Project Number : PS 7.1
Rotorcraft Fuselage Drag Study using
OVERFLOW-D2 on a Linux Cluster
PI:
Associate Professor EPN Duque
tel : 928-523-5842
www.cet.nau.edu/~end2
Northern Arizona University
Graduate Assistant/Research Engineer:
Nathan Scott
2004 RCOE Program Review
May 4, 2004
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College of Engineering and Natural Sciences
Mechanical Engineering Department
Background/ Problem Statement:
• Evaluate fuselage force
and moment prediction
capability of the
OVERFLOW2 and
OVERFLOW-D
• Utilize cost effective
computer systems
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College of Engineering and Natural Sciences
Mechanical Engineering Department
Technical Barriers or
Physical Mechanisms to Solve :
• Appropriate grid generation over specific
aircraft
• Lift and drag forces over simplified shapes
such as prolate spheroid
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•
Grid sensitivity studies required
•
Unsteady flow capturing on bluff bodies
College of Engineering and Natural Sciences
Mechanical Engineering Department
Task Objectives:
Using the OVERFLOW code
• Evaluate drag prediction on a prolate spheroid
• Evaluate drag prediction on a helicopter fuselage
• Evaluate and document effects of grid resolution
• Evaluate turbulence models upon predictions.
• 1-eqn, 2-eqn, DES
• Compare results with Penn State Methods
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Approaches:
• OVERFLOW2 Code
• Grid Generation
• Near body grid refinement in boundary layer
• Grid adaptation in the field for vortical flow
• Turbulence models
• Baldwin-Barth
• Spalart-Almaras
• k-w
• Mentor-SST
• include Detached Eddy Simulation (DES)
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College of Engineering and Natural Sciences
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Overview
Explain S-A and SST
Detached Eddy
simulation
 Discuss DES
Implementation in
OVERFLOW
 Circular Cylinder
results
 6:1 Prolate Spheroid
results

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College of Engineering and Natural Sciences
Mechanical Engineering Department
Experimental Data

Virginia Tech Stability Wind Tunnel
– Wetzel, Simpson, Ahn
1.37 m 6:1 Prolate Spheroid
 Free stream conditions

– α=20º, Re=4.2E6, Ma=0.16
Coefficient of Pressure (Cp), Skin
Friction (Cf)from Wetzel
Dissertation
 U/u*, y+ from Simpson’s Website

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Mechanical Engineering Department
CFD Methodology

Reynolds Averaged Navier-Stokes Equations
–
–
–
–
OVERFLOW-D code developed at NASA and Army
Uses detailed overset grids
Allows for detailed geometry definition
Captures viscous effects such as unsteady flow
separation
 OVERFLOW2
used for turbulence model study
and Implementation of DES
–
–
–
–
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Scalar penta-diagonal scheme
1st order difference in time
2nd or 4th order RHS (OVERFLOW2)
2nd and 4th order central difference dissipation terms
College of Engineering and Natural Sciences
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Detached Eddy Simulation
 First
Formulated by Spalart as a
modification to S-A model in 1997.
 Later generalized to any model by
Strelets in 2001.
 First step was to modify the S-A model
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S-A-DES formulation
 Change
distance to wall in S-A model dw to
– Ĩ=min(dw,CDES∆)
– ∆ is the maximum of the grid spacing in three
dimensions- ∆=max(δX, δY, δZ)
– CDES=0.65
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k-w-SST-DES Formulation
 Change
k-transport source term:
ρβ*kω=ρk3/2/Ĩ
– Ĩ=min(lk-ω,CDES∆)
– lk-ω=k1/2/(β*ω)
– ∆ is the maximum of the grid spacing in three
dimensions- ∆=max(δX, δY, δZ)
– CDES=(1-F1) Ck-ε+F1 Ck-ω
– Ck-ε=0.61, Ck-ω=0.78
• At equilibrium reduces to an algebraic
mixing-length Smagorinski type model.
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Implementation in OVERFLOW
 Determine
grid cell edge
lengths in J,K,L directions
– One sided difference at
boundaries
– Central difference otherwise
 Background
Cartesian
Grids - DES always
enabled
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College of Engineering and Natural Sciences
Mechanical Engineering Department
Circular Cylinder Test Case




Re=140,000, Ma=0.2
Fully Turbulent
S-A, S-A-DES, SST-DES turbulence
models
7.6 million grid points
– Near body 181 by 60 by 99
– Background 426 by 61 by 252
– Off Body grid resolution 0.05 the
diameter
– H type block grid extends 10 diameters
– 2 total grids

Methods
– 4th central difference in space
– 1st order Beam-Warming in time

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Inviscid wall Boundary Conditions
College of Engineering and Natural Sciences
Mechanical Engineering Department
Other DES work with Cylinder
 Travin, A,
Shur, M, Strelets, M, Spalart, P
– Re = 50,000 and 140,000
– Laminar Separation
» Laminar Separation
» LES in Background
– Turbulent Separation
» Run Fully Turbulent
» Compares to higher Re
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Iso-surface visualization
comparison Circular Cylinder
Travin-DES
OVERFLOW URANS
(Not Unsteady Yet)
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OVERFLOW S-A-DES
OVERFLOW k-w-SST-DES
Unsteady Pressure coefficient for 1 drag cycle
1.5
Average 1 drag cycle
Exp-Nunen-Re=7.6 E6
Exp-Roshko-8.6 E6
Scatter 1 drag cycle
1
0.5
Cp
0
-0.5
-1
-1.5
-2
-2.5
0
16
20
40
60
80
100
120
angle from windward side
College of Engineering and Natural Sciences
Mechanical Engineering Department
140
160
180
Average Pressure coefficient for 1 drag cycle
1.5
SA-DES
SA-RANS
DES-Travin
Exp-Nunen-Re=7.6 E6
Exp-Roshko-8.6 E6
1
0.5
Cp
0
-0.5
-1
-1.5
-2
0
17
20
40
60
80
100
120
angle from windward side
College of Engineering and Natural Sciences
Mechanical Engineering Department
140
160
180
Conclusions from Circular Cylinder
 S-A DES
in OVERFLOW looks promising
– More fine scale resolution
– Cross Flow on “2-D” cases
– Comparable comparisons to Experimental Data
 k-w-SST
DES in OVERFLOW also looks
promising
– SST has been shown to approximate separation
better so more desirable in shear layer
– More verification needs to be done
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College of Engineering and Natural Sciences
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6:1 Prolate Spheroid Test Case




Re=4,200,000, Ma=0.16
Trip to Turbulence at x/L=0.2
S-A, S-A-DES, SST-DES turbulence
models
7 million grid points
– Near body 361 by 310 by 45
– First off body Grid spacing 0.08 the length
– Remaining off body grids reduce in
resolution by half
– Off body grids extent to 10 times the length
– 61 Total grids
– Grid shown to be convergent in Previous
Study

Methods
– 4th central difference in space
– 1st order Beam-Warming in time
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College of Engineering and Natural Sciences
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Other DES work with 6:1 Prolate
Spheroid
 Rhee,
S. H. and Hino,T.
– Re = 4,200,000 Ma=0,16
– Run Steady and Unsteady
– Showed under prediction of Lift
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Surface Skin Friction and vorticty
contour comparison for 6:1 Spheroid
S-A
SST
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S-A DES
SST DES
Comparison Of Lift and Pitching
Moment for 6:1 Spheroid
 All
of the models fall
with error for
Pitching Moment
 All of the models
under predict lift
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College of Engineering and Natural Sciences
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Lift
Pitching
Moment
Experiment
0.61±0.03
0.23±0.04
SA
0.45
0.24
SST
0.48
0.23
S-A-DES
0.42
0.25
SST-DES
0.45
0.24
Rhee &
Hino
0.48
0.24
Axial Surface Pressure at x/L=0.77
0
-0.05
-0.1
Exp
S-A
SST
S-A DES
SST DES
Cp
-0.15
-0.2
-0.25
-0.3
-0.35
0
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20 40 60 80 100 120 140 160 180
angle from windward side
College of Engineering and Natural Sciences
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Velocity Profile at x/L=0.77 and 150º from
Windward side
25
20
U/u*
15
10
S-A DES
SST DES
S-A
SST
Exp
5
0 0
10
10
1
10
2
yplus
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College of Engineering and Natural Sciences
Mechanical Engineering Department
10
3
10
4
10
5
Axial Skin Friction at x/L=0.77
10
9
8
Cf
7
6
5
4
3
2
1
0
25
Exp
S-A
SST
S-A DES
SST DES
20 40 60 80 100 120 140 160 180
angle from windward side
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Streamlines on Leeside
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College of Engineering and Natural Sciences
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6:1 Spheroid Conclusions






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DES shown to work with
overset grids
DES did not improve
integrated forces
Skin friction remained the
same
Surface pressure showed
slight improvement
Velocity profiles remained the
same close to surface y+<10
Velocity profiles improved
farther away from surface
y+>100
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Accomplishments
 Summer
work with Roger Strawn and Mark
Potsdam at Ames
 Presented at AIAA 43rd Aerospace
Sciences Meetings.
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College of Engineering and Natural Sciences
Mechanical Engineering Department
Future Work
 Grid
Refinement Study
on 6:1 Prolate spheroid
and DES
 New research
engineer, explore new
LES
 Apply DES and LES to
helicopter fuselage
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College of Engineering and Natural Sciences
Mechanical Engineering Department