FLOW AROUND A CYLINDER - istiarto

– 2.5 –
The water circuit is a closed loop in which clear water is circulated. The flow is
generated from the general sump (1) by a 0.250[m 3 /s]-capacity pump (2), flowing
through the hydraulic circuit (3), passing the operating valve (4) and the electro-magnetic
flow-meter (5), entering the channel through the perforated pipe (6) and the channel inletbasin
(7). After the inlet basin, a type of honeycomb (8) is provided to distribute evenly
the flow over the upstream section of the channel. At the downstream of the honeycomb,
a floating plate (9) is installed to stabilize the water surface. The water flows along the
channel, passes through the working reach (10) at 10.5 [m] ≤ x L ≤ 14.25 [m] (x L is the
distance from the entrance) where the cylinder (11) is vertically installed at x L = 11 [m].
A basin (12) is provided at the downstream reach to trap the transported sediment. Before
leaving the channel and going back to the general sump, the water passes through a
motor-regulated tailgate (13), which is used to control the water-depth in the channel.
The mobile-bed is made of a uniform sand (14) having a mean diameter of d 50 = 2.1 [mm]
and a distribution ratio of � g = 1.3. The sand layer is 50 [cm] in the working reach and
10 [cm] along the rest of the channel. The sand layer is separated from the bottom of the
channel by a geotextile-sheet (15) and a layer of gravel (16). These layers avoid the
deformation of the sand layer when the channel is being slowly filled (at the beginning of
the experiment) or drained (at the end of the experiment) through the bottom pipes (17).
The measuring instrument, ADVP, is mounted on a measuring carriage (18) having a
three-axis positioning equipment allowing the instrument to be positioned at any point
along and across the channel.
2.2.2 Acoustic Doppler Velocity Profiler (ADVP)
Working principles
The Acoustic Doppler Velocity Profiler (ADVP) conceived and developed at LRH
(Lhermitte and Lemmin, 1994) is the main measuring instrument in the present work.
This non-intrusive instrument measures the instantaneous velocity vector at a number of
layers within the water column. The ADVP has been extensively exploited in the research
works conducted at LRH. Its modular installation allows a high flexibility in its use.
Several different configurations have been used in different measurements conducted
previously at LRH. This section describes the one used in the present work; given also
are the procedures of the measurement and of the velocity abstraction. Detail technical
descriptions of the instrument can be found in reports and publications elsewhere
(Lhermitte and Lemmin, 1994; Rolland, 1994; Hurther et al., 1996).
The ADVP measures the velocities in a water column based on the back-scattered
acoustic wave information. An emitting transducer sends an acoustic wave across the
water column. Upon hitting a target moving with the flow, the wave is reflected and
captured by one or several receiving transducers. The target can be air bubbles,
suspended particles, or density fronts due to temperature differences. Knowing the
frequency difference between the emitted and reflected ones, the so-called Doppler