The driving performance and driving stability of vehicles are influenced by the vehicles' aerodynamic properties. These properties are typically quantified using the aerodynamic coefficients for drag, axle-specific lift and lateral forces, and the resulting moments around the vehicle axes. The airflow interacting with the vehicle can be divided into the flow around the vehicle and the flow through the vehicle. As these flow components influence each other, they cannot be considered or optimized separately.
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The airflow around and through the vehicle significantly affects both driving performance and safety: The former suffers from air resistance, while the latter highly depends on the proper distribution of lift and lateral forces on the vehicle axles. A suitable design optimizes the airflow around the vehicle, lowering the air resistance, adjusting the lift distribution and reducing the sensitivity to crosswinds.
An important prerequisite for the optimization process is the realistic depiction of the road journey and the associated rotation of the wheels in both the wind tunnel test and in the numerical flow simulation (CFD).
FKFS is working on research projects concerned with optimizing testing and measurement equipment and on fundamental aerodynamic problems. We also support our customers in the aerodynamic design and optimization of vehicles and important vehicle components.
The airflow through the vehicle includes airflow through the engine compartment and airflow through the passenger compartment, the latter of which has a significant effect on passenger comfort. Airflow through the engine compartment ensures that heat is removed from thermally-loaded components, and is therefore necessary for maintaining the permissible operating temperatures for all sub-assemblies in the engine compartment. Thus the airflow through the vehicle forms an interface between aerodynamics and thermal management.
The cooling air flowing through the engine compartment is responsible for up to 15% of the vehicle's drag and therefore offers great potential for aerodynamic improvements. Both the on-demand control of the quantity of cooling air and the reduction of losses in its path are crucial in optimizing the airflow through the vehicle. FKFS has developed a method for measuring the volumetric flow of cooling air in the vehicle specifically for this purpose.
Other aspects concerning airflow through the vehicle are the guidance of cooling air over the brakes, and ensuring heat removal from other thermally-loaded components – such as the axle drive and transfer case.
When driving on the road, every vehicle is subject to the influence of other road users in addition to the natural wind. One example of this would be the changing flow conditions during overtaking maneuvers. In these driving situations, both the incident flow speed and direction change over time. This may result in an increase in wind noise and possibly changes in the vehicle's directional stability, thus these transient flow conditions reduce driving comfort and, in certain circumstances, may even impair the driver’s safety.
In particular, gusting crosswinds generate forces and torques on the vehicle which often cannot be simulated by stationary flow conditions in the wind tunnel. Typically, conventional measuring techniques in vehicle wind tunnels are also not able to record the resulting unsteady vehicle reactions.
In the wind tunnels operated by FKFS, we have the very latest tools for replicating transient flow conditions. The flow excitation system FKFS swing® was developed specifically for this purpose. Therefore, even today, we are able to offer our customers a test environment capable of generating transient incident flow and recording the associated unsteady forces and pressures on the vehicle. In addition to our experimental facilities, FKFS also has a validated simulation approach for depicting these effects in a numerical flow simulation.
This enables us to measure, evaluate and optimize vehicle reactions and performance changes resulting from unsteady flow conditions.
Clear visibility is extremely important, especially in adverse weather conditions which often occur in winter. For example, the visibility of the side view mirrow through the side window in a rainstorm is paramount for safety, as is the prevention of water running back from the windscreen wipers. The thermal wind tunnel can be used to conduct such investigations during the early stages of development – even on non-drivable prototypes without camouflage. This makes it possible to evaluate and optimize relevant vehicle geometries such as the A-pillar, the water channel on the A-pilar and the exterior mirror housing early on.
Additional examples for water management are self-soiling of the vehicle and brake fading under wet conditions. Wet brake fading refers to the reduction in stopping power due to fluid coating the brake disks, which significantly impairs vehicle safety.
To determine the effect of vehicle soiling on the braking behavior of vehicles, FKFS uses a specially-developed braking force measurement system. By holding the vehicle in a fixed position, the braking force of the vehicle can be determined reproducibly in different vehicle states (dry, wet). The measurements enable calculation of the friction coefficient between the brake pad and the brake disk. This can be used to determine the sensitivity of the brake system to water ingress, and to evaluate the effectiveness of corresponding design measures.
The objective evaluation of clear visibility and water management studies is performed after the test using DiVeAn©, the post-processing digital soiling analysis tool – developed and patented by FKFS. A photo of the area to be investigated is taken during the soiling test and subsequently processed using the DiVeAn© tool. Appropriate algorithms permit quantitative statements to be made about the soiled surface and the soiling intensity. This enables a quantitative comparison of different vehicles or configurations.