Journal of Wind Engineering & Industrial Aerodynamics
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Journal of Wind Engineering & Industrial Aerodynamics 206 (2020) 104301 Contents lists available at ScienceDirect Journal of Wind Engineering & Industrial Aerodynamics journal homepage: www.elsevier.com/locate/jweia Assessment of hybrid RANS-LES methods for accurate automotive aerodynamic simulations P. Ekman a,*, D. Wieser b, T. Virdung c, M. Karlsson a a Linkoping€ University, SE58183, Linkoping,€ Sweden b Technische Universitat€ Berlin, DE10623, Berlin-Charlottenburg, Germany c Volvo Car Corporation, SE40531, Goteborg,€ Sweden ARTICLE INFO The introduction of the Harmonized Light Vehicles Test Procedure causes a significant challenge for the auto- fi Keywords: motive industry, as it increases the importance of ef cient aerodynamics and demands how variations of optional CFD extras affect the car’s fuel consumption and emissions. This may lead to a huge number of combinations of DDES optional extras that may need to be aerodynamically analyzed and possibly optimized, being to resource- IDDES consuming to be done with wind tunnel testing merely. Reynolds Average Navier-Stoles (RANS) coupled with SBES Large Eddy Simulations (LES) have shown potential for accurate simulation for automotive applications for DrivAer reasonable computational cost. In this paper, three hybrid RANS-LES models are investigated on the DrivAer Notchback notchback and fastback car bodies and compared to wind tunnel measurements. Several yaw angles are inves- Fastback tigated to see the model’s ability to capture small and large changes of the flow field. It is seen that the models Yaw Wind tunnel generally are in good agreement with the measurement, but only one model is able to capture the behavior seen in Turbulence modelling the measurements consistently. This is connected to the complex flow over the rear window, which is important to capture for accurate force predictions. 1. Introduction (windless conditions), this makes the aerodynamics more important during the test cycle. For speeds over 80 km/h, the aerodynamic drag is Transports are responsible for almost 25 % of the greenhouse gas the main energy-consuming source Hucho and Sovran (1993). Secondly, emission in Europe and are the leading cause of air pollution in cities is that the exact configuration of the car that actually is sold to the European Commission (2016a). Of these 25 %, almost half of the carbon customer needs to be certified in terms of fuel consumption and emis- dioxide (CO2) emissions are emitted by passenger cars. Since 2009, the sions. The rationale for this is that customers should be able to have a European Commission has introduced legislation for reducing the emis- better understanding of what impact any specific configuration will have sions of new passenger cars. In 2015, a maximum limit of 130 g of on fuel consumption and emissions. This means that any combination of CO2/km was applied as regulation for all new vehicles, as a fleet-wide optional extras that the customer can add to the car must be certified with average. From 2021 the emission target will be lowered even further to WLTP. For example, the Volvo XC90 can theoretically be externally 95 CO2/km. For meeting these emission level criteria, a fuel consumption configured in more than 300 000 different combinations Ekman et al. of around 4.1 L=100km and 3.6 L=100km is needed for petrol and diesel (2019). Of all of these combinations, more than 200 specific combina- internal combustion engine (ICE) cars, respectively European Commis- tions may, in fact, have a significant impact on the aerodynamics and sion (2016b). therefore need to be analyzed and possibly optimized. For electric ve- In 2017, the Worldwide Harmonized Light Vehicles Test Procedure hicles, this has no impact on the emissions but instead directly affects the (WLTP) for measuring the emissions and fuel consumption of cars was range of the vehicle. The energy losses from aerodynamic drag is re- introduced WLTP Facts (2017). This test procedure has two major effects ported to be 4.4 times larger for electric vehicles (EV) than seen for ICE on the importance of aerodynamics for cars. Firstly, the average speed of vehicles Kawamata et al. (2016), resulting in even more importance for the test cycle is increased to 46 km/h, which is 12 km/h higher than in efficient aerodynamics. In 2019 Audi AG stated that 5 drag counts (one À3 the previously used test cycle (New European Driving Cycle). As aero- drag count ¼ ΔCD Á 10 ) corresponded to a 2.5 km in range of their fully dynamic forces increase with the square root of the vehicle’s velocity electric SUV Audi AG (2019). This may result in that optional extras, such * Corresponding author. E-mail address: [email protected] (P. Ekman). https://doi.org/10.1016/j.jweia.2020.104301 Received 23 December 2019; Received in revised form 1 July 2020; Accepted 4 July 2020 Available online xxxx 0167-6105/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). P. Ekman et al. Journal of Wind Engineering & Industrial Aerodynamics 206 (2020) 104301 as rims, spoilers, towbar and roof-rails, directly affect the range of the is achieved further downstream Mockett (2009). Fast transition between vehicle. the regions is, therefore, especially important for accurate predictions of Traditionally wind tunnel testing has been the main tool for the separating shear layers Menter (2016), being an essential flow feature of aerodynamic development of vehicles, but with the introduction of car aerodynamics. WLTP, which drastically increases the number of necessary aerodynamic In recent time several models have been proposed which attempts to design analysis, and the ever more importance for efficient aerodynamics solve these major challenges, such as the φ À f elliptic relaxation model of electric vehicles, Computational Fluid Dynamics (CFD) cannot merely Ashton et al. (2011), Stress Blended Eddy Simulation (SBES) Menter be a complement to wind tunnel testing during development. Today (2016) and the σ-DDES approach Nicoud et al. (2011), Fuchs et al. accurate CFD methods exist, as the Scale-Resolving Simulation (SRS) (2020). method Large Eddy Simulations (LES) have shown excellent agreement Historically most validation and sensitivity studies for CFD within the to measurements for flow around bluff bodies Krajnovic and Davidson automotive sector have been performed on simplified generic car bodies, (2004), Hinterberger et al. (2004), Serre et al. (2013). Unfortunately, LES such as the Ahmed body Ahmed et al. (1984) and the SAE body Le Good is, for most cases not feasible, due to the very high computational cost, and Garry (2004). However, such simplified car bodies do often only especially for the moderate to high Reynolds numbers that typically are resemble some of the aerodynamic features of ground vehicles. To fill the used during aerodynamic development of cars. Historically, Reynolds gap between the too generic bluff bodies and fully detailed production Average Navier-Stokes (RANS) simulations have been performed as a cars that are not public, TU Munich, in cooperation with car manufac- complement to wind tunnel testing during the aerodynamic develop- turers Audi AG and BMW, designed and released the generic DrivAer car ment. In RANS, the behavior of all the turbulence is modeled, which body Heft et al. (2012). The DrivAer design is a hybrid of the Audi A4TM makes it possible to use a significantly coarser spatial resolution than for and BMW 3-seriesTM and exists as a mid-size car and SUV Zhang et al. LES, resulting in lower computational costs. However, the flow around (2019) in three different rear end configurations, fastback, notchback, bluff bodies and vehicles are highly time-dependent and unsteady, and estate, respectively. The aerodynamic resemblance of a production typically causing RANS methods to struggle to capture details of the flow car has made it a popular validation and reference case for both wind accurately. Reasonable accurate drag predictions are possible with RANS tunnel measurements and CFD simulations. but can be very case dependent Guilmineau (2014), Ashton and Revell Although the DDES and IDDES models have shown good correlation (2015), Ashton et al. (2016), Rodi (1997), making it unreliable for ac- to wind tunnel measurements for the DrivAer car body (Ashton and curate aerodynamic development. For solving the high-cost deficits of Revell (2015); Ashton et al. (2016); Collin et al. (2016); Wang et al. LES and lower accuracy of RANS, hybrid RANS-LES methods have (2019)) there is still room for improvement, especially for the flow over become popular. The most common method is the Detached Eddy the rear part of the body. The fastback and notchback rear end configu- Simulation (DES) approach, where RANS is used to model the near-wall rations of the DrivAer car body have in wind tunnel measurements shown flow while LES is employed elsewhere in the domain Spalart (1997).As complex flow over the rear window with shallow separations affected by RANS is used near the wall, a coarse spatial resolution (compared to LES) A and C-pillar vortices Wieser et al. (2014, 2015a). This type of flow is is possible in the near-wall region, while a finer spatial resolution very challenging for hybrid RANS-LES models and includes features (compared to RANS) only is needed where turbulence is resolved. This where models with improved RANS-LES region transitioning have shown approach greatly reduces the overall mesh size and also the required significant improvements over the DDES model Fuchs et al. (2020). Ac- temporal resolution compared to LES. curate prediction of these complex and sometimes small details of the The original DES model proposed by Spalart et al. Spalart (1997) is flow field can be crucial for accurately predicting changes to the forces accurate for flow with thin boundary layers and massive separations with changes in the flow conditions.