Transcript summary_100608.ppt

Aspects of Transitional flow
for External Applications
A review presented by
Clare Turner
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Presentation Outline
• Review of T3 flat plate tests
• Conclusions from the flat plate tests
• Direction for transition prediction
• Current and future work
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T3 Flat Plate Tests
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Original Simulation Set-up
• Domain size taken from S. R. Likki :
• Inlet conditions:
S = 0.05m
k = 0.0536 m2/s2
L = 3.3m
ε = 1.35 m2/s3
H = 0.08m
U = 5.08 m/s
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y+ ≈ 1
Initial Results
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Anomalous Results
After Craft et al.:
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Possible Areas for Error
• Differencing scheme
• Grid density
• y+ value
• Residual tolerance
• Domain size
• Boundary conditions
• Cell distribution
• Near wall treatment
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Differencing Scheme
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Number of Streamwise Cells
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Y+ Value
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Number of cross-stream cells
• Grids have 23, 34 and 39 cells in the viscous layer respectively
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Domain Height
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Cell Distribution
• Several meshes generated to compare results and
convergence; largest cell distribution:
• More efficient to
concentrate cells at
the leading edge
• 39390
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Stream-wise Growth-rate
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Convergence
To the left: growthrate=1; to the right growthrate = 1.012
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Low Reynolds Number Models
Available in Star-CD: “Standard”
where
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Low Reynolds Number Models
Available in Star-CD: Suga’s
where
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Near Wall Treatment (1)
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Near Wall Treatment (2)
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Determination of Inlet Values
• Dissipation
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rate originally tailored to FSTI curve
Re-calibration of Inlet Values
• Free-stream taken to be at the edge of boundary layer
• Experimentally free-stream is at approximately 3 delta
• Turbulence intensities calculated with new definition
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Effect of Chosen Free-stream boundary
• Error was not in the calculation of the free-stream
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Inlet Values in Literature
• Inlet k and epsilon values differ to those in literature
• Differences may arise due to choice of U_inlet
• 3 different inlets are tested with U = 5 m/s:
1) Using a correlation for lengthscale from work of
Savill and interpolating FSTI at inlet
2) As above but with a higher FSTI
3) As 2) but with a lengthscale used by Chen et al.
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Choice of Inlet Velocity
• Original velocity set to 5.08
• Velocity at leading edge should be approx 5 m/s
• Turbulence model does not replicate acceleration at
the leading edge
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Effect of Chosen Inlet Values (1)
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Effect of Chosen Inlet Values (2)
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T3 Conclusions
• Largest contributing factor is the low Reynolds
number treatment
• Sufficient work has been done on the grid to
only require small alterations for future tests
• Non-linear eddy-viscosity models improve
transition onset prediction and running time is
not increased dramatically but no alterations can
improve transition length prediction.
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Approaches for a Physical Solution
rather than Correlation Based Models
3 possible approaches:
1) Adaptation of low Reynolds number RANS
models, eg. higher order of closure, tailored
to specific application.
2) An intermittency model derived using PDFs
3) A model using the concept of laminar kinetic
energy
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1) Low Reynolds Number RANS
Models
Advantages:
a) Large amount of in-house knowledge
b) Have models to develop, low risk
Disadvantages:
a) Nothing new to contribute
b) Low Re models do not accurately represent the
transition process
c) Higher orders of closure increase computational cost
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2) Physical Intermittency Model
Advantages:
a) New approach
b) Should be able to predict all modes of transition
Disadvantages:
a) No literature to refer to - risky
b) Have little expertise in PDFs
c) Little or no time would remain to apply to rear wing
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3) Concept of Laminar Kinetic Energy
Advantages:
a) Relatively new giving areas for improvement
b) Does not rely on diffusion dominated transition
c) Only requires one additional equation
Disadvantages:
a) Walters and Leylek model (2002) gives poor reaction
to large pressure gradients
b) Determination of the effective length-scale is only
based on distance from the wall
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Walters and Leylek Model (2003)
•
This is the starting point for 2k model development
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Walters and Leylek Model (2003)
•
Walters and Leylek assume large scales contribute to
the laminar kinetic energy and small scales to the
turbulent kinetic energy.
The cut – off point is the
effective length-scale :
•
Sveningsson uses a different definition:
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Effective Length-scale Along the Plate
At 400 mm (transitional region) :
δ99 = 18.5 mm
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Effective Length-scale Along the Plate
At 800 mm (turbulent region) :
δ99 = 24.1 mm
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Current Work
• Adjusting individual terms of Wilcox’s transport
equations for turbulent kinetic energy and specific
dissipation rate to those of Walters and Leylek
• Testing code by inserting fully turbulent values from
completed simulations
Aiming to ...
• Use code with Saturne assuming laminar kinetic
energy = 0
• Create new scalar variable KL to complete Walters and
Leylek Model
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Any Questions?
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Walters and Leylek Model 2002
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