Laboratory study of crossbeam structural design in control of asymmetric S- type jump of sudden expansion sections

Laboratory study of crossbeam structural design in control of asymmetric S- type jump of sudden expansion sections


Introduction
This paper presents an experimental study on a proposed dissipation structure, which consists of a series of cross beams, tested in different geometric configurations and hydraulic conditions. First, the effectiveness of this system was analyzed in terms of uniformity of flow and bed velocity and while, observing the dissipating mechanisms, in the next step, the system performance under variable tail water conditions by describing the three-dimensional flow patterns observed in the downstream channel with a gradual decrease in downstream level to 70%, 80% and 90% of tail water depth in the conditions the reference experiments were tested. Measurement of three-dimensional velocities to determine the parameters of flow uniformity, momentum and energy coefficients, and analysis of three-dimensional velocity distributions, turbulent kinetic energy, and supplementary studies on the development of isothermal line concentration and drop energy losses of reference experiments and optimal case compositions were examined. The results showed that in addition the similar qualitative trends of β and α, the flexibility of the dissipation structure has a high efficiency in the effective homogenization of the flow in the abrupt expansion channel, even in the downstream water level conditions.
Methodology
The experiments were performed in the hydraulic laboratory of Shahid Chamran University of Ahvaz and in a horizontal rectangular open channel with a length of 12 m and a width of 1 m with a height of 0.87 meters. Flow supply was provided through an open tank with dimensions of 7 m by 5 m at a height of 2.5 m. Hydraulic S-jump was performed with sudden expansion and design and construction of ogee weir. With the formation of S-jump, the conditions for the depth of hS downstream in the end control section were set, equal to 0.19, 0.15 and 0.11 m, respectively, to create three 7.4, 8.7 and 9.5 Froude numbers. Measurement of longitudinal velocity at a fixed height of 0.5 cm from the bottom of canal and longitudinal sections of 0.25 from each other in the first 2 m of downstream canal, and the other ones at distances of 2.5, 3, 4, 6 and 8 m from sudden expansion. Finding the best configurations to achieve a uniform flow and reduce the velocity distribution was done in most of the critical areas downstream of structure. For the levels of reference experiments, 54 geometries of the energy dissipation system with different configurations of beam distances, s, number of beams, N, hb height, position of the first beam from expansion, P, and slope of the system, θ, were investigated. Each component of the structure includes an I-shaped beam, with a flange width of 1.5 to 2.5 cm, depending on the hb, in the direction of the channel width and vertically in the direction of the main stream.
Results and Discussion
The results showed the values of βb and vmb2. βb calculated for three Froude numbers and different geometric parameters of the system, which means the absolute distance of 1.65, 1.85, and 2.55 meters from the expansion section for P = 0.4, 0.6, and 0.8 meters, respectively the effectiveness of the system (beam configurations) in homogenizing the flow and reducing the bed velocity is clearly evident, even for the worst performance settings. When using the structure, the mean βb is almost less than 1.1 with vmb2; the corresponding βb was measured to be approximately 0.1 m2/s2. According to the observed efficiency of the beam system, 3 of the best performance settings of the structures were selected for Fr = 9.5 (which was the most difficult situation regarding the energy characteristics of the input current) to the flow characteristics along with the flow and downstream of the structure. Energy dissipation should be fully described. According to research results, the qualitative trends of β and α are similar. Three-dimensional velocity distribution analysis showed that this type of structure has the flexibility to effectively homogenize the flow in abruptly expanding channels, even in the conditions of downstream water level varieties.
Investigation of the turbulence kinetic energy, smaller vortices that contribute to turbulence at the surface and promote mixing in the flow interface until they reach maximum value during the study period, which was from x = 0.3 m to x = 2 according to the definition of the ratio of energy losses to initial energy as a relative energy loss or jump efficiency (η), it was found that in all tested Froude numbers, the trend of increasing relative energy loss to a cross-section of 2.5 m after expansion section was increasing. Approximately X/Xexp = 4 reaches its peak and extends with a constant almost linear trend to the end of the section and the end.
Conclusion
In this research, a laboratory simulation application of a new structure with different geometric configurations of cross beams as a series of wide-wing beams to control asymmetric S-jump of sudden expansion sections was experienced. The use of different geometric configurations of cross beams shows the effectiveness of beams' contribution in homogenizing flow and reducing bed velocity.. The concentration of turbulent flow caused by the jet hitting the beam system, particularly the first beam, leads to a significant energy loss in the area between the first and 2nd beams. Before leaving the main system, the bubbles, leaving the system energy in the same evacuate the area and saw a calmer flow in after areas of the structure. The study of turbulent kinetic energy showed that the conversion rate of the high state from the mainstream after the beams (Conversion of mean flow) to turbulent flow occurs in some cases. Also, in all Froude numbers of reference experiments, the trend of increasing the relative energy loss to a cross-section of about 2.5 m after cross-section expansion is increasing.
Keywords
Sudden expansion, cross beams, hydraulic S-jump, flow patterns, energy dissipation, hydraulic structures, stilling basin.