Sharif Digital Repository

An experimental investigation was conducted to determine the effect of a uniform pitching motion on a axisymmetric body projectile undergoing excursions in angle of attack. Force and moment measurements were obtained for an axisymmetric body over a range of Reynolds numbers from 6.7×105 to 3.1×106 while varying the angle of attack from -3 to 12 degrees. The experiments included measuring the aerodynamic loads over a range of reduced frequencies, k = 0.0003 to 0.003. Steady state data were acquired and were used to provide a baseline for further analysis and comparison. The model was oscillated with amplitude of 2 and 4 degrees at two different mean angles of attack of 0 and 4. Shadowgraph... 

One of the issues that today is considered by planners and system operators in power systems, especially the Iranian power grid, is the high variation and the lack of uniformity of load curves in different hours of the day. This has led to the use of all installed capacity of the country's production only in peak hours, and in the low and intermediate times, a large amount of installed capacity is out of the circuit, which means Unused capital. This problem is more or less seen in world power networks with high load variation curves. Also, the production of renewable energies has always been fluctuating, and the production of power control to improve this fluctuation is very important. This... 

Fe-29Ni-17Co alloy with commercial name “Kovar” is an alloy with low expansion coefficient. The main specification of this alloy is to maintain this property at the high temperatures that is used in sealing of glass-metal. Achieving to desirable physical properties associated with suitable mechanical properties is the purpose of related industries. However the effect of cold work and heat treatment was studied on the physical properties of this alloy, there is no report about the effect of the hot work. Thus, in this investigation, the effect of the hot work on the physical and mechanical properties of Kovar was studied. At first, slabs of Kovar alloy were cast and remelted. Then,... 

In this research, we design a compatible waveform for the purpose of high-quality synthetic aperture radar (SAR) imaging in conjunction with sparse recovery methods for image formation. The goal is to make the imaging system tolerable against the wide-band and narrowband electromagnetic interferences. Actually, we consider minimizing the mutual interference between our radar and coexisting licensed emitters and minimizing the jamming signal (unlicensed emitters) power while enforcing some constraints over the waveform features like peak-to-average-power ratio (PAPR). For the constrained optimization problem to design a proper waveform, we introduce a new constraint to the optimization... 

In the present work, the numerical simulation of 2D inviscid compressible flows by using the spectral difference (SD) method on quadrilateral meshes is performed. The SD method combines the most desirable features of structured and unstructured grid methods to attain computational efficiency and geometric flexibility. Similar to the discontinuous Galerkin (DG) and spectral volume (SV) methods, the SD scheme utilizes the concept of discontinuous and high-order local representations to achieve conservation and high accuracy. The SD method is based on the finite-difference formulation and thus its formulation is simpler than the DG and SV methods ... 

In the present study, the numerical simulation of the compressible inviscid flow around helicopter rotor is performed using the solution of the preconditioned Euler equations. Three preconditioners proposed by Eriksson, Choi and Merkel, and Turkel are implemented in two- and three-dimensional upwind Euler flow solvers on unstructured meshes. The mathematical formulations of these preconditioning schemes for different sets of primitive variables are drawn and their eigenvalues and eigenvectors are compared with each others. For this aim, these preconditioning schemes are expressed in a unified formulation. A cell-centered finite volume Roe's upwind method is used for the discretization of the... 

Compressed Sensing (CS) technique refers to the digitalization process that efficiently reduces the number of measurements below the Nyquist rate while preserving signal structure. This technique was originally developed for the analysis of vector datasets. An x ∈R^n vector is transformed into an y ∈R^m vector so that n≪m. For a sufficient number of measurements, this transformation has been shown to preserve the signal structure. Therefore, the technique has been applied to machine learning applications.2D-CS was further developed for matrices (image datasets) so that they could be directly applied to matrices without flattening. X ∈R^(n×n) is transformed into Y ∈R^(m×m) via 2D-CD such... 

In general, the output power of the gas turbines decreases as the ambient temperature increases. One of the ways to deal with such a performance loss is to use a wet compression system, which reduces the air temperature rise throughout the compressor. In this system, the water droplets are injected into the compressor inlet and evaporated throughout the stages. As a result, the work of the compressor decreases, and the power and efficiency of the whole engine increase. Most of the researchers in this field have tried to develop mathematical models to investigate the effects of this system on compressor and gas turbine performance in steady-state conditions. However, it is necessary to... 

Shear modulus and confined modulus are important parameters for static and dynamic analysis in geotechnical applications. Elastoplastic and nonlinear constitutive models has been formulized by these parameters. Cyclic simple shear apparatus had been developed by mounting bender element and ultrasonic sensor to measure continuous shear wave and pressure wave velocities for determination of G_max and M.For this purpose two new pedestals have been fabricated on which HAE ceramic disc and sensors were mounted. A pressure control panel and a hanging column system have been designed to apply high and low matric suction with high accuracy.At first two small strain tests were conducted for... 

In this investigation, nanostructure Fe3Al was produced by mechanical milling initially. Pure iron and aluminum powders were milled with suitable ratio 86Fe-14Al by weight. Milling was done up to 120 h times and with BPR 10 and BPR 20. Morphological changes, powder density and size, phase and structural changes, grain size and lattice strain were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and laser particle size analyzer (LPSA). 40 h mechanical alloying with BPR 20 led to the corresponding intermetallic compound, Fe3Al with a nanocrystalline structure. Therefore Fe3Al/ZrO2 micro and nanocomposite powders were synthesized with manufactured by milling of... 

In this study, the solution of compressible flows is performed using finite volume lattice Boltzmann method (FVLBM). A model associated with 13 discrete velocity vectors and 2 energy levels is used and the Boltzmann transport equation is solved using a cell-centered finite volume on structured meshes. The values of distribution functions on each cell faceare determined by averaging from their values at the two control points located on the center of two neighboring cells. The fourth-order Runge-Kutta time-stepping scheme is applied to discretize temporal derivative term. The second- and fourth-order numerical dissipation termsareadded to the algorithm to stabilize the solution when solving... 

Silica nanoparticles is one of the most important mineral particles. In this study, Sodium silicate, Sulfuric acid and Sodium aluminate is used in order to synthesis of the modified Silica nanoparticles. The effects of temperature, reaction time and the molar ratio of regents were tested and optimized. Synthesized nanoparticles were characterized by use of XRD, FT-IR and SEM techniques. The results show that by controlling the reaction conditions, nanoparticles with an average size of 26.2 nm is obtained. The increase of temperature and reaction time alters the structure of the particles of amorphous state to a crystalline needles. The particle surface modification process was performed by... 

The idea of improving performance of internal combustion engines has always been important for researchers and manufacturers. In recent decades, due to limited resources of fossil fuels and introduction of vehicle emission regulations, researchers have undertaken more efforts to achieve goals such as increased horsepower, reduced fuel consumption and pollutant emissions reductions. Homogeneous charge compression ignition engine is a modern and efficient design that has attracted attention of many researchers. In[[ these engines, homogenous mixture of fuel and air enters into the combustion chamber and with a mechanism of self ignition, produces more power and has less fuel consumption... 

The remote sensing is the knowledge of gaining information about an event without having direct access to it, and synthetic aperture radars (SAR) have gained spectacular attention in this filed due to their wide applications and high efficiency. The performance of SAR, which are classified in the space-borne or space-borne radars is similar to that of pulse radars. The transmitted signals in SAR are generally chirp signals, and the received signal is two-dimensional which is scattered in two dimensions of range and azimuth called as raw data. Due to relative movement between the radar base and the target point, the distance between the radar base and the target point would not be fixed along... 

In the present work, the numerical solution of 2D inviscid compressible Magneto-hydrodynamic flow is performed by using the spectral difference (SD) method on quadrilateral grids. In this numerical method, similar to the discontinuous Galerkin (DG) and spectral volume (SV) methods, the concept of the discontinuous and high-order local representations is used to achieve conservation property and high-order accuracy. In the SD method, the test function or the surface integral is not involved and thus it has a simpler formulation than the DG and SV methods. In this numerical method, two sets of structured points, namely unknown points and flux points, are defined in each cell to support the... 

Collision attack is one of the usual attacks in cryptanalysis of hash functions. One method for applying this attack is linear approximation of equivalent compress function. Finding collisions for linearized compress function, having low Hamming weight, with a large probability could provide collisions for the main compress function. For this purpose, the problem of finding collision for used compress function is modeled by that of the linearized one, having low Hamming weight differences with a large probability. The problem of finding collisions with low Hamming weight differences is equivalent to the problem of finding codewords with low Hamming weights in linear binary code. Canteaut and... 

Nowadays, neural networks are used as the main method in most machine learning applications. But research has shown that these models are vulnerable to adversarial attacks imperceptible changes to the input of neural networks that cause the net- work to be deceived and predict incorrectly. The importance of this issue in sensitive and security applications of neural networks, such as self-driving cars and medical diagnosis systems, becomes much higher. In recent years, many researches have been done in the field of making neural net- works robust against this threat, but in most of them, higher robustness has been provided on the basis of larger and more complex models. Few researches have... 

Wireless Sensor Networks (WSNs) are comprised by number of sensor nodes, which collect data and transmit them to the sink node, the battery of sensor nodes is limited and this issue appeals the attention of researchers to attempt to improve the energy consuming of sensor nodes in order to prolong the lifetime of networks. Circumstance of routing affects the lifetime of network. Researches have proved the efficiency of Clustering in routing protocols in WSNs. It is notable that the importance of CH selection is undeniable. In this thesis we have worked on the CH election and CH rotation of hybrid method, which utilizes Compressive Sensing (CS) in inter-cluster transmission. We have proposed... 

Monitoring of soil moisture plays an essential role in correct decision making and implementation of any closed loop control system in precision agriculture. On the one hand, continuous using of a lot of moisture sensors and monitoring of soil moisture via direct measurement is restricted by practical prohibitions. On the grounds that activation of a lot of sensors continuously requires communication of a large amount of information and causes high power consumption for a moisture monitoring system. On the other hand, using remote sensing methods such as satellite methods, for moisture monitoring in arbitrary spatial and time resolution for control purposes, is not practically possible. The... 

In this thesis, we investigate the problem of OFDM channel estimation exploiting channel sparsity. Due to the sparse nature of the scattering objects, most of wireless multipath channel have a sparse impulse response. Therefore it is possible to use the methods in the field of sparse signal processing for the purpose of estimating such channels for better accuracy, and efficient spectrum usage.
First of all the problem of OFDM channel estimation using scattered pilots is stated. Then the so-called IMAT methods together with the other sparse signal processing methods are used for the purpose of estimating channel nonzero taps. Furthermore, since efficient use of spectrum in...