Search Results

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 potential of drag reduction on a generic model of a heavy vehicle using base flaps operated in combination with flow control devices is investigated experimentally. Base flaps are well known as drag reduction devices for bluff bodies and heavy road vehicles. However, for optimal performance their deflection angle should typically not exceed 12°. In this paper the primary goal is to increase the usable range of the deflection angles by applying flow control. The secondary goal is to find the most suitable method for flow control. A comparison is made between triangular vortex generators and fluidic oscillators as passive and active flow control methods, respectively. Vortex generators have the advantage of being very simple devices but produce drag. Fluidic oscillators are also quite simple devices but require additional air supply. Their advantages are that they can be activated when needed and that they do not generate additional drag.

The effect of an active flow control method is investigated on a 1:4 scale realistic vehicle model called “DrivAer” with notchback geometry. The wind tunnel experiments are conducted at a Reynolds number of Re=3.0·106. Fluidic oscillators are applied at the c-pillars and at the upper rear edge of the window. The actuators are installed inside the hollow designed model emitting a high frequency sweeping jet. The spacing of the actuators, the mass flow rate, and the position of actuation are varied. The effect of the active flow control on the car is investigated with force and surface pressure measurements. The surface trace pattern is visualized with tufts for the active flow control cases and the baseline case. A tuft algorithm analyzes provides statistical data of the flow angles. Moreover, particle image velocimetry measurements are performed in the plane of symmetry for β=0° to capture the flow field at the rear end and the wake.

The experimental investigation was conducted with a 25%-scaled realistic car model called “DrivAer” mounted in a wind tunnel. This model includes geometric elements of a BMW 3 series and an Audi A4, accommodating modular, rear-end geometries so that it represents a generalized modern production car. The measurements were done with two different DrivAer rear end configurations (fastback and notchback) at varying side-wind conditions and a Reynolds number of up to Re=3.2·106. An array of more than 300 pressure ports distributed over the entire rear section measured the temporal pressure distribution. Additionally, extensive flow visualizations were conducted. The combination of flow visualization, and spatially and temporally resolved surface pressure measurements enables a deep insight into the flow field characteristics and underlying mechanisms.

The realistic car model DrivAer is investigated experimentally up to a Reynolds number of 2.8 million in the closed-loop wind tunnel at the "Technische Universitüt" Berlin. This new open-source design-hybrid of an "Audi A4™" and a "BMW 3 series™" possesses more representative car features as the well-known generic "Ahmed-Body." Therefore, the study of realistic flow structures is enabled. The main focus of the experimental investigations is the analysis of unsteady flow phenomena in the near wake of the model with a fastback configuration. An internal six component force balance and 63 pressure sensors measured simultaneously the forces and surface pressures with a sample rate of 5 kHz. Furthermore, the velocity field in the plane of symmetry is visualized through Particle Image Velocimetry. In the trailing edge region of the roof and the beginning of the rear window a local low pressure peak likely caused by a vortex is detected.