOB slot at X=548mmExpected more robust separation delay

Summary

14

• A stabilizing effect directly downstream of the suction holes

• De-stabilizing effects on the downstream sides of the holes

• Both effects are beneficial for separation delay

• Staggered lines of suction holes create more effective and uniform effect

• Pulsed blowing alone produces unsteady spanwise and streamwise flow

field patterns which can be characterized by 2-3 finger-like structures

• Laminar-turbulent transition is promoted at the edges of the pulsed jets

• Adding Suction causes the jet to be more tangential to the surface and to an

increase of ~116% in skin friction (estimate from near wall velocity)

• Suction holes location and number is crucial for fingers’ separation and

degree of uniformity

High-EfficiencyActive Flow ControlledWind Turbine Airfoils

Avraham (“Avj”) SeifertFriedland, Dolgopyat and Shig

Meadow Aerodynamics laboratory

School of Mec. Eng.Tel Aviv University, ISRAEL

Talk Content

• Technology: Active flow control (AFC) for wind turbine blades• Current R&D:

– Wind tunnel experiments on Thick (31%, root) and– Medium (25%, mid blade) wind turbine blade sections

• Blade conditions: clean, contaminated, low speed, AFC• Models accounting for AFC effects developed (Qblade based)• Method for use of AFC for wind turbines defined• Next step: Apply to complete turbine in the field

The Technology: AFC

• Active flow control (AFC) uses small Piezoelectric or Fluidicactuators embedded within the turbine blades

• Inject small-scale finite circulation vortices into the flow• An appropriate choice of the vortices’ locations and scales

can prevent flow separation• Reattach separated flow• The effects of AFC surpass and

maintain also in fully turbulent flow

4

Experiment – Models:AH93-300 Airfoil

-120

-60

0

60

120

0 40 80 120 160 200 240 280 320 360 400 440 480X [mm]

Y [m

m]

P-taps

Slot 1 Slot 2 Slot 3 Slot 4 Slot 5

HF1HF2,3 HF4,5

HF6,7

HF8,9

End3End1End2

End5

End3

End4

Roughnes

AH-31%Root

section

DU-25%, mid blade section

AH93-300 Airfoil

-120

-60

0

60

120

0 40 80 120 160 200 240 280 320 360 400 440 480X [mm]

Y [m

m]

P-taps

Slot 1 Slot 2 Slot 3 Slot 4 Slot 5

HF1HF2,3 HF4,5

HF6,7

HF8,9

End3End1End2

End5

End3

End4

Roughnes

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028 0.03Cd Betz

Cl

CleanContaminatedContaminated + AFC

11/29/2016 48th IACAS 5

Results - Recovery of Lift and Drag – SJA 3 Rows

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-6 -3 0 3 6 9 12 15Alfa [deg]

CL

Clean

Contaminated

Contaminated + AFC

Lift

Lift

Drag

Incidence0.30.40.50.60.70.80.9

11.11.21.31.41.51.61.71.8

4 5 6 7 8 9 10 11 12 13 14

Alfa [deg]

AF

M1

2nd and 3rd slots at PSAFM1

Incidence

baseline

ac

c

DL

PDULU

AFM)(

1

SaOB AFC of t/c=31% Airfoil

6

• MSc Student: D. Sarkorov, AFLONext EU Project

• Clean AH93-300 airfoil installed in TAU Knapp-Meadow Wind Tunnel

• t/c=0.31, 12 Synchronized SaOb actuators at x/c~0.2, 96 press taps, wake rake

• Reynolds numbers 0.3-1.5x106

Actuator array close-up view

Flow Direction

05

10152025303540

0 2 4 6 8 10 12 14 16 18 20 22

Uje

t[m

/s]

Dist. box pressure [psi]

Open SuctionOne Suction RowTaped Suction

Actuators Array Bench-top Calibration

7

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35 40 45

U/U

max

Z [mm]

2.6 [psi]5.58 [psi]8.6 [psi]11.62[psi]14.6 [psi]

0

50

100

150

200

250

300

0 2 4 6 8 10 12 14 16 18 20 22

Freq

. [Hz

]

Dist. box pressure [psi]

Open suctionOne suction rowTaped Suction

Mean P.B. Velocity Oscillation Freq.

02468

1012141618

0 2 4 6 8 10 12 14 16

Usu

c [m

/s]

Dist. box pressure [psi]

One Suction Row

Open Suction

Mean Suc. Velocity

Normalized jet profile of singleactuator in still air

Measured in still air outside tunnel with hot-wires

8

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16

Cl

Alpha [deg]

Baseline

AFC Cmiu=0.0045

AFC Cmiu=0.0069-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

Cl

Cd Betz

Baseline

AFC Cmiu=0.0045

AFC Cmiu=0.0069

Effect of AFC on Lift and Drag

Lift and Drag at Re=1M, clean airfoil

25% lift increase, significant stall delay

10% increase in min drag

Effect of AFC Magnitude on Lift and Drag

9

• Relative change in Lift (left side) and Drag (right side) at =10• Reynolds number indicated in legend• Combined pulsed blowing and suction momentum coefficients• Scales lift but not drag, CµRe1/2 scales drag better

-0.1

0

0.1

0.2

0.3

0.4

0 5 10 15 20

ΔCl/

Cl_b

ase

Re^1/2(Cmiu+Csuc)

ΔCl% Re500K

ΔCl% Re700K

ΔCl% Re1M

ΔCl% Re1.5M-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 5 10 15 20

ΔCl/

Cl_b

ase

Re^1/2(Cmiu+Csuc)

ΔCdp% Re500KΔCdp% Re700KΔCdp% Re1MΔCdp% Re1.5M

Drag Reduction

j

jsucsuc

Ubc

UACsuc 2

2

_

i

ibi

Ubc

UAC 2

2

_

-0.050

0.050.1

0.150.2

0.250.3

0.350.4

0 0.005 0.01 0.015 0.02 0.025

ΔCl/

Cl_b

ase

Cµ+Csuc

ΔCl% Re500KΔCl% Re700KΔCl% Re1MΔCl% Re1.5M

Lift increment

10

Aerodynamic Figure of Merit

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.005 0.01 0.015 0.02 0.025

AFM

1

Cmiu+Csuc

AoA 10 deg

Re500KRe700KRe1MRe1.5M

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.005 0.01 0.015 0.02 0.025AF

M1

Cmiu+Csuc

AoA 14 deg

Re500KRe700KRe1MRe1.5M

Higher Efficiency at larger incidence and higher Reynolds numbers

The overall performance improved up to 27% at Cµ < 0.005

b

b

afcc

c

DL

PUDULAFM

1 AFC ON ONLY when AFM1>1

11/29/2016 11

AFC Devices - SJAPiezo Fluidic (Synthetic jet) ActuatorA cavity, two disks, Piezo material, 1st mode oscillationVolume change pressure change slot ZMF vorticesTwo coupled actuators, 10mm apart, 1.5 by 40mm slotsEject periodic vortex rings tangential to surfaceStrong interaction between devicesindividual Magnitude and Phase control3D “printed” Assembly bolted to airfoil

(Mohseni and Mittal)Piezo Fluidic Actuator

Inletpressure

<

SaOB Actuator Operation Principles

Ejector (suction pump) Bi-stable Fluidic Oscillator

(Marom, 2015)

Feedback tube

11/29/2016

13

AFC Devices - SaOBSaOB=Suction and Oscillatory BlowingInterface of the SaOB actuator with the Aerodynamic surface• 14 Suction holes, d=2mm each, 6mm distance, staggered• Interfaced with SaOB suction holes through a shallow cavity• Pulsed blowing slots connect to constant area nozzles• Nozzles eject oscillatory blowing tangential to surface, 1.5mm by 20mm• Feedback and Synchronization adapter, Synch Operation in an array• 3D “printed” Interface unit bolted to airfoil

Wind turbine blade Airfoil WithPiezo Electric and fluidic actuators

Suction and OscillatoryBlowing Actuator

Feedback andSynchronization

adapter

11/29/2016

14

AFC Devices – Instrumented AirfoilFour Arrays of AFC Actuators

• Every 20%c, from 20%c downstream• Staggered configuration• Besides 5th row (at x/c=0.8) only SJA due to space limitation• Forces and moments from: X and Z pressure Taps, wake rake• Sensors: Hot-films, unsteady pressures and Preston tubes

Wind turbine blade Airfoil WithPiezo Electric and fluidic actuators

Suction and OscillatoryBlowing Actuator

PiezoFluidic

Actuator

15

Fluidic Figure of Merit

Still air operation SaOB weight - 15 gm Control 10cm span Thrust = suction+blowing Note: PFM Log scale

Plasma: Kreigseis et al,JAP, 2013

Actuator Conversion Efficiency

Fa

p

P

FUFFM

0.0001

0.001

0.01

0.1

1

0 30 60 90 120 150 180 210 240

ETA

Up[m/s]

SOBXV-15 - EMPZE, NaimPZE, TimorPZE, Y&SPlasma - Kregseis

F

Actuators Bench-top TestsPiezo Fluidic (top right chart):Helmholtz f~1kHzPeak velocity of 25m/s per 1Watt.40m/s per 2.5 WElectricity driven, Max 50m/sSaOB (low right chart):Ub=24+Us(=13m/s) per 2.5 W (~same)Ub easy surpass 80-90m/sUs=Ub/2Frequency increases with Ub

Pressure driven

16

0

5

10

15

20

25

30

35

40

45

50

0 1 2 3 4 5 6 7

Up

[m/s

]

Power [Watt, 4 disks, 2 cavities]

0102030405060708090

0 10 20 30 40 50 60 70 80 90

U pb

U s

[m/s]

Input fluidic power [Watt]

Uj,pb

Usuc

Peak Velocity ofZero mass flux jet2 slots

Mean Pulsed BlowingVelocity

SuctionVelocity

11/29/201617

Baseline Airfoil Performance• Range of Re=0.4-1.5x106

• Clean, with all AFC open or taped over

-1-0.8-0.6-0.4-0.2

00.20.40.60.8

11.21.4

-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

Cl

AoA [deg]

Re=1mclose

Re=1mopen

11/29/2016

18

AFC Effect on Clean Blade Section• Data acquired at Re=1M• Using all actuators• At same TOTAL Cµ=0.001• Red line indicates optimal combination of

actuation• Lift increases from 2-4 deg AoA• Drag decreases only above AoA=6 deg

Increased Lift

Reduced DragAFC

Increased lift

0.25

0.5

0.75

1

1.25

1.5

0 5 10 15 20 25

Cl, L

ift C

oeff

.

[], Angle of attack

Re=1.2x106, Cµ=0.001

AFC

close baseline

11/29/2016

19

AFC Effect on Clean Blade Section

• Thick, turbulent airfoil, stalls from TE• With distributed AFC• Maximum lift was significantly increased

with AFC• Drag decreased• Size, speed effects well understood

INcreasedLift

Reduced Drag

AFC

Conclusions

• Application of AFC for Wind Turbines Airfoils:• Two kinds of actuator arrays• Applied to root and mid-radius airfoils• Considering energy efficiency, the two actuation concepts are

comparable• SaOB significantly more robust• With AFC performance can be improved 5-20%• Start-up velocity can be significantly (50%) reduced• Robust operation at turbulent conditions• Still, many application challenges exist