Development of Separation Phenomena on a Passenger Car
Doktorsavhandling, 2018

The general shape of a vehicle influences its aerodynamic performance through separation phenomena and flow structure development. Since these flow characteristics have a direct influence on the energy efficiency, safety and comfort, it is essential to study their formation and evolution into the freestream. The energy efficiency is determined by the aerodynamic drag, while the safety and comfort aspects are dependent on the noise generation, vehicle soiling, handling and stability.

The objective of this work is to achieve a more detailed physical understanding of the development of flow structures by analysing their surface properties and their evolution into the freestream. The concept of limiting streamlines is used to investigate and characterize the near wall flow, and surface properties such as the surface pressure, the wall shear stress and the vorticity are analysed and correlated with the flow patterns. The detachment of the flow from the surface and its development into the freestream are investigated using 2D streamlines and flow properties such as vorticity.

This study is based on numerical simulations of a detailed full scale passenger car of the notchback type. Results are compared to experimental flow visualisations and pressure measurements performed on a full scale vehicle. Special focus is put on the flow around the antenna, the flow over the rear window, the flow downstream of the front wheel and on the base wake flow.

Based on this analysis, it is found that the surface patten can be used to identify evolving flow phenomena. Analysing the limiting streamline pattern and 2D planes, together with the vorticity distribution, makes it possible to predict and study occurring flow phenomena. Flow structures developing in the main flow direction were the most dominant and are the least suppressed. It is shown that the only mechanisms, of flow detaching from the surface, must be either through singular points or along separation lines. The study of particular areas around the vehicle shows different flow phenomena and explains the formation of flow structures. Familiar phenomena such as the A-pillar vortex and the trailing vortices behind the vehicle are discussed. For instance, it is shown that, in the near wake an up-wash zone is created (crucial for contamination) and in the far wake, the two trailing vortices create a down-wash; both phenomena emanate from the vehicle base.

complex flows

vehicle aerodynamics

vortex formation

limiting streamlines

crossflow separation

flow separation

Författare

Sabine Bonitz

Chalmers, Mekanik och maritima vetenskaper, Fordonsteknik och autonoma system

Flow separation describes the detachment of flow from a surface. This phenomenon leads to the formation of recirculation zones and different kinds of vortices. The flow structures developing after a separation have an influence onto the geometry, the flow circulates around.

For a passenger car it is important to understand the flow around it as the created structures affect handling, stability, noise development, vehicle soiling and the aerodynamic performance. Handling and stability can lead to safety issues. The creation of noise is especially uncomfortable for the passengers and in the first place the driver, as it expedites mental fatigue and lack of concentration. Vehicle soiling is undesired from a customer point of view, but is also crucial in terms of safety aspects. Reduced driver visibility due to dirt and water can lead to dangerous situations, and the increased usage of sensors and radars for driving assistance systems requires a low level of contamination in order to ensure functionality. The aerodynamic performance is crucial as the aerodynamic drag directly affects the energy efficiency of the vehicle.

Hence, it is important to understand how the flow pattern around the vehicle is created and how it develops after it is separated. This thesis investigates the development of separation phenomena around a passenger car, starting by investigating the flow properties on the vehicle surface and in the near wall region to identify where separation originates. Based on the analysis of the surface pattern the flow development away from the surface is studied to understand the formation of flow structures.

Styrkeområden

Transport

Infrastruktur

C3SE (Chalmers Centre for Computational Science and Engineering)

Ämneskategorier

Farkostteknik

Strömningsmekanik och akustik

ISBN

978-91-7597-785-0

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4466

Utgivare

Chalmers tekniska högskola

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Senast uppdaterat

2019-05-03