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Technical Paper

Analysis of Wake Pattern for Reducing Aerodynamic Drag of Notchback Model

Notchback models generally have more complicated flow patterns than box models. This leads to intricate infuluence of rear geometry of Notchback on aerodynamic drag. Therefore, based on understanding of wake structure, flow phenomena for reducing the drag can be analyzed. This paper analyzes the influence of geometry of rear portion on the drag by means of 1/5 scale notchback models. For fastback models, at certain critical angle of the rear window the drag shows a sharp peak. For notchback models, it is found that some combination of the angle of rear window and the height of trunk deck shows simillar maximum in the drag. Moreover, the flow visualization and the detailed measurements of velocity fluctuation clarify typical vortex patterns of wake, which are an arch-type vortex behind the rear window and the trailing vortex behind the trunk deck.
Technical Paper

An Experimental Analysis and Estimation of Aerodynamic Noise Using a Production Vehicle

The wind-noise-level in the cabin of a vehicle depends on the magnitude of the aerodynamically generated noise and noise isolation characteristics. Therefore, one good way to reduce the wind-noise-level in the cabin is to minimise the acoustic noise itself generated by the turbulent air flow around the vehicle at high speed cruising. This paper describes the relation between the turbulent flow and the aerodynamic noise as well as how to estimate the magnitude of aerodynamic noise, especially around A-pillar of a production vehicle. First, the flow visualization and the detailed measurements of flow clarify the vortex structure generated around A-pillar and side window. Secondly, sound pressure fluctuations measured on the side window surface are discussed in relation to the vortex structure. Lastly, in order to estimate the order of the magnitude of aerodynamic noise we, propose physical parameters given by approximating the solution of Lighthill's equation.
Technical Paper

Influence of Geometry of Rear part on the Aerodynamic Drag and Wake Structure of a Vehicle

The substantial part of the drag of an automobile is the pressure drag. Therefore, the car must be designed as it produces minimum pressure drag. The present paper describes effects of geometrical configuration of the rear part of a car on the aerodynamic drag. Experiments were made on 1/5 scale models of fastback and notchback design. For the fastback car the drag depends heavily on the angle of a rear window. At a certain critical angle the drag shows a sharp peak. This peak drag can be reduced drastically by the tapering of plan form of the rear geometry. For the notch-back design some combination of the angle of rear window and height of trunk deck shows similar maximum in the drag. Methods of avoiding the large drag were also found. Our experiment was extended to the measurement of structure of wake by hot wire anemometers and total pressure tubes. The correlation between the wake structure and drag was clarified by the consideration of vorticity and circulation.
Technical Paper

Unsteady-Wake Analysis of the Aerodynamic Drag of a Notchback Model with Critical Afterbody Geometry

For both notchback-type and fastback-type models, it has been found that critical geometries which increase the aerodynamic drag exist, and the time-averaged wake patterns basically consist of an arch vortex behind the rear window and trailing vortices in the wake. The unsteady characteristics of the wake seem to be directly related to aerodynamic drag. However, the unsteady characteristics of these wake patterns for notchback and fastback cars were not clear. The purpose of present paper is to clarify these phenomena. We try to analyze experimentally the unsteady characteristics by measuring the velocity fluctuations in the wake, the pressure fluctuations on the trunk deck and the drag-force fluctuations acting on the model. At the same time, the analysis of the numerical simulation was made by using the same numerical model as the experimental model. The computed flow visualization behind the rear window showed a fluctuating arch vortex.
Technical Paper

Aerodynamic Pitching Stability of Sedan-Type Vehicles Influenced by Pillar-Shape Configurations

The present study investigated the aerodynamic pitching stability of sedan-type vehicles under the influence of A- and C-pillar geometrical configurations. The numerical method used for the investigation is based on the Large Eddy Simulation (LES) method. Whilst, the Arbitrary Lagrangian-Eulerian (ALE) method was employed to realize the prescribed pitching oscillation of vehicles during dynamic pitching and fluid flow coupled simulations. The trailing vortices that shed from the A-pillar and C-pillar edges produced the opposite tendencies on how they affect the aerodynamic pitching stability of vehicles. In particular, the vortex shed from the A-pillar edge tended to enhance the pitching oscillation of vehicle, while the vortex shed from the C-pillar edge tended to suppress it. Hence, the vehicle with rounded A-pillar and angular C-pillar exhibited a higher aerodynamic damping than the vehicle with the opposite A- and C-pillars configurations.
Technical Paper

Evaluation of Wind Noise Sources Using Experimental and Computational Methods

Experiment and CFD have been performed to clarify the distribution of wind noise sources and its generation mechanism for a production vehicle. Three noise source identification techniques were applied to measure the wind noise sources from the outside and inside of vehicle. The relation between these noise sources and the interior noise was investigated by modifying the specification of underbody and front-pillar. In addition, CFD was preformed to predict the noise sources and clarify its generation mechanism. The noise sources obtained by simulation show good agreement with experiment in the region of side window and underbody.
Journal Article

Flow Structures above the Trunk Deck of Sedan-Type Vehicles and Their Influence on High-Speed Vehicle Stability 1st Report: On-Road and Wind-Tunnel Studies on Unsteady Flow Characteristics that Stabilize Vehicle Behavior

This study shows an example in which the conventional aerodynamic evaluation method that focuses on “steady” aerodynamic lift coefficient is not necessarily sufficient to evaluate vehicle's straight-ahead stability at high speed, and proposes a new aerodynamic evaluation method for vehicle stability. In vehicle development, it is generally said that vehicle with lower aerodynamic lift coefficient has better straight-ahead stability at high speed. However, in some cases, straight-ahead stability differs between two vehicles with similar low aerodynamic lift coefficient. It is natural to think that this variation is caused by the difference of suspension characteristics or vehicle body rigidity. But from our experiences, different straight-ahead stability was observed between two vehicles having same suspension characteristics, same vehicle body rigidity and almost similar aerodynamic lift coefficient, but different vehicle configurations.
Journal Article

Flow Structures above the Trunk Deck of Sedan-Type Vehicles and Their Influence on High-Speed Vehicle Stability 2nd Report: Numerical Investigation on Simplified Vehicle Models using Large-Eddy Simulation

In the present study, two kinds of simplified vehicle models, which can reproduce flow structures around the two sedan-type vehicles in the previous study, are constructed for the object and the unsteady flow structures are extracted using Large-Eddy Simulation technique. The numerical results are validated in a stationary condition by comparing the results with a wind-tunnel experiment and details of steady and unsteady flow characteristics around the models, especially above the trunk deck, are investigated. In quasi- and non- stationary manner with regard to vehicle pitch motion, unsteady flow characteristics are also investigated and their relations to an aerodynamic stability are discussed.