Qualitative tuft flow visualization on the volvo S60 under realistic driving conditions

Paper in proceeding, 2016

The flow around a car is three-dimensional and turbulent. The main part of the dragis caused by pressure losses generated by various ow separations. In general a large anddistinct wake structure dominates the flow field at the base. Furthermore, diverse vertical structures are present at the backlight and the upper trunk region. The investigation ofthese owphenomena is important for the design and the optimization process of a car.Traditional ow visualization techniques, such as oil paint and tufts, are widely used andestablished for the study of surface friction lines. Aerodynamic investigations are usually conducted in sophisticated wind tunnels with a uniform freestream velocity and very lowturbulence intensities. However, this does not correspond to realistic road conditions. Inthis study, the applicability and the feasibility of a new tuft method is investigated. Theexperiments are conducted on a circuit test track and hence to realistic on-road conditions.A large amount of tufts are attached to the entire rear end section of a Volvo S60 passengercar. The movement of the tufts is continuously recorded as well as the freestream velocityusing a digital camera and a three-hole probe, respectively. Thus, every image can beassociated to a distinct inow conditions. A novel and Efficient image processing algorithmallows the extraction of the mean tuft angle afterwards. This data allows the calculationof the near wall streamlines and generates additionally statistical data of the directionalorientation of the tuft movement. The obtained time averaged surface traces indicate analmost symmetric flow field at the rear end of the car with head wind conditions. The effect of the streamwise vortices, which origins at the c-pillars, are evident on both sidesof the backlight. With side wind conditions the ow pattern at the backlight and trunkdeck shift slightly to the leeward side while the vorticalow structure at the base shiftsslightly windward. The analyses of the freestream velocity time series show Significantlyincreased turbulence intensities compared to wind tunnel tests. In summary, the new tuftow visualization and the usage of the three-hole probe in front of the car represent asimple and low cost tool for on-road test to study limiting streamline pattern.