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
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