Comparison of Vortex Identification Methods on a Complex 3-Dimensional Bluff Body Wake

In automotive base flows, slight changes in geometry can lead to significant changes in base drag brought on by large-scale changes in the wake structure. Properly identifying these structures and their differences can lead to insight into the causal mechanisms of these wake changes. Thus, designers can take advantage of this insight to better design automotive vehicles for reduced drag. However, properly characterizing aerodynamic structures has been a challenge for decades. In this work, we utilized Improved Delayed Detached-Eddy Simulations (IDDES) to simulate flow around an Ahmed body with a modified base to have both upsweep and downsweep simultaneously. This yielded a complex 3-dimensional flow field with multiple regions of separation, recirculation, and vortical structures. These structures were identified using several different vortex identification methods; ∆−, Q−, λ2−, and Ω− criteria. Of these methods, only the Ω− criterion was able to successfully separate the shear layer from the rotational structures in the wake. This allowed for the separation of the base torus from the shear layer entrainment into the streamwise oriented vortices. Additionally, the method proved successful in identifying weaker vortices further downstream in the wake. However, despite these advantages, the Ω−criterion failed to predict the rotational center of the base recirculation region determined by the streamline topology. One the other hand, the other methods produced a local maxima that aligned closely with the rotational center. Therefore, the determination of bluff body wake structures is subject to the vortex identification method used.