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Technical Paper

Transient Simulation of the Flow Field Around a Generic Pickup Truck

A complete transient, three dimensional simulation of the flow-field around a generic pickup truck geometry is carried out. A 1/12-scale replica of an actual pickup truck, with simplified features such as a smooth underbody, is considered in the study. The purpose of the study is twofold. First, it seeks to improve our understanding of the complex flow field around a pickup truck, which is predominantly a bluff body with a prominent wake. To this end a detail description of the time-averaged pressure distribution on the vehicle body as well as time-averaged velocities in the wake of the truck is provided. Secondly, the study seeks to judge the accuracy with which modern CFD techniques can predict complex, practical, bluff-body wake flows. This is accomplished by making a close comparison of the time-averaged wake velocity profiles predicted by CFD with analogous measurements made in a wind tunnel experiment using particle image velocimetry.
Technical Paper

The Immersed Boundary CFD Approach for Complex Aerodynamics Flow Predictions

Standard CFD methods require a mesh that fits the boundaries of the computational domain. For a complex geometry the generation of such a grid is time-consuming and often requires modifications to the model geometry. This paper evaluates the Immersed Boundary (IB) approach which does not require a boundary-conforming mesh and thus would speed up the process of the grid generation. In the IB approach the CAD surfaces (in Stereo Lithography -STL- format) are used directly and this eliminates the surface meshing phase and also mitigates the process of the CAD cleanup. A volume mesh, consisting of regular, locally refined, hexahedrals is generated in the computational domain, including inside the body. The cells are then classified as fluid, solid and interface cells using a simple ray-tracing scheme. Interface cells, correspond to regions that are partially fluid and are intersected by the boundary surfaces.
Technical Paper

Particle Image Velocimetry Measurements in a High-Swirl Engine Used for Evaluation of Computational Fluid Dynamics Calculations

Two-dimensional in-cylinder velocity distributions measured with Particle Image Velocimetry were compared with computed results from Computational Fluid Dynamics codes. A high-swirl, two-valve, four-stroke transparent-combustion-chamber research engine was used. Comparisons were made of mean-flow velocity distributions, swirl-ratio evolution during the intake and compression strokes, and turbulence distributions at top-dead-center compression. This comparison with the measured flows led to more accurate calculations by identifying code improvements including swirl in the residual gas, modeling of the gas exchange during the valve overlap, and improved numerical accuracy.
Technical Paper

Numerical Investigation of Road Vehicle Aerodynamics Using the Immersed Boundary RANS Approach

This paper describes the computational results of the flow field around two vehicle geometries using the Immersed Boundary (IB) technique in conjunction with a steady RANS CFD solver. The IB approach allows the computation of the flow around objects without requiring the grid lines to be aligned with the body surfaces. In the IB approach instead of specifying body boundary conditions, a body force is introduced in the governing equations to model the effect of the presence of an object on the flow. This approach reduces the time necessary for meshing and allows utilization of more efficient and fast CFD solvers. The simulations are carried out for an SUV and a pickup truck models at a Reynolds number of 8×105. Cartesian meshes (non-uniform) with local grid refinement are used to increase the resolution close to the boundaries. The simulation results are compared with the existing measurements in terms of surface pressures, velocity profiles, and drag coefficients.
Technical Paper

Modifying an Intake Manifold to Improve Cylinder-to-Cylinder EGR Distribution in a DI Diesel Engine Using Combined CFD and Engine Experiments

Improved cylinder-to-cylinder distribution of EGR in a 2-L Direct-Injection (DI) Diesel engine has been identified as one enabler to help reach more stringent emission standards. Through a combined effort of modeling, design, and experiment, two manifolds were developed that improve EGR distribution over the original manifold while minimizing design changes to engine components or interfering with the many varied vehicle platform installations. One of the modified manifolds, an elevated EGR entry (EEE) approach, provided a useful improvement over the original design that meet Euro-II emission standards, and has been put into production as it enabled meeting the Euro III emissions requirements a year early. The second revision, the distributed EGR entry (DEE) design, showed potential for further improvement in EGR distribution. This design has two EGR outlets rather than the one used in the original and EEE manifolds, and was first identified by modeling to be a promising concept.
Technical Paper

International Congress and Exposition Detroit, Michigan February 24-28, 1986Measured Steady Flow Velocity Distributions Around a Valve/Seat Annulus

Measurements were made of the three velocity components at the exit plane of the intake valve from an internal combustion engine. The velocities were measured using hot-wire anemometry in a steady-flow rig, and an assessment was made of the effects of flow rate, valve lift, cylinder bore diameter, and inlet configuration on the velocity distribution around the intake valve. The results showed that over the range of flow rates tested, the normalized velocity profiles are independent of flow rate. At a fixed flow rate, the velocity profiles around the valve periphery are found to be strongly dependent on proximity to the cylinder head. Close to the cylinder head, the profiles are skewed but become more uniform as the distance from the cylinder head increases. In addition, the results indicate that the profiles are sensitive to the valve lift and to the proximity of the cylinder wall to the valve axis.
Technical Paper

Intake-Generated Swirl and Tumble Motions in a 4-Valve Engine with Various Intake Configurations-Flow Visualization and Particle Tracking Velocimetry

Flow visualization and particle tracking velocimetry (PTV) were used to study the in-cylinder flow field produced by a 4-valve engine head during the intake process. Several intake configurations were considered, and their effects on the in-cylinder flow structures were assessed. The engine head was mounted on a special single cylinder water analog for the in-cylinder flow visualization and PTV experiments. The results for this study indicate that the in-cylinder flow field during the induction process is comprised of several large scale vortical motions on different planes within the engine cylinder. These large scale fluid motions are, in most cases, stable and highly repeatable from cycle to cycle. No significant in-cylinder tumbling motion was observed in the engine with both intake valves operating. The only well-defined tumbling flow structure was observed with shrouded intake valve configurations.
Technical Paper

Experimental and Computational Study of Unsteady Wake Flow Behind a Bluff Body with a Drag Reduction Device

Simple devices have been shown to be capable of tailoring the flow field around a vehicle and reducing aerodynamic drag. An experimental and computational investigation of a drag reduction device for bluff bodies in ground proximity has been conducted. The main goal of the research is to gain a better understanding of the drag reduction mechanisms in bluff-body square-back geometries. In principle, the device modifies the flow field behind the test model by disturbing the shear layer. As a consequence, the closure of the wake is altered and reductions in aerodynamic drag of more than 20 percent are observed. We report unsteady base pressure, hot-wire velocity fluctuations and Particle Image Velocimetry (PIV) measurements of the near wake of the two models (baseline and the modified models). In addition, the flows around the two configurations are simulated using the Reynolds Averaged Navier-Stokes (RANS) equations in conjunction with the V2F turbulence model.
Technical Paper

Experimental Investigation of the Near Wake of a Pick-up Truck

The results of an experimental investigation of the flow over a pickup truck are presented. The main objectives of the study are to gain a better understanding of the flow structure in near wake region, and to obtain a detailed quantitative data set for validation of numerical simulations of this flow. Experiments were conducted at moderate Reynolds numbers (∼3×105) in the open return tunnel at the University of Michigan. Measured quantities include: the mean pressure on the symmetry plane, unsteady pressure in the bed, and Particle Image Velocimetry (PIV) measurements of the flow in the near wake. The unsteady pressure results show that pressure fluctuations in the forward section of the bed are small and increase significantly at the edge of the tailgate. Pressure fluctuation spectra at the edge of the tailgate show a spectral peak at a Strouhal number of 0.07 and large energy content at very low frequency.
Technical Paper

Experimental Investigation of the Flow Around a Generic SUV

The results of an experimental investigation of the flow in the near wake of a generic Sport Utility Vehicle (SUV) model are presented. The main goals of the study are to gain a better understanding of the external aerodynamics of SUVs, and to obtain a comprehensive experimental database that can be used as a benchmark to validate math-based CFD simulations for external aerodynamics. Data obtained in this study include the instantaneous and mean pressures, as well as mean velocities and turbulent quantities at various locations in the near wake. Mean pressure coefficients on the base of the SUV model vary from −0.23 to −0.1. The spectrum of the pressure coefficient fluctuation at the base of the model has a weak peak at a Strouhal number of 0.07. PIV measurements show a complex three-dimensional recirculation region behind the model of length approximately 1.2 times the width of the model.
Technical Paper

Experimental Characterization of the Unsteady Flow Field behind Two outside Rear View Mirrors

The unsteady flow fields behind two different automobile outside side rear view mirrors were examined experimentally in order to obtain a comprehensive data base for the validation of the ongoing computational investigation effort to predict the aero-acoustic noise due to the outside rear view mirrors. This study is part of a larger scheme to predict the aero-acoustic noise due to various external components in vehicles. To aid with the characterization of this complex flow field, mean and unsteady surface pressure measurements were undertaken in the wake of two mirror models. Velocity measurements with particle image velocimetry were also conducted to develop the mean velocity field of the wake. Two full-scale mirror models with distinctive geometrical features were investigated.
Technical Paper

CFD Simulations for Flow Over Pickup Trucks

Computational fluid dynamics (CFD) was used to simulate the flow field over a pickup truck. The simulation was based on a steady state formulation and the focus of the simulation was to assess the capabilities of the currently used CFD tools for vehicle aerodynamic development for pickup trucks. Detailed comparisons were made between the CFD simulations and the existing experiments for a generic pickup truck. It was found that the flow structures obtained from the CFD calculations are very similar to the corresponding measured mean flows. Furthermore, the surface pressure distributions are captured reasonably well by the CFD analysis. Comparison for aerodynamic drags was carried out for both the generic pickup truck and a production pickup truck. Both the simulations and the measurements show the same trends for the drag as the vehicle geometry changes, This suggests that the steady state CFD simulation can be used to aid the aerodynamic development of pickup trucks.
Technical Paper

A CFD Study of Drag Reduction Devices for a Full Size Production Pickup Truck

Various drag reduction strategies have been applied to a full size production pickup truck to evaluate their effectiveness by using Computational Fluid Dynamics (CFD). The drag reduction devices evaluated in this study were placed at the rear end of the truck bed and the tailgate. Three types of devices were evaluated: (1) boat tail-like extended plates attached to the tailgate; (2) mid-plate attached to the mid-section of the tailgate and; (3) flat plates partially covering the truck bed. The effect of drag reduction by various combinations of these three devices are presented in this paper. Twenty-four configurations were evaluated in the study with the best achievable drag reduction of around 21 counts (ΔCd = 0.021). A detailed breakdown of the pressure differentials at the base of the truck is provided in order to understand the flow mechanism for the drag reductions.