Environmental Engineering Reference
In-Depth Information
=
obstacle, five dimensionless units downstream. For
D
1 (Fig.
4
a), corresponding
to the single wake of a large magnetic obstacle, the velocity signals oscillate in
antiphase. This is consistent with the fact that a large oscillating vortex structure
is formed behind the obstacle so that in the symmetrically located points where
the signals are registered, the velocity in the
x
-direction takes opposite values. For
D
5 which corresponds to the bistable flow, velocity oscillations do not present
a defined structure. This seems to be a characteristic feature of this regime as it has
been reported in the literature for the case of circular cylinders (Zdravkovich
1985
;
Peschard and Gal
1996
). Figure
4
c clearly shows in phase oscillations of the velocity
signals when
D
=
1
.
=
2 where even the amplitude of the oscillations coincides. Finally,
when
D
3 (Fig.
4
d), although velocity oscillations are in phase, amplitudes do not
coincide which indicate a weaker coupling of the wakes.
Important information can also be obtained from the Fourier analysis of the tem-
poral behavior of the velocity signals, particularly for determining the dominant
dimensionless frequency of the flow, that is, the Strouhal number. It is precisely at
this frequency at which the greatest amount of energy in the flow is transported.
Figure
5
shows the power spectrum obtained through the fast Fourier transform of
the corresponding velocity signals presented in Fig.
4
for different values of
D
.Only
the spectrum at one point is shown since it coincides with the one at the other
point. In Fig.
5
a(
D
=
1), a clear dominant characteristic frequency of 0.152 and its
corresponding harmonics are shown. This frequency is close to the ones obtained
=
(a)
(b)
(c)
(d)
Fig. 5
Power spectrum calculated by the Fast Fourier Transform of the velocity signals presented
in Fig.
4
.
Re
=
1,000.
a
D
=
1,
Q
=
2
.
7.
b
D
=
1
.
5,
Q
=
2
.
9.
c
D
=
2,
Q
=
2
.
4.
d
D
=
3,
Q
=
2
.
3
Search WWH ::
Custom Search