Vorticity Fields on Flow with Vortex System
Abstract: The flow around a
circular cylinder mounted normal to the channel bed is essentially a
three-dimensional one. The flow upstream undergoes a separation of the
turbulent boundary layer and rolls up to form a flow system, known as horseshoe
vortex, which is swept around the cylinder. This vortex system plays an
important role, if the bed material is movable. The shear stress of the vortex
system is responsible to the bed erosion. The shear stress depends on the
velocity gradient, may be presented as the vorticity. This study is done to
gain a better understanding of the
vorticity around a cylinder, especially where the system of vortices exist.
Flow measure-ments were used to study the vorticity fields on flow with a
horse-shoe vortex system around a pile. The velocity vector plots show the
presence of a primary vortex upstream of the cylinder. Based on these velocity
vectors, the vorticity can be analyzed
by using central finite difference approximations. Results of the vorticity
calcula-tion are presented as the vorticity contours. Results of study show the
greatest value of the positive-vorticity on the horse-shoe vortex system is in
the plane upstream of cylinder. This
value decreases in the planes down-stream, attaining the lowest value in the
plane downstream, where the wake vortex established. The strength of the
positive vorticity increases at larger Reynolds number. Underneath the field of
positive vorticity, stretching around the cylinder, it appears a field of
negative vorticity. This negative vorticity near the bed is high in front of the
cylinder, proportional to the bed shear stress, which is responsible to the
development of local scour at the movable bed. The line of zero vorticity is
plotted originated at the bed, being as the separation point. It is also concluded that the maximum
positive-vorticity is not necessary coincided with the center of the
vortex.
Author: Bambang Yulistiyanto
Journal Code: jptsipilgg090006
