Development and Validation of 3-Dimensional Tractive Force Measuring Instrument under High-Velocity Flow Conditions

다국어 초록 (Multilingual Abstract)

Tractive force is a crucial element in the river structure design and can be indirectly evaluated using a theoretical equation by measuring hydraulic elements. The theoretical equation is applicable when the flow type is steady and uniform; however, its application is not easy because it requires the measurement of turbulent terms in the flow direction and in both the lateral and vertical direction of the flow. Moreover, it requires the mean flow velocity and the mean value of the cross section of the flow for the calculation. Thus, deviations in local tractive force increase significantly according to the channel geometry and flow velocity distribution. River structures are carried away or destroyed by the local tractive force generated by high-velocity flow (Fr > 1) rather than low-velocity flow (Fr < 1). Thus, a tractive force measuring instrument that can directly measure local tractive force in a stable manner in low- and high-velocity flow is required.
In this study, a 3-dimensional tractive force measuring instrument that can directly measure local tractive force at the river bed was developed. The instrument can directly measure tractive forces in the flow direction (X) and in the lateral direction of the flow (Y) as well as the ±pressure in the flow vertical direction (Z) in a stable manner even at low and high flow velocities.
To verify the performance of the developed 3-dimensional tractive force measuring instrument, it was installed in an orifice type fixed bed open channel, and the tractive force was directly measured and analyzed in the velocity flow, Reynolds number, and Froude number ranges of 0.622–3.604 m/s , 33,756–188,285, and 0.646–4.217, respectively. The performance of the instrument was verified by comparing the tractive force values directly measured through hydraulic experiments with those calculated using the existing theoretical equation. In addition, the Reynolds stress and flow velocity of the same flow were measured using a particle image velocimetry (PIV) system.
The results of directly measuring the tractive force using the 3-dimensional tractive force measuring instrument are as follows. The tractive force measured in the flow direction increased in the form of a quadratic function graph as the Froude number increased and that measured in the lateral direction of the flow was close to zero. The plus (+) and minus (-) signs of the measurements in the lateral direction are vibrations that indicate the direction, and the value of zero implies that movements to the left and right are similar. The absolute values of the measurement data in the lateral direction represent the absolute sizes of the data. When the trend line of the average absolute value of the tractive force measured in the lateral direction was analyzed, it was found to linearly increase as the Froude number increased. Therefore, as the Froude number increased, the size of movements to the left and right also increased. This indicates that the tractive force in the lateral direction of the flow also affects the scour and erosion of the river bed material.
The tractive force directly measured using the 3-dimensional tractive force measuring instrument fell in the range of the tractive force that considered the Manning formula, and it also exhibited a range according to the Froude number. The tractive force had a range because the feed flow rate, flow velocity, and cross-sectional area of the flow in the channel were different depending on the experimental case. The Reynolds stress measured through PIV showed a larger range compared to other measurement methods, and it exhibited an error rate of approximately 30% with the measured tractive force value.
Because the tractive force that considered the Manning formula is proportional to the n-th power of the roughness coefficient, the error of the calculated tractive force becomes large owing to the roughness coefficient error. Therefore, the roughness coefficient was inversely calculated with the tractive force directly measured using the 3-dimensional tractive force measuring instrument, and the results were found to be 0.0165 and 0.0097 for subcritical and supercritical flows, respectively. According to Lee et al. (2010), the roughness coefficient decreases as the flow rate increases in an actual river. The results of this study also showed that the roughness coefficient decreased as the flow rate in the experimental channel increased.
Assuming that the suction phenomenon occurs in high-velocity flow owing to the pressure difference caused by the rapid water surface change, a pressure gauge (Z) was installed in the 3-dimensional tractive force measuring instrument, and the measurement results were as follows. In most experimental cases, the pressure head was measured to be approximately 10% higher than the water depth of the channel. When the standard deviation of the measurement data was analyzed, fluctuations in the measured bed pressure were found to be larger than those in the measured water depth as the Froude number increased. Further research is required to establish whether channel vibration was caused by the high-velocity flow or the actual bed pressure was high.
As the developed 3-dimensional tractive force measuring instrument well reflected the roughness coefficient and hydraulic characteristics, the tractive force and bed pressure in the flow direction and in the lateral direction of the flow were measured in real time. Based on this, it is necessary to directly measure local tractive forces.
It is presumed that the tractive force data directly measured in a fixed channel can be utilized in the numerical analysis correction procedure. If the shear plate of the 3-dimensional tractive force measuring instrument is produced to have a roughness similar to that of the river bed material, the tractive force at the bottom of an actual river can be directly measured. Based on the tractive force measured using the measuring instrument, one can inversely estimate the roughness coefficient, flow velocity, and flow rate, as well as evaluate the allowable and critical tractive forces of the installed material. The developed 3-dimensional tractive force measuring instrument can evaluate not only the single material at a laboratory scale but also the critical tractive force of river bed structures (e.g., revetments, river bed protection, and gabions) when installed at an actual site.

목차 (Table of Contents)

• Chapther 1. Introduction 1
• 1.1 Research Background 1
• 1.2 Research Purposes and Methods 4
• Chapther 2. Theoretical Background 10
• 2.1 Definition of an Open Channel 10
• 2.1.1 Classification of Flow in an Open Channel 12
• 2.2 Tractive Force and the Trend of Relevant Research 15
• 2.2.1 Tractive Force 15
• 2.2.2 Tractive Force Considering Manning Formula 17
• 2.2.3 Reynolds Stress Method 19
• 2.2.4 Trend of Research on Direct Tractive Force
• Measurement in Domestic 24
• 2.2.5 Trend of Research on Direct Tractive Force
• Measurement in Overseas 34
• 2.3 PIV (Particle Image Velocimetry) 54
• 2.3.1 Definition of PIV 54
• 2.3.2 Necessity of PIV 58
• Chapther 3. Hydraulic Experiments 63
• 3.1 Direct Tractive Force Measurement Instrument 66
• 3.1.1 Design and Development of the 3-Dimensional
• Tractive Force Measuring Instrument 66
• 3.1.2 A Static Calibration Experiment for the 3-Dimensional
• Tractive Force Measuring Instrument 74
• 3.2 Experimental Channel for Implementing
• High-Velocity Flow 89
• 3.2.1 The First Experimental Channel 89
• 3.2.2 The Second Experimental Channel 90
• 3.2.3 Third Experimental Channel 91
• 3.2.4 The Orifice-Type Fixed Bed Open Channel 93
• 3.3 Flow Rate Measuring Instrument 97
• 3.3.1 Ultrasonic Flow Meter 97
• 3.3.2 A Flow Rate at a Rectangular Weir 100
• 3.4 Flow Velocity Measuring Instrument 102
• 3.4.1 Acoustic Doppler Velocimetry 102
• 3.4.2 Particle Image Velocimetry 104
• 3.5 Water Depth Measuring Instrument 121
• 3.5.1 Ultrasonic level meter 121
• Chapther 4. Experimental Results and Analysis 123
• 4.1 Comparison and Analysis of the Tractive Force Results
• Measured Using the
• 3-Dimensional Tractive Force Measuring Instrument 138
• 4.2 Comparison and Analysis of the Bed Pressure Results
• Measured Using the
• 3-Dimensional Tractive Force Measuring Instrument 145
• 4.3 Comparison and Analysis of the
• Flow Rate Measurement Results 151
• 4.4 Comparison and analysis of the
• Flow Velocity Measurement Results 156
• 4.5 PIV analysis results 161
• Chapther 5. Conclusions 165
• 5.1 Conclusions 165
• 5.2 Future Research 169
• Appendix A. Experimental Results (PIV) 174
• REFERENCES 219