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High Lift CFD Simulations with an SST-Based Predictive Laminar to Turbulent Transition Model
Robin Steed
ANSYS Canada Ltd.
Waterloo, Ontario
Canada
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Outline
• About ANSYS
• ANSYS CFX Software and Solver Method
• SST g-Rq Transition Turbulence Model
• Model, Grids and Operating Conditions
• Results from SST g-Rq Transition Turbulence
Model
• Comparison to Fully Turbulent SST results
• Conclusions and Recommendations
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• Broad range of advanced
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CFX Solver
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• General purpose CFD solver– Single solution method for all
flow regimes
• Discretization– Fully implicit
– Element Vertex Finite Volume Method
– Bounded 2nd order upwind advection
• Solution Method– Psuedo-transient relaxation
– Coupled Mass and Momentum (u,v,w,p)
– Linear equations solved using Algebraic Multigrid
ANSYS CFX Software
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New Approach to Transition
Modeling
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Transition Mechanisms
• Natural Transition
• Bypass Transition
• Separation Induced Transition
Effect of Transition
• Increase in wall shear stress.
• Influence on separation behaviour.
• Separation induced transition on suction side determines reattachment point (controls stall, lift and drag)
• Dramatic increase in wall heat transfer
• Change in flow topology.
What is Transition?
Laminar
boundary layer
Turbulent
boundary layer
Transition
region
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Natural Transition
• Result of flow instability magnified in boundary layer (Tollmien-Schlichting instability)
• Occurs under low freestreamturbulence ( < 1%)
• Typical Examples:
• Wind Turbine blades
• Fans of jet engines
• Helicopter blades
• Any aerodynamic body moving in still air
Bypass Transition
• Occurs if flow outside the laminar boundary layer has a high level of turbulence (> 1%).
• Typical Examples:
• Compressor or Turbine blade, where upstream blades have generated large disturbances traveling with the freestream.
Separation Induced Transition
• Takes place after a laminar separation of the boundary layer.
• Leads to a very rapid growth of disturbances and to transition.
• Can occur in any device with a pressure gradients in the laminar region.
• If flow is computed fully turbulent, the separation is missed entirely.
• Typical Examples:
• fans, wind turbines, helicopter blades, axial turbomachinery
Transition Mechanisms
Wave leading to
instability
External disturbance
leading to instability
Strong Inflexional Instability
Produces Turbulence in the
Boundary Layer
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Transition Model Requirements
• Compatible with modern CFD code:
– Unknown application
– Complex geometries
– Unknown grid topology
– Unstructured meshes (no search directions)
– Parallel codes – domain decomposition
• Requirements:
– Absolutely no search algorithms
– Absolutely no integration along lines
– Local formulation
– Different transition mechanisms
– Robust
– No excessive grid resolution
Laminar Flow
Transitional
Fully Turbulent
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SST g-Rq Transition Turbulence
Model Formulation
• SST + two additional transport equations
– Intermittency (g)• Fraction of turbulent vs laminar flow
• Transition onset controlled by relation between vorticityReynolds number and Reθt
– Transition Onset Reynolds number (Rθ)• Used to pass information about free stream conditions
into boundary layer (e.g. impinging wakes)
• New Empirical Correlation
– Similar to Abu-Ghannam and Shaw, improvements for Natural transition
• Modification for Separation Induced Transition
– Forces rapid transition once laminar sep. occurs
– Locally Intermittency can be larger than one
Config1 @ 13 [deg]
Transition
Turbulence Intensity
Reθt = 260
)0,1Re193.2
Remax(~
t
vonsetF
q
Vorticity Re number profiles
y
uyv
2
Re
),(Re tTuft qq
01 gP
),(Re q Tuft
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The NASA Trap Wing Model
Validating Transition
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• Ideal test case for industrial application of transition model
• Low Reynolds number– 1e6 transition will be a factor
• Complex 3D wing– Opportunity for multiple, interacting
transition mechanisms
– Changes in flow topology can lead to large scale effect
• Difficult problem– Known difficulty capturing stall
• Could transition help?
• Validation– Good experimental data available
• Sharing and feedback– Excellent venue for gaining feedback
from expert users
HiLiPW-1 Trap Wing Model
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Trap Wing Model
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• Workshop grid
– Configuration 1
– Hex A-v1
• Operating Conditions
– Mach = 0.2
– Reynolds number = 4.3 million• based on mean aerodynamic chord
– Reference Temperature = 288.89 K
(530 R)
• Y+ < 1
Model, Grids and Operating
Conditions
Transition
α6 13 21 28 32 34 37
Extra-coarse
Coarse
Medium
Nodes Elements
Extra-coarse 6,068,737 5,957,632
Coarse 20,356,741 20,107,008
Medium 48,104,801 47,661,056
Fine 161,853,985 160,856,064
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SST g-Rq Transition Results
Lift Coefficient
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SST g-Rq Transition Results
Drag Coefficient
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SST g-Rq Transition Results
Moment Coefficient
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SST g-Rq Transition Results
Skin Friction @ α=28°
Laminar Separation Bubbles
Separation Induced Transition
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Comparison to SST Fully Turbulent SST
Lift Coefficient
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Comparison to SST Fully Turbulent SST
Drag Coefficient
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Comparison to SST Fully Turbulent SST
Moment Coefficient
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Comparison to SST Fully Turbulent SST
Laminar separation on slat
Laminar
Separation
Bubble
Normalized
Turbulence
Intensity = 0.17
a=34°20%
18%
16%
14%
12%
10%
8%
6%
4%
2%
0%
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Comparison to Fully Turbulent SST
results
Fully Turbulent Transitional
= 0.70
a=13°
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Comparison of Transitional, Fully Turbulent and
Experimental values @ α=13°, 28°, 34°
Separation
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Premature stall @ α=34°
Tra
nsitio
nal
Fully
Turb
ule
nt
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SST g-Rq Transition Results
Lift Coefficient
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Conclusions and
Recommendations
• Conclusions
– SST g-Rq transition model agrees well with
experimental data at all grid levels and most α
– Laminar separation leads to premature stall
prediction at high α
– Stall prediction improves with mesh refinement
• Transition optimized grid may be required
• Future work
– Investigate the influence of grid refinement
near transition zones
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Robin Steed
ANSYS Canada Ltd.
Waterloo, Ontario
Canada