Flow past a sphere up to a Reynolds number of 300
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The flow of an incompressible viscous fluid past a sphere is investigated
numerically and experimentally over flow regimes including steady and unsteady laminar
flow at Reynolds numbers of up to 300. Flow-visualization experiments are used
to validate the numerical results and to provide additional insight into the behaviour
of the flow. Near-wake visualizations are presented for both steady and unsteady flows.
Calculations for Reynolds numbers of up to 200 show steady axisymmetric
flow and compare well with previous experimental and numerical observations. For
Reynolds numbers of 210 to 270, a steady non-axisymmetric regime is found, also
in agreement with previous work. To advance the basic understanding of this transition,
a symmetry breaking mechanism is proposed based on a detailed analysis of the calculated
flow field. Unsteady flow is calculated at Reynolds numbers greater than 270. The
results at a Reynolds number of 300 show a highly organized periodic flow dominated
by vortex shedding. An analysis of the calculated vortical structure of the wake
reveals a sequence of shed hairpin vortices in combination with a sequence of previously
unidentified induced hairpin vortices. The numerical results compare favourably
with experimental flow visualizations which, interestingly, fail to reveal the
induced vortices. Based on the deduced symmetry-breaking mechanism, an analysis of the unsteady
kinematics, and the experimental results, a mechanism driving the transition
to unsteady flow is proposed.