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The transient surface pressure over a full-scale, operational compact automotive vehicle—a Volkswagen Golf 7—exposed to transient crosswinds with relative yaw angles of β = 22-45° has been characterized. Experiments were performed at the BMW side-wind facility in Aschheim, Germany. Measurements of the incoming flow in front of the car were taken with eleven five-hole dynamic pressure probes, and separately, time-resolved surface pressure measurements at 188 locations were performed. Unsteady characteristics (not able to be identified in quasi-steady modelling) have been identified: the flow in separated regions on the vehicle’s leeward side takes longer to develop than at the windward side, and spatially, the vehicle experiences local crosswind as it gradually enters the crosswind. The total effects of these localized unsteady pressure characteristics have also been predicted to affect the vehicle globally, with the evaluation of the transient forces and moments—through pressure integration—exhibiting unsteady characteristics of overshoots, undershoots, and temporal lag; relative to steady crosswind. These results provide new insight into both the characteristics and the causes of the unsteady crosswind response of a real, operational vehicle operating in real-world conditions. This insight can be used in the design of future vehicles to make them less sensitive to unsteady gusts. Further these results can be used to develop and validate numerical and reduced-scale experimental methods that can test and assess new vehicle geometry designs during the early stages of vehicle development.

Die auf offentlichen Autobahnen und der Teststrecke charakterisierten Anund Umstromungen des Fahrzeugs bilden die Grundlage fur die im folgenden Kapitel dargestellten Windkanaluntersuchungen. Ziel ist es, mit verschiedenen Hilfsmitteln die instationare Stromungssituation der Strase und der Teststrecke im Windkanal nachzubilden. Dazu wurden die in Kapitel 4 beschriebenen Windkanale und Fahrzeugmodelle verwendet.

Die Transporter-Hinterherfahrt wurde als reprasentatives individuelles Fahrmanover identifiziert, das die Strasenfahrt sehr gut abbildet. In diesem Kapitel werden die vorgestellten Oberflachendruckverteilungen und deren Schwankungen fur die Transporter-Hinterherfahrt in 50 m Abstand bei einer Fahrgeschwindigkeit von 27,8 m/s, mit den Ergebnissen der verschiedenen Untersuchungsumgebungen verglichen. Die Anstromgeschwindigkeit in den Untersuchungsumgebungen betrug ebenso 27,8 m/s, lediglich im Modellmasstab wurde eine Anstromgeschwindigkeit von 50 m/s verwendet.

Die Ergebnisse der Fahrversuche stellen die Grundlage zur Charakterisierung der instationaren Aerodynamik dar und werden verwendet, um die Randbedingungen fur Windkanalmessungen und numerische Stromungssimulationen zu quantifizieren. Dafur wurden die Untersuchungen unter den vorgestellten Randbedingungen auf den offentlichen Autobahnen und der Teststrecke ausgewertet. Die Ergebnisse sind im Folgenden dargestellt.

The transient incoming flow a compact car experiences whilst driving 10 m – 100 m behind a heavy vehicle on a runway has been characterised. The incoming flow was measured using a 2D array of 11 five-hole probes mounted 1 m in front of an operational, full-scale compact car. Additionally, 188 surface pressure taps were used to measure the effect of the incoming flow conditions on the compact car. The experiments were performed under ideal conditions on a 2.9 km long runway in Fasberg near the DLR in Trauen, Germany.

Als Grundlage fur Untersuchungen instationarer Anstromungen wurde zunachst der virtuelle Golf VII unter stationaren Anstrombedingungen mit Simulia PowerFLOW® aerodynamisch charakterisiert und anhand der Windkanalmessungen validiert. Dazu wurden die in Abschnitt 4.6 beschriebenen Randbedingungen verwendet. Die Ergebnisse der stationaren numerischen Stromungssimulation bei einer Anstromgeschwindigkeit von 27,8 m/s sind im Vergleich zu Messungen im Aeroakustik-Fahrzeugwindkanal in Abbildung 7.1 dargestellt.

Aerodynamic development for road vehicles is usually carried out in a uniform steady-state flow environment, either in the wind tunnel or in Computational Fluid Dynamics (CFD) simulations. However, out on the road, the vehicle experiences unsteady flow with fluctuating angles of incidence &beta, caused by natural wind, roadside obstacles, or traffic. In order to simulate such flow fields, the Forschungsinstitut f&uuml, r Kraftfahrwesen und Fahrzeugmotoren Stuttgart (FKFS) swing&reg, system installed in the quarter scale model wind tunnel can create a variety of time-resolved signals with variable &beta, The static pressure gradient in the empty test section, as well as cD values of the Society of Automotive Engineers (SAE) body and the DrivAer model, have been measured under these transient conditions. The cD measurements have been corrected using the Two-Measurement Correction method in order to decouple the influence of the unsteady flow from that of the static pressure gradient. The investigation has determined that the static pressure gradient in the empty test section varies greatly with different excitation signals. Thus, it is imperative to apply a cD correction for unsteady wind tunnel measurements. The corrected cD values show that a higher signal amplitude, as in, signals with large &beta, lead to higher drag forces. The influence of the signal frequency on drag values varies depending on the vehicle geometry and needs to be investigated further in the future.

Natural wind, roadside obstacles, terrain roughness, and traffic influence the incident flow of a vehicle driven on public roads. These transient on-road conditions differ from the idealized statistical steady-state flow environment utilized in CFD simulations and wind tunnel experiments. To understand these transient on-road conditions better, measurements were performed on German public highways and on a test site. A compact car was equipped with a measurement system that is capable of determining the transient airflow around the vehicle and the vehicle’s actual driving state. This vehicle was driven several times on a predefined 200 km long route to investigate different traffic densities on public highways in southern Germany. During the tests the transient incident flow and pressure distribution on the vehicle surface were measured. With the same test vehicle, individual driving situations were recreated on a test site under weather conditions similar to those of the tests on public roads. This paper presents a comparison of the aerodynamic characteristics measured on public highways and on the test site. Two driving situations were examined at the test site: one is driving the test vehicle without traffic and the other is driving behind a box truck in different distances. This paper compares the realistic properties of the turbulent flow structures and the surface pressure around the vehicle during on-road driving in public traffic with those measured on the testing site. The purpose of this study is to investigate the possibility of reproducing the representative driving on public highways with a simplified approach.

Vehicle aerodynamics and wind tunnel technology are progressing towards more realistic simulations of the real-world on-road environment. This paper presents an overview of the new systems which were implemented during the recent wind tunnel upgrade at Forschungsinstitut für Kraftfahrwesenund Fahrzeugmotoren Stuttgart as well as comparable computational fluid dynamics simulations. The fully interchangeable road simulation system features an interchangeable five-belt system and three-belt system in the same full-scale automotive wind tunnel. This system offers the efficiency of a five-belt system combined with the more sophisticated ground simulation technique of a wide belt system, which is necessary to assess the aerodynamic properties of sports cars and racing cars. In order to simulate on-road wind conditions, a side-wind generator can be installed to generate a turbulent flow field in the wind tunnel test section. It could be shown that the commonly determined drag coefficient at 0° yaw angle in the smooth flow environment of today’s wind tunnels is not representative of the drag found in real on-road wind conditions. Additionally, the investigations in unsteady side-wind conditions indicate that the commonly used approach to determine the side-wind sensitivity of a vehicle underestimates the forces occurring in turbulent flow conditions. A validated simulation model is presented. The simulation results are in good agreement with the experimental results and can be used as a complementary tool when assessing the unsteady aerodynamic behaviour of a vehicle; this behaviour can be coupled to a vehicle dynamics model for virtual road testing in the Stuttgart full-motion driving simulator. The unsteady-behaviour effects can be evaluated comprehensively, and the results allow a subjective assessment of the unsteady response of the vehicle. Furthermore, the aeroacoustic wind noise in on-road wind conditions is investigated during the development of the vehicle. The side-wind generator reproduces the natural stochastic cross-wind and allows the effect of these wind conditions to be investigated in the aeroacoustic wind tunnel. The results show similar ratings to those in on-road tests when compared with subjective listening tests. In summary, the techniques introduced open up new horizons in the field of vehicle aerodynamics and aeroacoustics, which are a step closer towards real-world conditions in automotive engineering.

The time-varying flow field a compact car experiences whilst performing an overtaking manoeuvre of a heavy vehicle has been characterized using an array of five, five-hole dynamic pressure probes mounted in front of an operational, full-scale compact automotive vehicle. In addition, the effect of these conditions on the vehicle’s surface pressure has been investigated through 188 pressure taps distributed over the compact vehicle in separate experiments. The experiments were performed in ideal conditions, on a 2.9 km runway of the Fassberg airfield next to DLR Trauen, Germany.