Mahdi Mohseni; ???? ????????
Volume 7, Issue 2 , September 2010, Pages 69-79
Abstract
There are a number of systems in which supercritical cryogenic fluids are used as coolants or propellant fluids. In some modern military aircraft, the fuel is pressurized above its critical point and used as a coolant to remove heat from the aircraft engine. Accurate prediction of heat transfer coefficients ...
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There are a number of systems in which supercritical cryogenic fluids are used as coolants or propellant fluids. In some modern military aircraft, the fuel is pressurized above its critical point and used as a coolant to remove heat from the aircraft engine. Accurate prediction of heat transfer coefficients to turbulent flows of supercritical fluids is essential in design of such systems. One of the most challenging parts in mathematical modeling of this phenomenon is the turbulence modeling. The turbulence modeling, like other aspects of the supercritical fluid flows, seems to be highly affected by the large variations of the fluid properties. A two dimensional CFD code has been developed in this study and a number of the Low Reynolds Number (LRN) k-e turbulence models have been examined. Both flow conditions corresponding to the heat transfer enhancement and deterioration have been studied. The results appear to be quite sensitive to the choice of the turbulence model, especially in the deteriorated regime of heat transfer. The turbulence model assisting the two-dimensional numerical model of the present study to best fit the experiments has been determined for both cases of the enhanced and deteriorated heat transfer. That is while the jump in the wall temperature occurring in the deteriorated regime of heat transfer is over- predicted by the present numerical code regardless of the turbulence model used.
maryam malekzadeh; Abolqasem Naghash; H.Ali Talebi
Volume 7, Issue 2 , September 2010, Pages 81-91
Abstract
Two robust nonlinear controllers along with a nonlinear observer have been developed in this study to control a 1D nonlinear flexible spacecraft. The first controller is based on dynamic inversion, while the second one is composed of dynamic inversion and µ-synthesis controllers. The extension of dynamic ...
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Two robust nonlinear controllers along with a nonlinear observer have been developed in this study to control a 1D nonlinear flexible spacecraft. The first controller is based on dynamic inversion, while the second one is composed of dynamic inversion and µ-synthesis controllers. The extension of dynamic inversion approach to flexible spacecraft is impeded by the non-minimum phase characteristics when the panel tip position is taken as the output of the system. To overcome this problem, the controllers are designed by utilizing the modified output re-definition approach. It is assumed that only one torque on the hub is used. In particular, the assumption that all sate variables are measurable is not realistic; hence sliding mode observers is used to estimate states. Actuator saturation is considered in the design of controllers. The performances of the proposed controllers are compared in terms of nominal performance, robustness to uncertainties, vibration suppression of panel, sensitivity to measurement noise, environment disturbance and nonlinearity in large maneuvers. To evaluate the performance of the proposed controllers, an extensive number of simulations on a nonlinear model of the spacecraft are performed. Simulation results show the ability of the proposed controller in tracking the attitude trajectory and damping panel vibration. It is also verified that the perturbations, environment disturbance and measurement errors have only slight effects on the tracking and damping responses.
mohammad Hassan Djavareshkian; A. Esmaeli; A. Parsania
Volume 7, Issue 2 , September 2010, Pages 93-106
Abstract
The prediction of aerodynamic characteristics and smart flap of airfoil under the ground effect are carried out by the integration of computational fluid dynamics. Considering different types of beams, a parametric bending profile of a smart flap is designed. Cantilever beam with uniformly varying load ...
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The prediction of aerodynamic characteristics and smart flap of airfoil under the ground effect are carried out by the integration of computational fluid dynamics. Considering different types of beams, a parametric bending profile of a smart flap is designed. Cantilever beam with uniformly varying load with roller support at the free end is considered in here. A pressure-based implicit procedure is utilized to solve Navier-Stokes equations and a nonorthogonal mesh with collocated finite volume formulation is used for simulation. The boundedness criteria for this procedure is determined by Normalized Variable diagram (NVD) scheme. The procedure incorporates the eddy-viscosity turbulence model. The SIMPLE algorithm is applied for turbulent aerodynamic flows around the airfoil with smart and conventional flaps. The results of two flaps are compared for different flap length, flap angle and ground clearance. It is found that: 1- The pressure coefficient distribution in smart flap is smoother than conventional flap. 2- Lift-drag ratio in smart flap is higher than conventional flap. 3- The maximum lift-drag ratio is at flap angle 7.5?. 4- The minimum ground clearance has the highest lift-drag ratio. 5- The increasing of flap length leads to the increasing lift-drag ratio.
Ali Ashrafizadeh; A. A. Hosseinjani
Volume 7, Issue 2 , September 2010, Pages 107-122
Abstract
The classical panel method has been extensively used in external aerodynamics to calculate ideal flow fields around moving vehicles or stationary structures in unbounded domains. However, the panel method, as a somewhat simpler implementation of the boundary element method, has rarely been employed to ...
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The classical panel method has been extensively used in external aerodynamics to calculate ideal flow fields around moving vehicles or stationary structures in unbounded domains. However, the panel method, as a somewhat simpler implementation of the boundary element method, has rarely been employed to solve problems in closed complex domains. This paper aims at filling this gap and discusses the numerical solution of the Laplace equation in bounded domains via the numerical panel method. It is shown that the panel method is an efficient and accurate computational algorithm for the solution of this class of problems. Several test cases in heat conduction and internal ideal flow are presented to show that the numerical panel method can be used in closed domains regardless of the complexities in the geometry and/or boundary conditions.
edris yousefirad; Mohammad Pasandidefard Fard
Volume 7, Issue 2 , September 2010, Pages 123-129
Abstract
In many applications the flow that past bluff bodies has frequency nature (oscillated) and it is not uniform. This kind of flow has effects on the formation of vortex shedding behind bluff bodies. In this paper the flow around a circular cylinder was numerically simulated. The effects of frequency variation ...
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In many applications the flow that past bluff bodies has frequency nature (oscillated) and it is not uniform. This kind of flow has effects on the formation of vortex shedding behind bluff bodies. In this paper the flow around a circular cylinder was numerically simulated. The effects of frequency variation at inlet flow on the vortex shedding frequency were investigated. The transient Two-Dimensional Navier-Stokes equations were employed to compute the unsteady laminar free stream flow over a circular cylinder. A time series of analysis for the formations of vortex shedding behind the circular cylinder was performed under uniform (Frequency=0) and oscillated flows (Frequency=0.1, 1, 10, 60 and 100Hz) at Re=300. Then the value of amplitude for one of the oscillated flows was changed. The global quantities such as drag coefficients and Strouhal number variables were compared for the above range of conditions for both uniform and oscillated flows. Results show that by increasing the inlet flow frequency, Strouhal number increases slightly and the oscillation amplitude of drag coefficient increases considerably. However the mean sizes of drag coefficients do not vary in all computations. Also the effect of variation in velocity amplitude on vortex shedding frequency and drag coefficient value were studied.
Mohammad Reza Dr. Soltani; M. Masdari; M. Seidjafari; Kaveh Ghorbanian
Volume 7, Issue 2 , September 2010, Pages 132-137
Abstract
Extensive wind tunnel tests are conducted to evaluate surface pressure distribution of a semi span swept wing. The wing section has a laminar flow airfoil similar to NACA 6-series airfoils. The investigations are conducted at various speeds and angles of attack. Surface pressure distribution over the ...
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Extensive wind tunnel tests are conducted to evaluate surface pressure distribution of a semi span swept wing. The wing section has a laminar flow airfoil similar to NACA 6-series airfoils. The investigations are conducted at various speeds and angles of attack. Surface pressure distribution over the wing upper surface is measured for both chordwise and spanwise sections. Statistical analyses are performed on the data to realize the transition point at each chordwise section. The 3D pressure profiles are compared to the corresponding 2D results at the same conditions. Calculation of the standard deviation, SD, of time variable pressure data shows that SD increases in the transition area and then decreases again when the flow becomes fully turbulent downstream. The measured transition points are further compared with 2D computational results.
Mohammad Reza Mosavi; Azadeh Nakhaei; Sh. Bagherinia
Volume 7, Issue 2 , September 2010, Pages 139-150
Abstract
Global Positioning System (GPS) is proven to be an accurate positioning sensor. However, there are several sources of errors such as ionosphere and troposphere effects, satellite time errors, errors of orbit data, receivers errors, and errors resulting from multi-path effect which reduce the accuracy ...
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Global Positioning System (GPS) is proven to be an accurate positioning sensor. However, there are several sources of errors such as ionosphere and troposphere effects, satellite time errors, errors of orbit data, receivers errors, and errors resulting from multi-path effect which reduce the accuracy of low-cost GPS receivers. These sources of errors also limit the use of single-frequency GPS receivers due to their less accurate data. Therefore, its important to reduce the effect of errors on GPS systems. In order to cope with these errors and enhance GPS systems accuracy, Differential GPS (DGPS) method can be used. The problem with this method is slow updating process of differential corrections. In this paper, three algorithms based on Kalman Filtering (KF) are proposed to predict real-time corrections of DGPS systems. The efficiency of proposed algorithms is verified based on the collected of actual data. The experimental results carried out in field tests assure the high potential of these methods to get accurate positioning data. The results show that KF with variable transition matrix is better than other methods; so that its possible to reduce the Root Mean Square (RMS) of positioning errors in low-cost GPS receivers to less than one meter.
