FLOW AROUND A CYLINDER - istiarto

– 6.3 –
The spatial distributions of the turbulence intensities and the Reynolds stresses were also
evaluated. The intensity of turbulence inside the scour hole is strong; an increasing
turbulence was detected approaching the cylinder and moving around the cylinder
towards the downstream (wake) region. In the wake region, where a separation evidenced
by a flow reversal takes place, the turbulence attains its strongest intensity. The kinetic
energy of the flow inside the scour hole, where a circulating flow is eminent, consists of
high turbulent energy, ranging from 10% to 90% of the total kinetic energy. The profiles
of the turbulent kinetic energy are characterized by distinguishable bulges along the
presumed separation line. Approaching and moving around the cylinder, those bulges
move downwards, increase, and enlarge.
The longitudinal distribution of the bed shear-stresses along the plane of symmetry shows
that the bed shear-stress is reduced upon entering the scour hole when compared to its
value in the approach flow. The shear stress along the upstream scour bed has negative
values, corresponding to the flow reversal in that region. These observations are
supported by the numerical simulation.
6.2.2 Numerical simulations
The flow simulations were performed by using a 3D numerical model, which is
developed based on the approximate solution of the Reynolds-averaged Navier-Stokes
equations for incompressible flows by using finite-volume method. The model uses the k-
� turbulence closure model to compute the turbulence stresses and the SIMPLE (Semi-
Implicit Method for Pressure-Linked Equations) method of Patankar and Spalding
(1972) 2 to link the velocity to the pressure. The water surface position is determined
according to the pressure along the surface boundary. The model solves the transient flow
equations, but is applicable only for steady flow problems.
The core of the present model is relatively standard and can be found in classical
textbooks. However, some detailed derivations and clarifications were elucidated about
the boundary conditions and the pressure-velocity coupling. These are seldom presented
in detail.
Preliminary simulation tests were carried out to ascertain the methods and modeling
techniques opted in the model; these serve as the calibration and verification of the
model. A simple and well-known uniform flow was selected. The model value of the
equivalent standard roughness of the bed was calibrated against the measurements.
Verification of the model was done by simulating the flow under different boundary and
initial conditions; the model could produce the uniform flow field and the agreement with
the measured one was nearly perfect.
2 Patankar, S.V., and Spalding, D.B. (1972). ―A calculation procedure for heat, mass and momentum
transfer in three-dimensional parabolic flows.‖ Int. J. Heat Mass Transfer, 15, 1787-1806.