On vortex shedding from bluff bodies with base cavities

Abstract

In an extension of a previous drag reduction study via geometric shaping, this paper describes a numerical study of a quasistreamlined body in which the trailing edge is modified to form a base cavity. Here, we seek to establish if any synergistic merits exist through a combination of a spanwise wavy trailing edge with a straight-edged base cavity. Both two-dimensional and three-dimensional (3D) simulations are conducted to assess the effect of Reynolds number, streamwise cavity length and spanwise waviness on the flow. Two dimensional simulations examining a variation in Reynolds number for a cavity of constant length reveal the presence of three different general wake patterns: the steady wake, the Karman wake, and an asymmetric, deflected wake. Most critically, the origins of the deflected wake pattern are traced to the presence of a vortex which resides within the base cavity. Similarly, for a constant Reynolds number the influence of the cavity length on the flow is also intricately related to this cavity vortex, giving rise to wake topologies which bear a strong resemblance to the above three shedding processes. Reductions in drag are observed for all the investigated cavity configurations and additionally it is found that the magnitude of the reduction obeys a direct relationship with the length of the cavity up to a certain asymptotic value. The results of the 3D simulations reveal that the current base-cavity arrangement appears to yield a potential for further reduction in form drag as compared to earlier studies where the entire base region is modified in the shape of a wave. However, for a fixed cavity length, introducing spanwise waviness reduces the fluctuation intensity of the form drag but offers no obvious added benefit in terms of the time-averaged values. We anticipate that the presence of the straight-edged cavity causes a loss of spanwise coherence of these structures such that further enhancements due to the introduction of waviness are of less consequence.