TURBULENT STRUCTURES IN THE TRAILING VORTEX WAKE OF A DELTA-WING

In the present study, we investigate the near- and far-wake instabilit ies in the wake of a delta wing with a view to understanding the basic instabilities, mixing, and decay rate of the wake. Extensive visualiz ation 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 t he fluctuating velocity in the immediate near wake not only demonstrat e the periodic shedding of the spanwise vortices in this ''braid wake, '' but also reveal a low-frequency oscillation nearer to the core of e ach primary vortex. The subsequent evolution of the wake is remarkable , as the scales of disturbances grow in what appear to be almost discr ete steps, until a large-scale instability of the primary pair develop s into a sequence of large-scale interconnecting vortex loops. Ultimat ely, the appearance is similar to a series of vortex rings. The stream wise wavelength of the ring-like structure was measured for these free -flight tests and for tests with the wing towed through water at a var iety of speeds and angles of attack, yielding a wavelength between 4 a nd 5 spans (intervortex spacing of the primary pair). Further investig ation is required before the relation of these results to theoretical analyses can be ascertained. Experiments in which the incident velocit y is perturbed at selected wavelength and amplitude reveal that the wa velength of the final structure is reasonably insensitive to either pa rameter unless the forcing becomes as large as 5% of free-stream veloc ity, provided the forcing wavelength is far from the natural wavelengt h. While investigating the vertical mixing of the fluid due to the des cending vortices, we found that the vertical extent of mixing is surpr isingly large owing to the formation of a ''curtain'' of vortical flui d left above the descending vortex pair. (C) Elsevier Science Inc., 19 97.