Visual study on wakes behind solid and slotted axisymmetric bluff bodies

A flow visualization study was conducted for various configurations of solid and slotted axisymmetric bluff bodies. The geometries also included those of solid and ribbon parachute canopy models. The structures of the unsteady near wake patterns were surveyed and compared. In addition, the effect of rapid model acceleration and deceleration on the wake was visualized. This article presents, although somewhat qualitatively, analysis of results from photographs and videotapes and an overview of various pertinent phenomena observed. CLEAR understanding of the wake flow behind a parachute is critical for the safe operation of parachutes. Though detailed knowledge of the wake structure is essential to predict, such phenomena as the wake recontact problem,1 analysis of the entire wake region at once appears to be inaccessible. For example, the wakes of axisymmetric bluff bodies are found to be fully three dimensional even for the simplest form of axisymmetric geometry. The flowfield imme- diately downstream of a ribbon parachute is particularly complex because of irregular merging of high speed jets through multiple narrow gaps. Moreover, the detailed study of the flowfield during a full-scale drop test appears to be a nearly inaccessible task at present. Flow visualizations in carefully designed laboratory environments, on the other hand, can not only provide excellent clues to the complex flowfield but also guide quantitative measurements and com- putation and analysis of the flowfield. It is also to be noted that some modern flow visualization techniques have started to provide detailed quantitative information with advance- ment of applied optics and image processings, as seen in Ref. 2. In the past, low-speed flow visualizations have been con- ducted to simulate complex high Reynolds number airflows in aircraft flight conditions, and somewhat surprisingly to some, the results compared well with flight test data.3 This means that many vortex-dominated flows are not as dependent on the differences in viscous effects. Naturally, additional com- pressibility effects must not be overlooked, and interpretation of flow visualizations in unsteady flowfields requires extra caution, as is often pointed out (see, for example, Gad-el- Hak4). In the present experimental investigation, the wake regions of the axisymmetric bluff bodies were observed using various flow visualization techniques in both air and water. Configurations studied included a circular disk, solid parachute model, slotted disk, and a ribbon parachute model. Flow visualization was also conducted to study the effect of rapid model acceleration and deceleration. Minnesota. The test section was 1.5m long and the cross section measured 1.37 x 0.97m. Freestream velocity up to 23 m/s can be obtained, though flow visualization was con- ducted at a substantially lower velocity, as noted in the figures in terms of the Reynolds numbers based on the model diame- ter. The tunnel flow condition5 was found adequate for the present study. The models to be described in the Results section were either sting-mounted or supported by thin wire along the centerline of the test section. The streamlined particle injector was fabricated and placed upstream of the model, and fine oil fog generated by a Rosco Model 1500 smoke/fog generator was introduced into the test section. The visualization in the empty test section as a control revealed no unsteadiness in the freestream nor any adverse effect of the injector. The diameter of particles, which are nontoxic accord- ing to the manufacturer, measured 2 jum on the average. Illumination was provided by an argon ion laser passed through a set of cylindrical lenses and front surface mirrors to produce a thin sheet of light aligned with the centerline of the model. The experiment in water was carried out in a towing tank at the St. Anthony Falls Hydraulic Laboratory, measuring 39 x 61 cm in cross section and 15.2m in length. In some tests, the tank was partitioned into a shorter segment for ease of operation. Normally, the model was towed in a stationary fluid. The towing carriage was driven by a variable speed a.c. motor. A 35-mm camera or a video camera and the lighting