Modares Mechanical Engineering
Year:2019 | Volume:19 | Issue:3 |Papers Count:26

In this study, the thermo-physical properties effects of the heat exchanger body on the adsorption chillers performance have been investigated. For this purpose, an adsorbent bed with rectangular finned flat-tube heat exchanger is simulated by employing a threedimensional control volume scheme. Furthermore, silica gel SWS-1L-water has been used as a working pair. In order to investigate the effects of thermo-physical properties of the heat exchanger body material, two main parameters including the thermal conductivity coefficient and the volumetric thermal capacity are examined. Also, the effects of these parameters along with variations of the fin height and fin pitch on the specific cooling power (SCP) and the system coefficient of performance (COP) are investigated. The results indicated that the SCP increases with the increase in thermal conductivity coefficient up to a certain value, which increases and decreases with the increase in fin height and fin pitch, respectively. The results also showed that the effects of the volumetric thermal capacity on the SCP are negligible such that it can be considered independent of the heat exchanger body material volumetric thermal capacity. Unlike the SCP, the COP is strongly influenced by the volumetric thermal capacity. The increase in volumetric thermal capacity results in decreasing the COP. The slope of the decrease in the COP decreases with increasing the fin height and pitch. Also, by increasing the thermal conductivity coefficient, the COP slightly decreases.

In this paper, a nonlinear theoretical solution is proposed to simulate thermoelectric generators. A thermoelectric generator (TEG) setup was designed and constructed to measure the thermoelectric properties of a specified TEG, and, then, to validate the simulation results. The setup is composed of four bismuth telluride based TEGs, which are placed between an electrical heater and water cooled heatsinks to generate power as the result of the temperature difference. In the first section, the thermoelectricity phenomenon is introduced and governing equations are presented in order to develop the finite element solution by weighted residual Galerkin method. The FEM code is written in MATLAB software. In the second section, the designed and fabricated setup is explained and it is investigated how to perform the experiments. The TEG properties including the Seebeck coefficient and internal electrical resistance were measured, which are, then, used for setup simulation. First, the thermal-fluidic parameters including temperature and velocity distribution are obtained by simulation in Ansys-Fluent software. Then, the thermoelectricity simulation is performed by means of both the proposed finite element solution, and Ansys-Thermal electric software; so, the output voltage, power, and efficiency are calculated. The results indicate the accuracy of the modeling. Also, using the proposed finite element solution, the impact of the geometrical dimensions and temperature conditions on the TEG performance is investigated.

In this study, convective heat transfer around a heated circular cylinder covered with an annular porous medium in a flat channel was numerically investigated. To enhance the heat transfer, the porous medium is chosen to have a high thermal conductivity, whereas it is equipped with two different dispersions to reduce the pressure drop through the channel. To create two different dispersions (bi-disperse porous medium), the cylinder is covered uniformly by multiple porous fins with a porosity of 0. 9. In this regard, the fin porosity will be the first levels of porosity (microscopic porosity) and the arrangement of fins will be referred to as the second levels (macroscopic porosity) of the porous medium. The main goal of this research is to investigate and optimize flow conditions to achieve the highest outlet temperature and the highest heat transfer rate, where the pressure drop is reduced to a minimum value. This optimization is carried out for flow Reynolds number of 60 to 120, the Darcy number of 10-3 to 10-5, macroscopic porosity of 0. 25 to 0. 75, and outer to inner fin ratios of 1. 5 to 2. Numerical simulations are conducted, using the lattice Boltzmann method and the validity of simulations is assessed by the use of numerical and experimental data available in the literature. To optimize, the response surface methodology (RSM) with a central composite design is used and numerical results indicate that predictions obtained by RSM are in good agreement with actual flow condition in the optimum configuration. This research can provide new insight into the optimization process in heat exchanger designs.

In the present study, the performance of zigzag demister has been numerically investigated for the separation of dispersed liquid droplets from the gas flow. In general, liquid droplets are dispersed from the gas flow in contact with the vane demister and the formation of the liquid film. Depending on the energy of the droplet collision to the surface, it is likely to occur splash drop into smaller droplets, which will reduce the separation efficiency of the system. In this study, by focusing on the flow collision regime near the surface, it is attempted to investigate the effect of the flow parameters and vane geometry on the separation efficiency and the pressure drop of flow. The Euler-Lagrange is used to simulate the flow and particle motion path. In this research, an experimental model is designed and constructed. Numerical solver results are validated, using the experimental data. The result of this study shows that separation efficiency decreased with increasing gas flow velocity, such that by increasing the 2. 5 times of gas velocity, the separation efficiency will lead to a 10% decrease. It was also found that increasing the diameter and increasing the droplet would increase the separation efficiency. On the other hand, choosing the geometry of vane has a significant effect on the amount of the pressure drop of the passing flow. In a way that, by increasing the 50% of the vane angle, the pressure drop will increase 5 times.

Total Hip Arthroplasty (THA) is one of the most successful orthopedic surgeries, which is advised by the specialist in cases which osteoarthritis worsens in the hip joint. In the long run, functionality of THA may be subject to problems such as wear, loosening, and displacement. Structural and mechanical mismatches of artificial joint with the patient’ s natural joint after THA leads to the changes stress distribution pattern on the bones in a way that the majority of the load is on the artificial joint and a small percentage is implemented on the patient’ s bone; in the long run, it reduces bone density and leads to loosening and displacement. One of the most important factors determining the stress distribution in the bone and prosthesis is the acetabularcup inclination in the acetabulum socket. In this study, a 24-year-old patient, who had been injured in the hip joint, is studied and the effect of the inclination angle on stress distribution in the acetabulum and acetabularcup is assessed. First, a 3D model of the patient’ s bone is obtained, using CT-scan imaging and its mechanical properties are found. Gait analysis is carried out on the patient and the movement pattern and muscle forces in a gait cycle are found, using OpenSim software. The hip prosthesis is designed and the mechanical analysis of the joint is carried out, using ABAQUS finite element software, and the appropriate inclination angle for the acetabularcup for this patient is derived. The results show that the acetabularcup implantation in 45 degrees of inclination leads to better prosthesis functionality and a longer life.

The aim of this study is to investigate hemodynamic parameters such as radial and longitudinal velocities, pressure gradients, and wall shear stress of blood flow through a time-varying radius tube with one end closed. Application of this research is in the intra (as AVICENA) and extra cardiac assist devices, in which their balloons can increase the blood’ s energy by its periodic inflation and deflation and it makes the blood to be pumped strongly into the aorta. The equation is considered as a two-dimensional model with axial symmetry and analyzed as an analytical solution for aorta. This research shows the continuation of the numerical analysis of the intra-and extra-aortic cardiac assist device in the past papers of the authors in an analytical and two-dimensional model. Results show that the longitudinal velocity is increased as we move from balloon inlet to the balloon outlet along the length of balloon. At a specific time as we move from the balloon walls towards to the centerline of the balloon, the radial velocity of blood flow decreases. It means that the blood flow radial velocity at the centerline of the balloon is close to zero. Pressure is decreased as we move from the end closed to the balloon outlet. Although the wall shear stress increases during contracting of balloon, its value is less than that of existing in aorta, thereby concluding that the chosen-balloon properties may be appropriate to be used for the balloon implanted in the aorta.

In the present study, the instability in the interface of two semi-infinite fluid layers with applying a shock is studied. To this end, the effect of various parameters such as fluid densities, velocities of fluids, and magnetic field on the instability is explored. By using the magnetohydrodynamic equations, a general equation is developed for the evolution of perturbation amplitude near the interface. Analytical and graphical results indicate that the time dependent part of perturbation amplitude is the same for both the constant and varying density cases and the instability depends on the growth rate. Remarkably, the growth rate depends on the characteristics of the fluids and magnetic field and can be real or imaginary; hence, the stability condition is determined with respect to this parameter. Furthermore, it is shown that the spatial part of the perturbation amplitude in the constant density case, even with different densities, is symmetric and independent from the layer densities and damps exponentially in the two sides of the interface. On the other hand, it is shown that in the varying density case, the function of the spatial part of the perturbation amplitude depends on the parameters of the environment and the fluid; so the spatial part of the perturbation amplitude in the two fluid damps asymmetrically. Moreover, the results attained in the constant density case match the findings of the previous studies.

Today, application of polymeric composites and sandwich panels has increased in the industry due to their lower weight to volume ratio and also better mechanical properties in comparison with metals used in automotive and marine industries in diverse structures. Detection of failure initiation and examination of failure mechanism in composites, especially for sandwich, panels are state of art. In this research, the Acoustic Emission (AE), as a non-destructive testing method, was applied to estimate the residual strength of the polyester/glass fiber sandwich pannel with polyurethane foam with 3 different lay-up techniques. Sandwich panels were placed in 3 different energy levels under a low velocity impact and, then, with a three-point bending test, their bending strength was evaluated using the acoustic Emission. By simultaneously analyzing the acoustic data and examining the force-displacement diagrams obtained from the bending test and their correlation, the remained strength of the sandwich panels, priorly damaged by impacts of different energy levels, is estimated. For this purpose, the accumulated acoustic energy during bending and strain energy from the force-displacement diagrams have been used to calculate the recently presented Sentry function of pre-damaged samples to compare with a virgin case without previous defect. The results show that there is a direct relationship between Sentry function data as a new indicator of residual strength and accumulated energy of acoustic data that contains the effects of various failure mechanisms. In the largest destroyed sample with fiber layout of 90 and 45 degrees with respect to bending direction containing a maximum pre-impact of 60 Jules, the highest strength drop was up to 27% compared to the virgin sample.

In this article, an exact analytical approach is presented to analyze free vibration of a thin piezoelectric spherical shell, using thin shallow shell theory. The piezoelectric spherical shell is modeled as a sensor or an actuator. The piezoelectric material is polarized through the thickness of the shell. Using the separation of variables method as well as some new potential functions, the equations of motion and Maxwell’ s equation are exactly solved, simultaneously. First, the equation of the transverse displacement of the shell is separately obtained and after extracting the transverse displacement, other unknowns such as the in-plane displacements and electrical potential function are obtained. Then, applying mechanical and electrical boundary conditions, the natural frequencies of the shell are obtained for the sensor and actuator cases. In order to validate the accuracy of the present method, the obtained results are compared to those obtained by a finite element analysis in ABAQUS software. Also, the effects of various parameters such as inner radius to radius of curvature of the shell ratio, thickness to inner radius ratio, and different boundary conditions on the natural frequencies are considered. Results show that piezoelectricity effect causes an increase in strain energy of the structure leading to increasing the natural frequencies for both sensor and actuator shells. Also, by changing the conditions from actuator state to sensor one, the structure experiences an increase in the natural frequencies.

In this study, the method of releasing strains for calculating residual stresses in hole drilling process has been considered. For this purpose, a thick piece of cylindrical aluminum of 5 mm thickness has been investigated. Stepwise and high-speed drilling was performed in several successive steps, and released strains were recorded by a rosette strain gauge. The distribution of released strains in 3 forms of functions in the depth of the hole has been studied to transform strains to stresses, a linear function, a second-order function, and a third-order function. For each case, the longitudinal, tangential, shear stresses, principle stresses, and principle angles in the thickness of the piece were calculated and the results of the convergence analysis by the Tikhonov regularization were evaluated. In the end, the results are evaluated and compared for 3 modes. The results of the comparison of stresses and the degree of curves have shown that the third-order curve is more suitable for evaluation of released strains and using to transform them to residual stresses, and the magnitude of the error in the second-order curve is greater than the two other modes.

Ejectors are as widely used as in food industries to refrigeration cycles and power plants. Since condensers of steam power plants are operated in vacuum conditions, there is a continuous air leakage, which results in metal corrosion and reduction in efficiency. Therefore, ejectors are used in these systems to remove the air. Over time, leakage increases, which requires more efficiency of ejector. Entrainment ratio (ER) is defined as the main criterion for ejector efficiency and leads to better performance if increased and also depends considerably on geometry of ejector. The aim of this research is to increase efficiency of ejector of Touss Power Plant by simultaneously changing nozzle exit position (NXP) and converging angle of mixing chamber. The main geometry of ejector was simulated by FLUENT and primary results were validated with experimental and computational data. Then, different geometries with simultaneous change in NXP and converging angle of mixing chamber were selected in the first step of Taguchi method and simulated by FLUENT. Geometries of the second step of Taguchi method were selected and designed based on the results of signal-to-noise ratio for the above-mentioned parameters and the values of entrainment ratio in the first step. An identical approach was followed for the third step. Final results showed 34% increase in entrainment ratio and also revealed that there is an optimum value for NXP and converging angle of the mixing chamber around which the value of entrainment ratio is maximum.

In this paper, a novel multi-generation system based on gas turbine-modular helium reactor cycle is presented. Integrated system consists of a Gas turbine-modular helium reactor cycle as a base cycle and from the combination of subsystems, hydrogen production, absorption refrigeration cycle, and desalination system. Thermodynamic comprehensive modeling (energy and exergy) was done on the suggested system. The effect of various system parameters, such as turbine inlet temperature, compressor pressure ratio, carbon dioxide to methane molar ratio, vapor generator temperature, and mass flow rate of the desalination system have been evaluated on the overall performance of the system. Also, optimization of the overall system using single and multi-objective optimization method has been investigated in terms of energy and exergy compared to the base case. The results showed that the maximum net power output and the energy efficiency and exergy of the overall system in compressor pressure ratio between 2. 3-2. 45 were 275 MW, 72. 05%, and 49. 35%, respectively, and with increasing turbine inlet temperature, heat production rate and energy and exergy efficiencies of overall system increases and the cooling production rate and freshwater decreases. In addition, the optimal point of the mass flow ratio of the desalination system for the energy and exergy efficiencies of overall system is 2. 857. According to the results obtained in the multi-objective optimization method, the energy and exergy efficiencies of overall system were 74. 41% and 50. 21%, respectively, and exergy destruction has been reduced to 0. 74% compared to base case.

Cavity receiver in solar tower concentrator usually experiences highly intense radiation. Due to asymmetric concentration of solar rays, non-uniform heat flux distribution occurs on the different parts of the cavity receiver. This non-uniform distribution leads to uneven thermal expansion and stresses in receiver, which affects the reliable operation and reduces life time of receiver parts. Therefore, it is necessary to reduce the non-uniformity of solar flux on the surface of the absorber tubes and different parts of the solar reactor. The aim of this study was to focuses on the distributions of concatenated solar flux over graphite tubes of a 50kW solar reactor, which was previously designed for methane thermal dissociation at the focus of a solar furnace. In this study, the absorbed solar power on the different parts of the reactor is determined by Monte Carlo ray tracing method. Moreover, the effect of aperture size and the absorptivity of receiver parts on the net magnitude and distribution of absorbed power in reactor are investigated. The results prove that the 16cm aperture absorbs the maximum power and leads to even better solar flux distributions. Replacing the absorbing walls by the reflective walls will also result in more power absorbed by the tubes and better uniformity of flux distribution around the tubes.

In this study, a three-dimensional (3D) Peano series solution is presented for the dynamic analysis of functionally graded (FG). Layered magneto-electro-elastic (MEE) plates resting on elastic foundations with considering imperfect interfacial bonding and the interfacial imperfection is modeled using a generalized spring layer method. Regardless of the number of layers, the equations of motion, Gauss’ equations (for electrostatics and magnetostatics), and the boundary and interface conditions are satisfied exactly. In this method, no assumptions on deformations, stresses, magnetic and electric fields along the thickness direction are introduced. Finally, the governing partial differential equations are solved using state-space method. The proposed formulation is validated through comparison with other available results. Effects of a two-parameter elastic foundation, gradient index, bonding imperfection, applied mechanical and electrical loads on the dynamic response of the functionally graded magneto-electro-elastic (FGMEE) plate are discussed The obtained exact solution can be used to assess the accuracy of the theorems for layered FGMEE plates and validating finite element codes.

Multidisciplinary shape optimization of a re-entry capsule with aero-thermodynamic, trajectory, stability, and the geometry considerations are presented in this research. The method is based on decomposition of the underlying problem into disciplinary routines performing separated analysis for each goal. The current research is separated into four main components, including shape parameterization of the re-entry capsule, aero-thermodynamic analysis, re-entry trajectory analysis, and optimization. The re-entry capsule that is studied here belongs to the family of the Orion-like capsule and its shape is composed of three analytic surfaces, including a spherical nose, a ring section, and a rear conical part. The objectives of the optimization are maximizing volumetric efficiency, minimizing longitudinal stability derivative, and minimizing the ballistic coefficient, subject to constraints on geometry, heating load, and deceleration. Utilizing a multi-objective genetic algorithm will result in a collection of non-dominated Pareto optimal solutions. Then, the multi-disciplinary multi-objective optimization process allows finding a Pareto front of the best shapes. Resulting optimal solutions obviously show the compromises among volumetric efficiency, longitudinal stability, and ballistic coefficient. In the end, the results containing the dimension’ s characteristics of the re-entry capsule is presented.

Tortuosity is an abnormality that may occur in some arteries such as carotid. It disrupts blood flow to the brain and, even in severe cases, causes ischemia and stroke. Tortuosity can be congenital or occurs due to hypertension and reduced axial pre-stretch of the artery that is buckling. Since atherosclerotic plaques disrupt the normal pattern of blood flow, and, thus, make the artery more susceptible to buckling, in this study, the effect of atherosclerotic plaques on arterial stability has been investigated, using a computational simulation of fluidstructure interaction under pulsatile flow and large deformation. Ideal and 3D geometry of normal and atherosclerotic carotid artery with different plaques (symmetric or asymmetric and in different percentage of stenosis) were constructed and used to simulate normal (1. 5) and reduced (1. 3) axial stretch ratio by ADINA Finite Element Analysis Software. The blood flow was assumed to be Newtonian and laminar. Arterial wall was considered as an anisotropic and hyperelastic material based on the Ogden’ s model. The results show that stenosis reduces the critical buckling pressure and arteries with asymmetric plaque have lower critical buckling pressure compared to the arteries with symmetric plaque. By reducing the axial stretch ratio from 1. 5 to 1. 3, the critical buckling pressure is reduced by 33-39%. Buckling increases the peak stress in the plaque and, thus, increases the risk of plaque rupture.

Waste heat recovery systems, which make use of waste sources for their input energy, have considerable importance in industry since they utilize streams, which will be disposed to nature if not employed. Ship’ s engines are one of the places, where a large amount of energy is wasted in different forms. In the present article, the idea of making use of these loss streams and consequently producing useful power in the outlet is proposed in the form of two systems. In the first system, the only stream of exhaust gases is utilized, while in the second system, the jacket cooling water is used together with the engine exhaust gases. Screening in the working fluids is conducted in order to select appropriate fluids, which have suitable characteristics in the physical, safety, and environmental aspects. The analyses indicate that using R600a presents the highest net power output, which reaches to the value of about 575 kW at the most. Comparison of the two introduced systems shows that preheating the working fluid by the jacket cooling water makes the better operation of the system and the power output is increased up to about 31-58% in different fluids. The lowest payback period in the systems is achieved through the use of R600a as the working fluid, which is about 3. 48 year in the second system.

The aim of this research is an analytical investigation of heat and mass transfer for the MHD nanofluid flow passed between non-parallel stretchable/shrinkable walls. In order to model nanofluid flow, effects of Thermophoresis, Brownian diffusion, and Joule heating are considered. The governing mass, momentum, and energy equations are solved analytically by applying Duan-Rach method, which caused to get a solution for the undetermined coefficients from conjectured profiles of variables without using numerical methods. Comparison between the current results with the numerical results of other references shows good agreement. The effects of the Reynolds number, opening angle parameter, and the Hartman number on the temperature, velocity, and concentration profiles have been investigated in the case of both convergent and divergent plates, either stretched or shrunk. Also, the effects of the Thermophoretic and Brownian parameters on the Nusselt number are obtained. This study indicates that increasing the Hartman number decreases the concentration profile and increasing in the temperature profile for divergent channels. In this case, as the opening angle parameter rises, the thickness of the thermal boundary layer increases. Also, for convergent and divergent channels, the increase in the thermophoretic parameter causes increases the Nusselt number. By applying an identical magnetic field to two divergent stretching and shrinking channels, the concentration profile in the stretching channel is more than the shrinking one. For convergent channels, this treatment of concentration profile is completely vice versa.

The aim of this study was to investigate the surface free energy on the surface of calcite rock and also on the surface of aged calcite in Surfapore Nanofluid using a contact angle measurement. For this purpose, calcite surfaces were prepared by cutting to an approximate size of 3×3×0. 4cm3 and grinded and polished to achieve different roughnesses. The purity and roughness of the samples were determined by X-ray diffraction and atomic force microscopy, respectively. Using the static contact angle on the surface of calcite and calcite aged in the Nanofluid, surface energy determined by three methods of geometric mean, arithmetic mean and Zisman plot showed surface free energy between 30-40mN/m, and polar forces overcome dispersion at calcite surface. After aging calcite surface in the Nanofluid, surface energy reached less than 12mN/m. This surface free energy reduction indicates an increase in the contact angle of the fluids on the aged calcite surface in the Nanofluid. The results of this study will help to better understand the surface properties of calcite in the presence of Nanofluid, as well as how to change its wettability to gas wet conditions, taking into account the static contact angle.

The application of computational fluid dynamics is being developed in recent years in order to evaluate the numerical impact of wind damage on high-rise buildings due to the increasing computing power of computers. With regard to the turbulent downturns around flexible, slender and long-winded buildings with relatively high Reynolds numbers, the study of aeroelastic behavior of tall buildings is essential. In this paper, the turbulent wind flow is simulated numerically with four different velocities around the high standard CAARC building. Large Eddy Simulation has been used to solve the turbulence effect in solving fluid flow equations and the response of tall buildings to wind forces is determined by solving the differential equation of motion. A two-way coupling method is used to transfer data between two areas of fluid and structural solution in each step of time. According to the results of the numerical simulation, the pressure coefficients, streamlines and instantaneous pressure field around the tall building are in good agreement with the common characteristics of the flow around the airborne objects. The critical speed corresponding to the lock in phenomenon in this problem is calculated using a Strouhal number equal to 100m/s. Also, the history of displacement of the roof of the building in the direction of the wind and perpendicular to its length have been extracted for different wind velocities and the mean and their standard deviations respectively have been calculated. The continuous increase in the range of the fluctuations of the building under the wind blowing at 100m/s is observed. This point indicates the efficiency and capability of the numerical process in detecting aeroelastic instability with a predicted speed.

The aim of this study is to investigate the life-span according to the damage caused by the main mechanisms of damage development in turbine blades and to model the growth of the damage. For this purpose, the low cycle fatigue test on martensitic 410 stainless steel was immersed in tempered glass at 565° C in three strain gauges 0. 8, 1 and 1. 5 with a constant temperature of 500° C and 15 seconds per cycle. The effect of creep-fatigue interaction on life and also damage to turbine blade in different conditions was investigated. The results showed that with the variation of the strain amplitude from 0. 8 to 1. 5, the life of the piece varies from 205 to 65 cycles and this is while the level of failure of the samples varies. In the next step, the modified Coffin-Manson model was used to indicate the damage and its simultaneous effect on the life of the piece. The results showed that decreasing the number of grain boundaries and its effect on the cavities created in the piece decreases the damage and thus the life of the turbine blade increases. High-temperature tensile tests and low-tensile fatigue-temperature control were also performed in different tempering modes for 410 and 420 steel stainless steel and The results showed that, under the same conditions, the temperature increase from 200 to 565° C resulted in a decrease in life from 2218 to 1952 cycles.

Coronary arteries play a vital role in heart nutrition, and if they get stenosis, they will be at risk of developing a heart attack. Coronary artery disease is a progressive disease that is caused by the accumulation of fat particles on the wall of the arteries, leading to thickening of the wall and the formation of layers of plaque on the wall of the arteries and ultimately causing stenosis. In the present study, in order to obtain the effect of percentage and position of stenosis on the pattern of flow and WALL SHEAR STRESS distribution, followed by the progression of atherosclerotic plaques, left coronary artery and its main branches, the anterior and anterior artery, in different conditions according to Medina classification, 50 and 75%, and three different positions of lesion locations based on their distance from carina relative to the center of the branching were modeled. According to the results, WALL SHEAR STRESS and flow ratio and the percentage of inflow into the lateral branch decreased with increasing percentage of stenosis. For example, in Medina type (1. 1. 1), in 50% diameter stenosis, the flow ratio was 41% of the main branch and it was 37% in 75% diameter stenosis. WALL SHEAR STRESS values are less than 1, even 0. 5 Pascal and in critical range in 75% diameter stenosis. Increasing the spacing of the plaque from the center of the branch, the WALL SHEAR STRESS and lateral branch flow ratio increase, and the likelihood of the expansion of the plaque decreases. Based on the development of stenosis severity, modal type (1. 0. 1) has the highest probability of developing atherosclerotic plaques and total vein occlusion compared to other types of medina.

When a flying vehicle is approaching a watery or earthy surface, the flow pattern around it is changed that is called the ground or surface effect. In this study, the phenomenon of ground effect and its effects on aerodynamic coefficients and flow pattern around NACA0012 and LH37 airfoils are numerically investigated. The analysis is done for statically and dynamically airfoils with plunging motion at subsonic incompressible flow regime. The Navier-Stocks governing equations are used with k-𝜔 SST turbulence model. At first the effects of ground effect on lift coefficient of airfoils are studied in various distance from surface, statically. Then at each position of airfoils from the surface the lift coefficient behavior of airfoils at sinusoidal plunging motion with the specified amplitude and frequency is investigated. the statically results show that the lift coefficient of airfoils and pressure distribution over them are changed when they approach the surface with respect to far from it, which is seen as decreasing to a certain height and then increasing it. Dynamically analyzes also indicate a change in the oscillation amplitude of the lift coefficient and the existence of a phase difference at the points of achievement of minimum and maximum lift, when the airfoils are an approach to the surface. The streamlines also showed the changes in flow field patterns around the airfoils, when they approach the surface.

In the present study, the electroosmotic and pressure driven flow of nanofluid in a microchannel with homogeneous surface potential is investigated by using the Poisson-Boltzmann equation and the flow field is assumed to be two-dimensional, laminar, incompressible, and steady. Distribution of nanoparticles in the base fluid is assumed to be homogeneous; therefore the nanofluid flow is modeled as a single phase. The thermal conductivity of the nanofluid is modeled by using the Patel model to account for temperature dependency. In order to validate the numerical solution, the results are compared with available analytical solutions and the comparison shows a good match with the results. Then, the effects of different parameters such as ion molar percentage, volume fraction, and nanoparticles’ diameter on the flow field and heat transfer are examined. The results show that by fixing the electric field and increasing the pressure gradient, the local Nusselt number will decrease, and by fixing the pressure gradient and enhancing the electric field, the Nusselt number increases. The average Nusselt number increases about 45, 35 and 25% while nanoparticles’ diameters are 100, 110 and 120nm, respectively. For Γ =0. 05, the average Nusselt number increases 10% while ion concentration changes from 10-4 to 10-2. Furthermore, the direction and magnitude of velocity and concavity of the velocity profile can be controlled by choosing a suitable phase angle between electrical and pressure driven flow parameters.

External disturbances and internal uncertainties with an unknown range, as well as the connection between the human body and robot, are major problems in control and stability of exoskeleton robots. In order to deal with disturbances and uncertainties with the known range of the system, the sliding mode controller is used as a robust approach. The chattering phenomenon is one of the drawbacks of sliding mode controller, which boundary layer is employed to reduce the effects of this phenomenon. In this case, not only the chattering phenomenon is not completely eliminated, but the robust characteristics of the controller are mitigated. In this paper, in order to cope with the disturbances and uncertainties with unknown range, and guard against chattering as a key ingredient of excessive energy consumption and convergence rate reduction, optimal adaptive high-order super twisting sliding mode control has been applied. The dynamic model of a lower limb exoskeleton robot is extracted using the Lagrange method in which four actuators on the hip and knee joints of the left and right legs are considered. To achieve optimal performance, controller parameters are determined using Harmony Search algorithm by minimizing an objective function consisting of ITAE and control signal rate. The proposed controller performance is compared with optimal adaptive supper twisting sliding mode and optimal sliding mode controllers which shows the superiority of the optimal adaptive high-order sliding mode controller rather than other designed controllers.