Frank Ullrich

Measurement of the spatial coherence of surface pressure in the wake of a car’s side mirror

Facing the rising comfort demand, car acoustic gains increasing importance. One of the main goals is the reduction of the wind noise, whereby the side window plays an important role especially because of its proximity to the driver’s ear. Its mechanical behavior and the resulting acoustic radiation into the cabin is mainly affected by the spatial coherence of the surface pressure. The surface pressure is a superposition of high hydrodynamic and low acoustic pressure so that the acoustic pressure is masked and cannot be measured directly. However, the problem is that the acoustic component is essential for the cabin noise for high frequencies and high traveling speeds. To gain information about the coherence characteristics of the surface pressure and to separate its hydrodynamic and acoustic components, a measurement of high spatial resolution is needed. For that reason a novel pressure transducer array with a minimum distance between two adjacent measurement points of only 2 mm was developed. The pressure transducers of the array are arranged sparsely on a grid. Thereby, all possible distances between the spots on the grid are covered. Due to this minimization of distance redundancy, the amount of microphones could be reduced from 1849 to 92 representing a virtual array of 43× 43 measurement positions. Because of its small dimensions, this array allows for measurements at various test areas on the side window, which is a major improvement compared to former investigations. Hence, the propagation direction of the acoustic waves can be determined at distinct positions. This enables the localization of acoustic sources and the reconstruction of the acoustic pressure level using a wavenumber filtering technique. Measurements using this new sensor array were carried out on a generic car model based on the SAE(Society of Automotive Engineers)-body for the two configurations with and without generic side mirror. Results of the localization of the acoustic sources are presented and wavenumber filtered acoustic pressure spectra in the wake of the side mirror are shown.

Neue Modulwaage im Akustikwindkanal von BMW

Das Windgerausch spielt eine entscheidende Rolle beim Innengerauschkomfort. Bis jetzt wird durch Feinschliffmasnahmen das Windgerausch so optimiert, dass keine subjektiv wahrnehmbaren Storgerausche auf die Insassen wirken. Um in der Konzeptgestaltung eines Fahrzeugs die entscheidenden Komponenten auszulegen, ist der erweiterte Einsatz von neuen Versuchs- und Simulationsmethoden erforderlich. Fur den Akustikwindkanal der BMW Group wurde dazu in Zusammenarbeit mit Horiba ATS eine Modulwaage entwickelt und installiert, die fur die Losung von aeroakustischen Aufgaben vollig neue Moglichkeiten eroffnet.

Experimental and numerical investigation of air dam aeroacoustics

Different vehicle manufacturers mount an air dam on the vehicle undercarriage just in front of the wheels. For better understanding of the noise generation mechanisms and flow induced vibrations directly at their source region, the acoustical effects in the interior of the vehicle will be correlated by the University of Applied Sciences Duesseldorf and BMW with the pressure fluctuations of the exterior of the vehicle. Steady state CFD calculations in the area of the rotating front wheel will be carried out for the positioning of the surface microphones and for a better understanding of the flow topology.

New balance platform module in the BMW acoustic wind tunnel

Wind noise plays a deciding role today in reaching a high level of interior comfort within premium vehicles. Until now, wind noise was optimized using small incremental changes in such a manner that no subjectively measured noise sources had a large influence on the passengers. In the concept phase of the development of a vehicle, while the layout of the vehicle geometry is being developed, the incorporation of new testing and simulation techniques is essential. Above all the correlation between flow effects and noise must be determined. The BMW Group acoustic wind tunnel was used in cooperation with the firm Horiba ATS to develop and install a balance module that opens entirely new possibilities to solve for aeroacoustic effects.

Flow-Induced Airborne and Structure-Borne Noise at a Simplified Car Model

Experimental investigations of flow-induced noise and the resulting interior acoustics are presented. The turbulent flow field next to the side window results in sound sources and, consequently, in the radiation of sound. The combination of these sound pressure fluctuations and the hydrodynamic wall-pressure fluctuations excites the side window and results in radiation of noise into the passenger cabin. To investigate these physical effects, a simplified model of an automobile was set up. The whole model is insulated acoustically in such a way that noise radiation to the interior is limited to the side window. To modify the flow around the model, a square bar at the a-pillar or a side mirror model was mounted. A microphone at the position of the driver’s ear detected the sound radiation to the interior. In order to quantify excitation of the side window, phase-resolved laser-scanning vibrometer measurements were performed. Furthermore, unsteady pressure measurements were carried out at the window surface. The investigations led to a better understanding of the physical mechanism of flow-induced sound generation and also the contribution of sound sources to the interior acoustics.

Experimentelle und numerische Untersuchung zur Strömungsakustik einer Staulippe

Die Staulippe befindet sich an Fahrzeugen unterschiedlicher Automobilhersteller am Unterboden jeweils vor den Radern. Ihre akustische Wirkung im Fahrzeuginnenraum wird im Rahmen einer Untersuchung der FH Dusseldorf und BMW mit Druckschwankungen an der Fahrzeugau:sBenseite korreliert, um den Ent stehungsmechanismus von Gerauschen und stromungsinduzierter Schwingungen bereits im Quellbereich besser zu verstehen. Fur die Positionie rung der Wandmikrofone und fur ein tieferes Verstandnis der Stromungstopografie werden stationare CFD-Rechnungen im Bereich des rotierenden Vorderrades durchgefuhrt.