Wheel Modelling and Cooling Flow Effects on Car Aerodynamics
Doctoral thesis, 2018
In recent years, the automotive industry has focused on computer simulations as an alternative to physical testing. This allows for early vehicle optimization, short turn around times, and significant cost savings. Aerodynamics is an important vehicle attribute where Computational Fluid Dynamics (CFD) simulations are being used to improve vehicle design. In this context, predicting the aerodynamic forces in CFD is challenging, given the complexity of both the geometry and the flow. Two main areas where discrepancies between CFD and wind tunnel tests are present are wheel aerodynamics and cooling flow.
Wheel aerodynamics has been the focus of CFD research for several years, especially with the expected introduction of the Worldwide harmonized Light vehicles Test Procedure (WLTP) regulations. Car manufactures will need to evaluate the drag of the vehicle for all different rim and tyre combinations sold with the car, in order to determine its official fuel consumption. Thus, accurate modelling of tyre and rims in CFD is a high priority for vehicle manufacturers, in order to optimize vehicle design without large increases in testing costs. This thesis investigates the effects of different wheel geometries, as well as different wheel rotation modelling techniques and their effect on overall vehicle forces. A hybrid approach for modelling tyre rotation, named MRFg, has been proposed and analysed. It showed significant improvements in numerical prediction, when compared to experimental results. MRFg can be applied on the loaded and deformed tyres with no significant cost increase. Investigations into the tyre geometry also showed that the CFD accuracy can be improved by reproducing the rain grooves' continuity at the contact patch in the virtual model. The rain grooves have been show to reduce overall vehicle drag in both the wind tunnel tests and the simulations.
The cooling flow is an important parameter to consider when validating simulations to experiments, as it changes the flow field around the vehicle and has a large impact on the aerodynamic forces. In this thesis, a simple and quick method for measuring cooling flow is introduced. It is a force based approach where the force acting on the radiator core is used to calculate the air mass flow. It allows for non-intrusive monitoring of the cooling flow and forces during aerodynamic development with good accuracy. A comparison of CFD and wind tunnel tests is also performed with focus on parameters influencing cooling flow and force measurements. The effects of wind tunnel blockage and measurement grid have been shown to have significant effects on mass flow predictions.
Wheel aerodynamics has been the focus of CFD research for several years, especially with the expected introduction of the Worldwide harmonized Light vehicles Test Procedure (WLTP) regulations. Car manufactures will need to evaluate the drag of the vehicle for all different rim and tyre combinations sold with the car, in order to determine its official fuel consumption. Thus, accurate modelling of tyre and rims in CFD is a high priority for vehicle manufacturers, in order to optimize vehicle design without large increases in testing costs. This thesis investigates the effects of different wheel geometries, as well as different wheel rotation modelling techniques and their effect on overall vehicle forces. A hybrid approach for modelling tyre rotation, named MRFg, has been proposed and analysed. It showed significant improvements in numerical prediction, when compared to experimental results. MRFg can be applied on the loaded and deformed tyres with no significant cost increase. Investigations into the tyre geometry also showed that the CFD accuracy can be improved by reproducing the rain grooves' continuity at the contact patch in the virtual model. The rain grooves have been show to reduce overall vehicle drag in both the wind tunnel tests and the simulations.
The cooling flow is an important parameter to consider when validating simulations to experiments, as it changes the flow field around the vehicle and has a large impact on the aerodynamic forces. In this thesis, a simple and quick method for measuring cooling flow is introduced. It is a force based approach where the force acting on the radiator core is used to calculate the air mass flow. It allows for non-intrusive monitoring of the cooling flow and forces during aerodynamic development with good accuracy. A comparison of CFD and wind tunnel tests is also performed with focus on parameters influencing cooling flow and force measurements. The effects of wind tunnel blockage and measurement grid have been shown to have significant effects on mass flow predictions.
HA3
Opponent: Carsten Repmann, Germany
Author
Teddy Hobeika
Chalmers, Mechanics and Maritime Sciences (M2), Vehicle Engineering and Autonomous Systems
Hobeika, T. and Sebben, S. Tyre Pattern Features and their Effects on Passenger Vehicle Drag
Force Based Measurement Method for Cooling Flow Quantification
SAE International Journal of Passenger Cars - Mechanical Systems,; Vol. 10(2017)p. 619-627
Journal article
Experimental and numerical investigations of cooling drag
Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering,; Vol. 231(2017)p. 1203-1210
Journal article
Investigation of the Influence of Tyre Geometry on the Aerodynamics of Passenger Cars
SAE International Journal of Passenger Cars - Mechanical Systems,; Vol. 6(2013)p. 316-325
Journal article
Study of different tyre simulation methods and effects on passenger car aerodynamics
International Vehicle Aerodynamics Conference,; (2014)p. 187-195
Paper in proceeding
CFD investigation on wheel rotation modelling
Journal of Wind Engineering and Industrial Aerodynamics,; Vol. 174(2018)p. 241-251
Journal article
Vehicle aerodynamics consists of a broad field that studies the displacement of air due to vehicle motion. The air movement requires energy from the vehicle and is reflected in an aerodynamic resistance force, known as drag. Reducing the vehicle drag is a key parameter in improving the vehicle’s energy efficiency and decreasing its fuel consumption. Over the past years, vehicle manufacturers have significantly reduced aerodynamic drag on vehicles, thus enabling them to meet the tight CO2 emissions regulations. This has been partially due to the increased focus on simulations in order to drive optimization at an early design stage, before physical models exist for testing.
This thesis investigates wheels aerodynamics and cooling flow from both numerical and experimental perspectives, providing a deeper understanding of both areas. It contributes to the field with two distinct methods: a numerical approach for modelling tyre rotation and as experimental approach for measuring mass flow.
It tackles the problem of modelling the rotation of detailed tyres in CFD, by suggesting a hybrid approach referred to as MRFg. The sensitivity analysis of the MRFg approach is performed and compared to experimental results. It showed significant improvements in predicting the drag contribution of tyre details in simulations.
When looking at cooling flow, the air flow through the car's radiator has been measured. Since measurements are typically performed in a test facility, a wind tunnel, its effects on the mass flow measurement as well as the effects of the measurement method have been quantified. This allows for a more accurate comparison to the on road conditions the vehicle experiences when in use by customers. Finally, an alternative method for measuring cooling flow is suggested and experimentally analysed.
This work contributes to the understanding of wheel aerodynamics and cooling flow. In addition, it provides accurate, quick, and reliable methods which assist in optimizing vehicles, increasing their efficiency, and reducing emissions.
This thesis investigates wheels aerodynamics and cooling flow from both numerical and experimental perspectives, providing a deeper understanding of both areas. It contributes to the field with two distinct methods: a numerical approach for modelling tyre rotation and as experimental approach for measuring mass flow.
It tackles the problem of modelling the rotation of detailed tyres in CFD, by suggesting a hybrid approach referred to as MRFg. The sensitivity analysis of the MRFg approach is performed and compared to experimental results. It showed significant improvements in predicting the drag contribution of tyre details in simulations.
When looking at cooling flow, the air flow through the car's radiator has been measured. Since measurements are typically performed in a test facility, a wind tunnel, its effects on the mass flow measurement as well as the effects of the measurement method have been quantified. This allows for a more accurate comparison to the on road conditions the vehicle experiences when in use by customers. Finally, an alternative method for measuring cooling flow is suggested and experimentally analysed.
This work contributes to the understanding of wheel aerodynamics and cooling flow. In addition, it provides accurate, quick, and reliable methods which assist in optimizing vehicles, increasing their efficiency, and reducing emissions.
Areas of Advance
Transport
Infrastructure
C3SE (Chalmers Centre for Computational Science and Engineering)
Subject Categories
Vehicle Engineering
Fluid Mechanics and Acoustics
ISBN
978-91-7597-687-7
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4368
Publisher
Chalmers
HA3
Opponent: Carsten Repmann, Germany