Sharif Digital Repository

The method of multiple scales is one of the common perturbation techniques for exploring nonlinear ordinary differential equations. Sitnikov has presented a mathematics model of a negligible mass body oscillation perpendicular to a plane in which two heavy bodies with equal mass orbit on itself Keplerian ellipse. This problem is well known as the Sitnikov problem. In this investigation, the method of multiple scales is applied to the Sitnikov problem and it presented an analytical response that is consistent with numerical solution. At first step, the circular form of the Sitnikov equation is approximated by MMS and then the obtained dynamic model from first step is employed to investigate... 

Abstract: Underwater vehicles when yawed to free stream flow, like submarines in a turning maneuver, produce large vortical separated regions. This vortical flow affects acoustic, body drag, control effectiveness and maneuverability. A suitable way to reduce the effects of this separated flow is to use vortex generators. The main goal of this study is to investigate the effect of the vortex generator on the flow field around a standard underwater model employing the vortex generators by using the oil flow visualization method in wind tunnel only in yaw direction (0° ≤ β ≤ 30° angles of attack, due to the experimental cost) and the CFD method in the pitch direction (0° ≤ α ≤ 30° angles of... 

In-flight aircraft center of gravity (COG) position estimation is investigated in this study based on the kinematics approach. The Quad-M basics of system identification requirements are carefully investigated for time-invariant and time-varying COG estimation during airdrop maneuver as a case study that contains both conditions. Modeling and simulation of airdrop maneuver are employed to prepare the required maneuver and measurement data for this investigation. The relative-acceleration equation, as a model structure, and parameter modeling of time-varying COG location and acceleration are introduced into the system identification and parameter estimation framework. The Kalman filter method... 

In-flight aircraft angular motion estimation based on an all-accelerometers inertial measurement unit (IMU) is investigated in this study. The relative acceleration equation as the representative of a rigid-body kinematics is manipulated to present the state and measurement equations of the aircraft dynamics. A decoupled scalar form (DSF) of the system equations, as a free-singularity problem, is derived. Mathematical modeling and simulation of an aircraft dynamics, equipped with an all-accelerometers IMU, are employed to prepare measurement data. Taking into account the modeling of accelerometer error, the measurement data have been simulated as a real condition. Three extended Kalman... 

Unlike a single-body approach, modeling based on a two-body approach has been employed to prepare the required system dynamic model as a time update equation in the applied filtering technology and measurement data for the estimation process. This more precise mathematical model enabled better understanding about the dynamics of the change in the aircraft mass properties during the airdropping operation. The problem is defined as estimation of the optimal mass properties parameters for the best possible fit of the model output to the real data. The parameter estimation problem is investigated by a nonlinear filtering methodology in two sequential steps. In the first step, the single extended... 

In this investigation, a novel numerical method is proposed for materials selection. This method is based on the well known weighting factor approach while combining non-linear normalization with a modified digital logic method. The proposed mathematical functions and their applicability to the materials selection process is verified by examining two case studies in mechanical design and comparing the results with those obtained from the classical weighted property method. It is concluded that the new approach is capable of providing more reasonable selections as opposed to those obtained from the existing method. © 2005 Elsevier Ltd. All rights reserved  

Subsonic wind tunnel experiments were conducted to study the effect of tripped boundary layer on the pressure distribution in the contraction region of the tunnel. Measurements were performed by installing trip strip at two different positions in the concave portion of the contraction. The results show that installation of the trip strips, have significant effects on both turbulence and pressure distribution. The reduction in the free stream turbulence and reduction of the wall static pressure distribution deferred signified with the location of the trip strip  

In this paper, the effects of turbulence on sound generation and velocity fluctuations due to pressure waves in a large subsonic wind tunnel are studied. A trip strip located at different positions in the contraction part or at one position in the diffuser of a large wind tunnel is used to investigate the aforementioned phenomenon, and the results indicate that the trip strip has significant effects on sound reduction. The lowest turbulence intensity and sound are obtained from a trip strip with a diameter of 0.91mm located either at X/L = 0.79 or at X/L = 0.115 in the wide portion of the contraction. Furthermore, the effect of monopole, dipole and quad- rupole sources of aerodynamic noise... 

In this research, the employment of an innovative but simple and cost effective method for turbulent reduction in a low speed wind tunnel is investigated and the effect of this turbulence reduction on the Power Spectrum and FFT-Based signals that obtained from Hot-wire Anemometry is studied. These terms represents the amount of power contained in the signals. Several situations are studied in this paper. One of these situations is when there are four screens in the settling chamber of the wind tunnel and after that by adding the trip strip in the contraction region of the wind tunnel the tests were repeated. In the next stage, three screens were removed from the settling chamber of the wind... 

This paper presents a novel approach for evaluating LBP in various settings. The proposed system uses cost-effective inertial sensors, in conjunction with pattern recognition techniques, for identifying sensitive classifiers towards discriminate identification of LB patients. 24 healthy individuals and 28 low back pain patients performed trunk motion tasks in five different directions for validation. Four combinations of these motions were selected based on literature, and the corresponding kinematic data was collected. Upon filtering (4th order, low pass Butterworth filter) and normalizing the data, Principal Component Analysis was used for feature extraction, while Support Vector Machine... 

A 2-hued coloring of a graph G is a coloring such that, for every vertex v∈V(G) of degree at least 2, the neighbors of v receive at least two colors. The smallest integer k such that G has a 2-hued coloring with k colors is called the 2-hued chromatic number of G, and is denoted by χ2(G). In this paper, we will show that, if G is a regular graph, then χ2(G)-χ(G)≤2log 2(α(G))+3, and, if G is a graph and δ(G)<2, then χ2(G)-χ(G)≤1+4 Δ2δ-1⌉(1+log 2Δ(G)2Δ(G)-δ(G)(α(G))), and in the general case, if G is a graph, then χ2(G)-χ(G)≤2+min α′(G),α(G)+ω(G)2  

This paper concerns the design of a neural state observer for nonlinear dynamic systems with noisy measurement channels and in the presence of small model errors. The proposed observer consists of three feedforward neural parts, two of which are MLP universal approximators, which are being trained off-line and the last one being a Linearly Parameterized Neural Network (LPNN), which is being updated on-line. The off-line trained parts are able to generate state estimations instantly and almost accurately, if there are not catastrophic errors in the mathematical model used. The contribution of the on-line adapting part is to compensate the remainder estimation error due to uncertain parameters... 

Optimum materials selection is a very important task in design process of every product. There are various materials selection methods like Ashby's method or digital logic methods such as DL and MDL. In the present research work the Z-transformation method is proposed for scaling the material properties to overcome the shortcoming of MDL method. The results show that despite the simple scaling function used, the ranking procedure is as powerful as MDL method and even it is superior to MDL when it ranks the less important materials existing among a list of candidate materials  

Material selection for mechanical designs is an important task. Different approaches have been proposed to fulfill this job, so far. However, most of them work well with only quantitatively measurable properties of materials. Also, due to a wide range of materials available, the selection space finds a fuzzy characteristic. Therefore, here a simplified fuzzy logic approach is introduced to provide a powerful tool for easy dealing with the qualitative properties of materials and the corresponding fuzzy space. With this approach the volume of mathematics involved with the conventional methods reduces, considerably. The results show an excellent match with the Manshadi's method. © 2008 Elsevier... 

In this study, a mixture-based sulfonate-based surfactant with a novel formula capable of functioning in real oil reservoir conditions was prepared and some microscopic parameters, like its compatibility state, emulsion behavior, interfacial tension, and temperature dependency in the presence of a heavy oil sample, and two of the most popular alkalis were analyzed; then the results were compared with those of a commercial sulfate-based surfactant. The designed surfactant provided an optimum three-phase region in relatively high salinity media; however, its optimum value decreased when adding any alkaline materials; this trend was reversed for the solutions made with the sulfate-based...