Under-body and Diffuser Flows of Passenger Vehicles

Doctoral thesis, 2013

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.