A numerical investigation of periodic actuation on bluff bodies in ground proximity

A detailed numerical study on the drag reduction effects of rear-edge periodic actuation on a flat-back 2.5D Ahmed body (a two-dimensional body extended in the third dimension) is presented, with particular focus on an optimum open-loop configuration. Actuation is applied simultaneously in both perpendicular and parallel directions from the upper and lower rear-edges, in phase. Large Eddy Simulations at Re = 23,000 show an optimum drag reduction range between St_{act} = 0.09 and St_{act} = 0.13. There is also a region St_{act} = 0.23-0.27, near the natural Strouhal number, which shows less efficient results. Both results agree with recent experiments completed by Pastoor (2008). A thorough transient wake analysis, including Dynamic Mode Decomposition, is conducted for all cases, with special attention paid to the Koopman modes in the flow and vortex progression downstream. Two modes are found to co-exist in the optimum cases: mode N, the natural vortex shedding mode, and mode A, which is characterised by synchronous vortices. The former is shown to be weakened for successful drag reduction cases. This, along with the creation of symmetric vortex shedding in the wake are shown to be the major mechanisms in lowering the drag (with opposite signed vortices annihilating one another and increasing the pressure in the near wake), while antisymmetry returns to the wake further downstream. Other configurations are also explored, with shear-layer actuation (perpendicular to the freestream only) and base actuation (parallel to the freestream only) tested. The former is shown to be successful in generating synchronous vortices, while the latter is successful at attenuating the natural instability (and can also create synchronous vortices if the momentum coefficient is large enough). Dual actuation (the original actuation method tested which combines the shear-layer and base configurations), is shown to be the most efficient for momentum coefficients under 0.008. At C_mu=0.016, and likely higher, the base actuation appears to offer potential for even greater drag reduction than is achieved via the dual configuration. However, when the body is moved towards a moving ground plane, a set up which better approximates flow over a moving vehicle, drag reduction is shown to be negligible. Analysis of the non-actuated flow in ground proximity reveals a far less prominent vortex shedding wake instability, which renders the creation of synchronous vortices through actuation in an effort to delay it redundant. This is once again confirmed through Dynamic Mode Decomposition.