دکتر محمدعلی رنجبر

Aerodynamic interference is one of the phenomena that occurs when two flying objects pass near each other. In this case, the change in the pressure distribution on objects passing near each other causes a change in the aerodynamic forces. In this research, the numerical investigation of the flow field between two narrow bodies at a close distance and the pressure changes along them at supersonic speeds is done. The simulation was done in two-dimensional and three-dimensional form, and the k-omega sst was used to model the flow turbulence. The two slender bodies were placed next to each other at speeds of Mach 1.5, 3, and 4.5 and at intervals of 2, 3, and 4 times the diameter of the body. they were examined in two modes with relative motion and without relative motion. The results show that as the free flow velocity increases, the shock wave deforms from bow to oblique and is reflected between the two bodies.  Also, as the flow Mach number decreases and the distance between two slender bodies increases, the reflected shock wave becomes weaker. At Mach 1.5 the shock wave reflection between two bodies is not very noticeable, while at Mach 4.5 the shock wave is well reflected between two bodies. In the state of relative motion of two slender bodies, separation occurs at the shock wave reflection points and the number of shock wave reflections decreases compared to the state without relative movement.

The main design goal of thermal control subsystem is to keep the temperature of the satellite equipment within specified margins. Satellite experienced extreme hot and cold conditions during the mission, so it is difficult to choose the materials and surface coatings for these conditions. If design is done to deal with hot conditions, satellite power for keeping warm in cold conditions is increased and this is while supply of required power for heating the satellite in cold conditions is one of the important design concerns. Due to restrictions in space systems for power supply, the value of power for thermal control subsystem should be optimal. This paper describes the process of designing a thermal control subsystem of small satellites using a simple model. In the present study, the thermal design is done by using the optimal choice for the radiator cover. This design has been done in a way that the least amount of heater power is required. For this purpose, optimization and linear programming have been used. Simulation results show that satellite temperature in warm and cold conditions is in the range and this method significantly reduces the amount of required power heater.

In this study, an anlytical exact solution for tow-dimensional steady-state heat transfer in cylindrical sandwich panel made of graphite-epoxy and poly urethane foam is presented. these sandwich panel is cylindrical shape and simulated a ground to air rocket. the symmetry heat transfer in the longitudinal and radial directions is presented. Finding the most possible solution based on complex boundary conditions and parametric studies is one of the innovative aspects of the present study. To find the most complete solution, Matlab and Abaqus software were used. For this purpose, first the cylinder was reliably analyzed in Abaqus software and then using the formulas of Strom-Liouville theory and Fourier transform heat transfer in Matlab software and the results were compared. Finally, the temperature distribution for each composite layer with the desired fiber angle between the temperature distribution for the zero and 90 angles is presented.

An existing solution for using tactical solid fuel rockets for space purposes is to add a blast tube to the nozzle. Equipping the nozzle with a blast tube, in addition to creating space to add the required subsystems, also makes it easier to stabilize and control the system. To get the best performance from a nozzle, various changes must be made to the nozzle structure.In the present study, the existing nozzle is first analyzed from a geometric and aerodynamic point of view using numerical solution in order to achieve performance and efficiency. Then in order to add the blast tube, two new geometries of the mentioned nozzle are designed and according to the prevailing conditions, the changes in velocity, pressure and temperature of the flow through the nozzle have been studied. The results show that the addition of blast tube causes a decrease in total pressure of less than 2%, which is acceptable considering the benefits obtained.

The use of new materials with suitable mechanical and physical properties in aerial structures can be a good alternative to materials that are commonly used (aluminum alloys, composites, etc). The use of magnesium alloys for various applications and predicting their behavior in simulation software requires complete and accurate identification of the properties and mechanical parameters of the material. One of these identification procedures is digital image correlation method. In this research, the first magnesium plate is prepared in different directions of rolling and then using two-dimensional digital image correlation method, the complete distribution of strain field during uniaxial tensile test in different directions of rolling is extracted and finally strain in three directions of length, width, thickness and anisotropy coefficient in terms of effective strain were calculated. Furthermore, for the first time using the virtual field method, elastic and plastic constants such as modulus of elasticity, Poisson ratio, strength coefficient, strain hardening exponent and yield stress for magnesium sheet in different directions of rolling were calculated.

The role of the nozzle in stability and guidance is very basic and the type of nozzle design has a great impact on the performance of the missile. In the present study, in order to create stability and easier control of the missile and to use the space created to add subsystems, it is intended to add blast tube and thermal insulation and calculate its thickness. For this purpose, a sample nozzle of the existing solid fuel rocket has been considered to add a blast tube and has been numerically analyzed. Then, two new designs of nozzles with and without using blast tube are presented and Using the existing relations analytically, the thickness of the insulators in each case is calculated and The simulation is based on it. Finally, a transient two-dimensional heat transfer analysis was performed in cylindrical coordinates for points inside the nozzle shell (behind the liner and insulation). The results show that increasing the thickness of insulation to a certain extent reduces the temperature of the nozzle shell. In the convergent-divergent nozzle designed with blast tube, this value is 0.11 in the convergent part, 0.07 in the blast tube, 0.068 in the throat and 0.11 in the divergent part. The results also show that unlike the convergent part of the nozzle, in the throat, blast and divergent parts, after passing the effective insulation thickness, the passage of time no longer has a significant effect on the nozzle shell temperature.

In the present study, optimum design of a semi-active oleopneumatic shock absorber equipped with MR valve in order to use in the airplane main landing gear will be discussed. In the first phase of design, based on the force-displacement diagram design criterion and considering a maximum value of gas pressure, dimensions of the main cylinder, piston cylinder, orifice width, valve radius, and length are determined in the passive state of shock absorber. In the following and in the second phase of design, by the use of obtained information from the first phase, and according to performance criteria of MR valve and the principle of using the maximum working capacity of MR fluid without magnetic saturation of MR valve, the optimum design of MR valve in the constant volume and the active state of shock absorber will be done. In the design procedure, the effect of geometric parameters on shock absorber force is also examined and will be shown that orifice width is the most effective and sensitive geometric parameter in this regard. Optimum design of airplane main landing gear MR shock absorber based on performance criteria of MR valve and investigating the effect of MR valve parameters on shock absorber forces and magnetic properties of valve are the novelties of this research. Due to the importance and the application of MR shock absorber, the results and the achievements of the present study can be used in other industries, in addition to the airplane landing gear as a specific case study.

Shape Memory Alloys (SMAs) are a group of intelligent materials which have distinct and superior properties compared to other alloys. The stress-strain behavior of these materials involves two specific nonlinear phenomena, namely the Shape Memory effect and the quasi-elastic effect. The behavior of the alloy whilst subject to these two phenomena is such that after it enters the plastic state, the material will regain its original form. The research is based on the modeling of these two behaviors of memory alloys in the ABAQUS software using UMAT code. In this study, the simulation of buckling and post-buckling of sandwich panels have been investigated. Preparing a UMAT memory alloy properties code, enables the designer to use any ABAQUS features to analyze the behavior of smart structures made with the shape memory alloy. Also, in this research, the first buckling critical force of the first five modes for different percentages of the alloy in the shell and core have been studied, and then the effects of volumetric percentage change of alloys, distance from neutral axis, fiber angle variations, and finally stability and post-buckling behavior of the memory alloy as the result of initial strain have been investigated. The investigations have shown large changes in the shape memory alloy due to the great recovery stress during the phase shift.

When the wing of the plane is placed in the air flow direction, due to surface effects, a boundary layer is created near its surfaces.  The boundary layer phenomenon affects Airfoil’s performance and has significant effects on the lift and drag coefficients, this phenomenon leads to restrictions that prevent the increase in wing performance. Therefore, in order to achieve optimal conditions, it is necessary to control the formed boundary layer by several techniques. In this paper, by the creation a groove perpendicular to the outstanding edge of the airfoil’s NACA0012, suction flow controlled and the airplane wing performance is investigated in two-dimensional and three-dimensional models and under attack angles and different flow rates. For this purpose, the necessary simulation was carried out using the fluent software, using the K-Kl-ω three-equation turbulence model. The width of the jet (created groove) is %2.5 of the length of the airfoil chord (%2.5C), the location of the groove is %10 of the length of the chord (from the leading edge of the airfoil) and the speed of suction is considered to be half the free speed. The results show that increasing the coefficient of lift can be achieved by creating the suction flow at the wing level and reduced the drag coefficient, which causes a delay the separation of the flow Subsequently, the angles of attack and the appropriate free flow speed rate can be selected to improve the flight of the Unmanned plan.

In this research, the combustion chamber of the liquid propellant engine has been simulated numerically by the computational fluid dynamics (CFD) method. After the simulation, several methods have been proposed to improve the combustion. These include using baffle inside the combustion chamber, increasing the equilibrium ratio and the nano catalyst. In each of these methods, the combustion temperature, mass fraction of fuel and oxidizer, mass fraction of combustion products and mass fraction of pollutants were calculated and compared with a simple chamber. The results showed that using these methods increases the combustion heat by 28.36%, reduces fuel mass fraction by 27.91%, increases mass fraction of complete combustion products including water and nitrogen, and reduces NOx pollutant mass fraction by 26.85 as it is known that NOx pollutant is a product of incomplete combustion and generally reducing it results in better combustion.

Among the available coolingmethods in the combustion chamber of liquid propellant engine, regenerative cooling is widely used due to its good performance used. In this paper, a combustion chamber of liquid propellant engine is simulated numerically and effect of increasing cooling channel surface roughness on cooling performance is studied. The results showed that maximum temperature of combustion chamber wall is in the throat and by increasing surface roughness in cooling channel, the temperature in throat decrease and the heat transfer from the combustion gases to the cooling fluid increases. The temperature drop in the throat is  and  when water and liquid hydrogen is used as cooling fluid respectively. Also in the roughness height between  and  by increasing the surface roughness, temperature drop in throat increase.

To reduce the adverse effects of cross winds on the performance of dry cooling towers in steam power plants, various methods such as installation of guide vanes cascades in wind direction have been used. This experimental investigation aims at evaluating the performance of guide vanes cascades under varying cross wind directions. For this purpose, effects of angle of flow at the entrance to the guide vanes cascade on the exiting flow quality and flow rate through the guide vanes cascade have been investigated using wind tunnel experiments. Results show that at zero degree angle of flow (with respect to the vane surface), the velocity distribution at the cascade outlet is uniform, the vanes' wake is small, and drop in the total pressure is about 2%. Moreover, with increase in the angle of flow at the entrance to guide vanes cascade (in the range 30° to 60°), flow uniformity diminishes and the drop in the total pressure increases, which is indicative of degradation in the performance of the guide vanes cascade. For example, at 60° angle of flow, the guide vanes cascade can only guide 20% of the total flow rate through the cascade. The results further show that for flow angles, at which cross wind strikes the back of the guide vanes cascade, installation of the cascade produces positive effects by changing the flow direction towards the cooling tower radiators, which improves the thermal performance of the radiators.

Having a strategy for the defense industry of any country is vital because of its complexity, cost and being time consuming. The way of planning and formulation of technology development policies and the existence of a comprehensive template and plan in this regard is one of the major issues in the defense industry. Managers and planners in this field are asked questions about how to analyze the internal and environmental conditions of the industry, how to assess the impact of future developments, and How to design and implement strategic planning steps. The purpose of this research is to identify and analyze the factors affecting the capabilities of defense industries and how to use them to take advantage of opportunities and control the impact of future threats through the development of effective strategies for the development of future defense technology. This research identifies the factors influencing the growth and development of technology by conducting extensive library studies, analyzing the achievements of national security policies and exploiting existing research, and by conducting a survey of experienced experts and planners of the defense industry in Delphi, extracts effective factors on the internal capability of defense industries and identify environmental factors affecting the development of defense technology and determine their significance. Then by using the SWOT analysis, the competitive, adaptive and defensive strategies affecting the growth and development of future defense technology were explained and the position of the defense industry was analyzed in order to employ the most effective strategies.

Guide vanes are used for directing flow in the desired path and reducing losses due to change in the flow direction. Guide vanes pitch (ratio of distance between the vanes to the vane chord) and the curvature of the vanes are the main parameters affecting the outlet flow quality. In this experimental study¡ different models with different vane pitches¡ ranging from 15 to 35%¡ and different curvature degrees¡ ranging from 45 to 90º¡ were constructed to investigate the effect of the above parameters. Results show that the most uniform velocity distribution with minimum pressure drop is obtained for a 25% pitch. The results also show that with an increase in the curvature degree¡ the velocity distribution becomes less uniform while the pressure drop increases. It is also observed that the wake of the guide vane reduces as Reynolds number increases from 4×〖10〗^4 to 1.8×〖10〗^5.

Wind is one of the factors influencing the performance of the dry cooling tower. According to research، wind will decrease the efficiency of dry cooling tower about 20%. Therefore، many studies have been done to improve the performance of dry cooling towers. Although various numerical methods and wind tunnel studies have been conducted، but the data obtained has not been properly validated and therefore requires that the appropriate field research done. To study the effect of wind on the performance of dry cooling tower، it is appropriate to model airflow around the cooling tower and the entrance of the deltas. In this research، which is the field research، air flow pattern around the tower and the deltas of Montazar ghaem plant cooling tower has been evaluated. The survey results found that the flow around the cooling tower has no separation. The sectors are positioned in front and back of the wind have the most efficiency and the least efficient sectors that are tangential to the wind. There is a vortex flow pattern in the critical deltas.