Quantitative Flow Visualization Applied to a Passive Wake Control Problem

Abstract

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.