Abstract
Tandem cylinder flow comprises several different flow regimes. Within the reattachment
regime, the development of the gap shear layers is of utmost importance to the flow, but
has received little attention so far. Through direct numerical simulations at Re = 104, for
a gap ratio of 3.0, we have discovered that the shear layers are significantly altered with
respect to a single cylinder. These differences include early onset of separation, crossflow
stabilising, delayed transition to turbulence and little meandering of the transition region.
Vortex pairing in the gap shear layers is reported for the first time. The interaction
between the recirculating gap flow and the shear layers was investigated. Asymmetrical,
large-scale gap vortices influence the position of transition to turbulence through direct
contact and through secondary flows. The occurrence of transition in the gap shear
layers has consequences for both the reattachment mechanism and the development of
the downstream cylinder wake. The reattachment points are unsteady with large amplitude
fluctuations on a fine time scale. Reattachment is seen to be a combination of impingement
and modification of the upstream shear layers, which causes a double shear layer in the
downstream cylinder near-wake. Buffeting by and interaction with the gap shear layers
likely cause transition to turbulence in the downstream cylinder boundary layer. This leads
to significant changes in the wake topology, compared with a single-cylinder wake.
regime, the development of the gap shear layers is of utmost importance to the flow, but
has received little attention so far. Through direct numerical simulations at Re = 104, for
a gap ratio of 3.0, we have discovered that the shear layers are significantly altered with
respect to a single cylinder. These differences include early onset of separation, crossflow
stabilising, delayed transition to turbulence and little meandering of the transition region.
Vortex pairing in the gap shear layers is reported for the first time. The interaction
between the recirculating gap flow and the shear layers was investigated. Asymmetrical,
large-scale gap vortices influence the position of transition to turbulence through direct
contact and through secondary flows. The occurrence of transition in the gap shear
layers has consequences for both the reattachment mechanism and the development of
the downstream cylinder wake. The reattachment points are unsteady with large amplitude
fluctuations on a fine time scale. Reattachment is seen to be a combination of impingement
and modification of the upstream shear layers, which causes a double shear layer in the
downstream cylinder near-wake. Buffeting by and interaction with the gap shear layers
likely cause transition to turbulence in the downstream cylinder boundary layer. This leads
to significant changes in the wake topology, compared with a single-cylinder wake.