Document Type : Original Article
Authors
1
Birjan University
2
Department of Water Science and Engineering, College of Agriculture, University of Birjand, Birjand, Iran
https://dx.doi.org/10.22034/iwrj.2025.15048.2651
Abstract
Introduction:
Hydraulic jump is an important phenomenon in free-surface flows that occurs when a supercritical flow transitions to subcritical conditions. This process involves a sudden change in flow velocity, which results in significant energy dissipation through variations in flow intensity, turbulence, and wave formation. The hydraulic jump plays a crucial role in the design and operation of hydraulic structures and channels, as it influences velocity distribution, water depth, and energy levels along the flow, ultimately affecting system stability and performance. The primary objective of this research is to introduce an innovative method for determining conjugate depths and hydraulic jump length. This method employs a rectangular water jet with rapid free characteristics to excite the flow and alter jump properties. By simulating real flow conditions through the application of this jet, the study aims to improve the accuracy of hydraulic jump modeling and provide insights for practical hydraulic design.
Methods:
This experimental study was performed in a rectangular glass-walled channel with dimensions of 10 m length, 0.3 m width, and 0.5 m height. To investigate the effects of jet angle, flow rate, and bed roughness on hydraulic jump characteristics, a total of 63 tests were conducted under varying conditions. Three different fall heights and three jet discharges (2, 2.5, and 3 L/s) were applied in combination with three Froude numbers (6.5, 8.3, and 9.5). Bed roughness was introduced at two different heights (0.03 m and 0.04 m) and at two distances (d and 2d) to simulate variable surface conditions. The independent variables included jet flow rate, angle, and bed roughness, which were specifically chosen to evaluate their influence on jump features.
Two complementary measurement methods were used to capture jump parameters. First, a direct method employed a fabric tape measure fixed along the channel wall to continuously record depth and length of the jump. This provided accurate data on the variation of water depth during the hydraulic jump. Second, an indirect method used high-resolution photography to capture detailed images of flow patterns, which were later processed with Grafar software to extract jump dimensions and analyze flow behavior. These combined approaches enhanced the precision of parameter measurement and ensured reliable evaluation of the effects of experimental variables.
Results:
The experiments revealed that the effect of jet angle on the hydraulic jump varied with flow conditions. At a specific threshold, termed the “ineffective angle,” changes in jet angle had no effect on jump position. Increasing the angle beyond this point caused upstream movement of the jump, until a maximum displacement angle was reached beyond which no further change occurred. Variations in jet angle and discharge significantly influenced jump length, secondary depth, energy dissipation, and shear stress on the channel bed. For example, at a Froude number of 6.5, a jet angle of 129°, and discharge of 3 L/s over triangular bed roughness, secondary depth increased by 21% compared with the smooth bed case. Similarly, at an angle of 63° with 3 L/s and Froude number 6.5, the secondary depth ratio rose by 20.96%. Conversely, under maximum Froude number and jet angle greater than the ineffective angle (145°), jump length decreased by 30.16%. Energy dissipation was also reduced under certain conditions; at a 60° angle with maximum Froude number and 3 L/s discharge, dissipation decreased by 22.4%. The largest reduction, 25.76%, occurred at 145° with maximum discharge and minimum Froude number on a rough bed. Furthermore, bed shear stress increased under maximum angle conditions, while at angles smaller than the ineffective angle, negative D values indicated higher secondary depths compared to a classic jump. The lowest D value (−21) was observed at 63°, 3 L/s discharge, and minimum Froude number in the smooth bed case. Positive D values at higher angles showed reduced secondary depths compared to classic jumps, with the maximum value (20.9) occurring at 129° and maximum discharge with minimum Froude number on triangular roughness.
Conclusion:
This study demonstrated that the use of a rectangular water jet significantly affects hydraulic jump behavior. When applied at angles greater than the ineffective angle, the jet reduced jump length, energy dissipation, and conjugate depths. Under maximum discharge and high Froude numbers, bed shear stress increased, altering flow velocity distribution and turbulence. These results highlight the importance of controlling jet parameters for effective energy management in hydraulic structures. The findings can be directly applied in the design and optimization of channels, spillways, and flood control systems to improve energy dissipation efficiency and enhance structural safety under intense flow conditions.
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