AEROSPACE KNOWLEDGE AND TECHNOLOGY JOURNAL
Year:2019 | Volume:8 | Issue:1 |Papers Count:10

Plasma sheath, as ion injection boundary of accelerator system, has direct effect on ion beam characteristics and consequently on ion thruster performance and lifetime. In most of numerical simulations of accelerator system of electrostatic ion thrusters, the effects of electrons on upstream plasma and plasma sheath's shape are simulated by treating them as fluid (Poisson-Boltzmann method) or charged particles. But in this study, plasma sheath are predicted using particle-in-cell (PIC) method and without any calculation for electrons. Predicted plasma sheath shows good agreement with experimental result and is more accurate; in comparison with numerical models that simulates electrons. Calculation results shows that, for specific beam current, nonconformity between test results and plasma sheath that predicted by Poisson-Boltzmann method, leads to ion beam diverges 3. 2 degrees or 18. 28 %, in comparison with the method that directly applied electron effect.

Using heat shield, especially in throat area has a significant effect on combustion chamber pressure and thermal efficiency of solid fuel engines, and so many studies have been made in this field. A precise prediction of regression in throat surface is essential to optimum design of high burning time engines. In this study, ablation of graphite nozzle in solid fuel engines is investigated numerically for a special ingredient composite fuel. Navier Stocks equations together with thermodynamic equations inside the engine as well as thermochemical and thermal conductivity equations on nozzle surface are derived and written in their suitable forms and solved to determine the regression rate of nozzle throat surface. Furthermore, a cartridge full size solid motor by polyester binder fuel was tested and the ablation rate was measured by using a 3D scanner. The experimental pressure-time and thrust-time curves were also derived and used as input data for numerical calculations. Comparison between numerical and experimental results shows a satisfactory agreement. The experimental results show that the ablation has maximum value in the inlet area, and in divergent section is approximately constant and its value is 0. 2mm. Because of important effect of ablation rate on the geometry of nozzle throat and so on the performance of the engine, the results of this study may have applicable usage in analyzing and designing solid fuel engines.

Weight loss, dimensions, and energy consumption are important issues in the aerospace industry (spacecraft and space station), which requires a high capacity cooling system and smaller dimensions. Nanofluids can play an important role in cooling systems. In this paper, natural convection of water-alumina nanofluid in a square cavity with a thin partition mounted at the middle of the cavity is studied. The cavity has different orientation angles with respect to the horizon. For the horizontal cavity, the top and bottom walls are adiabatic and the left and the right walls are considered to be hot and cold, respectively. At the center of cavity, a vertical baffle with negligible thickness is mounted. The nanofluid inside the cavity is under a magnetic field. Governing equations were discretized through control volume approach and were solved simultaneously applying SIMPLER algorithm. Based on obtained results from numerical method, the influence of pertinent parameters such as the orientation angle of the cavity, Rayleigh number, the volume fraction of nanoparticles and Hartman number on the flow field and heat transfer are investigated. The results show that maximum heat transfer occurs when the angle of hot wall with respect to the horizon is 45. Also, the existence of the baffle and increase of Hartman number reduce the heat transfer while the increase of Rayleigh number enhances the transfer of heat. Depending on Rayleigh number, the increase of nanoparticle volume fraction may increase or decrease the thermal performance.

Responses of airfoils unsteady aerodynamics have important role in rotary-wing and aeroelasticity problems. Numerical analysis of unsteady flows usually needs more time. Therefore, engineering use quasi-steady and analytical methods for solving oscillatory airfoils. In the present research, the responses of the analytical unsteady and quasi-steady methods are calculated for plunge and pitch motions at different reduced frequencies and Mach numbers. One of Mach numbers is incompressible and the other is compressible. Compressibility correction is also applied for compressible Mach number. A numerical inviscid code is developed based on central finite volume method to solve unsteady flow equations in the arbitrary Lagrangian-Eulerian formulation for moving boundary problems. The results of the quasi-steady and analytical unsteady methods are compared with numerical code results. An implicit dual time scheme is applied for time discretization in CFD code. The analytical unsteady method is chosen Theodorsen method. Results show the finite volume method is an accurate method and the analytical method is in good agreement with numerical code in incompressible flows. The compressibility correction improve the results in compressible flows.

In this study, shock wave mitigation technique was analyzed using qualitative observations of plasma discharge in Mach 2. 45 and atmospheric conditions testing. Plasma was produced in front of the aero-spike model by a 50 Hz, 50 mA, 30Kv electrical discharge and shadowgraph imaging technique at 300 frame per second and camera recording at 1000fps were used to record the qualitative results. Laboratory results show that increasing the magnetic field increases the frequency, stabilizes the glow discharge, changes the motion path of the charged particles from circular to cyclotron, improves plasma overlapping and also thickens the shock layer. Shadowgraph images at Mach 2. 45 show that combining magnetism with and increased by 7. 5 degrees at a shock wave angle and mitigates shock waves and removes the bow shock downstream of the spike. This is the most important result that indicates combining plasma and magnetism can remove shock waves at supersonic speeds and thus reduce wave drag.

Multi-wall carbon nanotubes (MWCNTs) which are mixed in polymer materials could be used as a piezoresistive strain sensor for the purpose of structural health monitoring in engineering structures. In this paper, stain sensing sensitivity of CNT-epoxy nanocomposite is presented with changes in electrical resistance. Nanocomposite sensor is sticked on the aluminum cantilever beam to apply strain on it. Initially, MWCNTs with varying content from 0. 01 wt% to 1. 5 wt% were uniformly dispersed in the epoxy matrix. Dispersion process was conducted with shear mixing device. Therefore, a smart material was created, which was suitable for strain sensing. The microstructure of the sensor was evaluated using scanning electron microscopy to characterize typical distribution of the MWCNTs inside the epoxy matrix and form conductive networks. The effect of the preparation method (type of initial mixing, curing temperature and MWCNTs weight percent) studied on the strain and electrical changes during mechanical loading. The results showed that, initial mixing of epoxy and hardener resulted in higher sensitivity of electrical changes. Also, nanocomposite was more sensitive to strains in cantilever beam when filler content of nanocomposite was closed to the percolation threshold. In addition, sample preparation at various temperatures of 80 and 100 oc showed that the samples in lower curing temperature were more sensitive to the applied strain.

Fatigue behavior of fiber reinforced composites is still difficult to analyze or understand to be determined. It is due to various parameters affecting and their complicated interactions which come from the constituents’ physical and mechanical behavior. Hence, conducting experiments and developing fatigue models are necessary in determination of fatigue behavior in many cases. On the other hand, complicated behaviors lead the application of composite materials to be accomplished with a number of experiments and/or including high safety factors in design calculations in both the process may not be cost effective. This paper introduces a new algorithm and model to determine fatigue response of damaged circular composite beam. The results are evaluated by experimental results. By using the proposed model, the number of experiments and the time needed to determine fatigue behavior of damaged circular beams are significantly reduced. To determine the constants introduced in the local fatigue damage model, cyclic tests are performed up to limited load cycles. The predicted results by the model and obtained from the experiments represent satisfactorily good agreements.

In this paper, Petri Nets tool is used to model logical operations, faults detection, controller designed and consequently significant growth in reliability for electromechanical actuator subsystem of UAV. In UAVs, actuators are used to control surfaces, therefore introducing a new way that can simulate the logical behavior and subsystem fault and control the system’ s probable errors, is beneficial. To design a controller based on Petri Nets, three stages have to be done; in the first stage, different sections and main features are modeled using Petri Nets. In the second stage, synchronization is done among resulted models, then using supervisory control in the third stage, in order to ensure the unsafe situation occurrence in the system, the all control procedure is guaranteed. In this research, analytical redundancy is used to prevent entering into unsafe conditions and process control. Results show that by using this method, the probability of fault detection increases and consequently the reliability has the same faith.

Developing aerospace products in the shortest possible time is one of the most important market competitive parameters. Meanwhile, inherent complexity of these products leads to large information cycles that increase completion time, significantly. As a result, the shortest completion time beside acceptable quality becomes an inevitable necessity. In this paper, “ Minimum-Risk Execution Plan (MREP)” is used to improve the conceptual design process of EH101 utility helicopter as the case study. The plan is formed around iteration management and reducing completion time in framework of Work Transformation Matrix (WTM). MREP is a work policy based on beginning with tasks with the highest couplings and by progressing the design process, adding the others in a gradual manner. The method can be described as the art of effective arrangement of design tasks while observing rework risk. Discrete-time simulation results show the supremacy of the proposed method over existing models in regard of completion time while the rework risk is maintained in the lowest possible level.

A small switching DC-DC power supply which provides different output level is a good alternative in navigation systems. Single inductor multi output (SIMO) converters can be good for existing parallel output configurations in these applications. In this paper, a multivariable control based on signal flow graph modelling is used to reduce the cross regulation problem of a step down single inductor multi output converter. The multi-stages operation of SIMO converters cause, to achieve the model which predicts their all behaviors, be more difficult than typical converters. By SFG method, all small-signal transfer functions can be derived. In the other hand, designing a controller to control each output independently needs an accurate model to predict all behavior of the converter. In this paper a buck/buck SIMO converter is modeled by SFG. An effort has been made to compare modelling results between SFG and state-space averaging method in a buck/buck structure. Then, a multivariable controller scheme based on SFG model is designed to eliminating the cross regulation of the outputs. Simulation results are included to show the validity of the obtained model and designed controller.