Date of Award

3-10-2021

Publication Type

Master Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

First Advisor

Ram Balachandar

Second Advisor

Vesseline Roussinova

Keywords

aerodynamics, ahmed body, bluff body, CFD

Rights

info:eu-repo/semantics/openAccess

Abstract

This thesis investigates the effects of approach flow conditions on the unsteady three-dimensional (3D) wake characteristics of a square back Ahmed body using improved delayed detached eddy simulations (IDDES). Two approach flow conditions were investigated: a reference uniform flow (Case A) and a thick turbulent boundary layer (TBL) (Case B), where the Ahmed body is fully submerged in the TBL. Case A has been studied extensively in the literature, whereas Case B is rarely studied, although it provides a more realistic flow condition for on-road vehicles. The present results showed that the wake structure of Case B is significantly different from that of Case A. In particular, the wake of Case A is dominated by a strong downwash flow from the top surface, but Case B exhibits a much stronger upwash flow, which is induced by the low momentum of the fluid that emanates from the clearance between the ground and vehicle. For Case A, conditional averaging of the flow field based on the sign of the drift force demonstrated the occurrence of the well-known wake-switching (bimodality) event in the spanwise direction. However, the bimodality was completely suppressed in the wake of the Ahmed body submerged in the TBL, indicating the influence of boundary layer thickness relative to body height on the wake-switching event. Both time-averaged turbulence statistics such as the Reynolds stresses and production terms, and time-resolved statistics including spectral analysis and temporal cross-correlations are used to explore the differences between the wake structure of the Ahmed body in a uniform flow and thick TBL.

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