Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.
Multiple cylindrical structures are widely seen in
engineering. Flow interference between the structures leads to
a very high fluctuating forces, structural vibrations, acoustic
noise, or resonance, which in some cases can trigger failure.
Recently circular pins in various arrays have been using as
fins to enhance the cooling effect. While the enhancement is
directly connected to nature of flow around the pins, no much
is known of physics of flow around the pins. The knowledge
of flow around two cylinders is insightful for understanding
the flow around an array of cylinders/pins. This paper presents
results of an experimental investigation into interactions
between flowing fluid and a cylinder that is neighbored by
another cylinder of the same diameter. Strouhal number (St),
time-mean and fluctuating forces on and flow structures
around the cylinder are investigated while the gap-spacing
ratio T/D is varied from 0.1 to 5 and the attack angle a from
0 to 180 where T is the gap width between the cylinders,
and D is the diameter of a cylinder. A flow visualisation test
was conducted to observe flow structures around the cylinders.
Based on forces, St, flow structures and fluid-cylinder
interaction mechanisms, 19 distinct flow categories in the
ranges of a and T/D are observed, including one quadristable
flow, three tristable flows and four bistable flows. The
quadristable, tristable and bistable flows ensue from
instabilities of the gap flow, shear layers, vortices, separation
bubbles and wakes, engendering a strong jump/drop in forces
and St of the cylinders. Six different interaction mechanisms
are observed, namely interaction between boundary layer and
cylinder, shear layer/wake and cylinder, shear layer and shear
layer, vortex and cylinder, vortex and shear layer, and vortex
and vortex. While the interaction between vortex and cylinder
results in a very high fluctuating drag, that between vortex and
shear layer results in a high fluctuating lift. On the other hand,
the interaction between shear layer/wake and cylinder
suppresses mean and fluctuating forces as well as weakens
flow unsteadiness for stationary cylinders but may cause
violent galloping vibration when the cylinders are elastic. The
interaction between boundary layer and cylinder also may
generate galloping vibrations.
