The importance of rear pillar geometry on fastback wake structures

Abstract

The wake of a fastback type passenger vehicle is characterised by trailing vortices from the rear pillars of the vehicle. These vortices strongly influence all the aerodynamic coefficients. Working at model scale, using two configurations of the Davis model with different rear pillar radii, (sharp edged and 10mm radius) the flow fields over the rear half of the models were investigated using balance measurements, flow visualisations, surface pressure and PIV (Particle Image Velocimetry) measurements. For a small geometry change between the two models, the changes to the aerodynamic loads and wake flow structures were unexpectedly large with significant differences to the strength and location of the trailing vortices in both the time averaged and unsteady results. The square edged model produced a flow field similar to that found on an Ahmed model with a sub-critical backlight angle. The round edged model produced a flow structure dominated by trailing vortices that mix with the wake behind the base of the model and are weaker. This flow structure was more unsteady than that of the square edged model. Consequently, although both models can be described as having a wake dominated by trailing vortices, there are significant differences to both the steady state and unsteady flow fields that have not been described previously. This also shows that the fastback wake structure described by Ahmed is not definitive. , trailing vortices, unsteadiness, Davis model, Ahmed model, fastback Wake

Influence of afterbody rounding on the pressure distribution over a fastback vehicle

“…At this Reynolds number, it was shown that this moderate rounding induces strong local modifications of the flow. However, as in Fuller and Passmore (2014), it was found that separation of the flow along the side and over the rounded C-pillars still occurred. For higher ratios of rounding (R 10 S >20 according to the preliminary numerical study), the side radius is large enough to prevent side flow separation, this case will be briefly described in the "Appendix" for a more realistic model.…”

“…Authors attributed this reduction to the fully attached flow over the backlight and the downstream motion of the structures developing in the near wake. Fuller et al (2014) analyzed the benefits of rounding the rear pillar geometry on the Davis (1982) model. They observed that rounded edges generated a different wake structure dominated by an interaction between the longitudinal vortices and the separated region, resulting in a drag reduction of 11 %.…”

Unsteady Aerodynamics of an Oscillating Fastback Model

“…There is a thorough investigation of the flow field around this model at 0º yaw reported in Fuller and Passmore [14]. Each side of the model was covered in 84 pressure tappings, shown in figure 1 and pressure data was collected under steady state and oscillating conditions.…”

Large Eddy Simulation of the Flow-Field around Road Vehicle Subjected to Pitching Motion

“…The flow field over road vehicle is very complex, and the vortices around the vehicle at the position behind 1.5L in the rear of the vehicle begin to settle down (Fuller et al 2014). Therefore, the grid topology is divided into several blocks in this paper to capture more details of flow information around the vehicle and save the computing resources as shown in Fig.…”