FLOW AROUND A CYLINDER - istiarto

5.2 Experimental data
– 5.3 –
5.2.1 Flow around a cylinder on a flat channel bed
The measured data of flow around a cylinder on a flat channel bed are obtained from the
previous work (Yulistiyanto, 1997). The data were produced from the measurements with
and without the cylinder in place. Given below is a brief description of the data; details of
the measurements and of the data can be found elsewhere (see Yulistiyanto, 1997).
The measurements were conducted in a rectangular metal-bed (smooth) tilting flume of
an effective length L = 38 [m] and width B = 2 [m]. The uniform approach flow was
established by: discharge Q = 0.250 [m 3 /s] (Q B � 0.125 [m 2 s]), flow depth
h ∞ = 18.5 [cm] (B h � � 10.8), cross-sectional-averaged velocity U ∞ = 0.67 [m/s]
(Re = 123,950 and Fr = 0.5), and bed slope S o = 6.25 �10 -4 . An acoustic Doppler
velocity profiler (ADVP), designed and conceived at LRH (see Lhermitte and Lemmin,
1994), was used to measure the instantaneous velocity vector. This non-intrusive
instrument measures instantaneously three-dimensional velocities at a number of layers
(in 5 [mm] intervals) within a water column (flow depth) based on the Doppler shift of
the backscattered acoustic signals. The ADVP was fixed at a compartment located below
the metal bed, at the center line, x L = 16 [m] from the entrance. A Mylar film, permeable
to acoustic waves, was used to separate the instrument compartment and the flow. A PVC
circular cylinder of diameter D p = 22 [cm] (B D p � 9 ) was installed vertically with
respect to the bed. The cylinder was attached to a movable carriage, allowing it to be
positioned at predetermined measurement stations around the ADVP. Vertical
distributions of 3D instantaneous velocities were obtained in five planes, � = 0°, 45°,
90°, 157.5°, and 180°. The vertical distributions of the time-averaged velocities, (u,v,w),
turbulence intensities, ( u �� u ��,
v �� v ��,
w �� w ��),
and the Reynolds stresses, (��u ��w
��,
��v ��w
��),
were subsequently deduced.
From the measurement in the uniform flow condition (the flow without the cylinder in
place), an equivalent roughness of k s � 0.54 [mm] was reported (Yulistiyanto, 1997).
This value was obtained by using the Colebroke-White equation. A reavaluation of the k s
by imposing a logarithmic distribution to the measured u-velocity profile, u(z), shows an
equivalent roughness of k s � 0.85 [mm]. Since the present mathematical model uses k s
as the input parameter in the wall boundary treatment, its value shall be calibrated to get
the suitable model value.
5.2.2 Flow around a cylinder in a scoured channel bed
Measured data of flow around a cylinder in a scoured channel bed are available from the
laboratory measurement presented in the previous chapter (see Chapter 2). A brief
description of the experimental work is recalled in the following paragraph.
The experiment was conducted in a rectangular channel of length L = 29 [m] and width
B = 2.45 [m]. The channel bed is made of uniform sand of mean diameter