ARL Penn State COMPUTATIONAL MECHANICS 1 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

ARL Penn State COMPUTATIONAL MECHANICS CONTENTS Introduction Computational Methods Results Summary 2

ARL Penn State COMPUTATIONAL MECHANICS 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 3

ARL Penn State COMPUTATIONAL MECHANICS 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 4

ARL Penn State COMPUTATIONAL MECHANICS 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 Numerical solution of mixture: mass, momentum, energy, additional phases, species, and turbulence models on moving or static, overset computational meshes. 5

ARL Penn State COMPUTATIONAL MECHANICS VALIDATION HIGHLIGHTS Cavitator Lift and Drag

ARL Penn State COMPUTATIONAL MECHANICS 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

ARL Penn State COMPUTATIONAL MECHANICS Cavity Size vs. Ventilation Rate VALIDATION HIGHLIGHTS

ARL Penn State COMPUTATIONAL MECHANICS Mesh showing flowpath, rotor, and stator in NSWC-CD Tunnel Normalized inlet total pressure Normalized Head Rise Normalized Power 9 VALIDATION HIGHLIGHTS

ARL Penn State COMPUTATIONAL MECHANICS 10 pump turbine stator pump turbine 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

ARL Penn State COMPUTATIONAL MECHANICS 11 Cavitating CFD current effort Single Phase CFD current effort a) b) CFD and test results on research converter. K-factor (RPM/[torque] 1/2 ) and torque ratio. Round Torus CFD Results MPa

ARL Penn State COMPUTATIONAL MECHANICS 12 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

ARL Penn State COMPUTATIONAL MECHANICS 13 Speed Ratio CFD results (red) diamond: cavitating square: 1-phase Dyno-135 N-m Dyno-250 N-m K-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

ARL Penn State COMPUTATIONAL MECHANICS MPa suction side pump and stator pressure-side pump and stator MPa 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

ARL Penn State COMPUTATIONAL MECHANICS 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 15

ARL Penn State COMPUTATIONAL MECHANICS Preconditioner Derivation simplified working in terms of mass fraction

ARL Penn State COMPUTATIONAL MECHANICS Preconditioner We choose: c’=min[ max( V cut-off, |V| ijk ), c ijk ] (c’=c ijk yields the unconditioned result) Introduces artificial sound speeds yielding good convergence/accuracy regardless of Mach number/density ratio