Hong-wu Tang

An improved PTV system for large-scale physical river model

To measure the surface flow in a physical river model, an improved system of Large-Scale Particle Tracking Velocimetry (LSPTV) was proposed and the elements of the PTV system were described. Usually the tracer particles of a PTV system seeded on water surface tend to form conglomerates due to surface tension of water. In addition, they can not float on water surface when water flow is shallow. Ellipsoid particles were used to avoid the above problems. Another important issue is particle recognition. In order to eliminate the influence of noise, particles were recognized by the processing of multi-frame images. The kernel of the improved PTV system is the algorithm for particle tracking. A new 3-frame PTV algorithm was developed. The performance of this algorithm was compared with the conventional 4-frame PTV algorithm and 2-frame PTV algorithm by means of computer simulation using synthetically generated images. The results show that the new 3-frame PTV algorithm can recover more velocity vectors and have lower relative error. In addition, in order to attain the whole flow field from individual flow fields, the method of stitching individual flow fields by obvious marks was worked out. Then the improved PTV system was applied to the measurement of surface flow field in Model Yellow River and shows good performance.

Parameter Identification for Modeling River Network Using a Genetic Algorithm

The simulation of a one-dimensional river network needs to solve the Saint-Venant equations, in which the variable parameters normally have a significant influence on the model accuracy. A Trial-and-Error approach is a most commonly adopted method of parameter calibration, however, this method is time-consuming and requires experience to select the appropriate values of parameter. Consequently, simulated results obtained via this method usually differ between practitioners. This article combines a hydrodynamic model with an intelligent model originated from the Genetic Algorithm (GA) technique, in order to provide an intelligent simulation method that can optimize the parameters automatically. Compared with current approaches, the method presented in this article is simpler, its dependence on field data is lower, and the model accuracy is higher. When the optimized parameters are taken into the hydrodynamic numerical model, a good agreement is attained between the simulated results and the field data.

A split-characteristic finite element model for 1-D unsteady flows

An efficient and accurate solution algorithm was proposed for 1-D unsteady flow problems widely existing in hydraulic engineering. Based on the split-characteristic finite element method, the numerical model with the Saint-Venant equations of 1-D unsteady flows was established. The assembled finite element equations were solved with the tri-diagonal matrix algorithm. In the semi-implicit and explicit scheme, the critical time step of the method was dependent on the space step and flow velocity, not on the wave celerity. The method was used to eliminate the restriction due to the wave celerity for the computational analysis of unsteady open-channel flows. The model was verified by the experimental data and theoretical solution and also applied to the simulation of the flow in practical river networks. It shows that the numerical method has high efficiency and accuracy and can be used to simulate 1-D steady flows, and unsteady flows with shock waves or flood waves. Compared with other numerical methods, the algorithm of this method is simpler with higher accuracy, less dissipation, higher computation efficiency and less computer storage

Local Scouring around Twin Bridge Piers in Open-Channel Flows

AbstractLaboratory experiments were carried out to investigate local scouring around twin cylindrical piers in open-channel flows. The experimental results of clear-water scouring around the twin piers show that the scour hole formed around the upstream pier at different pier spacing was almost the same as that around an isolated pier. However, the local scour depth around the downstream pier was less than that around the upstream pier. The local scour depth around the downstream pier with the velocity can be segregated into four regions: a no-scour region, synchronous-scouring region, transition region, and a radical-deviation region. Finally, a relationship between the deviation in the radical-deviation region and the spacing of the two piers was obtained.

Numerical study of hydrodynamics of multiple tandem jets in cross flow

The hydrodynamics of a single jet and four tandem jets in a cross flow are simulated by using the Computational Fluid Dynamics (CFD) software Fluent. The realizable k - ɛ model is used to close the Reynolds-Averaged equations. The flow characteristics of the jets, including the jet trajectory, the velocity field and the turbulent kinetic energy are obtained with various jet-tocross flow velocity ratios R in the range of 2.38-17.88. It is shown that a single jet penetrates slightly deeper than the first jet in a jet group at the same R, although the difference decreases with the decrease of R. It is also found that the way in which the velocity decays along the centerline of the jet is similar for both a single jet and the first jet in a group, and the speed of the decay increases with the decrease of R. The downstream jets in a group are found to behave differently due to the sheltering effect of the first jet in the group. Compared with the first jet, the downstream jets penetrate deeper into the cross flow, and the velocity decays more slowly. The circulation zone between the two upstream jets in the front is stronger than those formed between the downstream jets. The Turbulent Kinetic Energy (TKE) sees a distinct double-peak across the cross-sections close to each nozzle, with low values in the jet core and high values in the shear layers. The double-peak gradually vanishes, as the shear layers of the jet merge further away from the nozzle, where the TKE assumes peaks at the jet centerline.

Prediction of the future flood severity in plain river network region based on numerical model: A case study

Suzhou is one of China’s most developed regions, located in the eastern part of the Yangtze Delta. Due to its location and river features, it may at a high risk of flood under the climate change background in the future. In order to investigate the flood response to the extreme scenario in this region, 1-D hydrodynamic model with real-time operations of sluices and pumps is established. The rain-runoff processes of the urban and rural areas are simulated by two lumped hydrologic models, respectively. Indicators for a quantitative assessment of the flood severity in this region are proposed. The results indicate that the existing flood control system could prevent the Suzhou Downtown from inundation in the future. The difficulty of draining the Taihu Lake floods should be given attention to avoid the flood hazard. The modelling approach based on the in-bank model and the evaluation parameters could be effective for the flood severity estimation in the plain river network catchment. The insights from this study of the possible future extreme flood events may assist the policy making and the flood control planning.

Large-eddy simulation of suspended sediment transport in turbulent channel flow

The numerical simulation of the non-cohesive sediment transport in a turbulent channel flow with a high concentration is a challenging but practical task. A modified coherent dynamic eddy model of the Large Eddy Simulation (Les) with a pick-up function is used in the present study to simulate the sediment erosion and the deposition in a turbulent channel flow. The rough wall model is used instead of the Les with the near-wall resolution to obtain the reasonable turbulent flow characteristics while avoiding the high costs in the computation. Good results are obtained, and are used to analyze the sediment transport properties. The results show that the streamwise vortices play an important role in the riverbed erosion and the sediment pick-up, which may serve as guidelines for the sediment management and the water environment protection engineering.

Water flow and sediment transport at open-channel confluences: an experimental study

ABSTRACT The knowledge of the dynamics of urban channel confluences is insufficient as most past studies have focused on natural river confluences. This paper reports experimental observations on bed morphology and hydrodynamics of the urban confluences and their dependence to the discharge ratio. Typical hydrodynamic and morphological features such as the shear layer, helical cells, bars and scour holes are identified. Nevertheless, the presence of a region of low-velocity flow for a small discharge ratio differs from results obtained in previous studies. Two sand ridges associated with the corridors of the eroded sediment from the scour holes merge into a mid-stream bar. Results also show that the intense shear towards the bed by downwelling and upwelling flows involved in helical motions is responsible for the mid-stream scour hole.

Measurement of particle size based on digital imaging technique

To improve the analysis methods for the measurement of the sediment particle sizes with a wide distribution and of irregular shapes, a sediment particle image measurement, an analysis system, and an extraction algorithm of the optimal threshold based on the gray histogram peak values are proposed. Recording the pixels of the sediment particles by labeling them, the algorithm can effectively separate the sediment particle images from the background images using the equivalent pixel circles with the same diameters to represent the sediment particles. Compared with the laser analyzer for the case of blue plastic sands, the measurement results of the system are shown to be reasonably similar. The errors are mainly due to the small size of the particles and the limitation of the apparatus. The measurement accuracy can be improved by increasing the Charge-Coupled Devices (CCD) camera resolution. The analysis method of the sediment particle images can provide a technical support for the rapid measurement of the sediment particle size and its distribution.

Interaction between Tsunami Waves and Isolated Conical Islands

Abstract Chen, J.M.; Liang, D., and Tang, H., 2012. Interaction between tsunami waves and isolated conical islands. A newly developed computer model, which solves the horizontal two-dimensional Boussinesq equations using a total variation diminishing Lax-Wendroff scheme, has been used to study the runup of solitary waves, with various heights, on idealized conical islands consisting of side slopes of different angles. This numerical model has first been validated against high-quality laboratory measurements of solitary wave runups on a uniform plane slope and on an isoliated conical island, with satisfactory agreement being achieved. An extensive parametric study concerning the effects of the wave height and island slope on the solitary wave runup has subsequently been carried out. Strong wave shoaling and diffraction effects have been observed for all the cases investigated. The relationship between the runup height and wave height has been obtained and compared with that for the case on uniform plane slopes. It has been found that the runup on a conical island is generally lower than that on a uniform plane slope, as a result of the two-dimensional effect. The correlation between the runup with the side slope of an island has also been identified, with higher runups on milder slopes. This comprehensive study on the soliton runup on islands is relevant to the protection of coastal and inland regions from extreme wave attacks.