Computational investigation into the sensitivity of a simplified vehicle wake to small base geometry changes

Abstract

For vehicles with a squareback geometry, for example Sports Utility Vehicles (SUVs), base pressure drag is a large contributor to overall drag. Simple passive techniques, such as tapering, can reduce drag significantly but at a large aesthetic and functional cost. Therefore, very small base geometry changes have been investigated. An experimentally validated methodology has used Detached Eddy Simulations (DES) to obtain time-averaged and instantaneous data; allowing the effect of horizontal base slats on global forces and wake structures to be presented. The small geometry modifications have caused substantial changes to the base pressure distribution with the main mechanisms of change being identified and observed close to the model surfaces. A region of separation is seen below each slat corresponding to reduced pressure whilst high pressure regions attributed to stagnation are increased. The combined effect is a statistically significant drag reduction of 4 counts (1 count = 0.001 C D) when a slat is added at 3/4 of the base height. The results show the scope for very small changes to a simplified road vehicle, in areas that have not previously been explored, to reduce overall drag with minimal aesthetic penalties. This understanding provides the impetus for new approaches in real vehicle development.

Drag reduction mechanisms on a generic square-back vehicle using an optimised yaw-insensitive base cavity

“…22 where the differences are more apparent. The relative order of different geometries drag values and the near base crossflow is in agreement in this case, which is consistent with other work where low near-wall velocities were linked to decreases in drag (Luckhurst et al 2019;Varney 2020).…”

“…The wake surface, S wake , is defined as the region where the longitudinal velocity, v x , is smaller than 0.25v ∞ . These, and similar, quantities have been shown to correlate with vehicle drag in previous work (Luckhurst et al 2019;Urquhart et al 2020b).…”

On the recirculating flow of three-dimensional asymmetric bluff bodies

“…This effect is actually generally observed when a squareback base subjected to y-instability (either the Windsor or the Ahmed body) is modified with top and bottom rear, tapers (Perry et al 2016) or boat tails . For instance, the presence of the bottom diffuser of the Windsor body studied in Luckhurst et al (2019) has definitely selected a P z state compared to the square-back Windsor model (Perry et al 2016). Bonnavion and Cadot (2018) actually identify 2 mechanisms of wake orientation with body inclination, a wake adaptation when the modification respect the symmetry in the instability direction (as in Fig.…”

Numerical Study of the Unsteady Aerodynamic Performance of Two Maglev Trains Passing Each Other in Open Air Using Different Turbulence Models

“…In the hybrid method, the RANS method is used to model attached flows in the near-wall regions, whereas the LES method is used to resolve largely separated flows in the remaining computational domain regions [28,29]. Furthermore, this method can effectively solve the existing problems of classical detached eddy simulations or delayed detached eddy simulation models that contain model stress dissipation, gridinduced separation, and logarithmic layer mismatch [28,30]. Consequently, IDDES has been widely used to simulate external flows around a train, particularly small-scale vortex structures [12,18,19,22,23].…”