Session: 8.2 - Fluid Machinery

Paper Number: 155309

155309 - The Effects of Open and Closed Diffusers on the Wake Structures of an Ahmed Body 

Abstract: 

As automotive manufactures continue to apply passive aerodynamic devices to the geometry of vehicle bodies, it's important to study the effect they have on the wake structures. Understanding these wake structures are critical to creating effective designs to minimize the drag that are produced by the vehicles, increasing their overall efficiency. With electric vehicles (EV) having fewer mechanical systems, this provides the space in the rear portion of the vehicle to incorporate diffusers into the design of the body. The introduction of different geometries can have an affect on the distribution of energy in the wake.

 The purpose of this study is to perform a numerical analysis on the affect that a diffuser has on the distribution of energy within the wake structures behind an Ahmed body. To achieve this, a diffuser with varying angles between 0-20° was applied to the underside of a quarter-scale square-backed Ahmed body. A computational fluid dynamics (CFD) simulation was used to determine the time-averaged location of different coherent wake structures, such as streamwise vortices, recirculation zones, and separated flow in the diffuser. It was found that with the increasing diffuser angle, the streamwise vortices had an increase in strength. At an angle of 10°, a small recirculation bubble formed in the diffuser due to the separated flow being tripped on the transition. Using an Improved Delayed Detached Eddy Simulation (IDDES), the time dependent wake features were captured. This included the Kelvin-Helmholtz instability, shear-layer vortices, corner vortices, and the edge-tip vortices formed from the diffuser. Due to the presence of the diffuser, the typical corner vortex is diminished prior to the end of the Ahmed body. A new vortex generated on the edge of the diffuser, extending into the wake behind the Ahmed body. A Spectral Proper Orthogonal Decomposition (SPOD) analysis provides the distribution of energy between the different modes in the wake.

Presenting Author: Matt Molnar University of Windsor

Presenting Author Biography: