فصلنامه مکانیک هوافضا
پیاپی 1 (امرداد 1384)

In this paper, industerial falling film evaporators, which are used in sugar industries, are investigated. The heat transfer rate along the tube is computed by analytical as well as experimental methods. A computer program is provided to calculate the tube length. All important and relevant parameters of falling film evaporators are considered. The existing results are compared and confirmed with the standards. Then, the optimized tube length is compared with experimental results. Also, the static analysis is performed using ANSYS software and the mechanical strength of the tube structure is determined.

Powder suspended dielectric is one of the newest techniques for improving the effeciency of electro dischage machining. In this research, by adding Aluminium powder in less than 25mm size in dielectric, a variaty of machining parameters, like Gap size, material removal rate, and surface roughness were investigated and compared in finishining and semi-finishining regimes in two states of with powder and without powder. The results show that, due to adding powder to dielectric, a considerable improvement in quality machined workpiece is achieved.

One of the most suitable measurement tools in wind tunnels is the balance, on which strain gauge bridges are installed to suitably measure the applied strains in dynamic and static situations. This article describes a five component dynamic balance and its stimulation system and investigates their error sources when a standard dynamic model is installed on it. It calculates the system frequency response based on the practical tests to select the appropriate sampling frequency for the data acquisition system. In addition, it presents the design of a suitable filter to remove the noise from the original signal. It also presents a signal quality metric based on the minimization of the in-band noise to optimize the filter. This metric is also used to calibrate the system frequency and detect the signal phase and amplitude variations. The practical measurements in 1.5 Mach number on vertical force of the standard dynamic model show significant noise, phase, and amplitude variations, which indicate the side rotations of the model and the instability of the air current on the model during the test.

Nowadays, a great deal of efforts are being carried out to reduce NOx emission in gas turbines. NOx emission depends closely on the flame temperature. Water or steam injection reduces the pick temperature and NOx emission. In this paper, after modeling the combustion chamber, steam, with steam to fuel ratios of 0.5 and 1, is injected into the chamber and its effect on the NOx emission reduction is studied. The FLUENT software is used for solving the related reactive flow. It is concluded that, NOx emission decreases considerably, but CO emission increases.

Friction drag reduction in axisymmetric bodies with heating the surface in the turbulent boundary layer is investigated experimentally and numerically. For wind tunnel test, an axisymmetric model, nearly like an airplane body, with a length of 0.5 m and taper ratio of 10 is manufactured which, the quarter length of its tip and end has elliptical form and the others are flat. Also, axisymmetric form of the thin layer Navier- Stokes equations, using Baldwin-Lomax turbulence model is applied for the numerical simulation. Optimization of the heating area is performed by dividing the flat section of the model to five equal segments and therefore, the effects of temperature variation in various segments and numbers of heated segments are investigated. The experimental and numerical results show that, heating the first segment to, causes 24.4 (Exp.) and 23.8 (Num.) percent reduction in drag force, which show good agreements. The maximum drag reduction occurs in concentrated heating.

A turbulent supersonic flow over bodies of revolution, including the base flow, is investigated using multiblock grid to solve the thin layer Navier-Stokes (TLNS) equations. Patched method has been used near the interfaces. Our numerical scheme was implicit Beam-Warming centeral differencing, while Baldwin-Lomax turbulence modeling was used to close the Reynolds averaged Navier-Stokes equations. The results of this work have been compared with existing computational and experimental benchmark data and shows close agreements.

In this article, turbulent flow and heat transfer in a rotating duct is simulated numerically. The duct rotates about an axis perpendicular to its axis and causes secondary flows resulting from Coriolis and centrifugal forces. For rotation, the standard turbulent models are not able to predict the flow and heat transfer correctly. Therefore, modified turbulent models, including rotational effects, are implemented in the momentum and energy equations. The numerical results obtained for different models indicate that, Wilcox model is closer to the experimental available data.

The monitoring of tool wear is one of the important methods to improve the dimensional accuracy and economic aspects of machining. To achieve this, instead of measuring the tool wear, other parameteres, which are related to tool wear are used. In this paper, an intelligent system, based on neural networks, is presented. By this method, the tool wear is estimated on-line with measuring spindle motor parameters, such as current and the speed of motor. Thus, the current and the speed of motor in different condition of machining (feed, depth of cut and rpm of tool) and wear were measured with practical experiments and the effects of tool wear on current and speed of motor is analized. Based on the results, a back propagation (BP) neural network is developed and trained. Using this network, the tool wear could be estimated while machining in different conditions, measuring current and speed of motor. This system could be used to control and monitor the machining process.

Although the sliding mode controller usually results in an acceptable performance, chattering phenomenon may cause practical difficulties for actuators, particularly for on-off types as is the case in space applications. In this paper, a regulated sliding mode control law to fulfill stability requirements, robustness properties, and chattering elimination is proposed. Due to the activity of the regulating routine, proper positive values for the coefficient of sliding condition are determined. To this end, first the rotation dynamics of a typical satellite described in body coordinates is derived in terms of Euler quasi coordinate. Next, focusing on the chattering phenomenon, a new approach is proposed to alleviate the chattering trend. In order to set free the actuators from permanent activity, an Error Tolerance Margin (ETM) is defined. In this method, a passive time interval is defined, during which the actuator is turned off. Also, it is assumed that only On-Off actuators are available. To evaluate the energy consumed by the actuators, a ratio named Actuator Activity Factor (AAF) is introduced. The developed control law is applied to a given satellite during a rotational maneuver in presence of parametric uncertainties and noisy feedback signals to show its ability and merits.

Aircraft design is still a complex engineering process and many decisions are made based upon qualitative choice of the designer. The use of genetic algorithm (GA) as design and optimization methodology for Aircraft can help to reduce the number of qualitative decisions. The Fajr is a super short takeoff (T.O.) and landing (SSTOL) Aircraft, which is designed to fulfill the desire for new short runways such as heliports, center-city-to-center-city travel and carrier on-board delivery (COD) military Aircraft. The objective function for the optimization was a minimization of life-cycle-cost (LCC). Results of the design methodology are discussed, which shows the ability of the GA to perform Aircraft conceptual design. In this approach best SSTOL Aircraft configuration of word, such as a Boeing YC-14 and QSRA is evaluated by GA method and designer can follow a systematic approach to find the most appropriate configuration. Using this approach, the time and the cost of conceptual design can considerably be reduced.