فصلنامه مکانیک هوافضا
سال هجدهم شماره 1 (پیاپی 67، بهار 1401)

In this research, the fracture mechanics analysis for the 16th stage of a V94.2 gas turbine compressor blade under mechanical and thermal stresses due to rotation, pressure and temperature distributions in the full load steady state condition is studied using ANSYS finite element software. Modelling asymmetric sector of a balde-disk assembly with a ratio of 1 to 79, applying thermomechanical loading, boundary conditions and initial conditions, locations with high stress levels on the blade airfoil are recognized. Then, using ANSYS software to model some cracks with different sizes in two specified locations on the airfoil, the stress intensity factors are calculated. Moreover, by applying Paris relation, crack growth rate with respect to the stress intensity factors are drawn to estimate remaining life in the fatigue crack growth for the cracks with initial dimensions in these two locations. Finally, doing modal analysis, the operating frequencies of the blade are calculated and the Kampbell diagram, the interacting frequencies for the blade are also specified. The obtained results show that for these two cracks locations on the pressure side of the blade airfoil, by increasing the distance along the root of the compressor blade, the remaining life of the cracked compressor blade increases, accordingly.

In this article, the method of multiple scales is presented for solving nonlinear free and forced vibration equations of flexoelectric nanotube conveying viscous fluid under a temperature field located on a nonlinear elastic foundation using nonlocal strain gradient theory. By assuming simple Euler-Bernoulli beam theory and the nonlinear geometry of Van Carmen, the differential equation governing nonlinear vibrations was derived. An electrical voltage was applied to the upper surface of the nanotube, which created the electric field conditions of the closed circuit. Finally, the effect of different parameters such as temperature changes, electrical voltage, etc. on the real and imaginary parts of natural frequencies was investigated. Also, the effect of flexoelectric coefficient on primary, subharmonic and super harmonic resonance was investigated. The results show that the flexoelectric coefficient at the primary and super harmonic resonance initially causes the hardening behavior in the system and the jump phenomenon is quite clear; But as it increases, the system shows softening behavior.

The accurate performance of the braking system in all driving conditions is significantly effective in saving the life of the car's occupants. The disk and pad braking system is regarded as one of the friction braking systems. Employing the FGM disks and pads, the improvement of heat transfer and thermal properties of the braking system is investigated in this research. In order to specify the contact temperature distribution on the work surface of the brake, a three-dimensional analytical model is considered. Using the FEM, the temperature range of the disk with the appropriate thermal boundary conditions is determined based on the effects of the pad as a heat source. Following the power law of distribution, the material properties of the brake disk components change within the thickness and the effect of disk and pad properties on the thermal analysis results is examined. As stated in this research, selecting FGM structure only for the disk and once for the disk and pad with layers of the specified materials, the heat transfer rate can be raised, and the heat damage can be minimized. The maximum temperature in the FGM structure is 324 °C; however, in the previous research, this value is 272 °C and the slope of the temperature reduction in the FGM structure is more significant compared to the non-FGM structure which indicates the capability of the FGM structure to improve heat transfer resulting from braking.

In this paper, a shell and tube heat exchanger used to transfer heat between two different fluids is simulated in three dimensions.This converter consists of a shell with 90 U-shaped tubes inside. For further heat transfer, the tubes were simulated and compared once without fins and again with fins, which are produced longitudinally and integrally with the tube body.The current flowing in the shell is MIL-PRF23699 oil and the flowing fluid in the tubes is JP-4fuel.These two fluids flow in separate and opposite directions and exchange heat with each other through contact with the surface of the tubes. Using Aspen software, the design is done in such a way that the heat exchanger has a shorter length and weight to have a better and higher effect on the efficiency of the helicopter.To investigate the effect of tube geometry and oil mass flow on the rate of heat transfer between fuel and oil,simulation has been performed in ANSYSFluent program.In this simulation, a part of the whole heat exchanger is selected as the geometry and the effect of changing the geometry of the tubes, mass flow of fuel and oil on the heat transfer coefficient,Colburn coefficient, coefficient of friction and their ratio, and outlet temperature changes are investigated.The results of this simulation show that the heat transfer rate between fuel and oil for a heat exchanger with finned tubes is about 11%higher than without a fin.Also, reducing the mass flow of oil entering the shell increases the efficiency of the heat exchanger.

Tracking aerial objects using conventional 2-DOF pedestals requires excessive angular acceleration and velocity in the joints for some trajectories that pass through singular points, which is referred to as singularity. In this research, a 3-DOF redundant pedestal is proposed to solve such a problem. The redundant DOF increases the maneuverability and overall performance of the pedestal and makes it possible to pass through the conventional singularities in two-degree-of-freedom structures without the need to generate high speeds and accelerations in the movement axes of the pedestal. The challenge faced by the proposed three-degree-of-freedom pedestal is the lack of a unique solution to solve the inverse kinematics problem of this pedestal. In this article, to solve the problem of inverse kinematics of the pedestal with redundant three degrees of freedom, two optimization-based algorithms regarding limited joint velocities and limited joint accelerations are proposed and compared with the conventional quasi-inverse Jacobi method. In the limited joint velocities approach, a cost function of tracking error and joint velocities is formed and minimized. In the limited joint acceleration, on the other hand, joint velocities and accelerations are the elements that form the cost function. The criteria used to compare these methods are Integral of Squared Amplitude (ISA), Integral of Absolute Amplitude (IAA), Integral of Time-weighted Absolute Amplitude (ITAA), and Integral of Time-weighted Squared Amplitude (ITSA). The simulation results show the superiority of the limited joint acceleration method compared to the other two approaches.

In this study, sound transmission loss of a cylindrical shell made out of functionally graded materials with an arrangement of piezoelectric patches is investigated in the framework of the first-order shear deformation theory. Also, power law model is utilized to take into account material characteristics distribution along the thickness of the shell. It is noteworthy that both outer and inner piezoelectric patches are used as actuator and sensor. The structure is immersed in an acoustic medium of air and is subjected to acoustic waves with certain incident angle. The derivation of vibroacoustic equations in the form of coupled relations is realized by implementing Hamilton’s principle in conjunction with fluid/structure compatibility conditions. An analytical method is exploited to solve the coupled vibroacoustic governing equations with Fourier series. After validation study, parameter studies reveal the effects of the functionally graded index, incident angles, external electric voltage, and characteristics of piezoelectric patches on the sound transmission loss behavior of the structure in a certain frequency domain. Results indicated that with increasing the power law index, sound transmission loss decreases in the low-frequency region. Also, the applied voltage is able to improve the sound transmission loss especially in high-frequency region.

In this study, the vibrational behavior of V shaped AFM beam by supposing poly vinyl alcohol (PVA) nanofibers scaffolds with different formats and PVA concentrations as the soft samples has been investigated. Based on the importance of polyvinyl alcohol nanofibers and their application in medical sciences and industry, it seems necessary to consider an independent study on the vibrational behavior of atomic force microscope cantilevers considering polyvinyl alcohol nanofibers. After making of PVA nanofibers scaffolds with different formats and PVA concentrations by electrospinning system, the elasticity modules and adhesions of the samples have been earned in extend and retraction strokes as the most important step to study the vibrational behavior of the AFM beam. By increasing the PVA concentration, the elasticity modules increases, but adhesion decreases. Appling the extend strokes increases the elasticity modules, but decrease the adhesion. For theoretical dynamic behavior of V shaped AFM beam, Timoshenko bema theory has been applied. In the present study, the elastic modulus of poly vinyl alcohol (PVA) nanofibers scaffolds with different formats and PVA concentrations has been defined using AFM, then the vibrational behavior of V shaped AFM cantilever by supposing PVA nanofibers as the soft sample has been studied. The obtained results by FEM modeling show that increasing the elasticity modules of the samples increases the resonant frequency and amplitude of FRF of vertical movement of the beam. The results by theoretical modeling has been compared with experimental method. The results show excellent agreement.

The sled system is used to test anti-penetration structures, ejection seat, and spacecraft equipment that its technology is owned by a few developed countries. In this research, the dynamic analysis of applied forces on the system has been investigated. The effective forces on the sled include propulsive force, drag force, lift force, and friction force, which are all variable. To obtain the propulsive force according to the functional and geometric specifications of the designed sled, the grain engine design has been carried out to reach 0.85 Mach in a second. After extracting the governing equations for extracting the propulsive force, the changes of the propulsive force during the combustion time were obtained and formulated. At the next step, numerical simulation is used to obtain the lift and drag forces and after validating the numerical solution with experimental research, the values of drag and lift forces at different speeds are extracted and formulated. Next, due to sled mass variations during the combustion step, the friction force between the rail and the slipper is obtained. Finally, the differential equation and dynamic behavior of the system are analyzed. The results show that amount of friction force is negligible in comparison with other forces. Also, the highest pressure is applied to the inner part of the slipper, which can lead to wear and damage of the rail surface at high speeds and lead to the sled deviating from the rail track.

In the current research study, the large plastic deformations of similar and dissimilar single- and multi-layered metallic plates with the same areal density under repeated uniform impulsive loading have been investigated. The ballistic pendulum along a 200 mm length standoff blast tube was used to apply the blast load on the specimen. Forty testing specimen in three different layering configurations including single-, double, and triple-layered, and eight different arrangements were considered. For the same loading and experimental condition, 10 g plastic explosive was utilized and the dynamic response of structures was studied up to five consecutive loads. The experimental results indicated large plastic global deformation with thinning happening at the clamped boundary and also tearing for some experiments. The results also represented that the maximum permanent deflections of plates were increased by the number of blast loads and the progressive deflection of the plates at the center was decreased exponentially with increasing the number of blasts. Placing a thick front layer a thin back layer gives better blast performance to the tested specimen against repeated impulsive loading. The obtained result for multi-layered configurations with the same material is completely consistent with those structures made of dissimilar layers.

In this study, using Molecular Dynamics (MD) simulations, the effect of adhesion on the removal and separation of materials during the nano-scale scratching process has been studied numerically using the nano-scratching test. The virtual setup simulates the scratching of a flat substrate made of a single crystalline aluminum using a rigid conical indenter with a blunted spherical tip (radius of 20 nm) at four different scratching depths (i.e., 0, 3, 7 and 10 A) was studied. The classical Lenard-Jones interatomic potential is used to model and regulate the adhesion between the atoms of the pen and the substrate, which is 5 to 70% of the interatomic adhesion strength of aluminum atoms. This mimics the effect of the lubricant without modeling it. Although the effect of adhesion has been consistently neglected in previous studies, it has been observed that in scratches with shallower depths, the adhesive will have a significant effect on friction and wear. It is worth noting that in this study, because the effect of plasticity was the main mechanism of wear, according to experimental observations, friction and wear have a linear relationship with each other.

In this paper, a robust hybrid control approach for vibration and attitude control of a flexible rotating structure as a fully coupled rigid-flexible system is investigated. Two control algorithms as a combination of second-order super twisting-nonsingular terminal sliding mode control (SMC) considering external disturbances and system uncertainties are developed. The nonlinear dynamic equation of the motion is derived via the Lagrangian approach and the finite element method. The non-singular terminal SMC leads to high accuracy in target tracking, and convergence and the super twisting part significantly reduce the chattering phenomenon. The overall stability of the system has been guaranteed using the Lyapunov theory. The simulations in the form of a comparative study (with agility, accuracy, and high convergence criteria) show the feasibility of performing large-angle maneuvers and significantly reducing the vibrations caused by the flexible parts. Moreover, by controlling the chattering phenomenon, the system performance regarding high-frequency modes excitation is increased.

In this paper, the free vibrations of functionally graded porous beams with simple boundary conditions on an elastic foundation in a thermal environment were studied using the theory of third-order shear deformation. The properties of the material are temperature dependent and continuously change in the direction of the thickness of the beam and according to the power law distribution of the volume fraction of the material constituents. The uniform porosity distribution at the cross section is examined. Hamilton's principle was used to obtain the governing equations of motion. In order to discretize these equations, the generalized differential quadrature method has been used. Here, the effect of various parameters such as heat field type, temperature difference, power law index, porosity volume fraction, slenderness ratio and elastic foundation parameters on the natural frequencies of a functionally graded porous beam was studied for simple boundary conditions. The results, in addition to showing these effects on the thermomechanical behavior of the beam, also confirm the accuracy of the numerical method used.