Turbulent structures in the trailing vortex wake of a delta wing

Abstract

In the present study, we investigate the near- and far-wake instabilities in the wake of a delta wing with a view to understanding the basic instabilities, mixing, and decay rate of the wake. Extensive visualization of the wake as a delta wing glides in free flight through water reveals a “braid wake” between the primary vortex pair and shows it imposing its structure on the primary pair. Hot-wire measurements of the fluctuating velocity in the immediate near wake not only demonstrate the periodic shedding of the spanwise vortices in this “braid wake”, but also reveal a low-frequency oscillation nearer to the core of each primary vortex. The subsequent evolution of the wake is remarkable, as the scales of disturbances grow in what appear to be almost discrete steps, until a large-scale instability of the primary pair develops into a sequence of large-scale interconnecting vortex loops. Ultimately, the appearance is similar to a series of vortex rings. The streamwise wavelength of the ring-like structure was measured for these free-flight tests and for tests with the wing towed through water at a variety of speeds and angles of attack, yielding a wavelength between 4 and 5 spans (intervortex spacing of the primary pair). Further investigation is required before the relation of these results to theoretical analyses can be ascertained. Experiments in which the incident velocity is perturbed at selected wavelength and amplitude reveal that the wavelength of the final structure is reasonably insensitive to either parameter unless the forcing becomes as large as 5% of free-stream velocity, provided the forcing wavelength is far from the natural wavelength. While investigating the vertical mixing of the fluid due to the descending vortices, we found that the vertical extent of mixing is surprisingly large owing to the formation of a “curtain” of vortical fluid left above the descending vortex pair.