Numerical study of active flow control of wake induced loading on slender bluff bodies

Abstract

This study investigates the effectiveness of fluid-based aerodynamic modifications to slender bluff bodies. Computational fluid dynamics was used to simulate multiple cases of fluid forcing around a two-dimensional square prism.

The first set of numerical simulations focused on model validation, and investigated the effects of free stream turbulence on bluff bodies. A small circular rod was placed directly upstream of a square prism. It was shown that the fine-scale turbulence generated from the small rod was entrained into the separated shear layers and disrupted their coherency. This prevented the shear layers from rolling up into discrete vortices, greatly reducing the forcing on the square prism. The results showed that the drag reduction from the upstream rod favorably agreed with the wind tunnel experiment. Additionally, the reduction in the fluctuating lift force was similar to wind tunnel testing performed with uniform turbulent inflow conditions. This showed that small permutations to the flow field could create disruptions to the global flow field, and reduce loading.

A multitude of simulations using jets on the square prism were tested. The goal of the simulation was to add a small amount of energy from the jets to the flowfield and cause a significant reduction in the loading on the square prism. The location, angle, frequency, and velocity of the jet were tested to investigate which parameters were most influential in modifying the global flowfield. It was observed that steady jets were not effective at reducing the forcing on the square prism. However, by modifying the jet to pulse at different frequencies smaller momentum injections were needed to have the same or greater effects on the flow. The frequency at which the jet was pulsed was a key parameter in the jet's effectiveness. The most effective jet frequency corresponded to the natural frequency of the shear layer. In this case, the jet significantly changed the shear layer's coherency and prevented discrete vortex formation in the wake. This lead to large reductions in the fluctuating forces on the bluff body. Additionally, this study showed that the jet velocity was a governing parameter. A larger reduction in fluctuating forces was observed as the averaged jet velocity was increased.

The environment in which buildings are constructed is turbulent, which significantly effects the flow around bluff bodies. The final part of the study examined how effective the jets would be in the presence of free stream turbulence. The synthetic eddy method was employed to create turbulent inflow boundary conditions. This inflow matched with homogeneous isotropic turbulence generated in wind tunnels. The base results for turbulent flow around a square prism compared well with wind tunnel data. There was a large force reduction on the square prism with free stream turbulence present similar to the addition of a small rod upstream. The additions of unsteady jets were able to add to the effects of the free stream turbulence promoting partial reattachment of the separated shear layer. This decreased the RMS of the lift coefficient. This showed that the efficiency of the jets was not compromised by free stream turbulence and that unsteady jets can be used in turbulent situations such as a building in the atmospheric boundary layer.

A rapid population growth concentrated in urban areas is predicted to occur in the future. This increasing urban population will cause a significant rise in land prices and interior floor space prices. These trends will lead to taller, more slender buildings to be constructed which are very sensitive to wind, and are prone to vibrate under moderate wind events. Currently geometric-base aerodynamic modifications are used to change the shape of the building or add stiffness/damping to reduce vibrations. These modifications reduce valuable floor space and add to construction complexity. An alternative approach is to use fluid-based aerodynamic modifications in which air is exhausted from the building and interacts with the surrounding flow. This actuated fluid can significantly change the flow around the structure and reduce the wind induced vibrations.