Scientia Iranica
Volume:22 Issue: 2, 2015

The fluid flow through a bypass graft of 100% stenosed artery can substantially influence the outcome of bypass surgery. To help improve our understanding of this and related issues, unsteady flows for Newtonian/non-Newtonian fluids using Carreau’s shear thinning model are numerically simulated in an arterial bypass system with complete occluded host artery. The objective of this study is to deal with the influence of the non-Newtonian property of fluid, effect of different bypass-host artery angle θ, and flow pulsatility on flow configuration, wall shear stress (WSS), oscillatory shear index (OSI), and flow phenomena during the pulse cycle. Here we use θ= 30°, 45°, 60°, and 75° to study the effect of geometry. It is found that θ has strong influence on hemodynamic in distal femoral artery anastomoses. Results show that a bypass femoral anastomosis with a moderate θ, like θ =45° enhances its long-term performance. Also significant difference between the non-Newtonian and Newtonian pulsatile flows is revealed. This shows the necessity of using non-Newtonian model rather than Newtonian model for such flows. The unsteadiness of flow using the femoral pulsatile behavior of flow affect the dynamic behavior of the flow and shows complex flow characteristics, therefore the necessity of using unsteady analysis.

The eff ect of Knudsen number (Kn) on the nonlinear vibration and instability of double-walled boron nitride nanotubes (DWBNNTs) conveying fluid has been investigated, based on Fluid Structure Interaction (FSI). The embedded DWBNNT is simulated as a Timoshenko Beam (TB), which includes rotary inertia and transverse shear deformation. The electro-mechanical governing equations are derived using the nonlocal piezoelectricity theory and discretized by a Di erential Quadrature Method (DQM). Regarding the types of flow regime in FSI, including continuum, slip, transition and free molecular, the value of Knudsen number as a small size parameter is designated and utilized to modify the fluid velocity. Considering the slip condition for an internal nanotube, the e ects of Knudsen number on various vibration modes, small scale, and nonlinear frequency amplitude are also taken into account for a clamped-clamped boundary condition. Results indicate that, based on the slip flow regime, the Knudsen number is an important parameter in FSI that changes the critical flow velocity and instability of nano systems and, therefore, should be considered in nanotubes conveying fluid.

Gas turbine aerodynamic shape optimization has been the subject of several numerical studies in the past decades. In this research, numerical optimization of a one stage axial turbine is considered. A practical and e ective optimization method to improve efficiency and/or pressure ratio of the axial turbine is presented. Particular modi cations are accomplished with a limited number of optimization parameters, by stator and rotor blades re-staggering. A three-dimensional numerical model has been prepared and veri ed through experimental data, which are obtained from the reference gas turbine engine test rig. Coupling the veri ed numerical simulation solver and genetic algorithm, the eff ects of stator and rotor stagger angle distributions on the turbine performance are investigated. The optimization is carried out at the turbine design operating condition. To accelerate the GA convergence rate, the numerical mesh sizes are re ned in each generation. As a result, the overall computational time decreases by 20%. De ning two objective functions, this optimization approach resulted in the turbine stage isentropic eciency improvement of 0.85% and 0.86%, in the rst and second objective functions, respectively.

Researchers can reach important information about cell cycles such as migration, growth and muscle contraction, by studying the change of ion concentrations in animal cells. In the current work we have proposed three di erent techniques to study the passive ions motion in protein channels on di erent time scales. Molecular dynamics, Langevin dynamics and continuum models of mass transport are used to investigate ion transport from small to large time scales.

In this paper, we construct a framework for studying dynamic nonprehensile manipulation systems during which multi-link manipulators would manipulate multibody objects. The multibody object is a multilink one with some actuators in the joints witch complicates the manipulation process, because the control of the object configuration could not be decoupled from the control problem of the whole process. The manipulation problem includes a series of similar manipulators manipulating a multibody object. Both object and individual manipulator can be fully actuated, under actuated, or passive. The object has two contact surfaces that alternatively are in contact with the manipulators’ contact surfaces. Each manipulator carries the object during a contact phase and passes it to the next manipulator. The passing of the object from one manipulator to the other is an instantaneous phase, namely, the impact phase. Therefore, the whole process is a nonlinear process with impulse effects. After deriving formulation for the general problem, we solve three representative examples to show the concept. In these examples we study the manipulation of active and passive objects using active and passive manipulators. Dynamic, control, motion planning, and orbital stability at the presence of impact are the most important challenges in this work.

A high speed cutting process of bone such as drilling produces heat, and can result in thermal damage (death) of bone tissue. Temperature measurement in bone drilling is a primary step in establishing threshold level for thermal damage (necrosis). Advanced understanding of techniques for acquiring reliable thermal data on bone drilling is important to avoid traumatic incision. Currently, thermocouples are the main source in the experimental determination of temperature elevation during bone drilling. In an effort to overcome uncertainties in temperature measurements with thermocouples, a new approach was used which include inserting a thermocouple either attached to the cutting edge of a drill or inserted into the bone. The leading idea in this study was to investigate how temperature data obtained from the two mentioned systems are inherently different. Temperature measurements were observed to vary considerably with both types of measurement systems. Experimental resultsidentified drilling speed and depth of drilling as the critical parameters for inducing higher temperatures in bone drilling. The results presented in this study can be used to select the right method for acquiring temperature data for shallow and deep drilling in bone.

Increasing the performance of combustion engines has always been of interest to engine designers. In this regard, one approach has been to implement newly developed materials. Functionally Graded Materials (FGMs) have been shown to be heat treatable materials for high-temperature environments with thermal protection. Using these materials requires an understanding of the development of temperature and stress distribution in the transient state. This information is considered to be the main key in the design and optimization of devices for failure prevention. In this paper, a cylindrical Super Element (SE) has been employed to investigate the mechanical stress distribution of a cylinder-piston mechanism made of FGM for a combustion engine design. Analysis indicates that modeling a cylinder and piston with a few super elements gives the same results as a conventional nite element with 10052 elements. Materials are selected to be ceramics and metal with power low distribution. Results indicate that temperature distribution in the thickness of the cylinder does not vary, as the power of material distribution exceeds the value of one, while temperature distribution variation is sensitive to power distribution less than one. This has a high e ect on stress distribution in both FG cylinders and FG pistons.

In this article, the flow and heat transfer characteristics in an incompressible, non-Newtonian boundary layer flow of a viscoelastic fluid over a stretching sheet embedded in a porous medium with variable fluid viscosity and thermal conductivity including the eff ect of viscous dissipation has been examined. The fluid viscosity and thermal conductivity are assumed to be temperature dependent. Unlike the commonly employed thermal conditions of critical and prescribed surface temperature, the present study uses a convective heating boundary condition also along with the prescribed surface temperature condition. The di erential equations governing the problem have been transformed by a similarity transformation into a system of non-dimensional di erential equations which are solved numerically by Element Free Galerkin Method (EFGM). The e ect of various physical parameters like variable fluid viscosity, thermal conductivity, heat source/sink parameter, viscoelastic parameter, porosity parameter, Eckert number, Biot number on velocity, temperature, local skin friction and local heat transfer is studied for both the cases Prescribed Surface Temperature (PST) and Newtonian heating (NH) or convective heating. The present problem nds signi cant application in chemical engineering, material processing, solar porous wafer absorber systems and metallurgy.

Exoskeleton is a well-known example of an unconstrained robot for which the desired path is not predefined. Regarding these two effective features, a formulation for impedance control algorithm is presented and its prominence is demonstrated both mathematically and through simulation. Moreover it is essential to control this robot by an adaptive method because at least dynamic characteristics of the load are unknown. Unfortunately the existing methods do not address aforementioned traits or become unstable as inertia matrix becomes singular. Here an adaptive algorithm is generated based on the logic of Least Squares identification method rather than the Lyapunov stability criterion to tackle those limitations.

Multi-criteria optimization of processes parameters could be used to simultaneously achieve several conflicting goals such as increasing product quality and reducing production time. In this paper Grey Relational Analysis (GRA) and Taguchi method have been employed to optimize Electrical Discharge Machining (EDM) process parameters for AISI 2312 (40CrMnMoS86)hot worked steel alloy. The experimental data are gathered based on Taguchi L36 design matrix. The tests are conducted under varying peak current (I), voltage (V), pulse on time (Ton, pulse off time (Toff and duty factor h. The process output characteristics include Surface Roughness (SR), Tool Wear Rate (TWR) and Material Removal Rate (MRR). The objective is to find a combination of process parameters to minimize TWR and SR and maximize MRR. The three performance characteristics are combined into a single objective using grey relational analysis. The GRA was followed by the signal to noise ratio to specify the optimal levels of process parameters. The significance of the process parameters on the overall quality characteristics of the EDM process was also evaluated quantitatively using the Analysis of Variance (ANOVA) method. Optimal results were verified through additional experiments.

This research proposes a uni ed scheme to mathematically model and multiobjectively optimize the EDM parameters on tungsten carbide cobalt alloy (WC-6%Co), applying response surface methodology and a desirability function technique. Discharge current, pulse on-time, duty cycle and average discharge voltage have been chosen to be correlated with material removal rate, tool wear rate and surface roughness (Ra) as performance measures. The required experimental data were obtained in accordance with the face-centered central composite design. Signi cant parameters in the form of main, twoway interaction and pure quadratic e ects were carefully identi ed conducting a complete analysis of variance at 1%, 5% and 7% signi cance levels, and the adequacy of all tted second order regression models was con rmed. Parametric analysis was undertaken through direct and reciprocity e ect plots to fully reveal the di erent facets of ED-machinability characteristics. Finally, the optimization issue has been formulated as multi-objective from which the optimal parametric setting, yielding the most enviable conditions simultaneously, was then obtained in a compromised manner employing the notion of a desirability concept. The predicted optimal results were also interpreted and veri ed experimentally. The values of relative validation errors are all quite satisfactory (below 11%), which prove the ecacy and reliability of the suggested approach.

The thermo-elastic bending analysis of functionally graded ceramic-metal sandwich plates is presented in this study. The sandwich plate faces are assumed to be homogeneous and the core layer is constructed from FG material which its properties are varied through thickness according to the power-law equation. The hyperbolic shear deformation theory considering extension effect is employed for modeling the FG ceramic-metal sandwich plates. The presented theory is variationally consistent, does not require shear correction factor, and gives rise to transverse shear stress varying parabolically across the thickness. The governing equations are derived from principle of virtual work and the closed-form solutions are obtained using Navier method. The consideration of extension effect in presented formulation is examined and it is found that though it has no noticeable effect on transverse deflections and in-plane normal stresses, the obtained transverse shear stresses is quite affected by this term. Also the effects of thermal load, aspect ratio, thickness aspect ratio, thickness to side ratio and volume fraction index are investigated. It is observed that presented method is accurate and simple to use in comparison to other higher order shear deformation plate theories.