Impact of Longitudinal Acceleration and Deceleration on Bluff Body Wakes

Abstract

This study investigated the unsteady acceleration aerodynamics of bluff bodies through the study of a channel mounted square cylinder undergoing free-stream acceleration of ± 20 m / s 2 with Reynolds numbers spanning 3.2e4 to 3.6e5. To achieve this, a numerical simulation was created with a commercial finite volume unstructured computational fluid dynamics code, which was first validated using Improved Delayed Detached Eddy Simulation against experimental and direct numerical simulated results. Then, the free stream conditions were subjected to a periodic velocity signal where data were recorded and ensemble averaged over at least 30 distinct acceleration and deceleration data points. This enabled the comparison of body forces and flow field variations among accelerating, steady and decelerating free-stream conditions. Body force analysis determined that decelerating and accelerating drag forces varied −47% and 44%, respectively, in comparison to steady free-stream conditions. In addition, several differences were also observed and explored such as near-body flow structures, wake dynamics, Kármán vortices and vorticity production during the aforementioned conditions. The primary interest of this study was for the future application towards road vehicles for predictive dynamic modeling and aerodynamic development.

Keywords

• acceleration aerodynamics
• added mass
• ensemble averaging
• bluff body aerodynamics
• Computational Fluid Dynamics (CFD)
• Improved Delayed Detached Eddy Simulation (IDDES)