Comparison of Vortex Identification Methods in Highly Separated Base Wakes

In this work, the complex wake flow from a double-slanted Ahmed body with an upper slant of α = 25° and a standard single-slanted Ahmed body with a slant angle of 40° were used to evaluate vortex identification methods for automotive wake flows. Multiple three-dimensional (3D) vortex identification methods including Q−, λ ₂−, Ω− criteria, and Liutex method and the two-dimensional (2D) Γ₁− criterion were evaluated against the streamline topology as a pseudo-truth model. Of the 3D methods analyzed, none were found to produce wholly satisfactory results. The Q− and λ ₂−criteria were plagued by high threshold sensitivity and a failure to separate shear from rotation which led to inconsistent identification of the weak, lower-rotation vortices. While the Ω−criterion was able to mitigate the issues related to threshold sensitivity and separation of shear and rotation by consistently identifying the weak vortices, the identified structure did not align well with the streamline topology, producing mismatches at least three times greater than any other method analyzed. This phenomenon was found to be partially caused by geometry-induced solid body rotation (GISBR), in which the complex geometry interacting with the flow results in streamline curvature that produced local regions of solid body rotation without the presence of a vortex. Additionally, the localized non-dimensionalization of the Ω−criterion was found to exacerbate the effects of GISBR in mischaracterizing the weak, lower-rotational flow structures. Of the methods tested, the Liutex method offered the best compromise for 3D flows. Although the Γ₁− criterion was found to most consistently align with the streamline topology and was not impacted noticeably by GISBR, the method is currently relegated to 2D flows due to the need of assuming the axis of rotation. A possible extension of the Γ₁− criterion to 3D flows has also been proposed.