Aerodynamic performances of rounded fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models. Additional tests were conducted with geometry modifications as spoilers and underbody diffusers

WAKE STRUCTURE AND DRAG OF VEHICLES WITH ROUNDED REAR EDGES

The wake structure at the rear of road vehicle is known to be of prime importance in aerodynamics performances [3]: about 30% of the total pressure drag derives from the rear end of the vehicle. While production vehicle present significant curvature at the rear end, most of fundamental aerodynamic analyses were carried out around simplified car models presenting sharp edges at the rear. Since recently, very few papers addressed the question of rear edges curvature in aerodynamics performances. Thacker et al. Showed that rounding the edge between the end of the roof and the rear slant suppressed the separation over the rear window and resulted in 10% drag reduction. Fuller et al.Studied the effect on spatial stability and intensity of the pillar vortex when rounding the side rear pillars. For both of these works, the analysis was focused on the flow behaviour over the rear window: the impact of the rear end rounding on the near wake topology was not discussed. The current study aims to understand how the use of rounded pillars with respect to sharp edges modifies the flow field (over the body surface and in the near wake) and hence the global drag. Moreover, an “academic” and an “industrial” model will be characterized to discuss the applicability of simplified models to simulate properly the sensitivity of pillars rounding

T-PITM: a consistent formulation for seamless RANS/TLES coupling

In the frame of inhomogeneous turbulence, a consistent formalism to seamlessly bridge LES and RANS must be based on temporal filtering. Such an approach is presented, the temporal-PITM, based on a transport equations for the subfilter-scales. In order to obtain a balance resolved/modelled energy consistent with the cutoff frequency imposed by the local mesh refinement, a dynamical subfilter-scale model is used

On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high Reynolds number flow over an Ahmed body

We investigate a hierarchy of eddy-viscosity terms in POD Galerkin models to account for a large fraction of unresolved fluctuation energy. These Galerkin methods are applied to Large Eddy Simulation data for a flow around the vehicle-like bluff body call Ahmed body. This flow has three challenges for any reduced-order model: a high Reynolds number, coherent structures with broadband frequency dynamics, and meta-stable asymmetric base flow states. The Galerkin models are found to be most accurate with modal eddy viscosities as proposed by Rempfer & Fasel (1994). Robustness of the model solution with respect to initial conditions, eddy viscosity values and model order is only achieved for state-dependent eddy viscosities as proposed by Noack, Morzynski & Tadmor (2011). Only the POD system with state-dependent modal eddy viscosities can address all challenges of the flow characteristics. All parameters are analytically derived from the Navier-Stokes based balance equations with the available data. We arrive at simple general guidelines for robust and accurate POD models which can be expected to hold for a large class of turbulent flows.Comment: Submitted to the Journal of Fluid Mechanic

Analyse par imagerie laser des interactions entre une turbulence diffusive et un nuage de particules

Cette étude expérimentale porte sur la caractérisation des interactions entre un nuage de particules solides et un écoulement de jet d’air confiné, présentant une région de dynamique proche d’une turbulence homogène isotrope. L’originalité de l’expérience consiste à mesurer simultanément les champs de vitesse des particules et du fluide afin d’analyser le couplage entre les deux phases. Ces résultats sont obtenus en couplant la technique de PIV 2D2C (2 dimensions 2 composantes) classique et une méthode de PTV développée afin de permettre une mesure de vitesse des particules pour des écoulements globalement dilués mais présentant des zones de fortes surconcentrations en particules. Une analyse de la position instantanée des particules a mis en évidence un régime de concentration préférentielle qui modifie fortement la turbulence du fluide. La mesure simultanée des deux champs de vitesse a permis aussi de définir les statistiques conditionnelles des vitesses du fluide près des particules

Round Turbulent Air Jet Submitted to a Pulsed Coflow

Mean and turbulent properties of an unsteady round air jet submitted to a pulsed coflowing airstream were studied from laser Doppler Velocimetry measurements. The ejection velocity of the jet is kept at a constant value, whereas the coflowing stream is pulsed. These measurements revealed that the unsteadiness leads to a longitudinal partition of the jet. Near the jet exit, the flow is a quasi-steady jet flow. Farther downstream, the flow is unsteady with the creation of a large and propagative structure in the jet flow. The objective of the study is comprehen- sive understanding of the main physical mechanisms induced by the coflow unsteadiness. Consequences of the entrainment process are also discussed

Drag and crosswind sensitivity of rounded fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models

Influence of afterbody rounding on the near wake of fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with different afterbody rounding. Rounding the side pillars generated moderate changes in drag and important lift reductions. The minor effect on the drag force was found to result from opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to afterbody rounding