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The mobile flume has the advantages of not easy to be silted up and can be moved and reused in many places, etc. Compared with the fixed type, the streamlined shape of the measuring flume has good hydraulic performance, and the diffusion section of the flume has a great influence on the hydraulic performance, however, there is a lack of research on the design principle and method of the diffusion section at present. In order to study the influence of flume structure on hydraulic properties, and to provide theoretical guidance for the design of streamlined flume structure, This paper establishes a D40Ushaped channel simulation model, and then uses FLOW-3D to simulate the semi-elliptical with semi-elliptical, semi-elliptical with parabolic, semi-elliptical with arc, semi-elliptical with Myring curve, and fish-shaped flume under flow rates of 35, 45, and 55 L/s. Furthermore, the accuracy effect of numerical simulation is verified by prototype test, and then the design principle and method of diffusion section are discussed. Finally, based on the isight platform, the semi-elliptical with Myring curve and fish-shaped flume with better hydraulic performance are optimized. The results show that the relative error between the numerical simulation and the experimental water depth is 0.1%-1.3% and the relative errors of velocity in upstream, throat and downstream sections are 0.19%-6.63%, 0.06%-6.15% and 0.02%-6.22% at 45 L/s flow rate, respectively. The numerical simulation results have high accuracy. The Froude number in the upstream of the five flumes is less than 0.5, and the maximum backwater height is 5.71 cm, which meets the requirements of flow measurement accuracy and channel safety. Among them, the head loss and backwater height of the semi-elliptical with Myring curve flume are the lowest, the critical submergence degree of the semi-elliptical with semi-elliptical flume is the highest, and the Froude number is the smallest. The design principle of the diffusion section of the flume is: under the condition of slightly reducing the critical submergence degree, the first half of the diffusion section is designed as a curve with larger curvature to reduce the upstream backwater; the second half is designed as a curve with opposite curvature and smaller curvature to improve the flow state, smooth the water flow and reduce the head loss. After optimization, the curvature of the first half of the semi-ellipse with Myring curve and the fish-shape flume increases, and the curvature of the second half decreases, the backwater and Froude number are reduced, and the head loss is reduced by 5.5% and 6.3%, respectively. The study can provide theoretical guidance for the design and optimization of streamlined measuring tank structures. [ABSTRACT FROM AUTHOR]

Objectives: For global navigation satellite system (GNSS), the ranging performance of observation would influence the system service capability.Methods: Parabolic antenna with large⁃diameter could greatly reduce the impact of the ground equipment and the environment on signal acquisition. Based on data from parabolic antenna, observation characteristics and ranging performance of BeiDou⁃3 global satellite navigation system (BDS⁃3) are analyzed.Results: The results show that, regarding the pseudorange deviation on BeiDou⁃3 satellite, no systematic deviation of the same magnitude, compared with BeiDou⁃2, is found on the signal of B1I, B3I, B1C, B2a, and B2b. There are fluctuations on some signals. However, the correlation with elevation angle is not strong, and the average magnitude of all satellite fluctuations is 0.1 m. This paper analyzes the multipath variation and noise level of carrier observation for BeiDou⁃3 satellite of each signal using triple⁃frequency combinations. The fluctuations of carrier phase triple combination are of the millimeter level, and those fluctuations do not closely correlate with the elevation or time. The root mean square of the triple⁃frequency carrier combination from parabolic antenna is mostly less than 0.6 mm.Conclusions: The results indicate that no systematic bias occurs in code and phase observation of BDS⁃3, and the system has a good ranging performance. [ABSTRACT FROM AUTHOR]

The BeiDou global navigation system was officially launched in 2017 and will adopt a new BeiDou global ionospheric delay correction model(BDGIM). Using high-precision grid ionospheric data released by CODE and the Slant ionospheric delay derived from dual-frequency observations as a reference, the accuracy of the BDGIM model is analyzed and evaluated, and compared with the BeiDou Klobuchar and GPS Klobuchar models. The results show that in the region of China, the BDGIM model and the BeiDou Klobuchar model are comparable in accuracy and superior to the GPS Klobuchar model. On a global scale, the accuracy of the BDGIM model is better than that of the BeiDou Klobuchar and the GPS Klobuchar models. Different ionospheric models are implemented for single point positioning. The results show that the BDGIM model has a 13% improvement over the positioning accuracy of the BeiDou Klobuchar model, and a 7%-10% improvement over the GPS Klobuchar model. [ABSTRACT FROM AUTHOR]

Uneven frozen heave can cause some damage to the canal lining structure, particularly in the frozen soil in the north cold region of China. The frost-heaving damage of canal lining is closely related to the lining-frozen soil contact action. Some relations of contact consist of the contact surface, such as the freezing constraint, and friction slip. The lining stress can also depend directly on the separate conditions. However, the current simulation of the lining-frozen soil contact is mostly focused on the steady state. It is still lacking in the experimental verification of the mechanical parts. Taking the integral U-shaped canal frost heaving damage experiment in the Hetao irrigation area as the prototype, this research aims to construct the canal frost heave model with/without the contact relationship. Finite element (FE) software ABAQUS was selected to simulate the frostheaving damage process of the model. Two models were evaluated to compare the experimentally monitored data from the subsoil temperature field, layered frost heave displacement, and soil stress. A systematic analysis was made to determine the stress state and damage mechanism of the canal lining. The experimental and simulation were integrated to clarify the influence of lining-frozen soil separation and sliding process on the frost-heaving damage of the canal. The results show that the simulation considering the contact model was more consistent with the field test conditions, compared with no contact. There were the basically same distributions of temperature field between the experiment and simulation with/without contact. Specifically, the simulated freezing depths of the shady and sunny slopes increased by 2.44% and 1.68%, respectively, whereas, the freezing depth of the canal bottom decreased by 4.26%, compared with the experimental. Small errors were found between the experimental and simulated subsoil and lining normal frost heave displacement. The consideration of contact posed a weak limiting effect on the canal slope subsoil freezing. The average relative errors were 7.1% and 13.9% between the simulated and experimental data with/without contact, respectively. Moreover, the simulated stress peak at the side slope reached 3.3 times of the contact model. The voids were generated between the canal bottom lining and the foundation soil in the experimental and the considered contact model. By contrast, there were continuously increasing tensile stresses in the simulations without considering contact. The separation and sliding process between the lining-frozen soil contact surface were visualized dynamically to combine the considered contact model with the experiment. The lining at the bottom of the canal was the first to separate from the subsoil after the onset of frost heave, due to the inhomogeneity of the subsoil frost heave. There was no transferred contact stress. After that, relative sliding occurred between the lining and the frozen soil at the side slope. As such, the originally restrained lining stress was released. More importantly, the frost heave displacement of the lining was still increasing in the process of stress release. The sliding process changed the stress development trend from the "force increase" to the "force release". The peak values of normal stress were reduced by 903% and 164% on the upper and lower surfaces of the lining under the releasing force of the simulated slip in the considering contact model, compared with the simulation without considering contact. Meanwhile, the peak value of tangential force was reduced by 248% on the lower surface. All peak stresses occurred on the slightly shady slope at the bottom of the canal, where the frost-heaving damage was more likely to occur. The finding can provide a strong reference for the design and structural optimization of the frost heave prevention of canals in cold regions. [ABSTRACT FROM AUTHOR]

Objectives: The performance of the global navigation satellite system (GNSS) spaceborne atomic clock affects the entire navigation system, by having direct impact on GNSS measurement quality, ranging precision, clock prediction and satellite autonomous navigation capabilities. Different time synchronization systems may have different impact on the evaluation of spaceborne atomic clock. Precise clock bias data determined by inter-satellite link (ISL), two-way time transfer (TWTT) and orbit determination and time synchronization (ODTS) system are used to further evaluate the performance of BeiDou-3 on-orbit atomic clock. Methods: Quadratic polynomial model and total Allan / Hadamard variance are used to analyze the performance of the BeiDou-3 navigation satellite system (BDS-3) satellite clock data of the three different time synchronization systems. Results: The result shows that the frequency accuracy and frequency drift of the above three clock bias determination systems are consistent. The frequency accuracy of all satellites is within the range of (− 4~2)×10-11. Frequency accuracy of hydrogen clock is better than that of rubidium clock. The frequency drift based on the ISL system is slightly better than that of the ODTS, the three clock determination systems have their own advantages in evaluating the stability of the atomic clock, respectively. For short-term stability, the ODTS whose 3 000 s stability reaches 3×10−14 is better than that of ISL, and hydrogen clock is better than rubidium clock. For medium-term and long-term stability, when the averaging time is greater than 1×104 s, the result of ISL is closer to the actual condition of the BDS-3 spaceborne clock. For long-term stability more than 7 days, the broadcast ephemeris bias based on TWTT system can be used for rapid evaluation, which result is close to ODTS and ISL. Conclusions: The three clock bias determination systems can be used to evaluate the frequency accuracy and frequency drift of onorbit atomic clocks with basically consistent statistical results. The three clock bias determination systems have their own advantages in assessing the stability of the orbiting satellite clock when selecting different averaging time. [ABSTRACT FROM AUTHOR]