Sabine Bonitz

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.

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 reduced fuel 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 a full-scale passenger vehicle is investigated. The objective is threefold: i) Present the flow pattern on upper parts of the vehicle, ii) discuss the pressure gradients around selected areas and iii) link separation with the pressure field.

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.

Experimental Investigation of the Near Wall Flow Downstream of a Passenger Car Wheel Arch

The flow around and downstream of the front wheels of passenger cars is highly complex and characterized by flow structure interactions between the external flow, fluid exiting through the wheelhouse, flow from the engine bay and the underbody. In the present paper the near wall flow downstream of the front wheel house is analyzed, combining two traditional methods. A tuft visualization method is used to obtain the limiting streamline pattern and information about the near wall flow direction. Additionally, time resolved surface pressure measurements are used to study the pressure distribution and the standard deviation. The propagation of the occurring flow structures is investigated by cross correlations of the pressure signal and a spectral analysis provides the characteristic frequencies of the investigated flow. It is found that two main flow phenomena can be observed: one originates from flow exiting the upper wheelhouse and a second one resulting from a separation on the lower wheel house edge. The frequency spectrum reveals a dominant Strouhal number of 0.2. As the observed flow structures are attributed to the wheel-wheelhouse interaction, a closed wheelhouse configuration is also investigated and the results confirm that the fluctuations and observed flow structures are created by the flow interaction between the wheel, wheelhouse and the rotation of the wheel.

Quantitative Tuft Flow Visualization on the Volvo S60 under realistic driving Conditions

The flow around a car is three-dimensional and turbulent. The main part of the dragis caused by pressure losses generated by various ow separations. In general a large anddistinct wake structure dominates the flow field at the base. Furthermore, diverse vertical structures are present at the backlight and the upper trunk region. The investigation ofthese owphenomena is important for the design and the optimization process of a car.Traditional ow visualization techniques, such as oil paint and tufts, are widely used andestablished for the study of surface friction lines. Aerodynamic investigations are usually conducted in sophisticated wind tunnels with a uniform freestream velocity and very lowturbulence intensities. However, this does not correspond to realistic road conditions. Inthis study, the applicability and the feasibility of a new tuft method is investigated. Theexperiments are conducted on a circuit test track and hence to realistic on-road conditions.A large amount of tufts are attached to the entire rear end section of a Volvo S60 passengercar. The movement of the tufts is continuously recorded as well as the freestream velocityusing a digital camera and a three-hole probe, respectively. Thus, every image can beassociated to a distinct inow conditions. A novel and Efficient image processing algorithmallows the extraction of the mean tuft angle afterwards. This data allows the calculationof the near wall streamlines and generates additionally statistical data of the directionalorientation of the tuft movement. The obtained time averaged surface traces indicate analmost symmetric flow field at the rear end of the car with head wind conditions. The effect of the streamwise vortices, which origins at the c-pillars, are evident on both sidesof the backlight. With side wind conditions the ow pattern at the backlight and trunkdeck shift slightly to the leeward side while the vorticalow structure at the base shiftsslightly windward. The analyses of the freestream velocity time series show Significantlyincreased turbulence intensities compared to wind tunnel tests. In summary, the new tuftow visualization and the usage of the three-hole probe in front of the car represent asimple and low cost tool for on-road test to study limiting streamline pattern.

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.

Investigations of the Rear-End Flow Structures on a Sedan Car

The aerodynamic drag, fuel consumption and hence CO2 emissions, of a road vehicle depend strongly on its flow structures and the pressure drag generated. The rear end flow which is an area of complex three-dimensional flow structures, contributes to the wake development and the overall aerodynamic performance of the vehicle. This paper seeks to provide improved insight into this flow region to better inform future drag reduction strategies. Using experimental and numerical techniques, two vehicle shapes have been studied; a 30% scale model of a Volvo S60 representing a 2003MY vehicle and a full scale 2010MY S60. First the surface topology of the rear end (rear window and trunk deck) of both configurations is analysed, using paint to visualise the skin friction pattern. By means of critical points, the pattern is characterized and changes are identified studying the location and type of the occurring singularities. The flow field away from the surface is then analysed using PIV measurements and CFD for the scale model and CFD simulations for the full scale vehicle. The flow field is investigated regarding its singular points in cross-planes and the correlation between the patterns for the two geometries is analysed. Furthermore, it is discussed how the occurring structures can be described in more generalized terms to be able to compare different vehicle geometries regarding their flow field properties. The results show the extent to which detailed flow structures on similar but distinct vehicles are comparable; as well as providing insight into the complex 3D wake flow.

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

© Copyright 2016 SAE International. 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. The main advantages over other flow visualization methods, such as oil paint, is that experimental facilities are not contaminated and statistical data can be extracted. The investigated surface pattern shows a symmetric flow on the entire rear end section of the passenger car. The flow field on the roof, backlight, and upper trunk deck is attached almost everywhere. However, two small regions indicate the presence of two counter-rotating vortices at the lower edge of the backlight (rear window). Those vortices are also detectable in the distribution of the tuft angle standard deviation. A bifurcation line is present at each side of the trunk due to the streamwise vortices originating at the C-pillars. The tuft streamlines created with this novel tuft method are compared to a standard oil paint flow visualization to validate the calculated tuft flow pattern. A critical comparison between the methods confirms that the flow tuft analysis algorithm functions flawlessly as a highly detailed flow analysis tool without the mess of oil paint.