Numerical Simulation of Transient Characteristics of Start-Up Transition Process of Large Vertical Siphon Axial Flow Pump Station

In order to explore the transient characteristics of the large-scale vertical siphon axial flow pumping station during the start-up and exhaust process, numerical simulations were carried out on the start-up process of the axial flow pumping station under the two starting modes of pre-opening the vacuum breaking valve and keeping the vacuum breaking valve closed. The calculation results show that during the start-up phase of the unit, the flow separation phenomenon of the impeller channel of the pump device with the vacuum breaking valve closed is serious, the large-scale vortex in the guide vane blocks the flow channel, and the instantaneous impact on the blade surface is strong. The flow field of the pump device with pre-open vacuum failure valve is obviously less affected by the instantaneous impact characteristics during the start-up of the pump. The range of high entropy production area in the impeller channel is reduced, the duration of high entropy production area is significantly shortened, and the instantaneous impact on the blade surface is weak. Under the two starting modes, the internal flow field of the pump device is similar in the evolutionary law. The unstable flow phenomenon of the pump device is most prominent in the weir flow stage. The maximum instantaneous impact on the blade surface also mainly occurs in the weir flow stage. A very small part of the remaining gas in the siphon formation stage is difficult to discharge and takes a long time. After the pump device is exhausted and enters a stable operation state, the external characteristic parameters are in good agreement with the test results. Compared with the starting method in which the vacuum breaking valve is kept closed, the method of pre-opening the vacuum breaking valve reduces the maximum starting head by 20% and the exhaust time by 43%. The pre-open vacuum breaking valve effectively avoids the system instability caused by the start-up and exhaust of the pump device.

Particle Image Velocimetry Measurements of Flow Over an Ogee-Type Weir in a Hydraulic Flume

Laboratory tests of water flowing over a modified ogee weir are carried out in a wave–current flume for two different scales. A model of a weir representing a part of a spillway section of the existing Włocławek dam (Vistula River, Poland) is mounted in a wave–current flume. The Froude similarity law is used to simulate the flow over a real damming structure at 1:25 and 1:50 scales. Particle image velocimetry methods are employed to measure a flow velocity field over the crest of the weir model. The system is capable of providing high fidelity velocity fields at sampling rates of 10 Hz and 50 Hz. Detailed information on flow characteristics is extracted from the instantaneous velocity field measurements to provide a comprehensive description of the kinematics of a weir flow at discharges corresponding to hydrological events with return periods of 100 and 1000 years, revealing some interesting spatial features. The geometry of the weir results in the development of a characteristic circulation cell, which is relatively wide for the lower discharge. When the flow intensity increases, a triangular circulation develops behind the weir crest instead. Moreover, sudden changes in the flow regime lead to the rapid formation of vortex structures, which propagate downstream at speeds ranging from 0.3 to 1 m s−1. The origin of eddy formation is identified at the upstream and downstream ends of the weir crest for respective average velocities at the crest of approx. 0.6 m s−1 and 1.2 m s−1.

Pressure flow and weir scour beneath a bridge deck

In this study, a series of experiments in a flume was conducted to investigate the maximum scour hole depth that occurred due to the vertical contraction of the flow underneath a bridge deck model without a pier. The bridge model was tested under pressurized and weir type of flows governed by clear water conditions. Various parameters of flow, sediment, and geometric features of the bridge, such as approach flow depth, discharge, sediment size, girder depth and degree of submergence were varied to investigate their effects on the maximum depth of scour hole. A total of 102 experiments were conducted and two empirical equations were developed separately for pressure and weir flow types with the use of experimental data. Effects of the aforementioned parameters on the scour hole were also analyzed and discussed.

Simulation Analysis of Combined Flow over and under Semi-Cylindrical Structure

Combined flow over and under structure may solve the problem of the deposit of suspension materials in channels. Semi-cylindrical shape reduces the curvature of streamlines which reflected on its performance. To study how this shape performs, experimental and simulation has been done. The laboratory models were of four different diameters and four different gate openings. The same physical structures have been modeled in commercial software, FLOW-3D®, by employing RNG k- ε turbulence model. The verification has been based on measured flow profile and discharge. Simulation outputs indicate that a separation zone located at a distance from the structure became farther when the diameter and gate opening decreases, also the separation portions and their thickness are related to the incoming discharge. The location height of separation zone tends to be lower when there is an increase in flow discharge and it is located at half the total depth when two flow portions are equal. The weir flow in this system shows a better performance than traditional weir by at least 33%, while the gate out flow is less than free flow of the same total head by 70% to 90%. Within the limitation of this work, two mathematical models for predicting discharge coefficient have been proposed for the weir and gate respectively, moreover a model for predicting relative discharge of weir to gate, and one mathematical model for the dimensionless total discharge.