glasgow 2003 integrating cfd and experiment a detailed cfd and experimental investigation of a...
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GLASGOW 2003INTEGRATING CFD AND EXPERIMENT
A Detailed CFD and Experimental Investigation of a Benchmark Turbulent
Backward Facing Step Flow
Stephen Hall & Tracie Barber
University of New South WalesSydney, Australia
INTRODUCTION Turbulent Backward Facing Step - benchmark
flow
Compare Particle Image Velocimetry (PIV) and CFD
Not just to check CFD
Rather to use the best of each
What is the best of each? - first steps
TURBULENT BACKWARD FACING STEP
Mean flow structures Characteristic lines and points Wide velocity, turbulence range Good CFD and PIV test
x
y
(0,0)
Xr
Ys
Xs
Sp(x,y)
Ss(x,y)
Primary-Secondary interface
u=0 stagnation
u
Channel flow u=f(y)
Core flowShear layer
Recovery layerDividing streamline
Reverse boundary layer
Primaryvortex
Secondvortex
Top boundary layer
t
b
Xr
Top surface
Bottom surface
Outer layerInner layer
EXPERIMENTAL TEST RIG Dimensions and PIV access
SIDE VIEW
TOP VIEW
(0,0,0)
(0,0,0)
x
x
z
y
Laser light sheet
PIV Nd:YAG Laser
Bell inlet
2000 1500
Inlet ductTransition
Inlet channel Expansion channel
PIV CCD camera and lens
xy planes
Cross-sectional plane
Dimensions mm
Light Delivery System
Scatters
EXPERIMENTAL TEST RIG Clear acrylic – light access Low reflection base Full side view
EXPERIMENTAL TEST RIG High density PIV seeding – critical Particle – hollow polymer spheres Venturi jet seeder well upstream
HIGH RESOLUTION PIV Basic 2D arrangement Mean flow aligned in light sheet Camera perpendicular to light sheet
Seeding
Light Source
Light Delivery
Light Scattering Particles
LensCamera
Measurement Region
Light Sheet
Flow
x
y
z
HIGH RESOLUTION PIV
HIGH RESOLUTION PIV Constant intensity light sheet Adjustable width and thickness
PIV VERIFICATION USING CFD
Contours From Standard PIV Images of Okamoto and Watanabe (1999)
Contours From a PIV Analysis of the Standard PIV Images
PURPOSE OF CFD
Not only to validate CFD against experiment
But to reveal greater detail about phenomena
And supplement experimental results
Make full use of the reliability of modern CFD
CFD APPROACH 2D RANS finite volume (Fluent)
RNG k-, RSM turbulence closure (literature)
Second order interpolation
Ambient and boundary conditions – use experimental
CFD VERIFICATION ASSESSMENT Key functionals Xr, Xs, Ys, Sp, Ss, Lift, Drag
Grid convergence – literature 200 cells vertical
Iterative convergence
3D vs 2D – same grid resolution 3% on Xr
Turbulence modeling, is code correct? commercial code
CFD AND PIV COMPARISON Many quality CFD and Experimental data sets
Most missing complete boundary conditions not previously required
Perform both CFD and PIV myself “exactly equivalent”
Work in raw variables u, v, Vmag less need to non-dimensionalise
Contours – variation
Streamlines - direction
MEAN VELOCITY MAGNITUDE Velocity Mag. in CS Plane, (top) PIV, (mid.) RNG k-, (bot.) RSM
STREAMLINES Streamlines in CS Plane, (top) PIV, (mid.) RNG k-, (bot.) RSM
U HORIZONTAL VELOCITY Mean Horizontal Velocity in CS Plane, (top) PIV, (mid.) RNG k-,
(bot.) RSM
V VERTICAL VELOCITY Mean Vertical Velocity in CS Plane, (top) PIV, (mid.) RNG k-,
(bot.) RSM
VORTICITY Vorticity Mag. in CS Plane, (top) PIV, (mid.) RNG k-, (bot.) RSM
VELOCITY MAGNITUDE AND STREAMLINES BEHIND STEP
Characteristic Lines in CS Plane, (top) PIV, (mid.) RNG k-, (bot.) RSM
FOCUSED BEHIND STEP Mean Velocities in CS Plane, (left) PIV, (right) RSM tp=70 s
FOCUSED BEHIND STEP Streamlines in CS Plane, (left) PIV, (right) RSM, tp=70 s
CONCLUSIONS CFD and PIV reveal detailed flow structure
“Exactly Equivalent” simultaneous CFD and PIV Highlights limitations in each Improves confidence in results
Essential complimentary techniques
FUTURE DIRECTIONS CFD should be first part of any PIV?
Unsteady 3D CFD PIV (TR PIV, stereoscopic)
Total integration of CFD PIV PIV modifies turbulence closure model
throughout
CFD error checks PIV and fills in error regions
mathematically rigorous
PLANES OF INTEREST
x
y
(0,0,0)
Cross-Sectional Plane
zBottom Surface
x= 400
x= -100
Inlet Profile
z= 0
z= 117.5
y= 93.33
Inlet Line
HIGH RESOLUTION PIV Basic sub-systems Optimise performance of each
SeedingLight Source Light Delivery
Light Scattering Particles
FlowLight Encoding
Light Conditioning
Recording
Post ProcessingFilteringDistortion CorrectionMapping ImageStatistical Analysis
ProcessingImage TrimmingCross-correlationError Checking
Storage
t
Control
Analysis