نشریه دانش و فناوری هوافضا
سال یازدهم شماره 2 (پاییز و زمستان 1401)

In this paper the flow behavior in a radial inflow turbine impeller of aero gas turbine engine has been investigated for validate its aerodynamics design. To this aim, after importing the results of the developed program for aerodynamic design and meshing, the impeller is analyzed by using the k-ω-sst turbulence model and the flow behavior around the design point is investigated. For numerical analysis, a new method which is combination of stationary and rotating frames used. To investigate the flow behavior, the turbo post processing is used. By this method and define various surfaces and lines, flow parameters along the blade and also between blades are analyzed. Moreover, off design conditions for this turbine also are obtained. Then to validate of CFD analysis, experimental data from one existing impeller is used. Finally, the results of the aerodynamic design with CFD analysis have been compared. In all this steps good agreement is observed.

In this research, the placement of the split drag control system along the length of the UAV wing and its effect on the aerodynamic coefficients are numerically investigated. This control system consists of two plates on top of each other, which, when opened, creates a pressure drag in one wing. This system is used to create a yawing moment in flying wing airplanes. Flying wing airplanes have a high sensitivity for determining the location of control surfaces due to the presence of the swept back angle in the wings and the formation of the wing apex vortex at high angles of attack in this type of configuration. Here, for the installation and positioning of the split drag control system, from a static point of view, it is necessary to install the moving surfaces of the split drag at the end of the wing, because the maximum moment arm will be in this part, which causes the production of the maximum yawing moment; However, from the aerodynamic point of view, the placement of the control surface in this range always has disadvantages due to the existence of the wing tip vortex and the wing apex vortices. Therefore, here it has been trying to place the split drag system in 3 different opening angles in 3 longitudinal positions relative to the tip of the wing and check the resulting moments in different angles of attack. The aim of this research is to increase the yawing moment and decrease the rolling moment coefficients.

In the present research, the experimental and numerical investigation of the starting process of the second throat diffuser with a parabolic nozzle containing expansion ratio of 35 has been conducted at a relatively low total pressure. This investigation uses an experimental setup known as a small-scale high-altitude test facility with compressed air as the working fluid. Using numerical simulation, the physical phenomena occurring in each stage of vacuum generation have been identified and analyzed. The results show that the process of vacuum generation in the high-altitude simulator of the second throat type with the presence of Thrust Optimum Parabolic (TOP) nozzles included four stages. In the first stage, the vacuum was gradually created by using the nozzle in the free shock separation (FSS). In the second stage, which started as soon as the transition from the FSS pattern to restricted shock separation (RSS), the vacuum generation was slow and accompanied by oscillation. Vacuum generation was gradual in the third stage, which corresponds to the beginning of the shock separation with recirculation(SSR) and continues until the end of regular reflection (RR). And finally, the last stage also coincides with the structure of expanded, under-expanded conditions, the impact of the jet exiting the nozzle with the diffuser wall, and the establishment of start-up conditions, creating a vacuum at a slower rate than in other stages.

When designing complex products such as space thrusters, accurate simulation models are needed to evaluate and improve the design during development. The implementation of these accurate simulation models is often expensive and time-consuming. Surrogate models or metamodels are simplified models of accurate and expensive simulations that can be used to reduce some computational costs during studies or design optimization. The closer the surrogate model is to the real model, the more accurate the solution and the lower the percentage of error. These models with high accuracy are called metamodels. The purpose of this article is to design the metamodel of the liquid single-propellant thruster system using the kriging method, which can predict the behavior of the model to some extent. The purpose of this article is metamodeling of liquid monopropellant propulsion system by kriging method, which can predict the behavior of the model to some extent. Metamodel and distribution diagram of design points related to each of the subjects are extracted. In addition to the mass metamodel of each of the discipline, for the injector, the metamodel related to the mass flow rate of the fuel, for the catalytic bed, Also, four Gaussian, Exponential, Linear and Spherical functions with degree two were compared for each of the meta-models in the kriging method. In this comparison, it was observed that due to the same coefficient of determination, the Gaussian function has less error than other functions and, as a result, better accuracy.

Gel propellants are promising in the aerospace industry due to the benefits of both solid and liquid propellants. Unlike their base liquid propellants, the gel propellants have a non-Newtonian behavior that complicates their behavior in the propulsion system, especially in the atomization process. In this paper, the internal flow of a pressurized swirl injector for gel propulsion was studied using computational fluid dynamics based on the power law model. The simulations were performed in 3D mode and based on the volume of fluid model (VOF) in different values of the consistency and flow index of the fluid behavior at a constant inlet flow rate. The results showed that by increasing the value of fluid flow behavior index (n), the required pressure decreases for a same mass flow rate. The required pressures of the fluid flow of 0.063 kg /s were obtained as 1.8 and 0.45 MP for n = 0.2 and n = 0.6 respectively. The results also showed that increasing the consistency index, in terms of flow pattern, has the greatest impact on the injector swirl chamber so that the length of the air core formed along the injector axis decreases with increasing the conformity index.

In the present study, the effect of using a swirl nozzle and increasing the pressure of liquid fuel injection on the performance and emission of soot, nitrogen oxide and carbon monoxide combustion chamber of a propulsion system is investigated. The proposed solutions can change the intensity of cavitation and fuel spray characteristics.The effect of each of the proposed solutions with the help of a three-dimensional numerical model in EVL Fire software, which has been validated with experimental data in each section, on the performance of the diesel engine and its emissions has been investigated.The numerical results show that the creation of swirl inside the nozzle leads to a rotational flow of the fuel injector, an increase in the spray cone angle and the turbulence intensity inside the combustion chamber. Increasing the resulting spray cone angle improves the performance of the diesel engine by reducing fuel consumption and increasing power and torque, and the production of nitrogen oxide and carbon monoxide pollutants due to a better fuel-air mixture, increased turbulence intensity and average temperature inside the chamber.Combustion is reduced to an appropriate amount. Increasing the injection pressure can also improve the performance of the diesel engine.One of the negative points of increasing fuel injection pressure is the increase in nitrogen oxide production.By increasing the fuel injection pressure from 1350 bar to 2100 bar, fuel consumption decreases by 38% and its power and torque increase. Also in this case, carbon monoxide pollutants decrease by 65% and nitrogen oxide pollutants increase by 20%.

This paper proposes a guidance law design for trajectory tracking of a leader UAV and formation flight of pursuer UAVs. According to nonlinear kinematics for both problems, the feedback linearization theory has ben used. Therefore, using this theory, the nonlinear problem has been transferred to a linear one and using the linear control theory the control parameters has been determined to achieve the desired performance. In the trajectory tracking problem, assuming a constant velocity leader, a change of the independent variable from the time to the downrange has been performed and the problem is transferred to a single-input single-output control problem, consequently, the controller is designed. In the formation flight problem, assuming pursuers with the bounded controllable velocity, we have a two-input two-output nonlinear system. Supposing no communications between pursuers and the leader, a second order sliding mode observer is utilized to estimate the required states in formation flight controller. Results show that the settling time of trajectory tracking error of the proposed nonlinear guidance law is almost 30% better than the nonlinear trajectory tracking controller based on the proportional navigation guidance law. Also, the designed formation flight has a good performance in controlling the formation flight. Also, the sliding mode observer is able to estimate the leader states even in the presence of noise.

In this paper, dynamic modeling of flexible links manipulators is discussed. The modeling approach is based on the Lagrange equations and finite element discretization method. In order to obtain the closed form of dynamic equations for flexible links manipulators, symbolic calculation in MATLAB's symbolic mathematics toolbox is utilized, then the non-linear dynamic equations of a single-link manipulator have been obtained and compared with the results presented in other references. In this study, the nonlinear effects of centrifugal, Coriolis and gravity are Also considered which is rarely studied in other contributions. Then the equations of motion are solved by the Runge-Kutta method for different levels of excitation torque. The simulation results show that at low levels of excitation torque, the linear and non-linear models have the same results, while with the increase of the excitation level, the difference between the linear and non-linear models is considerable and the size of the elastic components in the non-linear model becomes smaller.

The aim of the present study was to investigate the adhesion and thermal resistance of epoxy / Novalac glass composite. Due to the use of phenolic resins, the pre-impregnation method has been used by combining different percentages of epoxy and novac resin. The two-piece composite samples of epoxy glass and novac glass were bonded once without glue and once with glue. In the following, the effect of different compounds of two types of epoxy resins and Novalak on adhesion and heat resistance has been investigated and evaluated. Using the results of differential scanning calorimeter test and gelling time, the sintering cycle of the samples is predicted. The results showed that the presence of curing agent of both epoxy and novac resin in the composition is essential. The results of interlayer shear strength test and three-point bending showed that the presence of phenolic resin in the samples reduced the mechanical strength of the sample. In the connection of two composites of epoxy glass and Novalac glass, the presence of exon adhesive has increased the strength of the samples compared to the direct connection of epoxy by 2.48%. Finally, the results of the oxyacetylene test indicate the need for pre-baking and a gradual increase in temperature in the cooking cycle.

A way to improve the accuracy of the three-body problem model is taking into account the eccentricity of primary attractors. Elliptic Restricted Three-Body Problem (ER3BP) is a model for studying spacecraft trajectory within the three-body problem such that the orbital eccentricity of primaries is reflected in it. As the principal cause of perturbation in the employed dynamical model, the primaries eccentricity changes the structure of orbits compared to the ideal Circular Restricted Three-Body Problem (CR3BP). In this paper, the orbit dynamics of a spacecraft in the ER3BP are exploited to find periodic orbits as the spacecraft is considered to be in planar orbits around Lagrangian points. Periodic orbits are repetitious behaviors in which spacecraft orbital dynamics repeated periodically, these periodic behaviors are the main interest of this study because they are beneficial for future mission designs and allow delineation of the system's governing dynamics. Previous studies laid the foundation for finding periodic orbits or analyzing the stability of the obtained orbits in the ER3BP regime. While in this paper, at first, initial guesses for correction algorithms were derived through the well-known family of orbits in the CR3BP, then correction algorithms were used to refine the calculated orbit. Periodic orbits are portrayed and compared to previous studies and simpler models. Finally, the periodic orbits for the systems with different values of eccentricity and mass ratios are compared to determine their effect on the shape of the orbits in the ER3BP.

According to the direct relationship between the deformation due to vibration and the voltage generated at the Unimorph piezoelectric Surface, it is important to investigate the role of the geometric dimensions of these components in the voltage generated in a constant force application. In this study, in order to optimize the geometric dimensions of the piezoelectric Micro Electro Mechanic Systems (MEMS), the tool of MATLAB software genetic algorithm was used. For this purpose, first the analytical relationship governing a unimorph piezoelectric MEMS in Euler-Bernoulli model was extracted and considered as the objective function in the desired optimization. Then the variables of beam length, piezoelectric layer length, beam width, piezoelectric layer thickness and the thickness of the beam was selected as the optimal variables to maximize the product voltage in this optimization. To ensure the accuracy of the analytical relationship presented in ABAQUS software, the unimorph piezoelectric MEMS was modeled, that showed a 88% compliance between the analytical relationship and the finite element model. It was also observed that optimization using the genetic algorithm tool of MATLAB software, if the suitable population size is selected, can increase the generated voltage by up to 59% and at the same time reduce the dimensions of the unimorph by more than 50%..

Uncertainties of the parameters cause the behavior of the systems to statistical distribution and change from what is expected. Therefore, in complex and sensitive systems, such as structures of satellite carrier and satellite connected to it, it is necessary to investigate the effect of uncertainty on dynamic response. In this paper, a novel finite element model of structures of satellite carrier and satellite is simulated to take parameters uncertainties of clamp band joint into account. This model is based on a method that calculates the distribution of axial and flexural stiffness equivalent of the clamp band joint considering all of the uncertainty. Then the vibrational behavior of the structure despite the uncertainties is investigated and a method for calculating the statistical distribution of the frequency response of the complete structure is presented. Finally, the frequency response distribution spectrum of the complete structure in both longitudinal and transverse (bending) directions is calculated. The results show that the standard deviation of the distribution in the frequency range close to the normal bending frequency due to the connection of clamp band joint has the highest value, which shows the effect of local stiffness of this connection on the dynamic response of the structure in the transverse direction. Also, the standard deviation of the distribution and consequently the effect of clamp band stiffness on the dynamic response of the structure in the longitudinal direction is small and negligible.

In this research, a new method based on model-free adaptive control is used for the suboptimal guidance and control of a hypothetical three-stage satellite carrier in order to place the satellite in a predetermined orbit. So, first, a pre-designed nominal guidance is considered for the nominal model of the system. Then, by using this method, at the same time by correcting the direction to a small extent, both an instantaneous model for the system is estimated and a control policy is extracted. Correcting the guidance and determining the control policy is done by taking into account the passing of the practical assumptions governing the problem. The simulation results show the efficiency of this method, its stability and resistance to uncertainty in input and output changes for this class of control systems and objectives.The simulation results show the efficiency of this method, its stability and resistance to uncertainty in input and output changes for this class of control systems and objectives.

Man's entry into outer space and specifically the use of satellites does not have much history compared to airplanes and ships. This has caused many uses of satellites in space to be associated with various legal uncertainties. The governments with satellite technology claim to use satellites in order to strengthen their interests and national security and based on this, they have prioritized military and weapons satellites; , they want the peaceful use of space for satellites and the prohibition of weaponization of this space. The main question in the present article, which is done in a descriptive-analytical way, is that in the legal conflict of peaceful use of satellites by weaponizing outer space and emphasizing security components, which one should prevail? The result of the current research is that the principle of peaceful use of satellites in the mentioned space has gradually decreased in importance and due to this, it is no longer possible to consider peacefulness as a basic security-oriented component in the field of using satellites. and this allows other governments, including Iran, to take advantage of their security interests in outer space by complying with their requirements according to the United Nations Charter and in order to legitimately defend their security interests. It seems that as a suggestion, it is possible to refer to the necessity of COPUS, which should prepare and approve a binding document regarding countering the weaponization of space through the use of military satellites. governments to deliver