A numerical investigation of the asymmetric wake mode of a squareback Ahmed body - effect of a base cavity

Numerical simulations of the turbulent flow over the flat backed Ahmed model at Reynolds number $Re\simeq 4\times 10^{5}$ are conducted using a lattice Boltzmann solver to clarify the mean topology of the static symmetry-breaking mode of the wake. It is shown that the recirculation region is occupied by a skewed low pressure torus, whose part closest to the body is responsible for an extra low pressure imprint on the base. Shedding of one-sided vortex loops is also reported, indicating global quasi-periodic dynamics in conformity with the seminal work of Grandemange et al. (J. Fluid Mech., vol. 722, 2013, pp. 51–84). Despite the limited low frequency resolution of the simulation, power spectra of the lateral velocity fluctuations at different locations corroborate the presence of this quasi-periodic mode at a Strouhal number of $St=0.16\pm 0.03$. A shallow base cavity of $5\,\%$ of the body height reduces the drag coefficient by $3\,\%$ but keeps the recirculating torus and its interaction with the base mostly unchanged. The drag reduction lies in a global constant positive shift of the base pressure distribution. For a deep base cavity of $33\,\%$ of the body height, a drag reduction of $9.5\,\%$ is obtained. It is accompanied by a large elongation of the recirculation inside the cavity that considerably attenuates the low pressure sources therein together with a symmetrization of the low pressure torus. The global quasi-periodic mode is found to be inhibited by the cavity.</jats:p

Asymmetry and global instability of real minivans' wake

© 2018 Elsevier Ltd Three minivan cars tested in real flow conditions are investigated through base pressure distribution, force balance and velocity measurements. Discontinuous transitions of permanent wake reversals and bistable dynamics between two well-defined states are observed varying ground clearance, pitch and yaw, with open and closed air-intake. These transitions prove that the cars undergo the same global z-instability as the square-back Ahmed body (M. Grandemange, M. Gohlke and O. Cadot, Physics of Fluids, 25, 2013). The contribution of the global instability to the lift coefficient is estimated to 0.012 and 0.024 depending on the wake state. Eventually, the potential of direct passive control of the z-instability is demonstrated by improving from 4.6% to 8.3% the drag reduction obtained by closing the front air-intake of the car