  A large eddy simulation (LES) study of the flow around a 1/4 scale squareback
Ahmed body at $Re_H=33,333$ is presented. The study consists of both
wall-resolved (WRLES) and wall-modelled (WMLES) simulations, and investigates
the bimodal switching of the wake between different horizontal positions.
Within a non-dimensional time-window of 1050 convective flow units, both WRLES
and WMLES simulations, for which only the near-wall region of the turbulent
boundary layer is treated in a Reynolds-averaged sense, are able to capture
horizontal (spanwise) shifts in the wake's cross-stream orientation.
Equilibrium wall-models in the form of Spalding's law and the log-law of the
wall are successfully used. Once these wall-models are, however, applied to a
very coarse near-wall WMLES mesh, in which a portion of the turbulent boundary
layer's outer region dynamics is treated in a Reynolds-averaged manner as well,
large-scale horizontal shifts in the wake's orientation are no longer detected.
This suggests larger-scale flow structures found within the turbulent boundary
layer's outer domain are responsible for generating the critical amount of flow
intermittency needed to trigger a bimodal switching event. By looking at mean
flow structures, instantaneous flow features and their associated turbulent
kinetic energy (TKE) production, it becomes clear that the front separation
bubbles just aft of the Ahmed body nose generate high levels of TKE through the
shedding of large hairpin vortices. Only in the reference WRLES and
(relatively) fine near-wall mesh WMLES simulations are these features present,
exemplifying their importance in triggering a bimodal event. This motivates
studies on the suppression of wake bimodality by acting upon the front
separation bubbles.
