Unsteady flow, Vehicle Surface Contamination and Aerodynamic Drag
I aim to understand the linkage between aerodynamics, water management and the build-up of dirt on the outer surfaces of cars. The result will be a process and methodologies that allow dirt deposition and vehicle aerodynamics to be simulated concurrently.
Why is this a problem?
The the build-up of water and dirt on the external surfaces of cars cause a range of problems for their users, and others on the road, including:
- reduced visibility,
- impaired drivers’ vision,
- degraded systems performance, and
- reduced aesthetic appeal (i.e. vehicle users get dirty hands & clothes; the vehicle needs frequent washing.)
This is a result of the environment that cars operate in frequently including rain and road surfaces accumulating a range of solid contaminants.
When it rains, some raindrops fall directly onto the external surfaces of cars (primary contamination), while others are driven onto these surfaces by wind and the air movement caused by other road vehicles.
Rain also falls onto road surfaces, where it combines with dirt (road soil.) A fraction of this dirty water is thrown back into the air as road spray by the wheels of other vehicles. As cars travel through this spray or mist, dirty water accumulates on its surfaces (foreign or third-party contamination.) Finally, a car’s tyres drive spray onto its body side and rear surfaces (self-soiling.)
Both rain and the spray of contaminants thrown up by the tyres of road vehicles will also be influenced by the aerodynamic disturbances generated by a moving car. Therefore, the aerodynamic characteristics of cars are intrinsic to this investigation.
Over time the build-up of dirt picked up from the road will obscure a vehicle's lights, reducing visibility to other road users. Water hitting screens and windows make it more difficult for the driver to see out of the car. As door handles, boot and tailgate surfaces become dirty, it is likely that people using a car will pick up dirt on their hands and clothes. Dirt accumulating on rear surfaces can obscure camera lenses and license plates.
It is inevitable that cars will accumulate surface contamination; however, where and how quickly this happens is the crux of the issue.
What could be done about it?
Manufacturers have taken a range of approaches to addressing this range of problems over the years, such as water and soiling manangement features (drainage channels for water, mudflaps and turning vanes for rear soiling control.) These have generally increased wind noise and aerodynamic drag. The latter is a key concern, as "70% of a car’s total resistance to motion at 110 km/h is attributable to aerodynamic drag" (Dávila et al., 2013) and overcoming this resistance to motion inevitably entails the generation of CO2. As manufacturers are under increasing pressure to reduce the CO2 emsissions of their vehicles, adding drag to combat soiling is no longer a viable approach.
Currently, these conflicting interests are balanced during the development process using mainly track and wind tunnel based experiments. Simulation methods are starting to be used, but these are currently under-developed.
My aim is to investigate one surface contamination issue: rear soiling for SUVs. I will use use advanced simulation approaches and experiments at reduced and full scale to understand the basic mechanisms and establish if rear soliling can be reduced without increasing drag. I will also design a new development process which combines both simulation and test-based approaches.
My Research Questions
As rear surface contamination is the key soiling issue for squareback vehicles, such as estate cars or SUVs, and the mechanisms of soiling in this area are poorly understood, I would like to establish:
(i) What is the relationship between unsteady flow and rear surface contamination for a squareback vehicle?
There is evidence of a strong relationship between aerodynamic drag and rear surface contamination. It appears to be adverse: i.e. improvement in one parameter leads to degradation of the other. It also appears to be non-linear. However, these indications arise from experiments using "turning vanes" to improve rear screen cleanliness, rather than an exploration of the relationship per se; so:
(ii) What is the relationship between aerodynamic drag and rear surface contamination for a squareback vehicle?
Finally, it is clear that rear surface contamination and aerodynamic drag require optimising within the vehicle development process. Given the extensive use of aerodynamic simulation, particularly during the early phases of vehicle development projects, I want to address the question:
(iii) How can rear surface contamination and vehicle drag be effectively optimised within the vehicle aerodynamics development process?
The questions will be explored using computation fluid dynamics as the main investigative tool, with experiment used to validate the simulation outcomes. This requires consideration of the accuracy of aerodynamic simulations for rear wake flow and sensitivity to assumptions made about the characteristics of contaminants to be addressed.
Dr Kerry Kirwan
kerry dot kirwan at warwick dot ac dot uk
Dr Duncan Lockerby
D dot Lockerby at warwick dot ac dot uk