Oliver Fischer

The Influence of a Horizontal Pressure Distribution on Aerodynamic Drag in Open and Closed Wind Tunnels

The influence on aerodynamic drag of a non-uniform, streamwise pressure distribution over the wake of an automobile model in both open-jet and closed-jet wind tunnels is considered in this paper. It has long been an unsolved issue in the theory of open-jet interference and is usually not important in closed-wall wind tunnels unless the model is very long. A new, semi-empirical approach is presented that is based on the observation that the drag changes due to a pressure gradient over a wake correlate with the empty-test-section pressure-coefficient difference between the base of the vehicle and the position of wake closure. A method is demonstrated that is able to remove the effect of the pressure gradient and that is not buoyancy related. This method is applied to a range of simplified and detailed automobile shapes at model scale and at full scale in various wind tunnels, as well as to normal flat plates. The paper presents a revision of previous approaches to the estimation of nozzle and collector blockage. Finally, and most importantly, the paper demonstrates that the differences measured between configurations in a wind tunnel with a gradient over the wake will be in error unless a correction is made for the wake distortion induced by the gradient.

Further Investigations on Gradient Effects

In automotive wind tunnels with modern road simulation installations boundary layer pre-suction is a widely-used technique for boundary layer control. The consequence of boundary layer pre-suction is an additional pressure gradient in front of the model. In order to investigate the effects of the additional pressure gradient on drag, experiments were conducted with two different models (scale 1:5) in the IVK Model Wind Tunnel. In these experiments the suction velocity of the boundary layer pre-suction served as a parameter to change the static pressure gradient along the test Section and was for this purpose adjusted higher and lower than the standard suction velocity. It is shown that the total drag increment due to boundary layer pre-suction consists of at least two parts: the ground simulation increment and the static pressure gradient increment. The ground simulation increment is due to a decrease in the boundary layer thickness and the resulting modified flow beneath the model. The static pressure gradient increment is due to the unintended additional pressure gradient. Furthermore a method is described to separate both increments and a correction method is proposed to correct the additional gradient effect.

Application and Investigation of the Correction Method

Within the aerodynamic community today, it is still discussed whether measurements in open-jet wind tunnels need to be corrected due to open-jet interference effects caused by the limited extent of the test section and the jet. Existing correction methods are still not applied in most open-jet wind tunnels to increase comparability. In this section, the correction method, as described in Chapter 4, is applied to measurements from different wind tunnels to show the usability and robustness of the method.

Wind Tunnel Interference Effects

Wind tunnels attempt to simulate the flow around a test object, which is located within an infinite stream. Primarily due to limited financial resources, the cross-section of the nozzle and the size of the test section are limited. The proximity of the surrounding wind tunnel geometry leads to a different flow field around the test object than within an infinite stream.

Computational Fluid Dynamics Investigations

In this thesis, CFD simulations were used to investigate the blockage effects in an open-jet test section, which significantly change the flow field and, thus, the acting forces on the vehicle. Hence, it is the goal to gain deeper insight into single interference effects and, finally, to gain deeper understanding into the physics behind the empirical correction method for open-jet interference effects. The main focus is on the horizontal buoyancy effect, which is the strongest of all open-jet interference effects in most known open-jet wind tunnels.

Aerodynamic Development Tools

In general, three methods are used today in the aerodynamic development of vehicles in the automotive industry. The dominating method is the aerodynamic measurement in a wind tunnel. Second is the road experiment method, which is used infrequently, as compared to the wind tunnel. CFD is the third method.

Correction Method for Interference Effects in Open-Jet Wind Tunnels

In contrast to wind tunnel corrections in closed-wall wind tunnels, the necessity of corrections of open-jet wind tunnel measurements for automotive applications has been a topic of discussion as they are significantly smaller [4].

Twenty years of automotive wind tunnels at the institute for combustion engines and automotive engineering of Universität Stuttgart 1989–2009

The transmission becomes more important in terms of evaluating emissions, drivability and comfort. These developments have caused an increase in the complexity of the transmission control units of automatically shifting transmissions (automatic transmissions, dual-clutch transmissions and automated manual transmissions) for more than twenty years. Additionally, the increasing popularity of these transmission concepts in all markets leads to more vehicle-engine-transmission combinations which have to be calibrated. The growing effort also means that more personnel and longer calibration times are required, which thus results in higher development costs. The Institute of Automotive Engineering of the Technische Universitat Braunschweig (Germany) describes methods and tools to reduce the effort for TCU calibration in terms of shift comfort by means of efficient transmission calibration on a roller dynamometer.

20 Jahre Fahrzeugwindkanäle der Universität Stuttgart am Institut für Verbrennungsmotoren und Kraftfahrwesen 1989–2009

Im Jahr 1989 nahmen der neue Fahrzeugwindkanal und der Modellwindkanal beim Institut fur Verbrennungsmotoren und Kraftfahrwesen (IVK) der Universitat Stuttgart in Stuttgart-Vaihingen den Betrieb auf. Beide Windkanale wurden in den vergangenen Jahren umfangreich technisch nachgerustet, und auch kunftig sind weitere Optimierungen geplant. Mit der Intensivierung der numerischen Stromungsberechnung begann die Entwicklung eines integrierten, aus Experiment und Simulation bestehenden Ansatzes zur aerodynamischen Fahrzeugoptimierung. Die Parallelitat und enge Vernetzung von Experiment und Simulation weist den Weg in die Zukunft der Automobilaerodynamik.