Computational Evaluation of the Cavitating Flow through Automotive Torque Converters
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Transcript of Computational Evaluation of the Cavitating Flow through Automotive Torque Converters
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Computational Evaluation of the Cavitating Flow through
Automotive Torque Converters
Acknowledgement: This work is supported by the General Motors Corporation
15 August 2012
J.W. Lindau F.J. Zajaczkowski M.F. Shanks R.F. Kunz
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CONTENTS
• Introduction
• Computational Methods
• Results
• Summary
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CAVITATION IN A TORQUE CONVERTER:•Working fluid is ATF, heat, extreme pressures•Torque converters have historically not suffered negative effects from cavitation •However, the trend is to smaller, lighter, etc•Minimum pressure/stator region may cavitate at high torque, low turbine speed•Concerns are performance, vibration, noise
Introduction: Automotive Torque Converter
Torque Converter from Wikipedia
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Methodology• First principals model of…
– Mixture of gases and liquids– Gas-liquid interfaces– Large scale gas cavities– Incompressible to compressible: disparate sound speeds– Shocks– Significant inherent unsteadiness (even in steady, planing configuration)– Energetic propulsion
• Chemistry and phase change– Liquid/vapor mass transfer (stiff)– Chemical reactions (stiff)
• Control Surfaces--6DOF: fully coupled to flow• Preconditioning (addresses stiff physical eigensystem)• Turbulence modeled and (where feasible, required) simulated• Numerical model: fully-conservative, unsteady, implicit, multiphase, preconditioned
finite volume form• Unsteady simulations with many millions of degrees of freedom are feasible/required
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DIFFERENTIAL MODEL
• Computational tool—• n-liquid+n-gas• preconditioned• all-Mach number• compressible• total energy conservation• any 2-variable eos/species• body forces/propulsors• mass-transfer=phase change and chemistry• shock-capturing• level-set—free-surface or cavity interface• multibody-control surfaces-6DOF• overset • 2-eq RANS/DES/transition
HFFQ
t
Q vjjjj
pc
,,
Numerical solution of mixture: mass, momentum, energy, additional phases,
species, and turbulence models on moving or static, overset computational
meshes.
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VALIDATION HIGHLIGHTS
Cavitator Lift and Drag
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Lift and drag values and comparison of experimental
and computational geometries and computed cavities (with
gas streamlines) from experiments of Waid and
Kermeen (1957).
VALIDATION HIGHLIGHTS
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0
0.2
0.4
0.6
0 0.1 0.2 0.3Cavitation Number, s
Air
En
tra
inm
en
t R
ate
, C
Q
Exp. Fr=26.7DESDES:w/strutURANSURANS:w/strutRANSSpurk Eq. 33
Cavity Size vs. Ventilation Rate
VALIDATION HIGHLIGHTS
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Mesh showing flowpath, rotor, and stator in NSWC-CD Tunnel
0.5 1 1.5 2 2.5 3 3.5 40.8
0.85
0.9
0.95
1
1.05
CFDEFD
Normalized inlet total pressure
0.5 1 1.5 2 2.5 3 3.5 40.8
0.9
1
1.1
CFDEFD
Normalized Head Rise
Normalized Power
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VALIDATION HIGHLIGHTS
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pump turbine
stator
pumpturbine
stator
Torque Converters: Computational Mesh
Round Torus: Research Converter
Thin Torus: Converter Approximating Current Designs Trends
BOTH A MIXING PLANE AND A BODY FORCE BASED COUPLING APPROACH ARE APPLIED
COMPUTATIONAL GEOMETRY REPEATED OVER FULL 360deg
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Cavitating CFD current effort
Single Phase CFD current effort
a)
1700 1800 1900 2000 2100 2200 2300 2400 25002.15
2.16
2.17
2.18
Tor
que
Rat
io
1700 1800 1900 2000 2100 2200 2300 2400 2500-4
-3.5
-3%
Err
or v
s. E
xper
imen
ts
RPM
b)
CFD and test results on research converter. K-factor (RPM/[torque]1/2) and torque ratio.
Round Torus CFD Results
MPa
0.8
0.0
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Through-flow Pump
Turbine
Stator
Grids for body force based method. Computational meshes, thin-torus torque converter (coarse). Solid surfaces are illustrated with black mesh. Periodic boundaries are illustrated with green mesh.
Thin Torus CFD Mesh
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Speed Ratio
CFD results (red)diamond: cavitating
square: 1-phase
Dyno-135 N-m
Dyno-250 N-mK-factor/100
Torque Ratio
• Computation and testing of Thin Torus TC. • Single-phase and cavitating.
Plot of K-factor/100 and torque ratio versus speed ratio. • Dynomometer: black marks with black lines. • CFD: red diamonds and dashed==single-phase, and • CFD: red squares and dashed ==cavitating
Thin Torus CFD Results
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MPa
1.1
0.0
suction side pump and stator
pressure-side pump and stator
MPa
0.5
0.0
Single-phase solution, pump at 3000RPM, turbine stationary, thin-torus torque converter.
Cavitating CFD solution, thin-torus unit. Elements repeated periodically for visual effect. Surfaces made translucent to better visualize stator and cavity. All surfaces colored by pressure. Isosurface of vapor volume fraction at 0.5. Pump at 3000RPM, stall condition
Thin Torus CFD Results
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SUMMARY
• CFD methodology validated for ventilated and natural cavitation, supercavitation, and turbomachinery
• Torque converters modeled using single blade passage, multi-blade row (steady, periodic assumption) CFD
• Mixing plane and body force coupling• Both approaches are problematic• Cavitation effects on pump torque captured• For high torque/large cavities and impact on
noise/vibration, a full 360deg unsteady analysis may be needed
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1pp Q
Q
k
i
Y
T
u
p
Q1
Preconditioner
Derivation simplified working in terms of mass fraction
l
ll
l
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YiTiijpi
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0
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k
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p
Q
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Preconditioner
• We choose: c’=min[ max( Vcut-off , |V|ijk ), cijk ] (c’=cijk yields the unconditioned result)
• Introduces artificial sound speeds yielding good convergence/accuracy regardless of Mach number/density ratio