مجله هیدرولیک
سال هفتم شماره 2 (تابستان 1391)

The purpose of this paper is to put forward a new method of Multi-phase Moving Particle Semi-implicit (MP-MPS) for solving continuity and momentum equations of viscous fluid flow. MP-MPS is a Lagrangian meshless method that solves the governing differential equations of gas, fluid and solid phases, simultaneously. This method has many applications in problems of free surface flow which is due to their complex conditions and non-linear equations. Among these complexities is the need to consider the effect of turbulence. Until now, MPS method has been used widely for inviscid fluids or those with constant eddy viscosity. In this paper, the effect of turbulence closure model is investigated using Prandtl’s mixing-length theory. The enhancement of MPS model in estimating free surface flow considering the effect of turbulence is demonstrated through comparing its results with those of fluid with constant eddy viscosity and inviscid fluid for a typical dam break problem. The results obtained from the developed model show that determination of eddy viscosity of each particle in each time step based on their location and velocity is important and has a major effect on the final hydrodynamic parameters computations. In addition, results obtained through the improvement made in the numerical model have a good degree of similarity with those obtained through experiments. Also, the results indicate that applying the turbulence nature of the flow helps in obtaining a more stable solution and therefore in this case, the type of Kernel function used in MPS method has no significant effect on its stability.

In this study armoring effect on local scouring under unsteady flow condition was investigated based on model experimentation. To obtain unsteady flow condition, ten different triangular hydrographs were considered. Experiments were performed in an 8.5 m long and 0.405 m wide laboratory-scaled flume. The bed materials were overlain by a thin armor layer of coarser sediment. Different combinations of uniform bed-armor layers were employed. Three circular piers of 22, 33 and 42 mm diameters were used. For each experiment the armor-layer thickness was maintained at 3da (da, is the armor-layer material mean size). The approaching flow velocity is restricted to the clear water scour condition with respect to the armor layer particles (u*/u*ca=0.8, where u* is the flow shear velocity, and u*ca is the critical shear velocity of the armor-layer particles estimated based on the Shields’ diagram). Results indicated that the equilibrium scour depth is independent of the hydrograph shape. In addition, increasing the hydrograph base time does not affect the scouring hole formed downstream of the pier. Observations showed that the effect of n hydrographs with the same base time of T is almost identical to that of a continuous hydrograph with the base time of nT.

When the river flow rate is larger than the capacity of the river, the adjacent areas are prone to flooding. Structural methods of flood control usually are used to reduce flood hydrograph peak. Rockfill dam is a type of structural methods that controls the flood. Levee, with no effective change in the flood peak, is another structure that controls flood. Simultaneous use of rockfill dams and levees will reduce the severity and power of flood and prevent flooding of the low lands adjacent to the river. In this way, the final options for flood control will be selected with more flexibility. The important point about dilatory flood control measures is suitable selection of their position. Depending on the topography and other environmental factors, location and design of rockfill dams and design of levees will be different. Accordingly, any kinds of proper design of these two methods can be effective in flood relief projects. Obviously, this combination is not the best with respect to the hydraulic and economic limitations. The purpose of this paper is to introduce a method in which the most appropriate design and location of dams and the most proper design for the levees in the residential area is taken into account. Other restrictions such as movement of the flood, total minimum cost of construction of dams and levees is also considered in this study. Reaching this goal requires a combination of basin simulation model with a nonlinear optimizer model, such as genetic algorithms.

In this paper available techniques of the bed shear stress measurement was reviewed and the three tube device, an efficient instrument with 85 Hz measuring frequency, was developed to measure the local bed shear stress around the river structures. The main advantage of the developed instrument is that it is not necessary to align the measuring ports in the local flow direction. Distribution of the bed shear stress around a single straight groyne in a flat and immovable bed has been measured using the three ports instrument. The maximum bed shear stress amplification at the groyne head, bed shear stress pattern along the outer side of the shear layer, distribution and general amplification and the zone where the scouring process initiates at the upstream of the groyne due to the general bed shear stress amplification were extracted. The measured data agrees well with the available experimental results and with the accurate numerical simulations as well.

Adaptive analysis is an important research subject in numerical methods. In recent decades, many research activities in this field have been performed. The ultimate goal of this analysis is to obtain the numerical solution with the desired accuracy and minimum computational cost. The adaptive refinement process consists of two main sections: the first one is error estimation and the second one is refinement procedure. One of the benefits of Meshless techniques is network flexibility in the adaptive refinement process. Common methods in refinement include moving and adding the new nodes. This paper presents a low-cost technique called adaptive weighing to raise the accuracy of the answers on the Meshless Collocation Discrete Least Square (CDLS). In the presented method, the remains are employed in order to estimate error and also used in definition of weighting function.

The annular flumes have been used to study the cohesive sediment transport and secondary flows. This research was carried out in an annular flume located at the hydraulics laboratory of Shahrekord University to study the flow and shear stress distribution. The results of velocity measurements by Acoustic Doppler Velocimeter (ADV) showed that the shear stress distribution at the flume cross section was almost uniform for the relative rotational speed of the upper ring to the flume equal to 1.1. It was also found that the flume velocity profile was in good agreement with logarithmic relationship.