On the scaling and topology of confined bluff-body flows

• An experimental study of bluff bodies in confinement is presented. Two Reynolds matched rigs (pipe diameters: D D 40 mm and D D 194 mm) are used to derive a picture of the flow topology of the primary-shedding mode (Karman vortex, mode-I). Confined bluff bodies create an additional spectral mode (mode-II). This is caused by the close coupling of the shedder blockage and the wall and is unique to the confined bluff-body problem. Under certain conditions, modes-I and II can interact, resulting in a lock-on, wherein the modes cease to exist at independent frequencies. The topological effects of mode interaction are demonstrated using flow visualisation. Furthermore, the scaling of mode-II is explored. The two experimental facilities span Reynolds numbers (based on the shedder diameter, d) 104 < Red < 105 and bulk Mach numbers 0 : 02 < Mb < 0 : 4. Bluff bodies with a constant blockage ratio (d = D), forebody shape and various splitter-plate lengths (l) and thicknesses (t) are used. Results indicate that the flow topology changes substantially between short (l < d) and long (l > d) tailed geometries. Surface flow visualisation indicates that the primary vortex becomes anchored on the tail when l & 3h (2h D d t). This criterion prohibits the development of such a topology for short-tailed geometries. When mode interaction occurs, which it does exclusively in long-tailed cases, the tail-anchored vortex pattern is disrupted. The onset of mode-II occurs at approximately the same Reynolds number in both rigs, although the associated dimensionless frequency is principally a function of Mach number. Accordingly, mode interaction is avoided in the larger-scale rig, due to the increased separation of the modal frequencies.

Subject headings

NATURVETENSKAP  -- Fysik -- Annan fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Other Physics Topics (hsv//eng)

Keyword

vortex shedding

vortex streets

Publication and Content Type

ref (subject category)

art (subject category)