An experimental study of a planar wake consisting of four vortices that simulate the trailing vortex wakes generated by transport airplanes in either takeoff or landing configurations is presented. The objective of this study was to examine naturally occurring wake instabilities. Specifically, the focus of the study was centered on bending wave instabilities of which the Crow instability represents a particular case. A unique method of generating a four-vortex wake was developed for this study. The four-vortex wake generating device permitted direct variation of the spacing between vortices as well as control over the vortex circulation strength. Two quantitative flow visualization experiments were instrumental in identifying wake configurations that were conducive to the rapid growth of bending wave modes and in the identification of the long-wavelength mode. Detailed experiments were also conducted to examine the flow structure in the near-field or roll-up region using a four sensor, hot-wire probe that could measure all three velocity components in the wake simultaneously. The results of both the flow visualization and hot-wire experiments indicate that the long-wavelength mode and the first short-wavelength mode likely dominate the far-field wake physics and may potentially be utilized in a wake control strategy.
An experiment is currently being conducted at the University of Notre Dame to investigate the interactions, and ultimately the instabilities, that persist in a four-vortex wake. A wake consisting of four trailing vortices was created in Notre Dame's atmospheric boundary layer wind tunnel. The advantage of this tunnel is the long test section, which permits the observation of wake interactions at distances up to thirty wake-spans downstream of the point of generation. The wake was created by four high aspect ratio wing models such that the circulation strength and the spacing between each vortex in the wake may be controlled. The principle aim of this study is to experimentally determine the optimal configurations, based on circulation strength and spacing, that are consistent with the rapid amplification of certain wake vortex instability modes. The primary method for evaluating the experimental facility has been flow visualization. Helium bubbles were introduced into the core region of individual vortices to visualize these wake structures. This paper presents some early results from the visualization of the four-vortex wake. These results suggest that the use of helium bubbles is a suitable method for visualizing wake vortices for the full length of the test section.
An investigation of the stability of a four-vortex wake has been conducted in the atmospheric boundary layer wind tunnel at the University of Notre Dame. Four independent wing models were used to create a generic four-vortex wake, such that the effects of circulation strength and vortex span could easily be studied. Helium bubble flow visualizations provide sufficient evidence of the persistence of a long-wavelength instability driven primarily by the strain field between the inboard-outboard vortex pair for a specific set of wake configurations. A quantitative investigation into the near-field development of these wakes via hot-wire anemometry further indicates the development of the elliptical form of short-wavelength instability. The experimental measurements of a short-wavelength perturbation frequency was found to compare favorably to that predicted by theory.
The use of flow visualization often plays a defining role in the understanding of many fluid flow problems. Through the use of direct injection flow visualization, the researcher may be afforded either a global view of a complex flow field or the enhanced understanding of a critical flow feature. The current article serves to introduce the reader to several commonly practiced direct injection flow visualization techniques including smoke injection, smoke wire, chemical reactions and helium bubbles. Several considerations for the use of each of these techniques are provided along with multiple examples of the use of direct injection flow visualization.
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