Dynamic Mode Decomposition of Flow around a Full-Scale Road Vehicle using Unsteady CFD

Abstract

The unsteady aerodynamic forces and flow structure around a full-scale road vehicle were investigated in this study. Special focus was given to the fluctuating lift force acting on the vehicle. It was proposed that the fluctuating lift force, especially at frequencies less than 5 Hz, affects the directional stability and the feel of the ride, because this frequency range is close to the natural frequency of the vehicle's mechanical system. The objectives were to clarify the flow structures that cause the low-frequency fluctuation of the lift force by using a mode decomposition technique and to suggest a method for improving the vehicle's aerodynamic stability. The target model was a full-scale production vehicle. The model had a complicated geometry based on available CAD data, so that the engine compartment and complicated underbody geometry were reproduced. An unsteady CFD technique was applied to obtain the transient aerodynamic forces and snapshots of the flow structure around the vehicle. Dynamic mode decomposition was used to extract the fluctuating flow features at a specific frequency. The results indicated that the lift force acting on the trunk deck and rear underbody contributed to the low-frequency fluctuating lift force. NomenclatureA = frontal area of the vehicle (= 2.189 m 2 ) CSGS = model constant of Smagorinsky model CD = drag coefficient (= 2Fx /ρu 2 A) CL = lift coefficient (= 2Fz /ρu 2 A) CLF, CLR = lift coefficient acting on the front wheel and the rear wheel 2 fd= Van-Driest-type function Fx, Fy = drag force and lift force acting on the vehicle i, j, k = directions of spatial coordinate p = static pressure Sij = strain rate tensor ui = velocity for i-th direction uτ = friction velocity on the solid wall y + = non-dimensional distance from the wall (= yp uτ/ν) Δ = spatial filter length (cube root of the numerical element volume) ν = kinetic viscosity ρ = density of the incoming fluid

Application of the DMD Approach to High-Reynolds-Number Flow over an Idealized Ground Vehicle

“…The reduction of the aerodynamic drag force remains a core objective of vehicle aerodynamic development and is motivated by the desire to reduce fuel consumption [1,2]. Researchers have attempted to achieve aerodynamic drag reduction through different types of passive flow control devices such as the front bypass ducts [3], rear bypass ducts [4], and various deflector designs [5][6][7].…”