Published online by Cambridge University Press: 04 April 2018
Oscillatory flow around a circular cylinder close to a plane boundary is numerically investigated at low-to-intermediate Keulegan–Carpenter ( $KC$) and Stokes numbers (
$KC$) and Stokes numbers ( $\unicode[STIX]{x1D6FD}$) for different gap-to-diameter ratios (
$\unicode[STIX]{x1D6FD}$) for different gap-to-diameter ratios ( $e/D$). A set of unique flow regimes is observed and classified based on the established nomenclature in the (
$e/D$). A set of unique flow regimes is observed and classified based on the established nomenclature in the ( $KC,\unicode[STIX]{x1D6FD}$)-space. It is found that the flow is not only influenced by
$KC,\unicode[STIX]{x1D6FD}$)-space. It is found that the flow is not only influenced by  $e/D$ but also by the ratio of the thickness of the Stokes boundary layer (
$e/D$ but also by the ratio of the thickness of the Stokes boundary layer ( $\unicode[STIX]{x1D6FF}$) to the gap size (e). At relatively large
$\unicode[STIX]{x1D6FF}$) to the gap size (e). At relatively large  $\unicode[STIX]{x1D6FF}/e$ values, vortex shedding through the gap is suppressed and vortices are only shed from the top of the cylinder. At intermediate values of
$\unicode[STIX]{x1D6FF}/e$ values, vortex shedding through the gap is suppressed and vortices are only shed from the top of the cylinder. At intermediate values of  $\unicode[STIX]{x1D6FF}/e$, flow through the gap is enhanced, resulting in horizontal gap vortex shedding. As
$\unicode[STIX]{x1D6FF}/e$, flow through the gap is enhanced, resulting in horizontal gap vortex shedding. As  $\unicode[STIX]{x1D6FF}/e$ is further reduced below a critical value, the influence of
$\unicode[STIX]{x1D6FF}/e$ is further reduced below a critical value, the influence of  $\unicode[STIX]{x1D6FF}/e$ becomes negligible and the flow is largely dependent on
$\unicode[STIX]{x1D6FF}/e$ becomes negligible and the flow is largely dependent on  $e/D$. A hysteresis phenomenon is observed for the transitions in the flow regime. The physical mechanisms responsible for the hysteresis and the variation of marginal stability curves with
$e/D$. A hysteresis phenomenon is observed for the transitions in the flow regime. The physical mechanisms responsible for the hysteresis and the variation of marginal stability curves with  $e/D$ are explored at
$e/D$ are explored at  $KC=6$ through specifically designed numerical simulations. The Stokes boundary layer over the plane boundary is found to be responsible for the relatively large hysteresis range over
$KC=6$ through specifically designed numerical simulations. The Stokes boundary layer over the plane boundary is found to be responsible for the relatively large hysteresis range over  $0.25<e/D<1.0$. Three mechanisms have been identified to the change of the marginal stability curve over
$0.25<e/D<1.0$. Three mechanisms have been identified to the change of the marginal stability curve over  $e/D$, which are the blockage effect due to the geometry setting, the favourable pressure gradient over the gap and the location of the leading eigenmode relative to the cylinder.
$e/D$, which are the blockage effect due to the geometry setting, the favourable pressure gradient over the gap and the location of the leading eigenmode relative to the cylinder.
 $hp$
                element method: investigation of time-stepping and parallelisation strategies. PhD thesis.Google Scholar
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                element method: investigation of time-stepping and parallelisation strategies. PhD thesis.Google ScholarAnimation of streakline in regime HA at (e/D, KC, β)=(0.25,10, 10).
Animation of streakline in regime GVS at (e/D, KC, β)=(0.5, 6, 20).
Animation of streakline in regime GVS-A at (e/D, KC, β)=(0.5, 6, 30).