Thomas Roesgen

Anomalous dispersion in atomic line filters applied for spatial frequency detection

The anomalous dispersion of an atomic line filter near a resonant transition is exploited for full-field frequency measurements. The influence of the line shape function on the dispersion in atomic vapors near resonance and the possibilities to increase sensitivity are discussed. From the model-calculated absorption of iodine vapor at frequency-doubled Nd:YAG laser wavelengths, the corresponding refractive index is obtained through the Kramers-Kronig relations. Both variables are used to assess the performance of a iodine vapor cell as a dispersive element in an interferometric setup for Doppler frequency shift detection. With good agreement, the predicted sensitivity of the setup is compared to an experimental calibration. Observed discrepancies are attributed to the assumption of a Gaussian line shape in the absorption model. The full-field Doppler frequency measurement capacity of the technique is demonstrated in a rotating disk experiment, and the measurement performance is assessed.

Quantitative Flow Visualization Applied to a Passive Wake Control Problem

The results of different approaches to quantitative flow visualization with large-scale capability are presented. The techniques were applied on the occasion of a measurement campaign in a medium-sized subsonic wind tunnel that addressed a passive wake control problem on an excursion boat. The obtained data was compared qualitatively with numerical results. Fluorescent tufts attached to the surface in the area of interest were filmed by a digital camera. Subsequent post-processing retrieved the local intensity variance and thus the tuft’s movement. This allowed for both, the identification of local flow direction and regions of detached flow on the surface of the model. Helium-filled soap bubbles were tracked using an asynchronous temporal contrast sensor or Dynamic Vision Sensor (DVS). The sensor is recording temporal changes in intensity only and has a high dynamic range which made it possible to track the bubbles in ambient light conditions. The recordings yielded local flow velocity and the streamlines around the model. The pressure field or local flow direction were recorded by tracking hand-held probes in 3D space with a stereo vision system. The system allowed free placement and fast measurements close to the model’s complex surface or in its wake. By moving the probe in the volume of interest and with suitable post-processing applied, a quick quantitative assessment of the pressure field and flow topology was possible. The application of the different techniques confirmed the potential of quantitative flow visualization in large-scale testing. The methods complemented each other in the sense that they served to extract surface flow, streamlines and pressure fields.

Experimental Considerations for Laser-Induced Thermal Acoustics (LITA) in Compressible Turbulent Flows

The accuracy of LITA measurements is limited by the signal–to–noise ratio as well as by the number and the depth of Doppler and Brillouin oscillations in the signal. The influence of the controllable experimental parameters on those is discussed. For highly diffusive fluids, e.g. highly turbulent flows, minimizing the laser spot size and the laser beam crossing angle maximizes the signal amplitude as well as the signal modulation. An improved, partially fiber–coupled experimental setup is presented, which reduces beam steering effects when using heterodyne detection for simultaneous speed of sound and flow velocity (Mach number) measurements.