FLOW AROUND A CYLINDER - istiarto

2.1 Measurement design
– 2.3 –
The flow measurement is aimed at collecting detailed data of the three-dimensional
velocity fields around the cylinder, from which a close investigation can be made to
characterize the alteration of the flow around the cylinder. A series of measurements shall
be performed around the cylinder in an equilibrium scour-depth under a clear-water scour
regime. An equilibrium scour-depth is defined as the depth of scour hole in front of the
cylinder that has no longer changed appreciably under a continuous (steady) flow. A
clear-water scour regime prevails when the scouring process is due solely to the
interaction of the flow with the obstruction (the cylinder) and that the sediment transport
does not exist in the approach flow. A live-bed scour prevails otherwise.
The ADVP shall be used to obtain the vertical distributions of the instantaneous threedimensional
velocities. The measurements are to be performed at stations distributed
along vertical planes around the cylinder, outside and inside the scour hole. The hydraulic
parameters of the experiment are selected according to certain criteria relating to the
scouring mechanism and the available laboratory facilities and instrumentation. In terms
of the scouring process, the flow has to maintain a clear-water scour and result in a
maximum scour depth for the given sediment and cylinder diameter. This can be
achieved with flows having a velocity close to but less than the sediment entrainment
velocity. From the instrument point of view, the scour-hole depth shall be small to keep
the entire flow depth in the range of the instrument measuring capability.
Given in the following sections are the detailed aspects of the measurements, from the
setup to the results of the measurements.
2.2 Experimental setup
2.2.1 Laboratory channel
The experiments were conducted in a 29 [m] long and 2.45 [m] wide rectangular
erodible-bed channel. This wide channel allows for flows with sufficiently large aspect
ratio in terms of the flow depth and cylinder diameter. Large ratios of channel width, B,
to cylinder diameter, Dp, guarantee that the flow around the cylinder is a manifestation of
the interaction between the approaching flow and the cylinder. In the present work, this
ratio is B D p = 16, being sufficiently higher than the generally agreed minimum ratio of
B D p > 8. The large ratios of the channel width to the flow depth, h ∞ , ensure the
approaching flow to be a quasi two-dimensional one where the side wall does not
influence the flow. In the present work, the ratio is B h = 13.6; this is higher than the
minimum value usually taken as the criterion, B h > 7. The general view of the channel
is shown in Fig. 2.1 and its principal elements are presented in the following paragraphs.