The separated flow past a zero-thickness flat plate held normal
to a free stream at
Re=250 has been investigated through numerical experiments. The
long-time
signatures of the drag and lift coefficients clearly capture a low-frequency
unsteadiness
with a period of approximately 10 times the primary shedding period. The
amplitude
and frequency of drag and lift variations during the shedding process are
strongly
modulated by the low frequency. A physical interpretation of the low-frequency
behaviour is that the flow gradually varies between two different regimes:
a regime H
of high mean drag and a regime L of low mean drag. It is observed that
in regime H
the shear layer rolls up closer to the plate to form coherent spanwise
vortices, while in
regime L the shear layer extends farther downstream and the rolled-up Kármán
vortices are less coherent. In the high-drag regime three-dimensionality
is characterized
by coherent Kármán vortices and reasonably well-organized
streamwise vortices
connecting the Kármán vortices. With a non-dimensional spanwise
wavelength of
about 1.2, the three-dimensionality in this regime is reminiscent of mode-B
three-dimensionality. It is observed that the high
degree of spanwise coherence that exists in
regime H breaks down in regime L. Based on detailed numerical flow visualization
we
conjecture that the formation of streamwise and spanwise vortices is not
in perfect
synchronization and that the low-frequency unsteadiness is the result of
this imbalance
(or phase mismatch).
Low-frequency unsteadiness in the wake of a normal flat plate
