Abstract—In this paper an alternative visualisation approach of the wake behind different vehicle body shapes with simplified and fully-detailed underbody has been proposed and analysed. This allows for a more clear distinction among the different wake regions. This visualisation is based on a transformation of the cartesian coordinates of a chosen wake plane to polar coordinates, using as filter velocities lower than the freestream. This transformation produces a polar wake plot that enables the division and quantification of the wake in a number of sections. In this paper, local drag has been used to visualise the drag contribution of the flow by the different sections. Visually, a balanced wake can be observed by the concentric behaviour of the polar plots. Alternatively, integration of the local drag of each degree section as a ratio of the total local drag yields a quantifiable approach of the wake uniformity, where different sections contribute equally to the local drag, with the exception of the wheels. Keywords—Coordinate transformation, ground vehicle, local drag, wake. I. INTRODUCTION ITH the increasing fuel prices and the more strict requirements in CO 2 exhaust levels, vehicle manufacturers are investigating intensively in methods to reduce the energy consumption of vehicles. Since aerodynamics plays an important role in the total driving resistance of vehicles, it is a logical step to focus in this area. A large part of the aerodynamic air resistance of bluff bodies can be contributed to the pressure drag caused by its wake, see [1], [5], [7] and [11]. Therefore, a better understanding of the flow structures in the wake is a necessity in order to develop methods for drag reduction. Numerous investigations on ground vehicle models have been conducted. Reference [7], for example, has investigated the different flow structures that are present in the near-wake of an automotive bluff body and characterised them with respect to vehicle type. Reference [9] introduced a new aerodynamic approach with the fluid tail technique (FTT) to search for a more balanced wake. Conditions required for a balanced wake are a circular or elliptical perimeter of the base, the flow separation had to be as close to the perimeter of the base as possible and it had to be transversal with respect to the vehicle’s motion of direction. However it is well-known that a vehicle’s wake does not reach this elliptical or circular cross- section, mainly due to wake interaction between wheel/wheelhouse wake and base wake [10]. L. Sterken is with the Environment & Fluid Dynamics Center, Volvo Car Corporation, Göteborg, Sweden (e-mail: lsterken@volvocars.com). S. Sebben, PhD is with the Environment & Fluid Dynamics Center, Volvo Car Corporation, Göteborg, Sweden. She is also with the Department of Applied Mechanics, Chalmers University Of Technology. (e-mail: ssebben@volvocars.com). Prof. L. Löfdahl is with the Department of Applied Mechanics, Chalmers University of Technology, Göteborg, Sweden. Starting with the momentum and continuity equations, an expression for the drag as a fucntion of the pressure and velocities in the wake of bluff bodies can be derived. This derivation can be found in a number of references, as [2], [3], [5] and [11]. Examples where local drag has been used to visualise the air resistance in the flow are [6], [8] and [10]. Reference [8] calls this expression microdrag. This paper prefers the term local drag since the flow properties are still determined on a macroscopic scale. In order to calculate the local drag, the primitive variables pressure and velocity at a plane 100 mm behind the vehicle body are exported for post- processing. In this study it has been chosen as definition of the wake size to follow the characteristic wake parameter, velocity deficit U D , usually used in axisymmetrical body wake analysis as a measure for the wake width [12]. The velocity deficit is defined by the difference between freestream and local velocity U ∞ – U. Reference [13] uses this parameter in his analysis for the far-field wake of bluff bodies to determine the wake size, but chooses to define the size in terms of half widths, which can be seen as the distance between points where the velocity deficit has reduced to half its maximum value. Here, it is opted to determine the wake size simply as the region where there is a velocity deficit. It is thought this is beneficial to investigate the uniformity of the wake since this definition incorporates more information into the wake analysis. Eventhough a vehicle’s wake does not obtain an elliptical or circular cross-section, this paper will introduce a coordinate transformation of the wake region into polar coordinates and rescale it to a unity circle. The main intention for this approach is to obtain a better insight in where the wake can be optimised with drag reduction as goal. Another aim is to facilitate the division of the wake into distinct zones that allows for their quantification. This quantification can then be used to identify and measure the main sources to the air resistance. The present analysis conducted in this paper has been for two vehicle types, an SUV and a sedan type, and for different underbody complexities. II. METHODOLOGY This section describes the applied methodology used to analyse the results. In Section II-A the different configurations that are investigated are explained. Section II-B describes in detail the numerical setup used for the CFD simulations. Finally, Section II-C explains the post-processing steps consisting of the coordinate transformation to polar coordinates and the definition of local drag. A. Configurations Three levels of geometric complexity of the models have been considered. They can be classified as: 1. Simplified models: The concept of coordinate transformation has initially been applied on a simplified model of a SUV-type vehicle with no wheels nor wheelhouses. L. Sterken, S. Sebben, L. Löfdahl Alternative Approach in Ground Vehicle Wake Analysis W World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:6, No:8, 2012 1370 International Scholarly and Scientific Research & Innovation 6(8) 2012 ISNI:0000000091950263 Open Science Index, Mechanical and Mechatronics Engineering Vol:6, No:8, 2012 publications.waset.org/868/pdf