Computational Simulation of a Heavy Vehicle Trailer Wake

Abstract

To better understand the flow mechanisms that contribute to the aerodynamic drag of heavy vehicles, unsteady large-eddy simulations are performed to model the wake of a truncated trailer geometry above a no-slip surface. The truncation of the heavy vehicle trailer is done to reduce the computational time needed to perform the simulations. Both unsteady and time-averaged results are presented from these simulations for two grids. A comparison of velocity fields with those obtained from a wind tunnel study demonstrate that there is a distinct difference in the separated wake of the experimental and computational results, perhaps indicating the influence of the geometry simplification, turbulence model, boundary conditions, or other aspects of the chosen numerical approach.

Investigation of the near-wake flow topology of a simplified heavy vehicle using PANS simulations

“…However, when the PANS equations are used with CDS, the flow topology is anti-symmetric to the experiments -flow state II ( Fig. 4(f)), but similar to the results of the LES of Rao et al (2018b) and Ortega et al (2004), and to the recent experimental investigations of Schmidt et al…”

“…Sitlani and Aung (2006) reported a 46% increase in the drag coefficient for a GTS model with a sharp lower frontal edge as compared to the standard case with the filleted edge. However, the flow topology at the rear of the model was found to be invariant of the frontal shape of the model as seen in the results of Ortega et al (2004), where an asymmetrical flow topology similar to Storms et al (2001) was observed in the wake of a truncated GTS model, albeit transposed across the lateral midplane. The schematic of the GTS model used in this study is shown in Fig.…”

“…Previous studies using RANS (Reynolds-averaged Navier-Stokes) turbulence models have failed to accurately predict the asymmetrical flow topology in the vertical midplane (Salari et al (2004), Roy et al (2006), Ghias et al (2008)). LES for a truncated GTS model by Ortega et al (2004) and detached eddy simulations (DES) by Unaune et al (2005) showed a flow topology in the vertical midplane which is anti-symmetric to that observed in the experiments, while the URANS (unsteady Reynolds-averaged Navier-Stokes) k À ε RNG turbulence model used by Gunes (2010) showed a flow topology similar to the experimental studies at Re ¼ 2 Â 10 6 . With the flow topology remaining invariant over a wide range of Reynolds numbers, well-resolved LES were undertaken by Rao et al (2018b) for a simplified GTS model, with modifications to the frontal A-pillars to obtain solutions on a purely hexahedral mesh, replicating the work of McArthur et al (2016).…”

“…It may be recalled that the height-to-width ratio of the GTS (1.392) is approximately equal to the width-to-height ratio of the squareback Ahmed body (1.35), thus indicating that the aspect ratio of the bluff body is a critical parameter in determining the occurrence of bi-stable flow states (Grandemange et al (2013a), McArthur et al (2016)). While the bi-stable states observed in the wake of squareback Ahmed body occur in the lateral midplane, the two flow states in the GTS wake are observed in the vertical midplane; flow state I in McArthur et al (2016) and Gunes (2010), and flow state II in Schmidt et al (2018), Rao et al (2018b), Ortega et al (2004) and Unaune et al (2005). The wake asymmetry (and the resulting bi-stable flow state) was found to originate at low Reynolds numbers for a squareback Ahmed body, where the flow transitions from a steady asymmetric flow state to an unsteady asymmetric flow state at Re ' 410 via an imperfect supercritical bifurcation (Grandemange et al (2012), Cadot et al (2015)), and is observed at Reynolds numbers well into the turbulent region of flow at Re ' 9:5 Â 10 4 .…”

Finding Computationally Inexpensive Methods to Model the Flow Past Heavy Vehicles and the Design of Active Flow Control Systems for Drag Reduction

“…To accurately predict the integrated forces over the GTS is particularly important to be able to model the unsteady flow behavior, vortex shedding frequency, and to obtain an accurate prediction of boundary layer separation. Various studies have been taking place using RANS, 7,8 Large Eddy Simulations (LES), 9 and hybrid RANS/LES methods. 10 Due to the complexity of the problem, LES has been shown to be the most reliable method when it comes to the predictions of the flow features and integrated forces.…”