  A simple method to increase the flow power extraction efficiency of a
circular cylinder, undergoing vortex-induced vibration (VIV), by confining it
between two parallel plates is proposed. A two-dimensional numerical study was
performed on VIV of a circular cylinder inside a parallel plate channel of
height H at Reynolds number 150 to quantify the improvement. The cylinder is
elastically mounted with a spring such that it is only free to vibrate in the
direction transverse to the channel flow and has a fixed mass ratio (m*) of 10.
The energy extraction process is modelled as a damper, with spatially constant
damping ration ((), attached to the cylinder. The simulations are performed by
varying the reduced velocity for a set of fixed mass-damping ({\alpha} = m*()
values ranging between 0 to 1. The blockage ratio (b = D/H) is varied from 0.25
to 0.5 by changing the channel height. The quasi-periodic initial branch found
for the unconfined cylinder shrinks with the increasing blockage. The extracted
power is found to increase rapidly with the blockage. For maximum blockage (b =
0.2), the maximum flow power extracted by the cylinder is an order of magnitude
larger as compared to what it would extract in an open domain with free stream
velocity equal to the channel mean velocity. The optimal mass-damping
({\alpha}c ) for extracting maximum power is found to lie between 0.2 to 0.3.
An expression is derived to predict the maximum extracted power from the
undamped response of a confined/unconfined cylinder. With the assumption
{\alpha}c = 0.25, the derived expression can predict the maximum power
extraction within +-20% of the actual values obtained from present and previous
numerical and experimental studies.
