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=40~mm and D=194~mm ) are used to derive a picture of the flow topology of the primary-shedding mode (Kármán 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 ( $2h=d-t$ ). 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.