Award Date
8-1-2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
First Committee Member
Yi-Tung Chen
Second Committee Member
Hui Zhao
Third Committee Member
Jeremy Cho
Fourth Committee Member
Melissa Morris
Fifth Committee Member
Monika Neda
Number of Pages
301
Abstract
In the field of automotive aerodynamics, the Ahmed body is a generic vehicle-shaped bluff body which is meant to generalize the rear end of fastback and hatchback vehicles. Specifically, an Ahmed body with a rear slant angle of 25° has seen common use due to its resemblance of common rear windshield shapes as well as its notoriously complex wake structure. This research focuses on simulating the flow over a 25° in various situations with an ultimate goal of drag reduction. First, a simple benchmark of the k-ω shear stress transport (SST), large eddy simulation (LES) with the Smagorinsky-Lilly subgrid scale model, LES with the wall-adapting local eddy viscosity (WALE) subgrid scale model, and improved delayed detached eddy simulation (IDDES) for the flow over a 25° Ahmed body traveling at 30 mph or approximately 13.44 m/s. Using the body height H as the characteristic length, this makes the Reynolds number ReH ≈ 2.65 × 105 In this benchmark, LES with the WALE was found to predict drag and lift forces both within 6% of previous experiments conducted at higher Reynolds numbers as well as consistent wake structures to higher Reynolds number experiments. This demonstrated a degree of Reynolds number independence for LES with the WALE model, making it suitable for use in lower Reynolds number simulations to reduce computational costs.
Next, simulations were performed for two platooning 25° Ahmed bodies to validate some previous experimental data as well as provide further information into the energy in the wake regions when driving in disturbed air. Regarding the aerodynamic forces, it was found that the first body in the platoon experiences a decrease in drag of up to 59.23% due to a drastic increase in pressure at its rear end when the second vehicle is following closely. For the second body, an increase in drag of up to 33.45% was observed for close following distances as the wake of the body ahead impinges on its rather blunt front end, leading to markedly higher pressures at the front end. However, although one body experienced an increase in drag, the total drag of the two vehicles in a platoon was found to always be less than the total drag of two bodies traveling in isolation, with the net drag being as low as 87.10% of that of two isolated bodies for close following distances. In addition to changes in drag, lower turbulent kinetic energies were observed in many portions of the wake of the second body in the platoon, with this being attributed to the lower upstream total pressure compared to that upstream of the body ahead of it. Using a proper orthogonal decomposition (POD) on fluctuating pressure components in a downstream plane, it was found that 80-83% of the total turbulent kinetic energy was found in the first mode, with this mode consisting of the C-pillar vortices and other vortical structures in the separation bubble behind each body. Using POD to identify high-energy modes of the flow gives an indication of which vortical structures should be targeted when implementing a drag reduction modification, as well as how these structures are altered during a close following scenario compared to driving in isolation.
Finally, a passive drag reduction modification was developed taking into consideration the POD modes and which flow structures contribute most to the overall drag force. Of the two designs tested, a novel bifurcating duct concept based on quadratic and cubic Bézier curves was found to improve the aerodynamic forces by a noticeable margin, where a reduction in drag of 2.77% and a reduction in lift of 14.75% were observed. Further improvements were made through both a more manual reshaping of the ducts and adjoint optimization, and the end result was a morphed bifurcating duct which reduced drag by 4.77% and lift by 15.90% compared to an isolated body. Additionally, the high-energy first POD mode had an overall energy content that was reduced by over 10%, with energy being transferred to other POD modes via the energy cascade, demonstrating the design’s success at reducing the energy content of high-energy POD modes. While a more complex optimization algorithm is needed to truly maximize drag reduction when the flow field is highly transient, the bifurcating duct showed great promise in drag (and lift) reduction, so further testing of this type of design is recommended for passive drag reduction on automobiles.
Keywords
Aerodynamics; Ahmed Body; CFD; Drag Reduction; LES; Platooning
Disciplines
Aerospace Engineering | Mechanical Engineering
File Format
File Size
8300KB
Degree Grantor
University of Nevada, Las Vegas
Language
English
Repository Citation
Polek, Skylar, "Development of a Passive Drag Reduction Modification for the 25° Ahmed Body using Large Eddy Simulation" (2024). UNLV Theses, Dissertations, Professional Papers, and Capstones. 5143.
https://digitalscholarship.unlv.edu/thesesdissertations/5143
Rights
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