An LES Investigation of the Near-Wake Flow Topology of a Simplified Heavy Vehicle

Abstract

Recent experimental investigations by McArthur et al. (J. Fluids Struct. 66, 293-314, 2016) in the wake of a simplified heavy vehicle or commonly known as the ground transportation system (GTS) model has shown that the flow topology is invariant over a large range of Reynolds numbers [2.7 × 10 4 − 2 × 10 6 ]. Numerical simulations are performed to investigate the initial flow topology at a Reynolds number of 2.7 × 10 4 , using well-resolved large eddy simulations (LES). In the vertical midplane behind the GTS, a flow state which is anti-symmetric to that reported in McArthur et al. (J. Fluids Struct. 66, 293-314, 2016) is observed here, thereby, confirming the possibility of occurrence of the complementary bi-stable flow state. The occurrence of this bi-stable state does not depend on the ground clearance between the GTS and the ground plane, as a similar flow topology is observed at both small and large gap heights. Furthermore, the flow topology in the vertical midplane is also found to be insensitive to the incoming flow for small yaw angles. However, complex flow behaviour is observed in the wake for larger yaw angles, where the flow topology in the vertical midplane becomes nearly symmetric, while an asymmetric flow topology is now observed in the lateral midplane in the near wake. Furthermore, the corner vortices which originate from either side at the front of the model merge in the far wake, leading to a large vortex structure nearly equal to the height of the model. The near-wake topology of the GTS is analysed and compared with previous studies for a range of scenarios, and the forces on the GTS are computed.

Simulations of the bi-modal wake past three-dimensional blunt bluff bodies

“…Simulations which accurately capture the wake unsteadiness at high Reynolds number, and which furthermore extend over the large time scales of bi-modal switching, are very costly. Unsteady simulations of simplified bluff body flows have been performed using partially averaged Navier-Stokes (PANS), simulations (Mirzaei, Krajnović & Basara 2015;Rao et al 2018a), unsteady Reynolds-averaged Navier-Stokes simulations (Khalighi, Chen & Iaccarino 2012), detached eddy simulations and large eddy simulations (Krajnović & Davidson 2003;Serre et al 2013;Aljure et al 2014;Östh et al 2014;Rao et al 2018b), some also employing a lattice Boltzmann formulation (Roumeas et al 2009;Lucas et al 2017 The present study achieves what we believe are the first simulations of wake bi-modality, capturing both asymmetric states, for a blunt bluff body. No artificial forcing nor inlet perturbations are applied to trigger bi-modality; the flow develops naturally from a steady inlet velocity condition, with bi-modal switches occurring randomly.…”

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

“…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 .…”

“…The GTS model is representative of a truck and a trailer with no intermediate gap, and is commonly used model in the transportation industry to investigate the aerodynamics of heavy vehicles. While a more detailed description of the near-wake flow topology of heavy vehicles has been presented in Rao et al (2018b), where well-resolved LES were undertaken on a simplified GTS model; a brief description of the literature reviewed is presented here.…”

“…Recent experimental investigations at Re ¼ 2:7 Â 10 4 by McArthur et al (2016) has shown that the flow topology in the vertical midplane of the GTS model is invariant over a large range of Reynolds numbers (Re, defined as the ratio of the inertial to the viscous forces), with a near identical flow topology being observed at Re ¼ 2 Â 10 6 (Storms et al (2001), Croll et al (1996)). The mean flow topology in this plane is asymmetrical, and consists of a large triangular-shaped vortex on one side, with an elliptical-shaped vortex located opposite to it, and a pair of counter-rotating vortices is observed in the lateral midplane (also see McArthur et al (2016) and Rao et al (2018b) for a detailed description of the flow topology). 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). The mean flow topology in the vertical midplane was anti-symmetric to that observed in McArthur et al (2016) for three meshes of increasing spatial resolution, and the flow topology was also insensitive to small yaw angles of up to 2:5 ∘ (also see Gentile et al (2017), Volpe et al (2014a)).…”