دکتر امیر حسین نیک سرشت

Determining the transfer function of the wave maker system in towing tank is one of the most important requirements for calculations of the wave maker system in towing tanks. This research analyzes and evaluates a towing tank’s wave maker system to determine a suitable function with specific inputs. Initially, after examining scientific sources and preliminary data collection using valid validation equations, theoretical calculations were conducted. Following, the validation of the resulting data, laboratory tests were carried out in the towing tank. In order to refine and reduce existing errors and ensure the information recorded by the oscilloscope system, multiple test repetitions were conducted. The oscilloscope system in question incorporates two mechanical sensors spaced apart, which is responsible for extracting the required laboratory data. Ultimately, through careful analysis of the experimental data and comparing with the theoretical results, a conversion function for the wave maker system of the traction basin were derived.

The impact problems associated with water entry have important applications in various aspects of naval architecture and ocean engineering. Also the calculation of impact force is favorable to many researchers. The purpose of this study is to simulate the impact problem of a wedge into the Newtonian and also Herschel Bulkley dilatant non-Newtonian fluids using the Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) method. Some non-Newtonian fluids, such as dilatant or Herschel Bulkley dilatant fluids can resist against the wedge entry due to their shear thickening effect. In this research a prediction and correction algorithm is used to solve the governing equations. Density correction and also artificial viscosity (which is used only in Newtonian fluids) are used to prevent the numerical instability. To show the validation, ability and robustness of the generated code to capture the free surface in Newtonian and non-Newtonian fluids, the dam break problem with the image boundary condition is simulated. After validating the code and the used method, the impact problem of a wedge with Monaghan repulsive force boundary condition in Newtonian and Herschel Bulkley Dilatant non-Newtonian fluids are investigated and the results of force, pressure coefficient and velocity of the wedge are presented and compared with experiments and also with each other. To save time, the initial values of hydrostatic pressure are imposed as an initial condition of the fluid.

In this study, considering a real model, numerical simulation of Aorta arch is performed using ANSYS-CFX software. Different aspects such as pulsatile behavior of blood flow and fluid-solid interaction are considered simultaneously in simulation of one and three-layer model for Aorta wall. The results show that the three-layer model leads to stress jumps in boundary of layers the stresses have higher values than one-layer model. For sake of more realistic condition, a hyperelastic model is used for Aorta wall behavior which in comparison to elastic model, the hyperelastic model results to lower stresses. The effect of Aorta arch curvature on the results is examined and it is observed that the von Mises stresses are bigger in Aorta arches with smaller curvature and the maximum value of stresses occurs in ascending part. After investigation of effects of various parameters on wall stresses, the areas with more possible risk of rupture are determined and a proper model for future researches is introduced.

Re-meshing in grid dependent CFD methods in large deformation problems has a high computational cost. Smoothed Particle Hydrodynamics (SPH) is a robust mesh-free method to deal with the problems of large deformation in fluid-solid interactions. Also SPH is a fully Lagrangian particle method، which has a good ability to capture the free surface in two phase flows. In this paper، at first، fluid flow under a gate is solved with the present SPH method and the results are compared with the VOF scheme. For validating the solid part of the code، vibration of a hypo-elastic plate is simulated. Comparing the results with the previous researches show a good agreement. Finally a coupled fluid flow under a hypo-elastic gate is simulated with the present SPH method and the interaction between fluid and solid is obtained with a proposed simple model. In this paper also a simple repulsive force which is independent of the pressure distribution is introduced to prevent the penetration of fluid particles into the solid parts. The results also show the high ability of the present SPH method to follow the fluid- solid interface accurately.

Modeling free surface flow is a challenging problem in hydraulic research. The objective of this research is to compare those methods for simulation of flow around hydraulic structures. Accordingly, Incompressible SPH (ISPH) methods, with two different projection methods to enforce incompressibility, are utilized to model flow under a gate and then compared to each other from mathematical and numerical viewpoints. Moreover, the effect of pressure term approximation on conservation equation of mass and momentum with two different relations is investigated by error index. For verification of ISPH results, VOF method is applied to similar gate problem. Results show that two methods of enforcing incompressibility are numerically and mathematically equivalent if proper approximation terms are used in SPH method. Furthermore, VOF and ISPH results are in excellent agreement. Besides, two pressure terms obey similar chaotic error trend for inner and all particles (inner plus free surface).

Smoothed Particle Hydrodynamic (SPH), a new generation of mesh-free numerical methods to model free surface flow, is of great interest among many researchers nowadays. In this paper, ISPH was applied to simulate free-surface flow around hydraulic structures in several classical cases including dambreak, flow over a sharp-crested weir, and simultaneous operation of weir and gate. Due to enforcing incompressibility of flow, Cummins and Rudmans (1999) projection method is used in these cases. Error curves are used to evaluate the sensitivity of ISPH to approximate pressure terms. Two pressure formulations for approximating the left-hand-side of the Poisson equation result in a similar and acceptable trend of error. Comparison of several classical flow cases results using ISPH with experimental data and FVM-VOF shows good agreement between them.

The river bed evolution in 180 degree alluvial channel bend is numerically simulated using FLUENT based on Eulerian two-phase model that implements Euler–Euler coupled governing equations for fluid and solid phases and a modified k–ε turbulence closure for the fluid phase. This two-phase model predicts sediment transport from more fundamental dynamical equations, thereby avoiding the use of purely empirical sediment transport formula. Such formulas have been found to be case-dependent, thus limiting their general use to cover a broad range of flow configurations. Both flow–particle and particle–particle interactions are considered in the model. During the simulations, the interface between sand and water is specified using a threshold volume fraction of sand, and the evolution of the bed form is studied in detail. The predictions of bed form evolution are in good agreement with previous laboratory measurements. In addition, the computational results indicate the presence of a new mechanism in sediment transport that can be called 'Laminated Load' hereafter.

The suitable design of the keel and skis is of great significance in reduction of impact load on the bottom surface of the flying boats and in hydrodynamic stability during take-off and landing. No theoretical tools are available to handle the effects of viscosity, gravity, and surface tension. Also, the experimental procedures in the laboratory are both time-consuming and expensive. In this paper, numerical simulation of two-phase flow in the water impact problem of a WIG with VOF method and finite volume methods are taken into account and the effect of changing the dead-rise angle on the slamming force is investigated. An algebraic relation based on the dimensionless parameters for the maximum slam force is developed. This is a new step towards choosing the optimum dead-rise angle of the keel and skis of flying boats, faster and easier and decreases the cost of numerical calculations.

Prediction of hydrodynamic loads during water impact is of great significance in the structural design of flying vehicles. Also the importance of the slam force of waves on the members of offshore structures cannot be overemphasized. No theoretical tool is available to handle this complicated phenomenon exactly and the experimental procedures in the laboratory are both time-consuming and expensive. In this paper, a computer program based on the VOF method is applied to evaluate the water impact outcome in a real situation. To asses the capabilities of the CFD code, two classical problems including water impact of a wedge and a circular cylinder traveling at constant speeds are studied. In a real situation the descend velocity is decreased by the impact loads known as “deceleration effect”. This change in speed is taken into account in an iterative procedure and the flow field around the base of a WIG craft is computed. All flow field computations in this study include the effects of viscosity, turbulence, gravity and surface tension. The comparison of results with experimental data shows the efficiency and correctness of this straightforward iterative method.