Comparison of Vortex Identification Methods on Base Wake Structures in Bluff Body Flows

<div class="section abstract"><div class="htmlview paragraph">In this work, a modified Ahmed body with both upsweep and downsweep was used to create a complex wake. The time-averaged streamline topology revealed that the wake was composed primarily of a torus past the vertical base and two pairs of streamwise-oriented vortices on the upper and lower slant edges. Several vortex identification methods including three-dimensional (3D) (<i>Q</i>−, <i>λ</i><sub>2</sub>−, Ω−criteria, and Liutex method) and two-dimensional (2D) (Γ<sub>1</sub>−criterion) methods were compared to determine the effectiveness in identifying complex wake structures. Of the 3D methods analyzed, none produced wholly satisfactory results. The <i>Q</i>− and <i>λ</i><sub>2</sub>−criteria were plagued by well noted issues; failing to separate shear from rotation and threshold sensitivity which led to inconsistently identifying the weaker torus. The Ω−criterion addressed all of these concerns, especially identifying the torus consistently. However, the identified torus structure did not reflect the physical structure observed using the streamline topology as a ground truth. This phenomenon was caused by Geometry Induced Solid Body Rotation (GISBR), in which the complex geometry resulted in streamline curvature producing local regions of solid body rotation even without a vortex. Therefore, the <i>Q</i>−,<i>λ</i><sub>2</sub>−, and Ω−criteria cannot be recommended for analyzing complex 3D wakes. Based on the current results, the Liutex method offers the best compromise of the 3D methods analyzed. However, it was also negatively impacted by the GISBR as were all 3D vortex identification methods. Only the Γ<sub>1</sub>−criterion successfully aligned with the streamline topology and was not impacted by GISBR. But, this method is currently relegated to 2D flows.</div></div>