Bahram Khalighi

Large Eddy Simulation of a Road Vehicle with Drag-Reduction Devices

The e ow around an idealized road vehicle at Reynolds numbers up to 10 5 has been simulated using large eddy simulation; the numerical technique is based on the immersed boundary approach, which allows efe cient calculations to be carried out on a Cartesian grid. The effect of the Reynolds number and the wake modie cations produced by drag reduction devices attached to the base have been analyzed and compared with available experimental data. Averaged drag coefe cient and mean velocity proe les are in good agreement with measurement. The effect of subgrid-scale modeling (Smagorinsky and dynamic model ) has also been studied.

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. 14 refs., 14 figs.

Particle tracking velocimetry: an automatic image processing algorithm

A quantitative full-field velocity measurement technique involving imaging of displaced particles in a fluid is described. In this paper a fully automatic image processing procedure is developed to extract velocity vectors from flow visualization images. Flow visualization images are generated using special illumination coding which allows determination of both velocity magnitude and direction. The utility of this technique for detecting well-defined flow structures in a flow is demonstrated.

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 (~3×10 5 ) 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. The velocity field measurements in the symmetry plane show that shear layers form at the top of the cab and the underbody flow region. The cab shear layer evolves more slowly than the underbody flow shear layer and does not interact strongly with the tailgate for the present geometry. Behind the tailgate there is no recirculating flow region in the symmetry plane believed to be due to downwash from streamwise vorticity in the near wake. There are small recirculating regions on the sides of the tailgate symmetry plane extending approximately one tailgate height downstream.

Unsteady-flow velocity measurements around an intake valve of a reciprocating engine

In the present work, measurements of the velocity profiles in the valve curtain area of an internal combustion engine were made using hot-wire anemometry. The three components of velocity were measured under a variety of engine speeds, valve lifts, and inlet pipe configurations. From an analysis of the results, it was found that during the intake stroke, a region spanning about 60 crank angle degrees and centered at the middle of the intake stroke could be identified where transient effects had little effect on the intake velocity profiles. In this region, the velocity profiles were fairly insensitive to engine speed and to the type of inlet pipe used, but were sensitive to the valve lift. Surrounding this region, in the early and late parts of the intake stroke, the profiles were found to be influenced by transient effects and were sensitive to engine speed, type of inlet pipe, and valve lift.

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. CFD simulations are carried out with an unstructured finite-volume method based Navier-Stokes solver in conjunction with the RNG k-e and LES turbulence models. Simulation results are compared with experimental data reported elsewhere in literature.

A Transient Water Analog of a Dual-Intake-Valve Engine for Intake Flow Visualization and Full-Field Velocity Measurements

It has been recognized that the fluid motion during the engine induction process has a significant effect on combustion in terms of efficiency and emissions. To understand the intake process, a transient water analog visualization engine has been developed. This model, which simulates a single cylinder engine, has a transparent dual-intake-valve cylinder head and a transparent cylinder and is used to study the in-cylinder and port flows both qualitatively and quantitatively during the induction process

Multidimensional In-Cylinder Flow Calculations and Flow Visualization in a Motored Engine

Multidimensional simulations of coupled intake port/valve and in-cylinder flow structures in a pancake-shape combustion chamber engine are reported. The engine calculations include moving piston, moving intake valve, and valve stem. In order to verify the calculated results, qualitative flow visualization experiments were carried out for the same intake geometry during the induction process using a transient water analog. During the intake process the results of the multidimensional simulation agreed very well with the qualitative flow visualization experiments. An important finding in this study is the generation ofa well-defined tumbling flow structure at BDC in the engine. In addition, this tumbling flow is sustained and amplified by the compression process and in turn causes generation of a high turbulence level before TDC. Many interesting features of the in-cylinder flow structures such as tumble, swirl, and global turbulent kinetic energy are discussed.

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. Turbulence properties are also reported, and the largescale turbulent structure in the near wake is investigated using Proper Orthogonal Decomposition (POD) methods. The results suggest that the more energetic modes in the symmetry plane correspond to a vortex shedding process, and the more energetic modes in the center horizontal plane are a lateral flapping motion of the wake and a breathing mode of the mean recirculation region.

Analysis of the Near Wake of Bluff Bodies in Ground Proximity

The effect of several drag reducing devices on the near wake of a generic ground vehicle body was investigated. Drag and base pressure measurements were conducted to identify the effects of the devices on the base drag. A Particle Image Velocimetry (PIV) study was conducted to determine changes of the near wake flow field. Averages of more than 200 PIV velocity vector fields were used to compute the mean velocity and turbulent stresses at several cross section planes. The results of the drag and base pressure measurements show that significant reductions of the total aerodynamic drag (as high as 48%) can be achieved with relatively simple devices. The results also indicated that models with base cavity have lower drag than their counter parts without it. The base pressure distributions showed a strong effect of the ground, resulting in decrease of pressure towards the lower half of the base. The PIV study showed that the extent of the recirculation region is not strongly affected by the drag reducing devices. The tested devices however, were found to have a strong effect on the underbody flow. A rapid upward deflection of the underbody flow in the near wake was observed. The devices were also found to reduce the turbulent stresses in the near wake. The turbulent stresses were found to decrease in magnitude with increasing drag reduction.Copyright