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A 20° SAE Notchback model was utilised in a comparative study between RANS methods with unstructured grids and Implicit DES methods with structured grids. Experimental data acquired in the Loughborough University wind-tunnel with this Notchback geometry was utilised in this investigation to gauge the accuracy of the two methods. The pressure coeffi...
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... . 2 shows the computational domain used in the second-order, unstructured study. The domain extends upstream of the model by 2 characteristic lengths and downstream of the model by 7 lengths. The extended domain behind the geometry is vital to fully capturing the wake regions and more importantly to prevent any reversed flow errors during ...
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... error due to wind-tunnel blockage. In the experiment conducted by Wood et al. [3] the model was mounted on four stingers, 40mm off the base of the wind-tunnel floor. These stingers were omitted but the model remains 40mm above the ground reference plane in the domain. To reduce computational expenses for the simulation, a refinement domain (Fig. 2) has been generated in close proximity to the model, as the wake region and immediate surface of the vehicle are the primary focus of this ...
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Citations
... In particular, the authors mention that small-scale motions completely disappear due to the Reynolds-averaged approach. Nader et al. [6] performed both RANS and Detached Eddy Simulations (DES) on the SAE body model experimentally studied by Wood et al. [7] although at a lower Reynolds number value of 250,000 instead of the 2.3 million value used in the experiment. Once again, a more important separation has been observed experimentally, with the flow severely detaching on the C-pillar and reattaching, which causes a strong impingement on the bootdeck, leading to a pressure peak that is not present in experiments. ...
A spectral/hp element methodology is utilised to investigate the SAE Notchback geometry with 20∘ backlight and 3∘ diffuser at Re=2.3×106. The study presented here considered two different mesh approaches: one focusing on classical h-type refinement with standard solution polynomial order (HFP3) and a second case considering relatively coarse mesh combined with high solution polynomial order (HCP5). For the same targeted number of degrees of freedom in both meshes, the results show significant differences in vorticity, flow structures and surface pressure. The first guidelines for hp refinement strategy are deduced for complex industrial cases. Further work on investigating the requirements for these hybrid techniques is required in order to maximize the benefits of the solution and mesh refinements in spectral/hp element method simulations.
... The inlet for the CFD simulations was placed 1 car length upstream of the nose and although this is close to the stagnation region at the nose of the car based on past bluff body investigations (23) , accurate inflow conditions were expected to minimise the impact on the results. High-resolution, turbulence-resolving simulations in existing literature, for example using an Ahmed body (6) also deemed 5L downstream distance to be sufficient.Preliminary studies conducted by the authors for a separate publication, also revealed that increasing the overall domain size to 11L had minimal influence on the results (24) . The top and lateral boundary surfaces were enlarged compared to the wind-tunnel test section and were defined as free-shear walls. ...
... The wind-tunnel model was mounted on four 8 mm diameter slender posts and a shallow diffuser on the car's underbody were removed from the domain to facilitate the creation of a high-quality structured mesh around the car. The influence of the diffuser was quantified previously in separate preliminary studies leading to a comparative article published on the notchback geometry (24) . Over the surface of the diffuser, the static pressure was 76% lower than the baseline configuration and this led to some additional downforce as expected although the pressure over the rest of the underbody remained nearly identical. ...
... It is important to note that to the best of our knowledge, this is the first comprehensive aerodynamic study of the SAE Notchback model and given the diverse flow features which are generated around this geometry combining both sharp-edged and subtle pressure-induced separation, this series of studies (24,13) seeks to add significant value to the research community. Results from the frequency domain analysis used a total of 1.02 million data points representing individual time-steps. ...
This research explores the modification and implementation of a Detached-Eddy Simulation (DES) in a high-order compressible solver and its application to automotive aerodynamics. This was conducted on a 20° SAE Reference Notchback Model with a Reynolds number of 2.23 × 10 ⁵ . This DES algorithm implemented within FLAMENCO, which is finite-volume research code operating over multi-block meshes, was used for all the simulations. The primary objectives were to capture unsteady flow features, separated coherent structures and also relax the meshing requirements to improve accessibility to turbulence-resolving methods for realistic configurations. This also aims to better understand the separated flow physics, especially around the base surfaces of the car. Simulations for three mesh refinement levels were compared to wind-tunnel measurements. Even on relatively coarse meshes (~7 m cells) for DES, time-averaged Cp was obtained with maximum errors of <8%.
Flow around a realistic car model (DrivAer fastback), was analyzed using Improved DelayedDetached-Eddy Simulations (IDDES) and the Lattice-Boltzmann Method (LBM). Two gridswere implemented, coarse and fine, with both methods predicting accurate drag on the lattergrid. IDDES showed more grid sensitivity, underpredicting drag by more than 10% on thecoarse grid. Drag buildups revealed discrepancies in drag accumulation on the nose, frontwheels, middle portion of vehicle length, and vehicle base between the two methods. Lowerpressure on the vehicle nose was predicted by IDDES which was rectified by increased gridresolution. Changes in the flow separation along the underside of the front bumper andwheels lead to differing drag accumulations along the underbody and wheel wells. Analysis ofthe base wake revealed stronger wall-normal vortices, determined via circulation, forming inIDDES leading to a higher accumulation of base drag. The increased circulation was partiallyattributed to IDDES predicting increased vorticity around the wheels being entrained into thewake vortices. Despite the Very Large-Eddy Simulation (VLES) of LBM capturing finer scalestructures around the vehicle in general, streamwise oriented planes of normalized turbulentkinetic energy matched well between the two methods. However, differing trends in the scalesof turbulent structures along with complex body interactions, lead to differing turbulent wakesaround the vehicle base and rotating wheels. Comparison of normalized surface pressurefluctuations with experiments did not identify either method as superior to the other forprediction of the unsteady flow field despite the noted discrepancies.
As the automotive industry strives to increase the amount of digital engineering in the product development process, cut costs and improve time to market, the need for high quality validation data has become a pressing requirement. While there is a substantial body of experimental work published in the literature, it is rarely accompanied by access to the data and a sufficient description of the test conditions for a high quality validation study. This paper addresses this by reporting on a comprehensive series of measurements for a 25% scale model of the DrivAer automotive test case. The paper reports on the measurement of the forces and moments, pressures and off body PIV measurements for three rear end body configurations, and summarises and compares the results. A detailed description of the test conditions and wind tunnel set up are included along with access to the full data set.
Islam, A., & Thornber, B. (2017). High-order detached-eddy simulation of external aerodynamics over an SAE notchback model. The Aeronautical Journal, 1-26. doi:10.1017/aer.2017.61
This research explores the modification and implementation of a Detached-Eddy Simulation (DES) in a high-order compressible solver and its application to automotive aerodynamics. This was conducted on a 20° SAE Reference Notchback Model with a Reynolds number of 2.23 × 105. This DES algorithm implemented within FLAMENCO, which is finite-volume research code operating over multi-block meshes, was used for all the simulations. The primary objectives were to capture unsteady flow features, separated coherent structures and also relax the meshing requirements to improve accessibility to turbulence-resolving methods for realistic configurations. This also aims to better understand the separated flow physics, especially around the base surfaces of the car. Simulations for three mesh refinement levels were compared to wind-tunnel measurements. Even on relatively coarse meshes (~7 m cells) for DES, time-averaged C p was obtained with maximum errors of <8%.
