  A dual-step cylinder is comprised of a large diameter cylinder (D) with low
aspect ratio (L/D) attached co-axially to the mid-span of a small diameter
cylinder (d). The fluid dynamics video presented in this investigation is used
to illustrate the effect of aspect ratio on dual step cylinder wake development
for Re = 2100, D/d = 2, and 0.2 < L/D < 3. In addition, the video provides
visualization of such flow phenomena as interaction of spanwise vortices,
development of streamwise vortex filaments, and formation of Kelvin-Helmholtz
rollers. The experiments were performed in a water flume at the University of
Waterloo. A hydrogen bubble flow visualization technique was employed to
visualize vortical structures downstream of each cylinder model.
High-resolution images of the flow were obtained with a high speed Photron
camera and post-processed using Adobe Photoshop CS4. The results show a
plethora of vortices developing in the wake of the dual step cylinder. For 1 <
L/D < 3, spanwise vortex shedding occurs from the large and small cylinders at
distinct frequencies. However, for smaller L/D, large cylinder vortex shedding
ceases and small cylinder vortices interact in the large cylinder wake.

Fluid Dynamics