Development of Separation Phenomena on a Passenger Car

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

Unsteady pressure analysis of the near wall flow downstream of the front wheel of a passenger car under yaw conditions

The flow around passenger cars is complex and is characterized by many different structures and interactions. The occurring flow phenomena around a car determine crucial vehicle properties such as the driving stability, the noise level, the aerodynamic performance and the vehicle contamination. Therefore, it is of high importance to increase the understanding of the developing flow phenomena. Generic models are widely used to investigate flow structures and their interactions, but cannot serve to derive a general flow field for detailed full-scale vehicle models. A particularly complex area is the flow around the wheels and its interaction with the vehicle geometry. Studies on wheel-wheelhouse flow focus mainly on the geometrical influence of the wheel size, rim and tyre on the aerodynamic drag and the flow field close to the wheel. The present work investigates the flow behind the front wheel arch of a full-scale passenger car. Time resolved surface pressure measurements were taken to study the near wall flow under different yaw conditions. Based on the data obtained, flow structures are identified and their propagation speed is calculated. Further, characteristic frequencies observed are discussed. It is found that coherent structures are present behind the front wheel arch, one above the wheel centre height and one below it. These remain even under large yaw angles, no matter if the vehicle is yawed to lee- or windward. The investigation further shows that two characteristic frequencies can be found, St=0.03 and St=0.2, whereby the latter is caused by the wheel rotation. The same frequencies also occur under yaw conditions, but yawing the measurement area to leeward results in less pronounced frequency peaks

Investigation of Wheel Ventilation-Drag using a Modular Wheel Design Concept

Passenger car fuel consumption is a constant concern for automotive companies and the contribution to fuel consumption from aerodynamics is well known. Several studies have been published on the aerodynamics of wheels. One area of wheel aerodynamics discussed in some of these earlier works is the so-called ventilation resistance. This study investigates ventilation resistance on a number of 17 inch rims, in the Volvo Cars Aerodynamic Wind Tunnel. The ventilation resistance was measured using a custom–built suspension with a tractive force measurement system installed in the Wheel Drive Units (WDUs). The study aims at identifying wheel design factors that have significant effect on the ventilation resistance for the investigated wheel size. The results show that it was possible to measure similar power requirements to rotate the wheels as was found in previous works. The magnitude of the measured ventilation resistance confirms the conclusion that this effect should be taken into account when designing a wheel. It was found that some of the rim design factors have greater influences on the ventilation resistance than others. It was also shown that one of the investigated rims had lower ventilation resistance than measured for the fully-covered wheel configuration

Investigation of Wheel Ventilation-Drag using a Modular Wheel Design Concept

Passenger car fuel consumption is a constant concern for automotive companies and the contribution to fuel consumption from aerodynamics is well known. Several studies have been published on the aerodynamics of wheels. One area of wheel aerodynamics discussed in some of these earlier works is the so-called ventilation resistance. This study investigates ventilation resistance on a number of 17 inch rims, in the Volvo Cars Aerodynamic Wind Tunnel. The ventilation resistance was measured using a custom–built suspension with a tractive force measurement system installed in the Wheel Drive Units (WDUs). The study aims at identifying wheel design factors that have significant effect on the ventilation resistance for the investigated wheel size. The results show that it was possible to measure similar power requirements to rotate the wheels as was found in previous works. The magnitude of the measured ventilation resistance confirms the conclusion that this effect should be taken into account when designing a wheel. It was found that some of the rim design factors have greater influences on the ventilation resistance than others. It was also shown that one of the investigated rims had lower ventilation resistance than measured for the fully-covered wheel configuration

Investigation of three-dimensional flow separation patterns and surface pressure gradients on a notchback vehicle

One main goal in the aerodynamic development of passenger vehicles is reducedfuel consumption. As vehicles are bluff bodies, drag is dominated by pressure drag,which is mainly caused by detached flow. To enable further reductions of the drag,it is of great importance to understand the physical phenomena behind separation.In this paper the influence of surface pressure gradients on the flow pattern of afull-scale passenger vehicle is investigated. The objective is threefold: i) Presentthe flow pattern on upper parts of the vehicle, ii) discuss the pressure gradientsaround selected areas and iii) link separation with the pressure field

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. Besides the discussion of the correlation between limiting streamlines and the surface pressure distribution, it is discussed how the surface pressure and limiting streamline development is formed. It is shown how vortices emanating from the antenna influence the surface pressure and therefore the limiting streamline pattern. Behind the front wheel arch it is explained how the separation bubble upstream influences the development of the limiting streamlines further downstream

Numerical investigation of crossflow separation on the a-pillar of a passenger car

The flow around passenger cars is characterized by many different separation structures, typically leading to vortices and areas of reversed flow. The flow phenomena in these patches show a strong interaction and the evolution of flow structures is difficult to understand from a physical point of view. Analyzing surface properties, such as pressure, vorticity, or shear stress, helps to identify different phenomena, but still it is not well understood how these are created. This paper investigates the crossflow separation (CFS) on the A-pillar of a passenger car using numerical simulations. It is discussed how the CFS and the resulting A-pillar vortex can be identified as well as how it is created. Additionally, the vortex strength is determined by its circulation to understand and discuss how the vortex preserves until it merges with the rear wake of the vehicle