Experimental investigations of automotive surface contamination

Sports Utility Vehicles are popular in the global automotive market due to their practicality. However, these geometries have a propensity to contamination of water and dirt that generates problems with visibility, driver assistance systems, lighting and customer dissatisfaction. Contamination issues tend to be discovered during prototype testing and are therefore expensive to resolve because the geometry is largely fixed. Identifying problems at an early stage in the design through numerical simulations would resolve many of the issues. To have confidence in the simulations they must be correctly initialised and validated objectively against experimental test cases.

An approach to quantifying surface contamination is identified for the first time and an extensive set of experimental controls determined. The approach uses an Ultra Violet dye to dope water and an Ultra Violet light to illuminate the fluid. The intensity of the emitted light from the dye is a function of the fluid thickness, illumination intensity and time. The approach is thoroughly explored and validated before being implemented in a pilot study that employs a quarter scale model of a simplified SUV and tested in a wind tunnel using a fully characterised spray to contaminate the base. The model base pressures and streamwise Particle Image Velocimetry planes of the wake are obtained and used alongside the results to identify mechanisms. The objective measure of mass deposition rate per area is calculated and compared to previously used measures to demonstrate its effectiveness.

Once deposited on the surface, wet contaminant forms drops, rivulets and thin films. These move across the surface of the car requiring wiper systems to maintain

visibility and management techniques to remove the contaminant. Uncontrolled flows can cause distractions to the driver and reduce the effectiveness of other systems such as cabin ventilation.

An essential requirement in currently available simulation methods for surface water flows is a representative contact angle model. The tilted plate method is used to obtain these at a range of Capillary numbers for different fluids and surfaces relevant to the topic. Model parameters for the simulations are identified and a novel validation method proposed.

This study demonstrates that fully characterised experiments and physical objective measures are absolutely essential to enable numeric simulations to be employed with the high levels of certainty demanded by the industry.