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Journal Article

Investigation of the Influence of Tyre Geometry on the Aerodynamics of Passenger Cars

It is well known that wheels are responsible for a significant amount of the total aerodynamic drag of passenger vehicles. Tyres, and mostly rims, have been the subject of research in the automotive industry for the past years, but their effect and interaction with each other and with the car exterior is still not completely understood. This paper focuses on the use of CFD to study the effects of tyre geometry (tyre profile and tyre tread) on road vehicle aerodynamics. Whenever possible, results of the numerical computations are compared with experiments. More than sixty configurations were simulated. These simulations combined different tyre profiles, treads, rim designs and spoke orientation on two car types: a sedan and a sports wagon. Two tyre geometries were obtained directly from the tyre manufacturer, while a third geometry was obtained from our database and represents a generic tyre which covers different profiles of a given tyre size.
Technical Paper

Analysis of Transient Compressible Gas Jets Using High Speed Schlieren Imaging

Transient compressible gas jets, as encountered in direct injection gas fuel engines, have been examined using Schlieren visualization. Helium has been injected into air in a pressure chamber to create the jets examined. The structure of the jets is studied from the mean and coefficient of variation of the penetration length, jet width and jet angle. The quantities are calculated by digital image processing of Schlieren images captured with a high-speed camera. Injection pressure and chamber pressure have been varied to determine whether they have an effect on the response variables. Design of experiments methods have been used to develop the scheme employed in performing the experiments. The mean normalized penetration length of the jets is found to scale with injection to chamber pressure ratio and is in agreement with a momentum conserving relation given in the literature. The dispersion of the penetration length has been found to be in agreement with a normal distribution.
Technical Paper

Effect of Rear-End Extensions on the Aerodynamic Forces of an SUV

Under a global impulse for less man-made emissions, the automotive manufacturers search for innovative methods to reduce the fuel consumption and hence the CO2-emissions. Aerodynamics has great potential to aid the emission reduction since aerodynamic drag is an important parameter in the overall driving resistance force. As vehicles are considered bluff bodies, the main drag source is pressure drag, caused by the difference between front and rear pressure. Therefore increasing the base pressure is a key parameter to reduce the aerodynamic drag. From previous research on small-scale and full-scale vehicles, rear-end extensions are known to have a positive effect on the base pressure, enhancing pressure recovery and reducing the wake area. This paper investigates the effect of several parameters of these extensions on the forces, on the surface pressures of an SUV in the Volvo Cars Aerodynamic Wind Tunnel and compares them with numerical results.
Technical Paper

A 1D Method for Transient Simulations of Cooling Systems with Non-Uniform Temperature and Flow Boundaries Extracted from a 3D CFD Solution

The current work investigates a method in 1D modeling of cooling systems including discretized cooling package with non-uniform boundary conditions. In a stacked cooling package the heat transfer through each heat exchanger depends on the mass flows and temperature fields. These are a result of complex three-dimensional phenomena, which take place in the under-hood and are highly non-uniform. A typical approach in 1D simulations is to assume these to be uniform, which reduces the authenticity of the simulation and calls for additional calibrations, normally done with input from test measurements. The presented work employs 3D CFD simulations of complete vehicle in STAR-CCM+ to perform a comprehensive study of mass-flow and thermal distribution over the inlet of the cooling package of a Volvo FM commercial vehicle in several steady-state operating points.
Technical Paper

Identifying Time-Consuming Human Modelling Tool Activities

The aim of this study was to identify and measure time-consuming human modelling tool activities. Five human modelling tool users at Volvo were observed for five days each. The results showed a wide distribution of both indirect and direct working tasks, as well as non-value added tasks such as waiting time. Most of the activities identified appear to be necessary to perform human modelling simulations of high quality. However, the time distribution could be questioned to some extent. There are many activities associated with communication, including a variety of contacts and meetings, where there appears to be potential to increase efficiency.
Technical Paper

CFD Modelling of Gasoline Sprays

A comprehensive model for sprays emerging from high pressure swirl injectors for GDI engine application has been developed. The primary and secondary atomization mechanism as well as the evaporation process both in standard and superheated conditions are taken into account. The spray modelling after the injection is based on the Liquid Instability Sheet Atomization (LISA) approach, modified to correctly predict the liquid sheet thickness at the breakup length. The effect of different values of the superheat degree on evaporation and impact on the spray distribution and fuel-air mixing is analyzed. Comparisons with experimental data show good agreements under atmospheric conditions and with different superheated degrees, while some discrepancies occur under higher ambient pressures.
Technical Paper

Modification of a Diesel Oil Surrogate Model for 3D CFD Simulation of Conventional and HCCI Combustion

This paper describes an analysis of the Diesel Oil Surrogate (DOS) model used at Chalmers University (Sweden), including 70 species participating in 310 reactions, and subsequent improvements prompted by the model's systematic tendency to under-predict the combustion intensity in simulations of kinetically-driven combustion modes, e.g. Homogeneous Charged Compression Ignition (HCCI). Key bases of the model are the properties of a model Diesel fuel with the molecular formula C14H28. In the vapor phase, a global reaction decomposes the starting fuel, C14H28, into its constituent components; n-heptane (C7H16) and toluene (C7H8). This global reaction was modified to yield a higher n-heptane:toluene ratio, due to the importance of preserving an n-heptane-like cetane number.
Technical Paper

Location of the First Auto-Ignition Sites for Two HCCI Systems in a Direct Injection Engine

To elucidate the processes controlling the auto-ignition timing and overall combustion duration in homogeneous charge compression ignition (HCCI) engines, the distribution of the auto-ignition sites, in both space and time, was studied. The auto-ignition locations were investigated using optical diagnosis of HCCI combustion, based on laser induced fluorescence (LIF) measurements of formaldehyde in an optical engine with fully variable valve actuation. This engine was operated in two different modes of HCCI. In the first, auto-ignition temperatures were reached by heating the inlet air, while in the second, residual mass from the previous combustion cycle was trapped using a negative valve overlap. The fuel was introduced directly into the combustion chamber in both approaches. To complement these experiments, 3-D numerical modeling of the gas exchange and compression stroke events was done for both HCCI-generating approaches.
Technical Paper

It's in the Eye of the Beholder: Who Should be the User of Computer Manikin Tools?

The aim of this study was to examine the influence of computer manikin users' background and knowledge for the results of a computer manikin simulation. Subjects taking part in the study were either production engineers or ergonomists. A manual task that presented production and ergonomics problems was used. The task was simulated prior to the subjects' sessions, using the computer manikin software Jack. During the sessions, the animated simulation was shown to the test subject. Results show that there are differences in how production engineers and ergonomists interpret results from a manikin simulation. Depending on the user's background, certain aspects that are difficult to visualise with the computer manikin were interpreted differently, regarding e.g. detected problems and holistic perspectives.
Technical Paper

Surface Flow Visualization on a Full-Scale Passenger Car with Quantitative Tuft Image Processing

Flow visualization techniques are widely used in aerodynamics to investigate the surface trace pattern. In this experimental investigation, the surface flow pattern over the rear end of a full-scale passenger car is studied using tufts. The movement of the tufts is recorded with a DSLR still camera, which continuously takes pictures. A novel and efficient tuft image processing algorithm has been developed to extract the tuft orientations in each image. This allows the extraction of the mean tuft angle and other such statistics. From the extracted tuft angles, streamline plots are created to identify points of interest, such as saddle points as well as separation and reattachment lines. Furthermore, the information about the tuft orientation in each time step allows studying steady and unsteady flow phenomena. Hence, the tuft image processing algorithm provides more detailed information about the surface flow than the traditional tuft method.
Technical Paper

Effect of Injection Parameters on Auto-Ignition and Soot Formation in Diesel Sprays

A validation study of the numerical model of n-heptane spray combustion based on experimental constant-volume data [1] was done, by comparing auto-ignition delays for different pre - turbulence levels and initial temperatures, flame contours, and soot distributions under Diesel-like conditions. The basic novelty of the methodology developed in [2] - [3] is the implementation of the partially stirred reactor (PaSR) model accounting for detailed chemistry / turbulence interactions. It is based on the assumption that the chemical processes proceed in two successive steps: micro mixing, simulated on a sub - grid scale, is followed by the reaction act. When the all Re number RNG k-ε or LES models are employed, the micro mixing time can be consistently defined giving the combustion model a “well-closed” form incorporated into the KIVA-3V code.
Technical Paper

The Structure of Cavitation and its Effect on the Spray Pattern in a Single-Hole Diesel Nozzle

The structure and evolution of cavitation in a transparent scaled-up diesel nozzle having a hole perpendicular to the nozzle axis has been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. Observations revealed that, at the inception stage, cavitation bubbles are dominantly seen in the vortices at the boundary layer shear flow and outside the separation zone. Cavitation bubbles grow intensively in the shear layer and develop into cloud-like coherent structures when viewed from the side of the nozzle. Shedding of the coherent cloud cavitation was observed. When the flow was increased further the cloud like cavitation bubbles developed into a large-scale coherent structure extending downstream of the hole. Under this condition the cavitation starts as a mainly glassy sheet at the entrance of the hole. Until this stage the spray appeared to be symmetric.
Technical Paper

A Model of Turbocharged Engines as Dynamic Drivetrain Members

An engine model for use in computer simulation of transient behavior in drivetrain and vehicle systems is presented. Two elements, important for deviation (e.g. turbo-lag) from steady state characteristics, are the inertia of the supercharging unit (turbo shaft) and the fuel injection control system. No extensive combustion calculations are carried out within the model. Instead it uses condensed results from existing combustion models and measurements. The model is semi-empirical. Some of the engine specific properties needed for simulation are (e.g. for a turbocharged diesel): engine data in steady state operation, mappings of compressor and turbine performance, inertia of the engine components condensed to the crankshaft, turbo shaft inertia, displacement, compression ratio and the essentials of the fuel injection control strategy. Input parameters to the computer program based on the model are accelerator pedal position and external torque acting on the flywheel.
Technical Paper

Oxidation of Hydrocarbons Released from Piston Crevices of S.I. Engines

This work presents a numerical method for predictions of HC oxidation in the cold turbulent wall jet emerging from the piston top land crevice in an S.I. engine, using a complex chemical reaction model. The method has been applied to an engine model geometry with the aim to predict the HC oxidation rate under engine - relevant conditions. According to the simulation a large amount of HC survives oxidation due to the long ignition delay of the wall jet emitted from the crevice. This ignition delay, in turn depends mainly on chemical composition and temperature of the gas mixture in the crevice and also on the temperature distribution in the cylinder boundary layer.
Journal Article

Structures of Flow Separation on a Passenger Car

The phenomenon of three-dimensional flow separation is and has been in the focus of many researchers. An improved understanding of the physics and the driving forces is desired to be able to improve numerical simulations and to minimize aerodynamic drag over bluff bodies. To investigate the sources of separation one wants to understand what happens at the surface when the flow starts to detach and the upwelling of the streamlines becomes strong. This observation of a flow leaving the surface could be captured by investigating the limiting streamlines and surface parameters as pressure, vorticity or the shear stress. In this paper, numerical methods are used to investigate the surface pressure and flow patterns on a sedan passenger vehicle. Observed limiting streamlines are compared to the pressure distribution and their correlation is shown. For this investigation the region behind the antenna and behind the wheel arch, are pointed out and studied in detail.