Sharif Digital Repository

To help walking, using assistive devices can be considered to reduce the loads caused by weight and to effectively decrease the propulsive forces. In this study, a mobility Saddle-Assistive Device (S-AD) supporting body weight while walking was evaluated on two healthy volunteers. This device is based on the support of body weight against gravity with the help of a saddle, which is not used in other passive mobility assistive devices. To prove the efficiency of this device, the experimental results obtained while walking with this device were compared with those related to walking without the assistive device. The results showed that this device could significantly reduce the forces and... 

This study seeks to explore the ability of unsteady Reynolds-averaged Navier-Stokes (URANS) simulation approach for resolving two-dimensional (2D) gravity currents on fine computational meshes. A 2D URANS equations closed by a buoyancy-modified k-ε turbulence model are integrated in time with a second-order fractional step approach coupled with a direct implicit method and discretized in space on a staggered mesh using a second-order accurate finite volume approach incorporating a high resolution semi-Lagrangian technique for the convective terms. A series of 2D simulations are carried out for gravity currents from both discontinuous and continuous sources. Comparisons with experimental... 

The effect of Alkali-Aggregate Reactions (AAR) in concrete dams was investigated using the finite element method. Two models have been presented to assess the effects of the AAR in concrete dams. The stress dependency of AAR strains has been taken into account in the models and the degradation of concrete properties during the reaction has been implemented in the finite element program. The results of the isothermal and nonisothermal analyses of a gravity section of a Beauharnois power plant have been presented and compared with the measured data and the results are in close agreement. The results of the analysis demonstrate the importance of the AAR in concrete dams. High stresses and large... 

HATS-18 b is a transiting planet with a large mass and a short orbital period, and is one of the best candidates for the detection of orbital decay induced by tidal effects. We present extensive photometry of HATS-18 from which we measure 27 times of mid-transit. Two further transit times were measured from data from the Transiting Exoplanet Survey Satellite (TESS) and three more taken from the literature. The transit timings were fitted with linear and quadratic ephemerides and an upper limit on orbital decay was determined. This corresponds to a lower limit on the modified stellar tidal quality factor of $Q_star ^{, prime } gt 10^{5.11 pm 0.04}$. This is at the cusp of constraining the... 

The problem of cooling two hot blocks in a novel geometry using magnetohydrodynamic flow of Al2O3–water nanofluid has been studied utilizing a D2Q9 Lattice Boltzmann Model. While the Hartmann number (Ha) takes 0, 50, or 100 values, the Richardson number (Ri) varies between 0.02 and 20. Four variations of the geometry are selected. The gravity angle is set to be either (Formula presented.), (Formula presented.), or (Formula presented.). Results reveal that the Nusselt number (Nu) increases as Ri increases for all cases. Furthermore, the Hartmann number has a deteriorating effect on the Nusselt number except for low Ri numbers. In addition, the results indicate that while the geometrical... 

This study investigates the performance of geogrid-reinforced unbound pavement layer. In this study, geogrid was used in various layouts and numbers using three different gradation of granular materials. Light weight deflectometer (LWD) device is known as a useful tool to evaluate the stiffness of unbound pavement layers. In this study, the LWD was utilized to experimentally investigate factors affecting the performance of geogrid reinforcement, including the number of geogrids, geogrid layout, and the gradation of unbound layer. There are several parameters which affect the LWD test results including the hammer weight, the falling height, and the surface stress. The effect of these... 

In this paper, optimal trajectories of a spacecraft traveling from Earth to Moon using impulsive maneuvers (ΔV maneuvers) are investigated. The total flight time and the summation of impulsive maneuvers ΔV are the objective functions to be minimized. The main celestial bodies influencing the motion of the spacecraft in this journey are Sun, Earth and Moon. Therefore, a three-dimensional restricted four-body problem (R4BP) model is utilized to represent the motion of the spacecraft in the gravitational field of these celestial bodies. The total ΔV of the maneuvers is minimized by eliminating the ΔV required for capturing the spacecraft by Moon. In this regard, only a mid-course impulsive... 

Purpose - This paper aims to present the design and implementation of VirSense, a novel six-DOF haptic interface system, with an emphasis on its gravity compensation and fixed-base motors. Design/methodology/approach - In this paper, the design and manufacture of the VirSense robot and its comparison with the existing haptic devices are presented. The kinematic analysis of the robot, design of the components, and manufacturing of the robot are explained as well. Findings - The proposed system is employed to generate a Virtual Sense (VirSense) with fixed-base motors and a spring compensation system for counterbalancing the torques generated by the weight of the links. The fixed bases of the... 

In this paper, particle deposition in the upper airways and five lobes of a human lung is simulated. The simulation is based on a stochastic lung model, derived from detailed morphometric measurements. Pathways are simulated using Monte Carlo methods consequently the whole structure changes both stochastically and statistically in each simulation. In this investigation the termination phenomena is a function of each daughter's diameter which best satisfies the lung's morphometry. Complementary to the previous available assumptions, i.e. flow divisions according to the ratio of daughter's cross sections or distal volumes, in this investigation flow rates are computed in an upward manner... 

Not only should dams be evaluated for seismic shaking, but their capability to survive potential fault displacement in their foundations should also be assessed. Safety reviews of existing dams suggest that geological-seismic evaluation of some dam sites has failed to recognize the existence of possibly active faults. In this study, the nonlinear seismic behavior of concrete gravity dams, due to relative fault dislocation occurring in foundations, has been investigated. Two types of fault movement, including normal-slip and reverse-slip, have been considered. These two types, combined with the location of fault lines, with respect to the toe, middle, and heel of the dam base, angle of fault,... 

In the adolescent idiopathic scoliosis (AIS) treatment, a brace is prescribed to the patients who have 20 to 45° curves on their spines to prevent the disorder's advancement. For the analysis of Milwaukee brace effects during time, finite element models (FEMs) of the spine (the thoracolumbar region) and the ribcage (contained 10 pairs of the ribs and the sternum) were prepared for two patients. For modeling the spine part, a new element was used in which a disc (as viscoelastic 3D beam) and a vertebra (as rigid link) were modeled as an element and the ribs and the sternum modeled by 3D elastic beams. The gravity, Milwaukee brace constraints and the forces of the brace's different regions... 

In fractured reservoirs, a large variation of permeability due to the presence of fractures leads to changes in the production mechanism compared to conventional reservoirs. Hence, an appropriate model with the ability to describe the reservoir properly can provide a more confident prediction of its future performance. One of the features of a representative model is the number and height of the matrix blocks. The determination of these two parameters is one of the decisive steps in the calculation of an accurate amount of oil production from these reservoirs. In fact, matrix height shows its effect as a gravity force, which is one of the driving mechanisms. If the matrix height is less than... 

Density currents (DCs) or gravity currents are driven by gravity in a fluid environment with density variation. Smoothed Particle Hydrodynamics (SPH) has been proved to have capabilities such as free surface modeling and accurate tracking of the immiscible-fluids interface that can be useful in the context of gravity currents. However, SPH applications to gravity currents have been limited to often-coarse simulations of high density-ratio currents. In this work, the SPH projection method is tried to solve currents with very low density-ratios (close to one), at a resolution, that captures the Kelvin-Helmholtz instabilities at the fluids interface. Existing implementations of the SPH... 

Gravity drainage is the main production mechanism in the gas invaded zone in naturally fractured reservoirs. However, there are large ambiguities and complexities, resulting from the dynamic of oil depletion from matrix blocks toward the fracture network. Visualization of drained oil at pore scale using glass micromodels provides the opportunity to better understand the effects of different parameters which might affect oil recovery from fractured reservoirs. In this work a micromodel apparatus generated by laser etching is used to perform some gravity drainage tests on the network patterns. The experiments were performed on double block systems using crude oil. The block to block... 

In this work, a boundary element formulation for 2D linear viscoelastic solid polymers subjected to body force of gravity has been presented. Structural analysis of solid polymers is one of the most important subjects in advanced engineering structures. From basic assumptions of the viscoelastic constitutive equations and the weighted residual techniques, a simple but effective boundary element formulation is implemented for standard linear solid (SLS) model. The SLS model provides an approximate representation of observed behavior of a real advanced polymer in its viscoelastic range. This approach avoids the use of relaxation functions and mathematical transformations, and it is able to... 

In this study, a coupled numerical approach based on Lattice Boltzmann Method (LBM) and Discrete Element Method (DEM) is employed for two-dimensional simulation of fluid flow in deformable particulate media comprising of movable circular particles. The developed LB-DE code is validated against the results of a bi-axial shear test as well as two well-known benchmark problems including settling of a circular particle under gravity force inside a viscous fluid, and motion of a neutrally buoyant particle released in a Poiseuille flow. The verified code is then utilized for simulation of "Sand Production" phenomenon which is of importance for oil producing wells in weakly cemented sandstone... 

Nonlinear dynamics of an extensible cantilevered pipe conveying pulsating flow is considered in this paper. The fluid flow fluctuates harmonically and exhausts via a nozzle attached to the end of the pipe. Taking into account the extensibility assumption, the coupled nonlinear lateral–longitudinal equations of motion are derived using Hamilton's principle and discretized via Galerkin's method. The adaptive time step Adams algorithm is applied to extract the time response, and then the bifurcation, power spectral density and phase plane maps are plotted for some case studies. Effects of some geometrical parameters such as flow mass, pulsating flow frequency, gravity, nozzle mass and nozzle... 

The aim of this paper is to model and evaluate the performance of intrusion detection systems (IDSs) facing black-hole and grey-hole attacks within wireless ad hoc networks (WANETs). The main performance metric of an IDS in a WANET can be defined as the mean time required for the IDS to detect an attack. To evaluate this measure, two types of stochastic models are used in this paper. In the first step, two different continuous time Markov chains (CTMCs) are proposed to model the attacks, and then, the method of computing the mean time to attack detection is presented. Since the number of states in the proposed CTMCs grows rapidly with increasing the number of intermediate nodes and the...