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- Marklund, Ture Jesper, 1970, et al.
(författare)
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Performance of an Automotive Under-Body Diffuser Applied to a Sedan and a Wagon Vehicle
- 2013
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Ingår i: SAE International Journal of Passenger Cars - Mechanical Systems. - : SAE International. - 1946-3995 .- 1946-4002. ; 6:1, s. 293-307
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Tidskriftsartikel (refereegranskat)abstract
- Reducing resistance forces all over the vehicle is the most sustainable way to reduce fuel consumption. Aerodynamicdrag is the dominating resistance force at highway speeds, and the power required to overcome this force increases by the power three of speed. The exterior body and especially the under-body and rear-end geometry of a passenger car aresignificant contributors to the overall aerodynamic drag. To reduce the aerodynamic drag it is of great importance to have a good pressure recovery at the rear. Since pressure drag is the dominating aerodynamic drag force for a passenger vehicle,the drag force will be a measure of the difference between the pressure in front and at the rear. There is high stagnationpressure at the front which requires a base pressure as high as possible. The pressure will recover from the sides by a taperangle, from the top by the rear wind screen, and from the bottom, by a diffuser. It is not necessarily the case that anoptimized lower part of the rear end for a wagon-type car has the same performance as for a sedan or hatch-back car. Thisstudy focused on the function of an under-body diffuser applied to a sedan and wagon car. The diffuser geometry waschosen from a feasibility stand-point of a production vehicle such as a passenger car. The fluid dynamic function andtheory of the automotive under-body diffuser working as a drag reduction device is discussed. The flow physics of theunder-body and the wake was analyzed to understand the diffuser behaviour in its application to lift and drag forces on avehicle in ground proximity. This work is mainly a numerical analysis that uses the traditional CFD approach from theautomotive industry. Results from this study show a potential to reduce aerodynamic drag of the sedan car approximately10%, and the wagon car by 2-3 %. The possible gain was much bigger for the sedan vehicle and the optimum occurs at ahigher diffuser angle. This was most likely due to the fact that the sedan car in its original shape produced more lift force than the wagon, a wagon usually produces very little lift or even down-force. Lift forces were also reduced with the use of under-body covers with diffuser. The down-force increased, or lift force decreased, linearly with increased diffuser angle, and the trend was the same for both sedan and wagon rear ends. Flow analysis of the wake showed the importance of how the wake is balanced.
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| 2. |
- Marklund, Ture Jesper, 1970
(författare)
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Under-body and Diffuser Flows of Passenger Vehicles
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Energy efficient vehicles will be required to meet future emission and fuel consumption requirements. Customers require reduced fuel consumption due to increasing fuel prices and the environmental issues, are drivers to reduce CO2. It is essential to improve the drivelines, but improving resistance forces of the vehicle is also an efficient and sustainable way to improve energy efficiency. Aerodynamic drag is the dominating resistance force for passenger and commercial vehicles at highway speeds.A passenger car is a bluff body aerodynamically, with pressure forces at the rear that dominate the aerodynamic drag. This is due to a relatively square shape, with a length / height ratio of approximately three, and a truncated rear-end that generates a wake. About 60 % of the aerodynamic drag forces of a passenger vehicle are related to the exterior body, upper and under-body; the rest being related to wheel, wheel house and cooling drag.This work focuses on the aerodynamics of the rear-end and under-body of bluff bodies in general, but also applied to passenger cars. Firstly, simplified bluff bodies, that represent different vehicle types, were used to study and map the general behaviour of the bodies. The findings were then tested and applied to full–size vehicles, with the focus on under-body flows and the effect of under-body diffusers. Both experimental and numerical tools were used, and scale model as well as full-size test bodies have been investigated.A unique feature with road vehicle aerodynamics are the boundary conditions: ground proximity and moving ground; relative the body. Also, rotating wheels and a cooling flow that re-distributes the flow around the body have to be considered. The Chalmers L2 wind tunnel is equipped with a moving ground system, and the simulations were set up with moving ground, rotating wheels and a cooling flow. The rotating wheels were simulated with the MRF approach and the cooling flow was tuned by measuring the cooling flow of a full-sized car and using this data in the simulations.A significant difference in the flow in an under-body diffuser, depending on upper body, was noticed in the bluff body experiments. In particular, drag was reduced more for a sedan or fastback upper body, compare to a wagon or square-back. This difference was confirmed in simulations of full–size vehicles, under road-vehicle boundary conditions, with under-body diffusers applied. It was found that it is very important to have flow symmetry around the vehicle and especially at the wake, to optimize pressure recovery at the rear end and reduce drag.
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