جستجوی مقالات مرتبط با کلیدواژه « نسبت منظری » در نشریات گروه « مکانیک »

Three airfoils NACA0015, NACA0018 and NACA0021 were selected and in the Q-Blade software, the coefficients of shear, post and the maximum ratio of the exponential coefficients were determined and, finally, the Airfoil Nakata 0015 at a speed of 5 and 10 meters per second, was selected and best. Fluent software was used to solve it, based on the finite volume method. For numerical analysis, the turbulence method K-ω SST was validated with experimental results. The wind turbine schematics was designed in Catia software and the height of the blades was 35 and 75 centimeters, the radius of the blade was 18.5 cm and the length of the airfoil 6.4 Cm is. The results show that in order to operate the turbine of porous blade windings at 35 cm height at speeds of 1, 2, 3, 4, 5, 7, 7.45, 8.25, 8.5 m / s 50% 50%, 33%, 50%, 50%, 60%, 57%, 55%, 50%, and for setting up a porous blade wind turbine at a height of 75 cm at a speed of one, two, three, four, Five, seven, 7.45, 8.25, 8.5, 9, 9.5 m / s 66.6%, 75%, 80%, 71.4%, 66.6%, 76.9%, 80%, 82%, 89%, 100% of the launching force Smooth wind turbine is required at the same height.

In the present work a three-dimensional simulation of the slip flow regime for a small-sized channel of a fuel cell with lower porous wall. This study is used to determine the variations of average velocity, pressure difference, and friction coefficient considering the penetrated mass from porous wall, the wall and channel cross-sectional area, and aspect ratio of channel. Furtermore, the results are comprised with Continuous flow regime. In this solution, the equations of mass and momentum for continuous and slip regimes evaluated with developed Fortran code, and Validated with papers.The results with different channel area for two fluid regimes are obtained. Considering the aspect ratio equal to one, the friction coefficient average velocity, and the pressure difference are increased with decreasing at Knudsen number. Considering the slip regime into the channel, the friction coefficient, pressure difference and average velocity values are calculated more accurate (at best 5%), than non-slip regimes. Furthermore, when Knudsen number and aspect ratio are respectively equal to 0.001 and 5, changing the flow assumption from non-slip regime to slip one is occurred to increasing the expected pressure difference and average velocity.

The lubricant's ability to maintain the dynamic stability of rotor particularly in special conditions such as operating at critical speeds and instantaneous turbulences in loading or lubricant properties is always one of the most prominent characteristics of the journal bearings. Aspect or length to diameter ratio of bearing is an important factor that in different loading conditions will have an obvious effect on the performance of the trapped lubricant film between the rotor surface and bearings shell. So, the effects of aspect ratio on the damping of rotor disturbances with linear and nonlinear dynamic analysis approaches are studied in this research. Initially, the static equilibrium point of the rotor center in noncircular two, three and four lobe bearings space is obtained using the governing Reynolds equation of micropolar lubrication for different values of aspect ratio. Later, assuming the rotor perturbation as the limit cycle oscillations around the equilibrium point, critical mass and whirl frequency ratio are determined as the linear dynamic stability indexes for recognizing the converging disturbances. In nonlinear analysis model, the simultaneous solving of the lubrication and the rotor motion equations in successive time steps with Runge-Kutta method is done to differentiate the converging or diverging rotor perturbations. Results show that decreasing the aspect ratio improves the stability and the chance of controlling disturbances and returning the rotor center to static equilibrium position. Comparison of linear and nonlinear dynamic analysis results also indicates more cautious behavior and limited stability range of linear model in most of investigated cases.

In this study, a microgenerator is designed to supply the energy needed for electrical circuits of a MAV using piezoelectric materials. For this purpose, a composite airplane wing including all structural elements such as the ribs, spars and skins was designed in COMSOL multiphysics software. On the spar of this wing, a piezoelectric piece is modeled. The wing is modeled as a cantilever beam that its end is excited in an oscillatory manner with given frequencies and amplitudes. During the oscillation, the stress and strain of the wing elements are obtained using the finite element method and the amount of the generated voltage is calculated by coupling the piezoelectric governing equations with the strains. Next, an experimental model is created with the same characteristics of the numerical model and tested. The results of the numerical solution are compared with the results of the experimental tests for the verification. Afterwards, the effects of parameters such as the aspect ratio, the size of piezoelectric materials and the spar thickness on the generated voltage are studied. Finally, the results have been discussed.

In this research, the plunging motion of wing in unsteady incompressible and laminar flow is simulated by a numerical method based on finite volume technique and using dynamic mesh. The effects of amplitude, reduced frequency and aspect ratio on the trust generation in MAVs and Fishes Reynolds number region, are investigated. For simulating this oscillating wing, dynamic mesh with rigid body motion is applied. The simulation results are compared with published data to confirm the validity of this research. In this simulation, the effect of reduced frequency, amplitude, their simultaneous effect of them and the effect of aspect ratios investigated. These simulations are in Reynolds 3000 and some aspect ratios, reduced frequencies and amplitudes. This simulation results show that by increasing reduced frequency and amplitude, relative angle of attack increases, which increase pressure deference around the wings. Results showed that increase in reduced frequency is of more importance than the increase in the amplitude because of more acceleration. Low aspect ratio of the wing has negative effect on the production of trust force due to backflow of wing tips.

In the present paper, a numerical simulation is performed to analyze aspect ratio effects of a rectangular stirrer along with two other parameters, frequency and amplitude of stirrer on the mixing efficiency inside a straight microchannel by LBM. Results showed that in a low frequency amplitude, AR variations are not effective on the efficiency in low values of St, however, in the intermediate values, the efficiency increases with the increase of AR. Moreover, in high values of St, the rise in AR up to 0.5 leads to the efficiency reduction, but further increase of AR, increases the mixing efficiency too. In a larger amplitude than the previous case, in high values St, increasing AR up to 0.7 rises efficiency, but further increase of AR, decreases the mixing efficiency. In low and intermediate values of St, the mixing performance in low values of AR has been more efficient than high values of AR. Furthermore, it was shown that in a low frequency, the mixing efficiency decreases and then increases with rise in AR for all values of K so that the closer to 1 the AR, the better the mixing efficiency. In a higher frequency, the efficiency decreases and then increases with the growth in AR in small value of K, while this trend become inverse in large values of K. Generally, in intermediate values, the larger and nearer to 1 the AR, the larger the mixing efficiency.

In recent years, design of small aircrafts are studied and extensively developed. One of the most important of these aircrafts is micro aerial vehicles (MAVs). As part of aerodynamic design of MAVs, the wing design is important. In fact, the main part of this design is to find an appropriate shape for the wing. In this work, influence of the wing shape at low Reynolds number flow is investigated using several planforms. Results show that as aspect ratio decreases, the lift-curve slope decreases as predicted by aerodynamic theory. The performance of the inverse Zimmerman and rectangular planforms are higher than elliptic and Zimmerman planforms. Also, the inverse Zimmerman planform is less influenced with the side flow. Effects of the Reynolds number and the wing edges on aerodynamic coefficients are discussed. The CFD computations are performed, and verified with the experimental data and aerodynamic theory.

Carbon nanotubes (CNTs) demonstrate unusually high stiffness, strength and resilience, and are therefore an ideal reinforcing material for nanocomposites. However, much work has to be done before the potentials of CNT-based composites can be fully realized. Evaluating the effective material properties of such nanoscale materials is a very difficult tasks. Simulations using molecular dynamics and continuum mechanics models can play significant roles in this development. Currently, the continuum approach seems to be the only feasible approach for such large scale analysis. In this paper, effective mechanical properties of CNT-based composites are evaluated using a square representative volume element (RVE) based on the continuum mechanics and Finite Element Method (FEM). Formulas are derived based on the elasticity theory to extract the effective material constants from solutions for the square RVEs under two load cases. Next, CNT aspect ratio effects on the nanocomposite mechanical properties are investigated. Results indicate that increasing CNT aspect ratio results in an increase in nanocomposite longitudinal modulus and a decrease in nanocomposite transverse modulus. Also, increasing the CNT aspect ratio resulted in a decrease in nanocomposite Poisson’s ratio in the x-y plane and an increase in nanocomposite Poisson’s ratio in the x-z plane.

In this work a rectangular micro-heat exchanger with constant hydraulic diameter at various aspect ratios was considered and its pressure drop and thermal parameters versus pumping power in laminar flow at constant heat flux of 0.3 MW/m2 were analyzed and compared. Also, the effect of temperature-dependent fluid properties (viscosity and conductivity coefficient) on the pressure drop and thermal resistance of micro heat exchanger were investigated.Finally, a new criterion, based on pumping power and transmitted heat per temperature gradient, was proposed as a tool to optimize the aforementioned micro-heat exchanger.