Affiliations: Department of Mechanical Engineering, The Hong Kong
Polytechnic University, Hung Hom, Kowloon, Hong Kong. E-mail:
[email protected] | Visiting scholar, The State Key Laboratory of
Mechanical Transmission, Chongqing University | School of Mechanical Engineering, University of
Adelaide, S.A. 5005, Australia
Abstract: Interference is investigated between a stationary cylinder wake and
that of a downstream streamwise oscillating cylinder. Experiments were carried
out in a water tunnel. A laser-induced fluorescence technique was used to
visualize the flow structure behind two inline circular cylinders of identical
diameter d. The downstream cylinder was forced to oscillate harmonically at the
amplitude of 0.5 d and the frequency ratio
f_e/f_s=1.8, where f_e
is the oscillation frequency of the downstream cylinder and
f_s is the vortex shedding frequency from an isolated
stationary cylinder. The investigation was conducted for the cylinder
center-to-center spacing L/d=2.5 ~ 4.5. Two flow regimes have been
identified, i.e. the 'single-cylinder shedding regime' at L/d less than or equal to 3.5 and the
'two-cylinder shedding regime' at L/d > 3.5. At small L/d, the upstream
cylinder does not appear to shed vortices; vortices are symmetrically formed
behind the downstream cylinder as a result of interactions between the shear
layers separated from the upstream cylinder and the oscillation of the
downstream cylinder. This is drastically different from that behind two
stationary cylinders at L/d less than or equal to 3.5, where vortices are shed alternately from
the downstream cylinder only. At L/d=4.5, both upstream and downstream
cylinders shed vortices. This is true with or without the oscillation of the
downstream cylinder. The flow structure is now totally different from that at
L/d=3.5. The vortices are shed alternately from the upstream cylinder; a
staggered spatial arrangement of vortices occurs behind the downstream
cylinder.
Keywords: Interference, LIF, Tandem, Streamwise Oscillation, Wake