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Engineering, Test & Technology
Boeing Research & Technology
Application of Active Flow Control for Download
Alleviation in Rotorcraft
Y. Yadlin, A. Shmilovich and R. Narducci
The Boeing Co.
1
14th Overset Grid Symposium
College Park, MD
October 1-4, 2018
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The problem:
▪ High download/drag for tiltrotor airplane in hover
The solution:
▪ Flap deflection
oReduced frontal area
oReduced download force
o Flow separation
Apply Fluidic Oscillators
2
Introduction
thrust
Apply AFC to reattach the flow:
Zero-mass-flux
Weight
vibrations
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Outline
3
o Introduction
o Numerical Method
o Computing Strategies for Wing in Hover
2D Simulation
Infinite Wing
o Modeling an Isolated Fluidic Oscillator
Low pressure input
High pressure input
Calibration
o Simulation of Wind Tunnel Model
Computing Jets Tares
Active Flow Control
o Fluidic Oscillator
o Traverse Actuator
o Summary
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• OVERFLOW – NASA developed with Boeing’s modifications
• Unsteady RANS
• Overset grid system
• Upwind scheme
• Multiple turbulence models available
• Time-varying boundary conditions
• Domain Connectivity
• PEGASUS
• Domain Connectivity Function (DCF)
• X-ray method for hole cutting
• Off-body Cartesian grids
• Geometry Manipulation Protocol (GMP) files
4
Numerical Method
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Outline
5
o Introduction
o Numerical Method
o Computing Strategies for Wing in Hover
2D Simulation
Infinite Wing
o Modeling an Isolated Fluidic Oscillator
Low pressure input
High pressure input
Calibration
o Simulation of Wind Tunnel Model
Computing Jets Tares
Active Flow Control
o Fluidic Oscillator
o Traverse Actuator
o Summary
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Flow characteristics:
• Unsteady
• Bluff body
• High angle of attack
• Separated regions
• Large wake
Incremental study of grid systems and numerical schemes
6
Flow Over a Wing in Hover
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Two Dimensional Model (1/3)
7
grid.in :
•near-body grids
x.save :
•near-body grids
•off-body Cartesian grids
•hole cutsFull domain
Wing characteristics:
• Constant chord
• Extract section @ mid-wing
• Flap @ 75°
• Sealed gap
• Use Domain Connectivity Function
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f = 4.3
Time accurate
Time accurate simulation
8
Two Dimensional Model (2/3)
• Initial solution in local time-stepping
• Time accurate to limit cycle (at ~0.5 sec)
Wake oscillation similar to
flow over a long cylinder
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9
Two Dimensional Model (3/3)
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Infinite Wing (1/5)
Highly separated flow—
▪ Is RANS sufficient?
▪ LES computationally prohibitive
10
Hybrid scheme: Delayed Detach-Eddy Simulation (DDES)
RANS near the body
LES off body and wake
• Simplified wing and flap (one block)
• Pre-defined boxes and far-field grids
• Span-to-chord ration ~2
• Periodic boundary conditions in span
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11
Infinite Wing (2/5)
Time accurate
DDES
• Initial solution in local time-stepping
• RANS as initial condition for DDES
No periodicity in the DDES solution
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Need a “wider” wing
Infinite Wing (3/5)
DDES: Flap is always separatedURANS: Flap is separated and reattaches periodically
large variation of pressure on bottom of the wing
DDES URANS
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Infinite Wing (4/5)
13
original span ~ 2 chordsnew span ~ 4 chords
Time accurateVortex shading frequency = 5Hz
• Double span-to-chord ratio
• Results are similar
• Same frequency
• Different force amplitude
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Future calculations will use RANS formulation
14
Infinite Wing (4/5)
URANSDDES
DDES resolves smaller structure of wake at larger computational expense
Wake structure is compatible
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Outline
15
o Introduction
o Numerical Method
o Computing Strategies for Wing in Hover
2D Simulation
Infinite Wing
o Modeling an Isolated Fluidic Oscillator
Low pressure input
High pressure input
Calibration
o Simulation of Wind Tunnel Model
Computing Jets Tares
Active Flow Control
o Fluidic Oscillator
o Traverse Actuator
o Summary
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Isolated Actuator – NASA Test
16
Simulated instantaneous flow field (PR=1.2)
CFDMach number
Time-averaged longitudinal velocity profiles
(PR=1.2)
• Validation with data for low PR (up to 1.4)Vatsa, Koklu, Wygnanski, and Fares, AIAA 2012-3239
• Reasonable agreement in velocity profiles and
frequency of actuation
Exp.~254 Hz, CFD~274 Hz
Lateral Coordinate
|q|,
ve
lm
ag
nit
ud
e
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.50.00
0.15CFD
EXP
|q|=
1is
so
nic
Exit width
3 psig12 mm probe
NASA modelGrid 8.4 million
~1700 time steps per cycle
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Isolated Actuator– Caltech Test
17
• Experimental data up to PR=5.5− Seele, Graff, Lin, and Wygnanski, AIAA 2013-0411
• Simulations– Choked flow at PR>2
– Swiveling motion reduced with increased PR
PR = 2.0 PR = 4.0 PR = 5.5
Instantaneous
Time-averaged
Grid 6.3 million
~800 time steps per cycle
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Validation – Caltech Test
18
Simulations, PR=2Density gradients
Experiment, PR=2Schlieren
• Validation with data up to PR=5.5
• Data limited to Schlieren flow viz
• Good qualitative agreement
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Actuator Characterization – Caltech Test
• Actuation parameters should be based on compressible flow– Exp C calculated from gas dynamics– Enables consistent exp-to-CFD comparison
• Good agreement is achieved over the subsonic/supersonic range
19
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P1ASME P2ASME
84.5298 14.2191
110.46 21.1216
130.05 26.7064
158.676 34.8964
184.549 42.2981
208.098 49.0241
Boundary conditions
are imposed here;
What P should we
use?
20
Calibration of Actuator in the Wind Tunnel (1/2)
• The problem:
• CFD wind tunnel model does not include high-pressure air supply network
• Limited measurement at plenum
• Definition of momentum coefficient (C )
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21
Calibration of Actuator in the Wind Tunnel (2/2)
• Limited measurement at actuator exit
shows pressure loss of ~40% between
measurement location and actuator
plenum
• Set of runs at increasing inlet pressure
Bad correlation of Momentum Coefficient
(C ) as function of pressure ratio
Using a consistently-define variable, mass
flow (Cq) → good correlation.
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Outline
22
o Introduction
o Numerical Method
o Computing Strategies for Wing in Hover
2D Simulation
Infinite Wing
o Modeling an Isolated Fluidic Oscillator
Low pressure input
High pressure input
Calibration
o Simulation of Wind Tunnel Model
AFC OFF
Computing Jets Tares
Active Flow Control
o Fluidic Oscillator
o Traverse Actuator
o Summary
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23
CFD Model of Wind Tunnel Test
• Boeing V/STOL Wind Tunnel (BVWT)
• 60% scale half span wing and flap
• 2 rows of Fluidic Oscillators (FO’s)
• 32 FO’s in each row
CFD Model:
Wing mounted on floor
Flap at 75°
1 row of FO’s
27 near-body zones
288 FO’s zones
65 off-body zones
~141 million grid points
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Good agreement at lower angles
24
AFC OFF
• No FO’s on CFD model (taped WT model)
• RANS mode
• High-order discretization
• Time-accurate mode
• Dt = 7.5x10-5 with 5 Newton iterations
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• Need to isolated jets forces from total forces on the wing
• Turn ON FO’s at M∞ = 0.0 at various inlet pressure ratios
• Compare with inlet conditions with wind-tunnel calibration test
Comparing results based on common variables and definitions
25
Computing Jets Tare Forces
Once correcting for
pressure loss and
appropriate C definition,
tare forces compare well
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CFD Model:
Wing mounted on floor
Flap at 75°
a = 80°
M∞ = 0.0992
C sweep
Numerical Process:
• local time stepping for initial conditions (5000 it)
• DTPHYS = 0.02
• 10 Newton iterations
CFD tool is capable of download alleviation simulation
26
Active Flow Control ON
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Effect of FO’s on Flow Field
27
AFC ONAFC OFF
• Reattach flow over flap
• Reduce wake width
• Reduce wake oscillations
~30% reduction in
download force
AFC OFF AFC ON
separation
bubble
PR = 3
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Fluidic Oscillator – periodic perturbation at discrete location
Traverse – array of jets traversing across the aerodynamic surface
Shmilovich and Yadlin, “Traverse Actuation Method”, AIAA Paper 2016-3309, 2016
28
Alternative Actuation Method – Traverse Actuator
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Fluidic Oscillator – periodic perturbation at discrete location
Traverse – array of jets traversing across the aerodynamic surface
Shmilovich and Yadlin, “Traverse Actuation Method”, AIAA Paper 2016-3309, 2016
Promising options; require further study
29
Alternative Actuation Method – Traverse Actuator
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Questions?
30
Summary
• Systematic buildup of CFD model for download simulation
• Physics-based gas-dynamics approach in flow characterization
• Consistent CFD-to-experiment comparison at actuation levels of practical
interest
• Capture flow characteristics of isolated FO correctly
• Reasonable agreement with test data for download alleviation
• Discrepancies due to turbulence modeling of time accurate simulation, highly
separated flow and experimental setup uncertainties
• Traverse actuator as an alternative low-input flow control device
• Temperature effects
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