  The Ahmed body is one of the most studied 3D bluff bodies used for automotive
research and was first proposed by Ahmed in 1984. The variation of the slant
angle of the rear upper surface on this body generates different flow
behaviours, similar to a standard road vehicles. In this study we extend the
geometrical variation to evaluate the influence of a rear underbody diffuser
which are commonly applied in high performance and race cars to improve
downforce. We perform parametric studies on the rear diffuser angle of two
baseline configurations of the Ahmed body: the first with a 0 degree upper
slant angle and the second with a 25 degrees slant angle. We employ a
high-fidelity CFD simulation based on the spectral/hp element discretisation
that combines classical mesh refinement with polynomial expansions in order to
achieve both geometrical refinement and better accuracy. The diffuser length
was fixed to the same length of 222 mm similar to the top slant angle that have
previously been studies The diffuser angle was changed from 0 to 50 degrees in
increments of 10 degrees and an additional case considering the angle of 5
degrees. For the case of a 0 degree slant angle on the upper surface the peak
values for drag and negative lift (downforce) coefficient were achieved with a
30 degrees diffuser angle, where the flow is fully attached with two streamwise
vortical structures, analogous to results obtained from Ahmed but with the body
flipped upside down. For diffuser angles above 30 degrees, flow is fully
separated from the diffuser. The Ahmed body with 25 degrees slant angle and a
diffuser achieves a peak value for downforce at a 20 degrees diffuser angle,
where the flow on the diffuser has two streamwise vortices combined with some
flow separation. The peak drag value for this case is at 30 degrees diffuser
angle, where the flow becomes fully separated.
