Assessment of hybrid RANS-LES methods for accurate automotive aerodynamic simulations

P Ekman,D Wieser,T Virdung,M Karlsson

doi:10.1016/j.jweia.2020.104301

P Ekman, D Wieser + Show 2 more

Open Access

https://doi.org/10.1016/j.jweia.2020.104301

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Abstract

The introduction of the Harmonized Light Vehicles Test Procedure causes a significant challenge for the automotive industry, as it increases the importance of efficient aerodynamics and demands how variations of optional extras affect the car’s fuel consumption and emissions. This may lead to a huge number of combinations of optional extras that may need to be aerodynamically analyzed and possibly optimized, being to resource-consuming to be done with wind tunnel testing merely. Reynolds Average Navier-Stoles (RANS) coupled with Large Eddy Simulations (LES) have shown potential for accurate simulation for automotive applications for reasonable computational cost. In this paper, three hybrid RANS-LES models are investigated on the DrivAer notchback and fastback car bodies and compared to wind tunnel measurements. Several yaw angles are investigated to see the model’s ability to capture small and large changes of the flow field. It is seen that the models generally are in good agreement with the measurement, but only one model is able to capture the behavior seen in the measurements consistently. This is connected to the complex flow over the rear window, which is important to capture for accurate force predictions. • Three hybrid RANS-LES models are compared to wind tunnel measurements for two DrivAer car body configurations. • The turbulence models RANS shielding and transition to LES region properties are essential for achieving consistent accuracy. • The SBES model is the only model to capture the complex flow over the rear window of the notchback car configuration. • The DDES and IDDES models suffer from insufficient shielding of the RANS region and premature switch to WMLES, respectively.

Highlights

Transports are responsible for almost 25 % of the greenhouse gas emission in Europe and are the leading cause of air pollution in cities European Commission (2016a)
In this paper the Delayed DES (DDES) and Improved DDES (IDDES) models are compared to the recently released Stress Blended Eddy Simulation (SBES) model for the flow around the notchback and fastback DrivAer car bodies, to see if stronger Reynolds Average Navier-Stoles (RANS) shielding and faster transition between the RANS and Large Eddy Simulations (LES) regions improves the aerodynamic accuracy simulation of cars, and is to be recommended for use during aerodynamic development
This is followed by comparison, between measurement and simulations, of the rear wake of the notchback vehicle, which accounts for a significant part of the drag for vehicles

Summary

Introduction

Transports are responsible for almost 25 % of the greenhouse gas emission in Europe and are the leading cause of air pollution in cities European Commission (2016a). Of these 25 %, almost half of the carbon dioxide (CO2) emissions are emitted by passenger cars. Since 2009, the European Commission has introduced legislation for reducing the emissions of new passenger cars. From 2021 the emission target will be lowered even further to 95 CO2/km For meeting these emission level criteria, a fuel consumption of around 4.1 L=100km and 3.6 L=100km is needed for petrol and diesel internal combustion engine (ICE) cars, respectively European Commission (2016b)

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