Comparing the Scour Upstream of Circular and Square Orifices

摘要

Scour pattern upstream of square and circular orifices with equal opening areas of 38.45 cm2 are compared experimentally. A total of 16 tests for four constant heads, two initial conditions of fixed bed versus mobile bed and two non-cohesive sediments used as the bed layer are performed. The bed profile is surveyed by an acoustic bed profiler and the velocity distribution is measured using an ADV instrument. In the fixed-bed tests the thickness of sediment layer below the orifices' invert is 2 mm and the observations show that the influence zone, which is the zone exposed to excess shear stress in which particles are removed, is elliptically shaped in both orifices with larger ellipse radius in transverse direction. The influence zone area upstream of square orifice is 1.46 times larger than the circular orifice. Observations of the mobile-bed tests indicate that the geometrical scour parameters, namely maximum depth of scour, length and width of the scour hole increase with the head of water upstream of both orifices. It is also found that the maximum scour depth upstream of square orifice is approximately two times deeper than that of circular orifice. Meanwhile, scour length and width, upstream of square orifice are, respectively, 10-15 and 20-33% larger than those of circular orifice. Finally, the measurements of the longitudinal velocity profiles along the centrelines of the orifices for both orifices are performed. The results indicate the existence of three separate zones of velocity distribution upstream of the orifices.