Energy Losses And Turbulence Characteristics Through Hydraulic Structures Using Laser Doppler Velocimetry(LDV)

Abstract

This paper deals with the experimental investigation of energy losses and turbulence characteristics through hydraulic structures in a rectangular channel using Laser Doppler, measurements include turbulence intensity components and mean velocity components. Experiments were conducted with different contraction ratios at different expansion angles for different bed slopes. The results show that, the rate of variation of the energy loss increases till expansion angle about 30°. This rate of increase decreases above this value of angle of expansion. The energy loss is quite high at a contraction ratio of 0.7. Also, the results clearly show that, gradual expansion decrease the turbulence intensities in the wall and free surface regions compared to the sudden expansion. The maximum values of the turbulence intensities occur either close to the bed or at the free surface, with minimum values occurring within the core region. The turbulence intensities, however increases sharply at the free surface due to the free surface waves effect, and is the largest in sudden expansion. The information regarding the turbulence characteristics in the transitional structures is somewhat scanty. Paradoxically enough, the problem of separation of the main stream of flow at open channel transitions or at an abrupt change of the boundary attracted the attention of investigators since the earliest time and yet it remains one of the least understood and the most critical problems of fluid dynamics today. Open channel transitions are commonly used in hydraulic structures in variety of situation to serve as link with minimum possible energy loss. Open channel transitions have been studied extensively because of their use in water resources engineering and their efficacy in reducing the energy loss in hydraulic structures. Transitions are provided, whenever the size or the shape of the cross section of an open channel changes. Such changes are often required in natural and artificial channels for water structures economically as well as for practical reasons. The transitions may be vertical or horizontal, contracting or expanding, sudden or gradually which are required for subcritical or supercritical flows. The change in the cross section disturbs the flow in the contracted reach and near it from both upstream and downstream. The change in the cross section, slope, and/or alignment over a specified reach is termed local transition, such channel transition is used mainly to avoid or minimize the excessive energy loss, to eliminate the cross waves, the resulting turbulence and to ensure safety of both the structure and the downstream channel reach. In the design of hydraulic structures, designers do their best to avoid sudden transition of the flow by sudden contractions to ensure smooth flow with minimum energy loss and to reduce turbulence pattern. As the flow passes through a bridge, a channel transition in the form of contraction and subsequent expansion is involved. Since these transitions are meant for continuous use, their role in minimization of the energy loss and attenuation of turbulence assumes significance. It is indispensable in hydraulic engineering to investigate structures of turbulence behind of multi vents water structures in the expansion zone in