FLOW AROUND A CYLINDER - istiarto

3.2.1 Vertical distribution of the velocity
– 3.4 –
Shown in Fig. 5.1 are some selected velocity profiles, u(z), v(z), and w(z), measured in
the planes � = 0°, 45°, 90°, 135°, and 180°. Indicated also in the figure are the
normalized axes, u U � and z h � , where U ∞ = 0.45 [m/s] and h ∞ = 18 [cm] are the cross
sectional velocity and flow depth of the (far-field) approach flow. Observing individually
the three velocity components in each plane, the following is to be remarked:
� In the plane � = 0°, approaching the cylinder. The u component hardly changes its
magnitude until r = 42 [cm], slightly decreases until r = 30 [cm], and progressively
diminishes over the entire flow depth closer to the cylinder. Noticeable is the negative
u-component at the lower layer of r ≤ 30 [cm], indicating a reversed flow. The v
component is practically negligible in all measured profiles, showing that the flow is
practically symmetrical about the center line. The w component considerably grows
into a downward flow (negative values) with a maximum value of w U � � �0.6 close
to the cylinder. A similar magnitude of downward velocity was reported by Melville
(see Melville and Raudkivi, 1977). In the flat channel bed measurement, however, a
lower downward peak was obtained, w U � � �0.3 (see Graf and Yulistiyanto, 1998).�
� In the plane � = 45°, approaching the cylinder. The u component slightly decreases in
the upper layer, but progressively increases in the lower layer; negative u-component
is observed close to the bed, but with a lower intensity than that in the plane � = 0°.
The v component remains nearly constant until r = 30 [cm], and afterwards shows
decreasing positive values in the lower layer, v U � � 0.1, and increasing negative
values in the upper layer, v U � � �0.4 . The w component exhibits a similar pattern
as that in the plane � = 0° where an increasing downward flow is observed, but with a
slightly lower intensity. A maximum downward flow of w U � � �0.5 was measured
close to the cylinder.
� In the plane � = 90°, approaching the cylinder. The u component moderately grows,
notably after r ≤ 30 [cm]; a weak negative velocity is observed close to the bed. The v
component is small in r > 30 [cm], shows afterwards increasing negative values (the
flow is away from the cylinder) in the upper layer and increasing positive values (the
flow is towards the cylinder) in the lower layer. An important v component is
observed close to the bed and next to the cylinder. The w component always exhibits
an increasing downward flow, but in a smaller intensity than that in the planes � = 0°
and 45°, being w U � � �0.35. The w(z) profiles show concave distribution with zero
values at the bed and surface. This is unlike in the flat channel bed measurement
where the w velocity is maximum near the surface and is decreasing towards zero at
the bed (see Graf and Yulistiyanto, 1998). The concave w(z) profiles are also
observed in the previous planes (� = 0° and 45°).