Project number: DFG, RA 569/16-1
Project leader: Dr. Armin Raabe
Investigators: Dr. Manuela Barth, Michael Wilsdorf

The main objective of the project was the enhancement of a method for simultaneously determining three-dimensional distributions of temperature and flow properties in a predefined air volume.

The speed of sound in air mainly depends on temperature and flow properties along the propagation path of acoustic signals. Thus, measuring the travel-time between a sound source and a receiver whose positions are known exactly one can deduce averages of these meteorological quantities along the propagation path. Using tomographic techniques, spatially averaged distributions of temperature and flow can be calculated from a combined treatment of single-line measurements along different paths through the investigation area.

Within the framework of this project, a measurement system for two-dimensional applications was improved and tested in such a way that it is now possible to investigate a special volume with up to 16 sound sources and 16 receivers. It was shown that the system is capable of detecting three-dimensional flow and temperature distributions and their evolution in time simultaneously.

Concerning hardware adaption for three-dimensional measurements, special loudspeakers were designed and constructed which ensure a homogeneous three-dimensional sound emission into one half-space. Furthermore, algorithms for calculating three-dimensional distributions of temperature and flow fields were developed and implemented into the measurement system. In order to investigate the reconstruction accuracy sensitivity studies with simulated fields were carried out. It was shown that a good agreement between simulated and reconstructed distributions could be achieved.

Afterwards, the system was tested under different conditions within a laboratory (no wind) and within the low-speed wind tunnel of the Technische Universität Dresden (Germany). During calm conditions a thermal layering within the measurement volume was observed which corresponds to measurements with additional temperature sensors. As expected, such thermal layering could not be observed during wind tunnel experiments.

For an undisturbed air flow within the wind tunnel, tomographically reconstructed flow properties agree very well with preset flow properties. Besides homogeneous flow investigations, further measurements were done with different obstacles within the air flow to generate flow inhomogeneities. For comparison, flow was measured at several distinct points using a hot-wire probe and it was visualized using fog. Also in this case there was a good agreement between acoustically gained data and alternative methods.

Hence, it was shown that the enhanced tomographic system is suited to remotely detect spatial (three-dimensional) distributions of temperature and flow properties within a measurement volume simultaneously.