School of Engineering, The University of Queensland
Abstract
Recent preliminary experiments have indicated that a neutrally buoyant tethered sphere develops a large diameter quasi-circular trajectory, unlike the oscillations observed for non-neutrally buoyant tethered spheres. This shows similarities to the path of buoyant bubbles, which may follow zig-zag and/or helical paths depending on the Reynolds number. The current study explores the behaviour using well resolved numerical simulations. The forces like tension, buoyancy and fluid force are considered. It is found that there exist six different flow regimes within the range of the Reynolds number = [50, 800] according to the sphere response. Regime I (Re = [50, 205]) and Regime II (Re = [210, 260]) are characterised by steady axisymmetric flow structure without body movement except the loss of axisymmetry in Regime II. The sphere starts to vibrate from Regime III (Re = [270, 280]). Regime IV (Re = [300, 330]) shows suppressed body oscillation and steep decrease of off-centered distance in the plane normal to streamwise direction (yz plane). In Regime V (Re = [335, 550]), the sphere oscillates around (0, 0) in yz plane. The sphere of Regime VI (Re = [600, 800]) oscillates rather irregularily. The transitions are compared with those for a fixed sphere. In addition, the effect of moving away from neutral buoyancy is examined
