Geoscience Reference
In-Depth Information
OXYGEN TRANSFER BY FLOW CHARACTERISTICS
AT STEPPED DROP STRUCTURE
KIM JIN-HONG
Department of Civil Engineering, Chung-Ang University
72-1 Nae-ri, Daeduck-myeon, Ansong-si, Kyeonggi-do, Korea
jinhkim@cau.ac.kr
This paper deals with oxygen transfer by air entrainment by flow character-
istics at the stepped drop structure. Nappe flow occurred at low flow rates
and for relatively large step height. Dominant flow features included an air
pocket, a free-falling nappe impact and a subsequent hydraulic jump on the
downstream step. Skimming flow occurred at larger flow rates with formation
of recirculating vortices between the main flow and the step corners. Oxygen
transfer was found to be proportional to the flow velocity, the flow discharge,
and the Froude number. It was more related to the flow discharge than to the
Froude number. The stepped drop structure was found to be e
cient for water
treatment associated with substantial air entrainment.
1. Introduction
Drop structure is useful for air entrainment and energy dissipation by the
stepped type of the downstream part of the flow section. Air entrainment
by macro-roughness is ecient in water treatment
1
because of the strong
turbulent mixing associated with substantial air entrainment. It will be
built along polluted and eutrophic streams to control the water quality,
since it is used in water treatment for reoxygenation, denitrification and
volatile organic component (VOC) removals.
2
The flows over the stepped drop structures are characterized by the
large amount of self-entrained air. The macro-roughness of the steps leads
to a sharp increase in the thickness of the turbulent boundary layer. Where
the boundary layer reaches the free surface, air is entrained at the so-called
inception point of air entrainment. Dissolved oxygen will be made and with
this effect, plenty of algae, aquatic insects and fishes can inhabit down-
stream of the stepped drop structures.
Recently, river environmental works considering ecological habitats have
been thought to be important, and the hydraulic studies on the ecological
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