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Analysis of the Vorticity in the Near Wake of a Station Wagon
Abstract
A computational study of the turbulent flow around a modified station wagon vehicle is presented in order to predict aerodynamic forces and to understand some details of the near wake. The geometrical model was obtained by a three-dimensional scanning process but excludes some details of the vehicle such as the engine bay and the underfloor was considered flat. A hybrid mesh (prisms layers and tetrahedral) was generated with refinements close to the vehicle surface (including the wheels) and the near wake. The simulations were performed in the commercial software ANSYS FLUENT at a Reynolds number of 2.7x106 based on the wheelbase. Numerical results of the drag coefficient predict values of 0.431 which is considered in fairly good agreement with the experimental result (based on a coastdown test) of 0.404. Contours of velocity, pressure and eddy viscosity field show some important features of the separated flow in the rear part of the vehicle. A detailed study of the near wake was performed in which the evolution of the vorticity was analyzed in the downstream direction, showing several pairs of counter-rotating vortices generated from the upper part of the A-pillar, wing mirrors and the fender of the vehicle.
... 4, from this box the volume occupied by the vehicle is subtracted. In order to simplify the model and save on computational cost, it takes advantage of the symmetry of the flow around the vehicle [15], therefore only half of the vehicle is included in the computational domain. Based on the information found in the literature [16], the dimensions used in the present study for the computational domain are those shown in tab. 3. ...
... As can be seen in fig. 13, two bubbles of recirculation or detachment (A and B) appear in opposite directions in the rear of the vehicle, one with greater intensity than the other, as it is said in the theory [15]. Additionally, a recirculation is observed in the front part of the vehicle (C), due to the straight shape of the fairing that directly impacts the flow, which generates an acceleration of the flow in all directions. ...
... Finally, with the analysis of obtained results, we proceed to identify the vorticity fields around the vehicle, for this, it is applied in the vortex visualization method known as "Qcriterion", which is defined as the fluid region that it has the second invariant of the positive velocity tensor [15]. When observing these same iso-surfaces of vorticity but with the intensity of the TVR, it is concluded that this last mentioned vortex, presents a great intensity and influence in the general aerodynamics of the Baja SAE. ...
The aerodynamic analysis of vehicles using Computational Fluid Dynamics (CFD) is a powerful tool that is used to explore and investigate how air flow behaves around the vehicle, gives a qualitative idea of aerodynamic behavior and provides engineers with the information necessary to design the pieces that will be tested in the wind tunnel. This technique allows to find the values of forces and moments to get an idea of the drag resistance and lift forces on the vehicle. It is also possible to determine the position of the center of pressure that exerts a vital importance in the definition of the directional stability of the vehicle in its lateral dynamics. On the other hand, it offers the advantage of being able to see many variables of the problem that are very difficult to access in reality, for example to see current lines, vortex shedding, fields of pressures around the vehicle. Although the use of CFD has become a common practice in the automotive industry in recent decades, it is still not considered as a single technique, since one of the limitations is related to the dynamics of a turbulent flow is quite complex so which analytical and computational study depends on what is called Turbulence Models.
This article aims to show the aerodynamic design process by CFD simulation of a Baja SAE vehicle chassis using Ansys software, widely used for simulation of static charges, thermodynamics, fluid mechanics, multiphysics systems and vibrations and to provide information that can be validated experimentally in the future.
... Accordingly, the dimensions for computational domain are illustrated in Table 7. These dimensions satisfy the requirements found in [40]. ...
The trade-off between the fuel consumption and drag coefficient makes the investigations of drag reduction of utmost importance. In this paper, the rear-end shape optimization of Ahmed body is performed. Before changing the geometry, to identify the suitable simulation method and validate it, the standard Ahmed body is simulated using k − ω shear stress transport (SST) and k-epsilon turbulence models. The slant angle, rear box angle, and rear box length as variables were optimized simultaneously. Optimizations conducted by genetic algorithm (GA) and particle swarm optimization (PSO) methods indicate a 26.3% decrease in the drag coefficient. To ensure the validity of the results, a numerical-experimental study is conducted on the optimized model. Thereafter, the velocity profiles and flow structure in the boundary layers of the original geometry were compared to those of the optimized geometry at different sections. The results indicate that there are points where the velocity profile in the boundary layer can exceed the free stream velocity and return to it again, an overlooked observation in the previous studies. In addition to the streamlines, to better understand the formation of three-dimensional vortexes, the Q-criterion factor is computed and illustrated.
... It can, under certain circumstances, be found along inclined sharp and rounded edges as for instance the A and C-pillar [12][13][14][15][16][17][18] or behind the antenna [19][20][21]. While the rear end flow of generic and realistic vehicle geometries is studied quite widely (for instance in [18,[22][23][24][25][26]), the flow around the A-pillar has received less attention. ...
The flow around passenger cars is characterized by many different separation structures, typically leading to vortices and areas of reversed flow. The flow phenomena in these patches show a strong interaction and the evolution of flow structures is difficult to understand from a physical point of view. Analyzing surface properties, such as pressure, vorticity, or shear stress, helps to identify different phenomena, but still it is not well understood how these are created. This paper investigates the crossflow separation (CFS) on the A-pillar of a passenger car using numerical simulations. It is discussed how the CFS and the resulting A-pillar vortex can be identified as well as how it is created. Additionally, the vortex strength is determined by its circulation to understand and discuss how the vortex preserves until it merges with the rear wake of the vehicle.
The present research study examines the aerodynamic behavior of a Suzuki Forsa vehicle with a racing airfoil. The ANSYS Fluent software and the RANS K-ϵ turbulence model are used to perform a numerical simulation. A coarse and a refined mesh are used to design the airfoil. The data is validated with the refined mesh by using a normalized wall distance (y+=5) and an inflation rate of 1.20. This configuration is used to perform computational fluid dynamics simulations with different angles of attack (5º, 10º, 15º, and 20º). The simulation time was 10 hours for the proposed angles of attack. The results show that the 20º angle of attack provides an adequate negative sustentation force (aerodynamic force) to improve tire grip with tread surface and achieve the most efficient aerodynamic. It is concluded that the 20º angle of attack configuration provides the most downforce and the most efficient aerodynamic configuration for Suzuki Forsa vehicles.
During July 2016, an on-road study was conducted in and around the Toronto, Canada region to investigate the spatial variation of vehicle-induced turbulence on highways. The power spectral density of turbulent kinetic energy (TKE) while following on-road vehicles is significantly enhanced for frequencies greater than 0.5 Hz. This increase is not present while driving isolated from traffic, demonstrating that TKE is enhanced considerably on highways in the presence of vehicles. The magnitude of normalized TKE is found to decay following a power-law relationship with increasing normalized distance behind on-road vehicles, which is most pronounced behind heavy-duty trucks. The results suggest that the TKE in the vehicle wake is maximized in the upper shear layer near the vehicle top. An extended parametrization is outlined that describes the total on-road TKE enhancement due to a composition of vehicles, which includes a vertical dependence on the magnitude of TKE.
Experimental and numerical investigations were performed to determine the pressure distributions and the drag forces on a passenger car model. Experiments were carried out with 1/5th scale model FIAT Linea for 20% and 1% blockage ratios in the Uludag University Wind Tunnel (UURT) and in the Ankara Wind Tunnel (ART), respectively. Computational fluid dynamics (CFD) analysis for 1/5th scale model with 0%, 5%, and 20% blockage ratios was performed to validate various blockage correction methods supplementary to the experimental results. Three-dimensional, incompressible, and steady governing equations were solved by STAR-CCM code with realizable k-e two-layer turbulence model. The calculated drag coefficients were in good agreement with the experimental results within 6%. Pressure coefficients on the model surfaces have shown similar trends in the experimental and numerical studies. Some of the existing blockage correction methods were successfully compared in this study and predicted drag coefficients were within 6 5%. The authors propose the continuity and the Sykes blockage correction methods for passenger car models because they are very simple and practical and they can be used economically for engineering applications.
When analyzing the topology of turbulent flow systems for alternative vehicle configurations, conventional wind-tunnel tests may not be sufficient. Vehicle development teams investigating this kind of physical phenomenon are increasingly interested in numerical simulation, but reliable implementation of computational techniques in automotive aerodynamics requires to confirm that such methods are capable of accurately reproducing experimentally proven physical behaviour. We address this issue by comparing the results of experimental wind-tunnel tests with simulated values for aerodynamic drag coefficient and vortex wake flows at different angles of car rear window inclination, using an Ahmed reference model [S. Ahmed and J. C. Tannehill, AIAA J. 30, No. 2, 302-303 (1992; Zbl 0825.76289)].
A transport equation for the turbulent viscosity is assembled, using empiricism and arguments of dimensional analysis, Galilean invariance, and selective dependence on the molecular viscosity. It has similarities with the models of Nee and Kovaszany, Secundov et al., and Baldwin and Barth. The equation includes a non-viscous destruction term that depends on the distance of the wall.
Recent studies of turbulent shear flows have shown that many of their
important kinematical and dynamical properties can be more clearly
understood by describing the flows in terms of individual events or
streamline patterns. These events or flow regions are studied because
they are associated with relatively large contributions to certain
average properties of the flow, for example kinetic energy, Reynolds
stress, or to particular processes in the flow, such as mixing and
chemical reactions, which may be concentrated at locations where
streamlines converge for fast chemical reactions (referred to as
convergence or C regions), or in recirculating eddying regions for slow
chemical reactions. The aim of this project was to use the numerical
simulations to develop suitable criteria for defining these eddying or
vortical zones. The C and streaming (S) zones were defined in order to
define the whole flow field. It is concluded that homogeneous and
sheared turbulent flow fields are made up of characteristic flow zones:
eddy, C, and S zones. A set of objective criteria were found which
describe regions in which the streamlines circulate, converge or
diverge, and form high streams of high velocity flow.
This study presents an implementation of delayed detached-eddy simulation (DDES) on a full-scale passenger vehicle for three configurations with the use of commercial software harpoon (mesher) and ansys fluent (solver). The methodology aims to simulate the flow accurately around complex geometries at relevantly high Re numbers for use in industrial applications, within an acceptable computational time. Geometric differences between the three configurations ensure significant drag changes that have a strong effect on the wake formation behind the vehicle. Therefore, this paper focuses on the analysis of the base wake region. At first, the paper evaluates the performance of the DDES, where it verifies the different operating conditions of the flow around the vehicle with respect to the DDES definition. In a second step, the numerical results are correlated with force measurements and time-averaged flow field investigations, conducted in the Volvo Cars aerodynamic wind tunnel (WT). The comparison confirms a good agreement between the experiments and the simulations. The resolved flow scales obtained by DDES give a further insight into differences in the wake flow characteristics between the configurations related to their contribution to drag.
The Ahmed car body represents a generic vehicle exhibiting key aspects of the 3D flow arising due to standard automobile designs. It is recognised to be a challenging test case for the turbulence modelling community; combining strong separation with a pair of counter-rotating vortices, which interact to produce a downstream recirculation region. In recent years this case has been extensively studied using a range of methods, with varying success. In general, conclusions have been made on the basis of the standard form of each model, while in the present work we focus on variants of the common Delayed Detached-Eddy Simulation (DDES) approach, in order to demonstrate its sensitivity to commonly varied aspects of its usage. We demonstrate that variations in the usage of a single approach can easily be of the order of those observed when using different approaches.
The paper reports a study about the prediction of flows around simplified vehicles using Partially-Averaged Navier–Stokes (PANS). Numerical simulations are performed for two simplified vehicles: a slanted-back Ahmed body at Re = 30,000 and a squared-back Ahmed body at Re = 300,000. A comparison of the resolved and modeled physical flow scales is made with corresponding LES and experimental data for a better understanding of the performance of the PANS model. The PANS model is compared for coarse and fine grid resolutions and it is indicated that even a coarse-grid PANS simulation is able to produce fairly close flow predictions to those from a well-resolved LES simulation. The results indicate the possibility of improvement of the predictions by employing a finer grid resolution.
In the present paper, Computational Fluid Dynamics (CFD) simulations of the aerodynamics of a station wagon using DES (Detached Eddy Simulation), based on the Spalart-Allmaras turbulence model, are discussed and compared with experimental results. DES is a non-zonal hybrid turbulence model that uses both Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES), enabling a better equilibrium between the accurateness and the computational cost of the solution. Simulations were run in parallel using the commercial software ANSYS/FLUENT v13.0, and required a computational grid of approximately 50 million cells. Some flow characteristics such as boundary layer separation, recirculation zones, and the entire pressure and velocity field were also obtained and analyzed. Computational results of drag force were compared with experimental results based on the SAE J1263 recommended practice (which describes a way to calculate the resistive loads that act on a vehicle during non- forced deceleration). Changes were made in the experimental estimation of the drag force, compared to the procedure stated in SAE J1263 (e.g., a differential formulation of the system was used, and the mass of the vehicle was altered by loading extra bodies into the vehicle). A selection among different sets of experimental conditions (vehicle velocities and vehicle weights) was made to improve the accuracy of the prediction. Even though computational and experimental approaches involved different models and simplifications, results from each of them showed a fair agreement. Congruence was driven by the iterative improvements made on the boundary conditions and meshing parameters on the CFD approach, and the careful experimental design performed. The aerodynamic drag coefficient calculated by CFD was of 0.460 compared to 0.404 when calculated by experimental means, which represents a 13.8% difference or 56 drag counts.
External car aerodynamics study has great importance in overall car efficiency and ride stability, being a
key element in successful automotive design. The flow over car geometries shows three dimensional and
unsteady turbulent characteristics. Additionally, vortex shedding, flow reattachment and recirculation
bubbles are also found around the bluff body. These phenomena greatly influence the lift and drag
coefficients, which are fundamental for ride stability and energy efficiency, respectively. The aim of
the present study is focused on the assessment of different LES models (e.g. VMS or SIGMA models), as
well as to show their capabilities of capturing the large scale turbulent flow structures in car-like bodies
using relative coarse grids. In order to achieve these objectives, the flow around two model car geometries,
the Ahmed and the Asmo cars, is simulated. These generic bluff bodies reproduce the basic fluid
dynamics features of real cars. First, the flow over both geometries is studied and compared against
experimental results to validate the numerical results. Then, different LES models are used to study
the flow in detail and compare the structures found in both geometries.
The lasting high fuel cost has recently inspired resurgence in drag reduction research for vehicles, which calls for a thorough understanding of the vehicle wake. The simplified Ahmed vehicle model is characterized by controllable flow separation, thus especially suitable for this purpose. In spite of a considerable number of previous investigations, our knowledge of flow around this model remains incomplete. This work aims to revisit turbulent flow structure behind this model. Two rear slant angles, i.e., α = 25º and 35º, of the model were examined, representing two distinct flow regimes. The Reynolds number was 5.26 × 104 based on the model height (H) and incident flow velocity. Using particle image velocimetry (PIV), flow was measured with and without a gap (g/H = 0.174) between the vehicle underside and ground in three orthogonal planes, viz. the x–z, x–y and y–z planes, where x, y, and z are the coordinates along longitudinal, transverse, and spanwise directions, respectively. The flow at g/H = 0 serves as an important reference for the understanding of the highly complicated vehicle wake (g/H ≠ 0). While reconfirming the well-documented major characteristics of the mean flow structure, both instantaneous and time-averaged PIV data unveil a number of important features of the flow structure, which have not been previously reported. As such, considerably modified flow structure models are proposed for both regimes. The time-averaged velocities, second moments of fluctuating velocities, and vorticity components are presented and discussed, along with their dependence on g/H in the two distinct flow regimes.
Preface. Acknowledgments. 1. Preliminaries. 2. Overview of Turbulent Flow Physics and Equations. 3. Experimental and Numerical Methods. 4. Properties of Bounded Turbulent Flows. 5. Properties of Turbulent Free Shear Flows. 6. Turbulent Transport. 7. Theory of Idealized Turbulent Flows. 8. Turbulence Modeling. 9. Applications of Turbulence Modeling. 10. Large Eddy Simulations. 11. Analysis of Turbulent Scalar Fields. 12. Turbulence Theory. Author Index. Subject Index.
This introduction to aerodynamic aspects of motor vehicle design will be of use both to vehicle designers and students of automobile engineering. Content covers vehicle systems, ventilation and aerodynamic design to reduce drag and increase stability of cars, commercial vehicles and PSVs. Topics considered include automobile aerodynamics; some fundamentals of fluid mechanics; performance of cars and light vans; aerodynamic drag of passenger cars; driving stability in sidewinds; operation, safety and comfort; high-performance vehicle aerodynamics; commercial vehicles; engine cooling systems; heating, ventilation and air conditioning of motor vehicles; wind tunnels for automobile aerodynamics; measuring and testing techniques; and numerical methods for computation of flow around road vehicles.
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