16 research outputs found
Fluid force and symmetry breaking modes of a 3D bluff body with a base cavity
International audienceA cavity at the base of the squareback Ahmed model at Re 4 Ă— 10 5 is able to reduce the base suction by 18% and the drag coefficient by 9%, while the geometry at the separation remains unaffected. Instantaneous pressure measurements at the body base, fluid force measurements and wake velocity measurements are investigated varying the cavity depth from 0 to 35% of the base height. Due to the reflectional symmetry of the rectangular base, there are two Reflectional Symmetry Breaking (RSB) mirror modes present in the natural wake that switch from one to the other randomly in accordance with the recent findings of Grandemange et al. (2013b). It is shown that these modes exhibit an energetic 3D static vortex system close to the base of the body. A sufficiently deep cavity is able to stabilize the wake toward a symmetry preserved wake, thus suppressing the RSB modes and leading to a weaker elliptical toric recirculation. The stabilization can be modelled with a Langevin equation. The plausible mechanism for drag reduction with the base cavity is based on the interaction of the static 3D vortex system of the RSB modes with the base and their suppression by stabilization. There are some strong evidences that this mechanism may be generalized to axisym-metric bodies with base cavity
Optimal low-dispersion low-dissipation LBM schemes for computational aeroacoustics
Lattice Boltmzmann Methods (LBM) have been proved to be very effective
methods for computational aeroacoustics (CAA), which have been used to capture
the dynamics of weak acoustic fluctuations. In this paper, we propose a
strategy to reduce the dispersive and disspative errors of the two-dimensional
(2D) multi-relaxation-time lattice Boltzmann method (MRT-LBM). By presenting an
effective algorithm, we obtain a uniform form of the linearized Navier-Stokes
equations corresponding to the MRT-LBM in wave-number space. Using the matrix
perturbation theory and the equivalent modified equation approach for finite
difference methods, we propose a class of minimization problems to optimize the
free-parameters in the MRT-LBM. We obtain this way a
dispersion-relation-preserving LBM (DRP-LBM) to circumvent the minimized
dispersion error of the MRT-LBM. The dissipation relation precision is also
improved.And the stability of the MRT-LBM with the small bulk viscosity is
guaranteed. Von Neuman analysis of the linearized MRT-LBM is performed to
validate the optimized dispersion/dissipation relations considering
monochromatic wave solutions. Meanwhile, dispersion and dissipation errors of
the optimized MRT-LBM are quantitatively compared with the original MRT-LBM .
Finally, some numerical simulations are carried out to assess the new optimized
MRT-LBM schemes.Comment: 33 page
A new directional splitting approach for grid refinement in the lattice Boltzmann method
International audienc