Modares Mechanical Engineering
Year:2019 | Volume:19 | Issue:10 |Papers Count:28

This study aims to investigate the ability of ultrasonic method by using critically refracted longitudinal (LCR) wave for measuring stress in the elastic phase of an iron-base alloy and a an equation includes of acoustoelastic constant was provided. For this purpose, extract detail of metal alloy components was achieved by use of quantometer analysis testing and. In order to send and receive the LCR wave into the samples, the investigation leads to design a unique type of ultrasonic fixture. The fixture was made based on Snell’ s law that only in one part. In the next step, different amounts of stress were applied to the specimens by using a uniaxial tensile testing machine and record stress-strain curve data. To this end, more than three metal samples were used in the study. Measurement of longitudinal applied stress by ultrasonic method was done by using 2MHz probes based on Acoustoelasticity theory and close to the surface of the samples. After conducting the experimental tests, the results indicated that there was a significant relation between stress and time of flight and wave speed in the elastic phase of the used sample. Every material has a unique acoustoelastic constant that can determine stress value by having times of flight wave. The conclusions of the study provide a gradient of a line that known as an acoustoelastic constant. Finally, by comparing the results of the used method with other researchers results, findings showed that there were good agreements between them which shows the good capability of acoustoelasticity theory in the measurement of stress.

Natural gas characteristics make it an attractive choice for replacing with oil fuels which causes climatic problems and environmental pollutions in the world. Generally, using natural gas in an internal combustion engine leads to lower volumetric efficiency of the engine, but gas direct injection technology would improve volumetric efficiency. Furthermore, more research is essential for improving the effectiveness of direct injection engines. A partially stratified charge is an appropriate idea for combustion efficiency improvement in direct injection engines. In the present study, a port injection engine is converted to direct injection engine and feasibility and condition of partially stratified mixture formation are investigated. Also, its effects on combustion and efficiency of the engine, with regards to location and injection timing of injector are noticed. Numerically simulation of current study shows that the formation of partially stratified charge, because of using air-guided method and located injector between air intake valves, is so hard and inaccessible. The high momentum of CNG jet makes a rapid motion of injected gas fuel and is not able to perform an appropriate mixture of air and fuel. Accordingly, an increase in air and fuel ratio is locally seen in the combustion chamber as this causes a drop in combustion efficiency. Although the increase in flame propagation and heat release can be expressed as results of this study, however, the benefits of rapid burning of CNG combustion due to the problems that are mentioned are not so impressive.

Gas turbines have a wide range of application in different industries. There are different models of the gas turbine for its analysis and diagnosis. In this paper, a hybrid model is considered for the gas turbine. This model combines thermodynamic relations and data-based equations which cause to eliminate dynamic loops of thermodynamic relations. Also, the compressor performance curve is considered in the proposed model which leads to noticing physical and geometric characteristic of a gas turbine. The model is dynamic and nonlinear that cause to adapt to a different condition and increase the accuracy of modeling. The model is accurate, simplified and nonlinear state space form. For these reasons, the model is suitable for analyzing of controllers and observers. The proposed controller is a new sliding model controller for implementing in the model. The controller is based on the l_1 norm and frequency analysis. Since the sliding mode is robust and the l_1 norm is optimizer than the l_2 norm, the controller tracks fuel command with acceptable accuracy and minimizing the control fluctuations. Also, the data that is used in this paper is the data of an industrial gas turbine (IGT25) of Iran’ s national turbine which is logged in different ambient and functions conditions.

Condition assessment is one of the most significant techniques of equipment health, repairs, maintenance, and management. Prognostics and Health Managemen (PHM) methodology cycle is a developed form of Condition Based Maintenance (CBM). Condition assessment is the most important step of this cycle. In this study, based on the presented model, the Remaining Useful Life (RUL) is estimated using equipment condition assessment. Using the simulation and forecasting of a new feature for the vibration of the equipment (Kurtosis-Entropy) by Autoregressive Markov Regime Switching (AMRS) method, equipment health condition is determined. Prior to forecasting the condition of the equipment, the equipment degradation state is determined by the fuzzy C-means clustering method. Based on the current state of equipment and pre-determined state of degradation, the remaining useful life of the equipment is estimated. In order to evaluate the model, the experimental data from the FEMTO-ST Institute, which is provided to estimate the remaining useful life of the bearing, was used and the results of the study are compared with the rival models. The innovation of this paper is the use of fuzzy C-means, a new approach to evidence theory for data fusion, and the use of the Markov switching model for prediction.

With the recognition of wind energy in the world, wind farms are expected to become more extended and cover increasingly larger surface areas. The most important issue in large wind farms is increasing power and efficiency. When wind turbines are deployed in large arrays, their efficiency decreases due to complex interactions among themselves and with the atmospheric boundary layer (ABL). When the length of the wind farms exceeds the height of the ABL by over an order of magnitude, a “ fully developed” flow regime can be established. Since the length of the farm is larger than the atmospheric boundary layer thickness, changes in the streamwise direction can be neglected and the relevant exchanges occur in the vertical direction. In the wind farms established in the fully developed atmospheric boundary layer, the kinetic energy extracted by the wind turbines is transported into the wind-turbine region by vertical fluxes associated with turbulence. Surface roughness is one of the most important factors affecting this phenomenon. In this research, the effect of surface roughness on the efficiency of the large wind farm in the ABL by large eddy simulation is investigated. For this purpose, the large eddy simulation (LES) is applied to solve the turbulence flow equations. In this article wind turbines are modeled using the classical drag disk. The various surface roughnesses are modeled by logarithmic wall functions applied to the bottom of the domain. The results show that the efficiency and power of the wind farm are decreased by increase of surface roughness.

Microbeams are one of the most important members of microelectromechanical systems (MEMS) which contrast of electrical and mechanical forces in them cause pull-in instability. One of the proposed mechanisms for controlling this instability and enlarging the stable range of system are initially curved microbeams. Despite studying various pull-in instability in straight elastic or viscoelastic microbeams, the instability of curved microbeams has been investigated only within the range of elastic behavior. Therefore in the present study, assuming a clampedclamped viscoelastic initially curved microbeam, the effect of viscoelastic behavior on the instabilities called snap-through and pull-in, was investigated. The viscoelastic behavior was simulated by the standard anelastic linear solid model. The governing differential equation was obtained based on the modified couple stress theory and by use of Hamilton’ s pull-in instability principle. By using the Galerkin method, the governing equation was converted to a nonlinear ordinary differential equation and solved by MATLAB sofware. The structure behaviors are compared in two extreme situations before and after the viscoelastic relaxation by drawing diagrams. The results show when the time of structure relaxation increases, viscoelastic behavior causes more decreasing in instabilities voltage, but its effect on the position of instability will depend on the axial load. In this way, in the presence of tensile load, viscoelastic behavior increases the snap-through position and decreases the pull-in position, but in the presence of compressive load, snap-through occurs at smaller deflections and pull-in occurs at larger deflections.

Nowadays with the increase of the power of electronic components, their heat generation rates have also increased therefore, therefore it is necessary to use new methods to cooling different parts. One of the solutions to cool the high-power components is the use of vapor chambers. The vapor chamber consists of three sections, the evaporation, the middle and the condensation section, which are flattened and can transfer a significant amount of heat without the need for external power and only by using a fluid phase change. In this study, two vapor chambers with a length and width of 120 mm and a height of 15 mm were made to cool the high-power printed circuit board, where the evaporation section of one of them was roughened and the condensation section is cooled down by the fin and through the air. In this research, the effect of roughening the evaporation section, the angle of the vapor chamber relative to the horizon, different heat input and the geometric deformation of the heat source in the fixed area, as well as changing the location of the heat source in the evaporation section, on the thermal performance of the vapor chamber, is experimentally reviewed and compared. The results of the experiments show that increasing the heat input and roughing the evaporation section improves the performance of the vapor chamber and the thermal resistance of the vapor chamber is also the function of changing its angle relative to the horizon, deformation, and location of the thermal source.

The aim of this study is the modeling of the solar chimney for achieving the relation between turbine output power and geometrical parameters. In this regards, 9 different models are determined based on the variety of chimney height and diameter for investigating the effects of geometrical parameters on the turbine performance. As well as, in order to improvement of system performance, the hydrophobic surfaces were evaluated with consideration of friction reduction by verification of slip condition on walls. The k-ε turbulent model was used to modeling turbulence flow and reverse-fan model was employed for simulating the turbine. For this purpose, the extracted data from the mass flow rate and velocity changes were validated with prior studies and then were compared in different pressure jumps in order to better comprehension of the performance of the turbine. The optimization was done through the defined models and it was observed that to have a better and optimized design, the geometrical parameters should have been considered in the system design simultaneously. Meanwhile, the chimney diameter should have been paid more attention as one of the most important design parameters. Also, the precise correlation was represented to estimate the turbine output power with respect to the height and diameter of the chimney. Furthermore, based on the applying of slip condition on walls for simulating hydrophobic surfaces, shear stresses reduction was done and it was revealed that the hydrophobic surfaces could have a positive effect on the performance of SCPP up to 5 percent.

Turbine blades are exposed to mechanical and thermal stresses due to their operation in critical conditions that lead to various damages such as fatigue and wear. These factors reduce the blades life cycle by accelerating the cracking process. In this paper, the effects of three geometric parameters including the contact length, the contact angle, and the surface friction coefficient on relative slip amplitude and contact pressure values in the turbine blade root were investigated using a two-dimensional finite element model. Comparing the results of the analysis with the actual blade damages by use of scanning electron microscopy shows acceptable consistency between predicted damage site and the actual blade damages. The results of the blade analysis indicate that by moving from the top of the contact edge to the bottom, the contact pressure increases gradually and its maximum occurs near the lower edge of the contact. According to the results, the prescribed increments in the coefficient of friction, the contact angle, and the length of contact, respectively decrease the slip amplitude by 26%, 19%, and 10% and also decrease the contact pressures by 35%, 15%, and 5%. In addition, increasing contact angle and coefficient of friction increase the opening region length at the upper edge on both sides of the blade root. While increasing the contact length has no considerable effect on the length of this region.

In this research, a dimensionless model was developed based on the geometric parameters for the deep drawing process to reduce the manufacturing cost of square cup deep drawing in the large scales. In the following, a series of groups were found for dimensionless ratios based on the geometric parameters of the square cup by Π-Buckingham dimensional analysis method in the two states of circular and square sheets. In order to find the best group of dimensionless geometric parameters, three scales of cups were numerically evaluated by commercial finite elements software. The results were validated by an experimental test. After analyzing all the effective geometric parameters, a fittest dimensionless equation was obtained. The st12 metal sheet was used for experimental validation in the room temperature. Moreover, the results and tearing force as target parameter were compared in simulation states, experimental tests and the proposed dimensionless model based on Π-Buckingham theory. By comparing the results in the two states of the circular and square sheets, it can be concluded that the geometric characteristics of the main scale sample can be predicted by a sample in a small size through the proposed dimensionless model. Comparison of the results of the dimensionless model and experiments show that the proposed model has high accuracy in predicting the tearing force and geometric parameters in the square cup deep drawing process.

Turbofan engines are widely used in modern aircraft. Low-pressure turbines are the heaviest components of turbofan engines, and reduction of their weights is very effective in improving the specific fuel consumption and overall efficiency of these engines. One of the methods of decreasing the engine weight is to decrease the number of blades which is accompanied by an increase of the blade loading. For this purpose, high-lift airfoils can be used. As the occurrence of flow separation is very probable in high-lift blades, the recognition of the location and size of the separation bubble is important to assess the energy loss of flow. In this research, T106DEIZ high-lift cascade is simulated by two-dimensional Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with Shear Stress Transport (SST) turbulence model and γ-Re_θ transition model in two Reynolds numbers 200, 000 and 60, 000 at a constant isentropic exit Mach number of 0. 4, which represent a typical flow condition in low-pressure turbine. The results show that when Reynolds number is high, the separation bubble remains small on the suction side and the separated shear layer returns to the blade surface, and the energy loss of flow decreases. On the other hand, at a low Reynolds number, the separation bubble grows and energy loss increases. Separation bubble is not directly detectable in an evaluation of pressure distribution. However, proper orthogonal decomposition of the pressure field provides the capability to identify flow structures including vortex stretching, the onset of flow separation, and flow reattachment. When the separation bubble is long, large vortical structures are formed on the suction surface. Release of these large vortices can increase the profile loss by more than 50 percent.

Improvement of behavioral indicators of oil journal bearings has particular importance due to the increasing development of their application as support of rotary components in industrial machinery. Creation of regular roughness (texture) with various geometries on the inner surface of a bearing shell is one of the newest methods proposed by the lubrication researchers to enhance the performance of the hydrodynamic journal bearings. In this study, the comparison of the performance of circular bearings with variable cubic, cylindrical and ellipsoid textures of different depths arranged in a different zone of the shell has been evaluated. For this purpose, the governing Reynolds equation on hydrodynamic lubrication of oil journal bearing was modified considering the changes of the film thickness affected by the geometry and position of the textures. This equation was solved by finite element numerical method, applying the assumption of the Reynolds boundary condition for determining cavitation zone. After obtaining the lubricant pressure profile, the parameters of steady-state performance of the bearing with different texture types were calculated and compared together. Results indicate that the creation of textures with any geometry reduces the lubricant pressure and changes the parameters of the bearing performance. Also, the placement of textures in the maximum pressure area leads to significant changes in performance components while their positioning in the lubricant cavitation region has a weak effect on the bearing behavior. Further, the results show that the difference in characteristics of bearing performance with shallow textures is more considerable and with the increase of textures depth the effect of geometry form on the performance will be reduced.

In plasma cutting, a noble gas at high speed is blown from the nozzle and ionized with the help of a frequency spark at high voltage and an electric arc is created which cause the gas changes to the plasma state. Plasma cutting is an ideal process for cutting of the hard metals. In this research, the effect of the input parameters and their optimization in plasma cutting of AISI 309 stainless steel were studied. By conducting the different experimental tests, the effect of input parameters including amperage, gas pressure and the cutting speed of torch on the three output parameters of the width of cut (Kerf), heat-affected zone (HAZ) and surface roughness (Ra) were investigated. Analysis of the results showed that the amperage, cutting speed and gas pressure have the highest impact on the output parameters, respectively. The artificial neural network (ANN)-genetic algorithm was used to predict and optimize the output parameters. The results indicate that the artificial neural networks model trained by the genetic algorithm are able to predict the output parameters accurately. Finally, the optimization of output parameters to achieve the best cutting conditions was carried out using the genetic algorithm. The artificial neural network models were considered as the objective function and also, the parameters of the heat-affected zone, surface roughness, and the width of cut were introduced as inputs of the algorithm. According to results, a combination of the neural network and genetic algorithm is an efficient method for optimization of the plasma cutting process. This method can be easily modified and utilized for other advanced cutting methods.

The superhydrophobic surfaces have many applications, including skin friction reduction, antiicing, anti-fouling, and self-cleaning surfaces. Also, with the precise design of these surfaces, it is possible to increase the heat transfer coefficient in the condensation heat transfer. In recent years, a variety of methods have been proposed for the fabrication of the superhydrophobic surfaces, some of which are very complex and not applicable for industrial uses. In this paper, a nanocomposite superhydrophobic coating is produced in a simple and applicable way for large surfaces. Using this method, a superhydrophobic surface with surface structures in multi-scale and with a sliding angle of less than 5 degrees is obtained. After evaluating the specification of superhydrophobic surfaces, slip length measurement of the coating is performed using a fabricated measurement system. It should be noted that the slip length of the superhydrophobic surface is a characteristic feature of these surfaces and always its measurement is associated with challenges. In this research, the slip length of the created coating was measured by use of the proposed measurement system. The results show that the slip lengths of about 40-500 microns can be achieved by use of the proposed measurement system.

In this paper, a 3D model is proposed for investigating the performance of HGMS filters. This filter consists of a matrix of iron rods arranged in a channel with a square cross-section and subjected to an external magnetic field. The flowing fluid is the amine solution which contains the FeS micro-particles. In the presented model, first, the capture performances of magnetic particles for 2D geometries are calculated numerically at various conditions using COMSOL Multiphysics software through finite element method. Using these results, a database of capture performance has been established for different speeds of the flow, diameters of the particle and arrangement of the rods. By use of the processing of this database, the capture performance of semi-sized particles for a 3D problem is calculated through the integration of captured particles along the length of the rods. Finally, the amount of total particles captured on the rod matrix is obtained for a group of particles with various diameters assuming Gaussian distribution. The results indicate that in HGMS filters, the particle capturing is directly related to the particle diameter but inversely depends on Reynolds number and the vertical distance between the rods. Also, at the same conditions, the filtration of the triangular arrangement of rods is greater than the rectangular arrangement. However, the performance difference of these two arrangements decreases with increase in the flow velocity or increase in the distance between the rods or decrease in the diameter of the particles. These results can be used to optimize the filtration of particles in the magnetic filters at different conditions.

Adsorption simulation of vancomycin antibiotic is done using molecular dynamics. The simulation results show the adsorption behavior of vancomycin on a functionalized biosensor. Regarding the importance of vancomycin, its molecular function is simulated using multiscale discipline. Adsorption to a single assembly monolayer is considered according to vancomycin’ s in-vivo function. A selected biosensor is a non-symmetrically functionalized microcantilever which undergoes deformation as a result of changes in surface tension regarding functionalized surface. Multiscale simulations implemented to calculate microcantilever deformation. Molecular models in a vacuum and aquatic media are taken into account. Energy parameters related to surface tension is studied versus the distance of target molecules to the surface of the biosensor. To calculate the distance between receptor molecules in single assembly monolayer, an algorithm is proposed based on experimental results.

Cardiovascular diseases are the major cause of death in industrialized countries. Recent attempts in computational modeling of the human heart in normal and diseased conditions made it possible to find a way to predict the behavior and test the cures virtually with less harm for the human body. Ventricular hypertrophy that occurs in response to blood pressure and volume overload in ventricles can change its property and function and finally lead to heart failure. In this research, concentric left ventricular hypertrophy of the human heart was modeled in silico. The left ventricle (LV) model was implemented into the commercial nonlinear finite elements (FE) software ABAQUS/STANDARD through the user-defined subroutine UMAT based on continuum mechanics. We tried to determine the fibers distribution with more accuracy and considered the fibers and sheets dispersion in the anisotropic hyperelastic growing model. When the ventricular pressure and the resultant wall stress increased, the sheet growth multiplier started to increase from the endocardium to the epicardium and the ventricular wall became thicker. Residual stresses were observed in the model after unloading. Sheet growth multiplier changes versus stress showed that sheet growth multiplier increased dramatically near the maximum pressure while the stress remained almost constant.

Shape memory alloys (SMAs) are a new generation of smart materials which was the subject of researches in recent years. In this study, SMAs are employed to improve the vibrational and structural behavior of composite beams. A numerical solution was presented for natural frequency analysis of the clamped-clamped beam and the obtained results were validated with results of available references. Two main goals were followed in this study: first, analysis the influences of effective design parameters of embedded SMA wires on natural frequencies of composite beams and second, optimal design of SMAs to improve the vibrational and structural behavior of composite beam. In the first step, the effect of design parameters of shape memory alloy wires including the number and the diameter of wires on natural frequencies and total mass of structure was studied. In the second step, maximization of the first natural frequency of the structure and minimization of the total weight of the structure was the objective function of multi-objective optimization process which was performed by employing the genetic algorithm and weighted sum optimization approach. The obtained results of optimization processes confirmed the high efficiency of the proposed approach to improve the vibrational and structural properties of Shape memory alloys composite beam.

Flexible and lightweight unmanned aerial vehicles (UAVs) have shown their widespread applications in recent years and hence attracted so much attention of various aerospace communities. Due to their high flexibility, the interactions of aerodynamic loading and structure deformations are the dominant factor in their design process. Aerodynamic loading causes a set of deformations in the structure which consequently alters aerodynamic coefficients. In the current study, the effect of UAV flexibility on aerodynamic derivatives and lateral stability of the vehicle was investigated and an efficient method is proposed to provide an accurate estimation of the aerodynamic coefficients. This method is based on fast aerodynamic calculations as well as the modal formulation of elastic beams and is given for a full free-free airplane. Vehicle analysis is conducted by using the Modal beam formulation (through finite element mode shapes) and aerodynamic calculations based upon the 3D panel method (source– doublet combination). The final aero-elastic coupled formulation for the whole system is also given in terms of matrix operators. Verification studies are conducted for a special type of UAV and flexibility effects on derivatives are evaluated in the two states. In a first evaluation, the lift load factor is altered and after trimming the airplane, various aerodynamic derivatives are computed while in the second evaluation, with varying the wingspan length, the aerodynamic derivatives are obtained at each aspect ratio of the wing. Results show that flexibility can enhance some of the stability derivatives of the UAV up to several times.

The helicopter rotor blade flapping results in a helicopter rotor symmetry lift and has a significant impact on stability and control. In this paper, the modeling of helicopter flapping in the presence of aerodynamic forces and moments and the effect of offset, blade torque, hinge resistant spring, blade geometry, natural frequency effect, and forward ratio to achieve reliable relief from flapping was investigated. In the simulation, the effects of small and large flapping angles and the role of offset on the momentum entered on the blade, as well as the role of the forward ratio in moments were investigated. Different models of flapping dynamics and equations for the flight of a hover and cruise helicopter are fully presented and all of the important issues are examined for a numerical example. Also, the effect of non-uniform flow in the flapping equations of the blade is the effect of the natural frequency of the flapping motion with the blade offset. This leads to increasing the accuracy in modeling the phenomenon of flapping on a helicopter. Simulation results show the importance and impact of offsets, moments and forces imposed on the blade in the motion of the flapping, which leads to an increase of accuracy in modeling.

The Nd: YAG pulsed laser welding process with different speed and shielding gas was applied on 2205 duplex stainless steel. The effects of different parameters on the microstructural evolutions and mechanical properties were investigated. Four different zones with different secondary austenite contents were observed in the weld microstructure. By changing the shielding gas from argon to nitrogen, the secondary austenite percentage was not significantly varied. The secondary austenite fraction was showed about 38% reduction with increasing the welding speed. The weld penetration depth decreased with changing the shielding gas from argon to nitrogen (about 26% and 14% reduction at speed of 3. 8 and 8. 3 mm/s, respectively) and increasing the welding speed (about 43% and 34% reduction under shielding gas of argon and nitrogen, respectively). The variations in microhardness values along the weld line were correlated to the microstructural characterizations. Changing the welding speed had no significant effect on the microhardness variations, but changing the shielding gas from argon to nitrogen caused a significant increase of microhardness.

Supersonic inlet has a key role in the performance of air-breathing engine special in ramjet engines. The function of a ramjet engines diffuser is to decelerate the air velocity from its freestream at the intake to a combustor velocity which is compatible with the available flame velocity. Accurate design of inlet has an important effect on the performance of a ramjet system and its main parameters. In this study, an axisymmetric external compression supersonic inlet designed at Mach 3. Flow behavior for this type of inlet is more complicated than the two-dimensional inlet. Because of this character, a computer program for solving the equations of supersonic flow through the cone, and the relationship between the parameters and the principles of the axisymmetric inlet has developed which integrates the final geometry, including center body geometrical details, inlet cowling and subsonic channel as output. Since viscous forces cannot be ignored because they cause complex shock patterns and flow separation, the simulation and flow behavior pattern is modeled and validated by computational fluid dynamics software. For this purpose, the RNG K-ɛ model was used for flow turbulence modeling and second-order accuracy was used for discretization of convection term. The results show that the designed geometry can meet the desired performance requirements.

Incremental forming is considered as one of the rapid prototyping methods and has a high degree of flexibility and cost-effectiveness at low production volume. Meanwhile, the lack of technical knowledge has challenged the use of this method in the industry. One of the things that can help the actual usage of this process is the suitable process window; a window used to determine maximum tearing depth of the sheet with respect to the material, thickness and wall angle. In this study, firstly, the formability of low-carbon steel sheet, St12, with the thicknesses of 1. 25 and 1. 50 mm in single point incremental forming of a truncated pyramid with different constant wall angles has been investigated experimentally. Then, it is compared with the formability of the truncated pyramid with variable wall angles under two different wall geometries. Based on the experimental results, the process windows are presented in terms of the maximum depth and wall angle and compared to each other under different circumstances. The results showed that the critical wall angle for St12 sheet in incremental forming of a truncated pyramid with a fixed wall angle differs from the pyramid with variable wall angle, but doesn’ t depend on the size of the pyramid base. The critical wall angle for the fixed and variable wall angle pyramids was obtained 67⁰ and 75⁰ , respectively. For a pyramid with a fixed wall angle, the thickness distribution of the wall is almost constant, while for a pyramid with a variable wall, it varies along the path.

Among semi-empirical dynamic stall models, the boeing-vertol model uses relatively few dependent parameters determined from experimental data. Despite its simple formulation and appropriate performance, the model does not precisely predict aerodynamic lift and moment coefficients in some geometric angles of attack. Moreover, unsteady effects of flow sequences in downstream of the cross section have not been included in the model similar to most of the dynamic stall models. In order to increasing the precision of the aerodynamic coefficients, modification and development of the boeing-vertol model is the main goal of this research witj considering the unsteady wake effects of flow sequences. Hence, unsteady aerodynamic theory based on Wagner function was used to consider the unsteady wake effects and to introduce an effective angle of attack including airfoil degrees of freedom and their derivatives for both bending and pitching oscillations. The aerodynamic lift coefficient of the Boeing-Vertol model was improved by using the introduced effective angle of attack and flow apparent mass effects. Also, a new pitching moment coefficient is introduced and is replaced in the model. The introduced aerodynamic coefficients are validated and verified by experimental data and also compared with the original model. The obtained results represnt the correction of the lift coefficient in linear region of the static lift curve, improvement of maximum lift coefficient, corresponding angle of attack and improvement of moment coefficient in boeing-vertol model. Also, the results show that the proposed formulation enhances the boeing vertol model to predict moment coefficient in dynamic condition. In addition, a parametric study is conducted to investigate the effects of reduced frequency on effective angle of attack and it is shown that while reduced frequency increases to 0. 36, unsteady wake effects on effective angle of attack of an airfoil reach to its maximum value. Moreover, for reduced frequencies upper than 0. 1, pitch axis location changes the characteristics of the effective angle of attack of the airfoil caused by unsteady effects of flow sequences.

This study aims to extract the dynamic parameters of a viscoelastic material sample. The mechanical model considered for viscoelastic material is the standard linear solid model. To extract the parameters of the model, first, a sample of the viscoelastic polymer was made. Then, it was subjected to a constant initial strain value and immediately pressure variations were measured over time. Then, using the governing relations of the standard linear solid viscoelastic model, by comparing theoretical and experimental relaxation functions, the dynamic properties of the viscoelastic material, such as storage and loss modulus, and its damping property were identified in terms of frequency. To investigate the effects of time passing on the dynamic mechanical properties of the viscoelastic material, the studies were repeated at a different time, which was ten times more than the first study. Also, the effects of constant strain amplitude on the dynamic mechanical properties and damping characteristic of the viscoelastic sample were investigated with three different levels of strain. These values for the relaxation function in the first test for the displacement of 2 mm were E(0)=523177. 2 N/m2 and after 70 days this value was equivalent to E(0)=666060. 8 N/m2. In the same test, the values of the relaxation function for the first test are equivalent to E(∞ )=458717. 9 N/m2 and in the second test, the value is E (∞ )=573029. 7 N / m2. Also, the results show that, in smaller constant strains, the efficacy of the viscoelastic material is greater in energy dissipation. In addition, the intrinsic parameter of the Young Modulus is obtained in the experimental estimate of 0. 89 MPa.