Introduction:

This study investigates, through laboratory experiments, the effects of a central baffle structure on the hydraulic characteristics of flow within a rectangular compound channel under two distinct hydraulic regimes: subcritical and supercritical. The primary objective is to analyze and evaluate the influence of flow obstruction caused by the central baffle on the flow pattern and its key parameters. Unlike previous research, which has predominantly focused on channels with simple cross sections, this study employs a rectangular compound channel equipped with a central baffle, thereby simulating more realistic conditions typically encountered in hydraulic engineering systems such as water conveyance channels, rivers, and water resources management structures. A deeper understanding of flow behavior in these more complex environments provides the scientific basis necessary for the optimal design of baffled structures, leading to improved hydraulic performance, reduced erosion, and enhanced efficiency of water systems. Focusing on both subcritical and supercritical regimes enables a comprehensive investigation of flow pattern variations and hydraulic parameters under different discharge and energy conditions.



Materials and Methods:

Experiments were conducted in the hydraulic laboratory of Shahrekord University using a rectangular compound channel equipped with a central baffle at a 1:50 scale. The flume measured 20 meters in length, 60 centimeters in width, and 60 centimeters in height, featuring transparent fiberglass walls and a metal bed. The experimental channel and central baffle were constructed with precise dimensions and carefully installed in the flume. The total length of the channel and the characteristics of its compound cross section complied with the experimental design. The central baffle, with specified geometric dimensions, glass material, and accurately defined installation location along the channel, was fixed using clamps and connectors to ensure stability without vibration or displacement. The water supply system and return pathway to the reservoir were configured to maintain steady selected discharges. The experimental program included five subcritical and five supercritical discharges. Three water depth measurement points were designated for each test: the beginning of the baffle, the center of the baffle, and the end of the baffle. For each discharge, water depths at these three points were recorded to examine depth distribution along the channel and the effect of the baffle on the water surface profile. In this research, parameters such as discharge, flow depth, average velocity, Froude number, and energy loss were accurately measured and recorded in the presence of the central baffle across different discharge ranges for both subcritical and supercritical regimes. The experiments were organized into two distinct categories based on flow regime: the subcritical regime (Froude number Fr < 1) with discharges ranging from 0.01057 to 0.025496 m³/s, and the supercritical regime (Fr > 1) with higher discharges ranging from 0.04896 to 0.108 m³/s. The central baffle, designed with standard geometric dimensions, was installed at a designated location within the channel. Measurements were performed using precise hydrometric instruments, including an Acoustic Doppler Velocimeter (ADV) for velocity measurements and water surface level sensors.



Results and Discussions:

The results of this study revealed the significant impact of the central baffle structure on the flow pattern within the rectangular compound channel across both hydraulic regimes. In the subcritical regime, as the discharge increased from 0.01057 to 0.025496 m³/s, the flow depth increased proportionally, indicating an increase in the total flow energy with higher discharge. The most significant depth increase occurred in the upstream regions due to backwater effects, while this trend was attenuated in shallower or lateral regions (at the outlet and the throat). In this regime, the baffle induced a backwater effect (h_1>h_3) and substantial hydraulic resistance, resulting in significant energy losses ranging from 74.2% to 77.9%. This energy dissipation was primarily attributed to form drag, intense turbulence, and local friction around the baffle, demonstrating its effectiveness in dissipating energy in low-velocity flows. The Froude number in this regime ranged from 0.140014 to 0.255311 (Fr< 1), and its mild increase with discharge indicated flow stability in the presence of the baffle. In contrast, in the supercritical regime, as the discharge increased from 0.04896 to 0.108 m³/s, despite the increase in depth and velocity, energy dissipation by the baffle decreased significantly (25.73% to 35.08%), and the flow tended toward more stable conditions. Phenomena such as standing waves and flow separation were observed in this regime. The Froude number ranged from 1.098702 to 1.25347 (Fr > 1) and decreased with increasing discharge. Velocity profile analysis indicated an intensification of the three-dimensional nature of the flow, increased velocity around the baffle, vortex formation downstream, and, ultimately, increased energy dissipation. In the supercritical regime, the channel was capable of passing much higher discharges, and the accelerating effect of the throat was less pronounced than in the subcritical regime. Behind the baffle (downstream zone), flow separation induced recirculating regions or “dead zones” characterized by low velocities or even reverse flow (vortices), which are evident in velocity profiles as significant velocity reductions or vector reversals at the structure’s exit. These zones play a crucial role in energy dissipation and increased turbulence. Overall, changes in the velocity gradient and increased turbulence led to the transformation of velocity profiles and an increase in the system’s total energy loss.



Conclusion:

This research clearly demonstrated that the central baffle structure exerts significant hydraulic effects on flow in rectangular compound channels. In the subcritical regime, the structure induces increased flow depth and high energy dissipation, whereas in the supercritical regime, it acts as a moderator and stabilizer, resulting in lower energy losses. Collectively, these effects contribute to increased depth, reduced average velocity, and a lower Froude number, proving effective in controlling high velocities and mitigating downstream erosion. The findings of this study provide a deeper understanding of energy dissipation mechanisms and flow behavior in the presence of hydraulic obstructions, offering a scientific basis for the optimal design of structures aimed at flow control and erosion reduction.