FLOW AROUND A CYLINDER - istiarto

2.6 Summary and conclusions
– 2.57 –
Presented in this chapter are the measured data of the three-dimensional flow fields
around a vertical cylinder, installed in an established (equilibrium) scour depth, under a
clear-water regime. The acoustic-Doppler velocity profiler (ADVP) was used to measure
instantaneously the vertical distributions of the three velocity components in vertical
planes around the cylinder. The ADVP, conceived and developed at LRH, is a nonintrusive
instrument, capable of measuring the three components of the instantaneous
velocity in a number of points (in a 3 to 5 [mm] interval) over a vertical. The measured
flow field is presented as the vertical distributions of the (time-averaged) velocities, the
turbulence intensities, and the Reynolds shear-stresses in the planes � = 0°, 45°, 90°,
135°, and 180°. Presented also are the vertical distributions for the uniform approach
flow, i.e. the flow without the cylinder being installed.
The measurement in the approach flow, without the cylinder being installed, reveals that
the approach flow is uniform, with a slight tendency of being a decelerated one, and is
hydraulically (incomplete) rough. The vertical distribution of its longitudinal velocity
component, u�z�, can be well explained by the logarithmic law-of-the-wall for the inner
region, z h � 0.2 , and by the Coles law-of-the-wake for the entire depth. The vertical
distributions of the longitudinal component of the turbulence intensities can be
satisfactorily presented by the semi-empirical expression in the form of
u ��u
�� u� � Du exp��C u z h�
(see Nezu and Nakagawa, 1993). A similar expression, with
slightly different values of the constants D and C, describes the transverse component.
The vertical component of the turbulence intensities, however, cannot be satisfactorily
described by that type of expression. The vertical distribution of the Reynolds shearstresses
shows a linear distribution as expected, being zero at the water surface and
increasing towards the bed. The profiles, however, indicates diminishing values in the
inner layer, z ≤ 0.2h.
The velocity measurements, with the cylinder in the scour hole, have confirmed the
complex picture of the three-dimensional pattern of flow around a cylinder. The flow in
the plane � = 0°, being the plane of symmetry (confirmed by negligible v-component
except close to the cylinder and the bed where a weak transverse velocity is detected), is
characterized by a flow circulation and a strong downward flow at region close to the
cylinder. Towards the downstream planes, � = 45°, 90° and 135°, the flow circulation
remains, but with a diminishing strength. Similarly, the downward flow close to the
cylinder also remains and shows a decreasing intensity. Downstream of the cylinder, in
the plane � = 180°, there is a flow reversal towards the cylinder close to the surface. The
v-component, as in the plane � = 0°, is always negligible except at regions close to the
cylinder and to the bed where small values are detected. As the flow moves downstream,
leaving the scour hole, the flow reversal diminishes and the flow is recovering to the
approach flow condition. In all planes the measurements show that the flow in the far
region, beyond the scour-hole circumference, does not change with the presence of the
cylinder. The flow is practically altered only in the scour-hole region.