Johannes Gürtler

High-speed light field camera and frequency division multiplexing for fast multi-plane velocity measurements

Non-intrusive fast 3d measurements of volumetric velocity fields are necessary for understanding complex flows. Using high-speed cameras and spectroscopic measurement principles, where the Doppler frequency of scattered light is evaluated within the illuminated plane, each pixel allows one measurement and, thus, planar measurements with high data rates are possible. While scanning is one standard technique to add the third dimension, the volumetric data is not acquired simultaneously. In order to overcome this drawback, a high-speed light field camera is proposed for obtaining volumetric data with each single frame. The high-speed light field camera approach is applied to a Doppler global velocimeter with sinusoidal laser frequency modulation. As a result, a frequency multiplexing technique is required in addition to the plenoptic refocusing for eliminating the crosstalk between the measurement planes. However, the plenoptic refocusing is still necessary in order to achieve a large refocusing range for a high numerical aperture that minimizes the measurement uncertainty. Finally, two spatially separated measurement planes with 25×25 pixels each are simultaneously acquired with a measurement rate of 0.5 kHz with a single high-speed camera.

Berührungslose Messung des lokalen Schalldrucks mittels Hochgeschwindigkeits-Holographie / Non-invasive measurement of the local sound pressure using high-speed holography

Zusammenfassung Für die Lärmreduzierung von Flugzeugtriebwerken kommen perforierte Bleche mit dahinterliegender Kavität (sogenannte Liner) zum Einsatz. Das aeroakustische Dämpfungsprinzip dieser Elemente beruht auf der Wechselwirkung von Schall und Strömung. Bisherige physikalische Modelle der Dämpfungsprozesse erlauben jedoch nicht die notwendige Entwicklung effizienterer Liner, da die aeroakustische Wechselwirkung noch nicht vollständig verstanden ist. Daher ist die Kenntnis relevanter Größen, wie die lokale Schalldruckverteilung, entscheidend für das Verständnis der lokalen Dämpfungsprozesse. Dazu präsentieren wir ein kamerabasiertes Messsystem für die zweidimensionale nicht-invasive ortsaufgelöste Schalldruckbestimmung. Abstract In order to reduce the noise emission from airplane turbines perforated sheet with a cavity behind it (so called liner). The dampening principle of these elements are based on the interaction between sound and flow. Current physical models of this aeroacoustical sound absorption do not allow the development of more efficient liners, since the aeroacoustic interaction has not been yet fully understood. Therefore all relevant factors, like the local sound field distribution, have to be measured to fully comprehend the local sound absorption processes. For this purpose we present with this work a camera-based measuring system for the two dimensional non-invasive measurement of the local sound pressure with a high spatial resolution.

Non-invasive seedingless measurements of the flame transfer function using high-speed camera-based laser vibrometry

The characterization of modern jet engines or stationary gas turbines running with lean combustion by means of swirl-stabilized flames necessitates seedingless optical field measurements of the flame transfer function, i.e. the ratio of the fluctuating heat release rate inside the flame volume, the instationary flow velocity at the combustor outlet and the time average of both quantities. For this reason, a high-speed camera-based laser interferometric vibrometer is proposed for spatio-temporally resolved measurements of the flame transfer function inside a swirl-stabilized technically premixed flame. Each pixel provides line-of-sight measurements of the heat release rate due to the linear coupling to fluctuations of the refractive index along the laser beam, which are based on density fluctuations inside the flame volume. Additionally, field measurements of the instationary flow velocity are possible due to correlation of simultaneously measured pixel signals and the known distance between the measurement positions. Thus, the new system enables the spatially resolved detection of the flame transfer function and instationary flow behavior with a single measurement for the first time. The presented setup offers single pixel resolution with measurement rates up to 40 kHz at an maximum image resolution of 256 px x 128 px. Based on a comparison with reference measurements using a standard pointwise laser interferometric vibrometer, the new system is validated and a discussion of the measurement uncertainty is presented. Finally, the measurement of refractive index fluctuations inside a flame volume is demonstrated.

Volumetrische Strömungsmessungen mittelsplenoptischer Hochgeschwindigkeitskamera

Zusammenfassung Um für komplexe, hochdynamische Strömungsprozesse wie bei Einspritzungen in Verbrennungsmotoren eine hohe Datenrate zu erreichen, d. h. eine große Voxelanzahl im dreidimensionalen Messvolumen und eine hohe Messrate, wird der Aufbau einer Lichtfeld-High-Speed-Kamera sowie deren Anwendung für ein frequenzmoduliertes Doppler-Global-Velozimeter (FM-DGV) vorgestellt. Die Lichtfeldauswertung gestattet hierbei die 3D-Rekonstruktion des Messvolumens für jedes aufgenommene Bild. Das Messsystem wurde zunächst für zwei simultan beobachtete Messebenen aufgebaut und es wird die Untersuchung eines instationären Sprays präsentiert.