Influence of Free and Forced Disturbances on the Shear Layer Separating from a Square Prism

This project investigates the use of active flow control on square bluff bodies relevant to tall buildings. The research seeks to ameliorate the unsteady loading by controlling the formation and development of the von-Kármán vortex street, which is known to be the cause of undesirable building motion. More specifically, a fluidic jet is periodically excited at each of leading edges of a square prism where the shear layers separate and evolve into the large-scale vortex shedding in the wake. Thus the perturbation influences the evolution the shear layers, which in turn, influences the formation and growth of the larger fluid structures. This research additionally aims to explore the effectiveness of this control technique in the presence of high levels of freestream turbulence. To this end, an additional experimental campaign was conducted which aimed to understand the influence of small scale freestream turbulence on the development and evolution of the bluff body shear layers separating from a square prism. This work observed a link between the altered dynamics of the shear layer transition (a bypass transition) and the formation of the von-Kármán vortex street shedding.

Schematic of the active flow control model used for test the shear-layer control strategy. An internal rotor supplies the unsteady actuations at the separation edges by pulsing compressed air.

The influence of freestream turbulence on the transitional characteristics of a 2D square prism shear layer is considered experimentally.

Decomposition of the velocity field into a coordinate system which is locally perpendicular the shear layer axis.

Instantaneious Q criterion of the square prism shear layer demonstrating the spatial development of the Kelvin Helmholtz (KH) instability, including pairing of KH coherent structures.