Showing 15 results for Wave
N. Talebbeydokhti and M. Nikmanesh,
Volume 20, Issue 2 (4-2001)
Abstract
In this paper, longshore sediment transport in littoral zones is investigated. For investigation of sediment transport in the nearshore zone, the effects of waves, currents and topographical conditions of coast are considered. Linear wave theory was used for the investigation of the wave behaviour. Governing equations of littoral current are continuity and momentum ones. For calculating concentraion profile of suspended sediments at depth, the convection-diffusion equations must be solved.
A computer program called “PLSTP” (Prediction of Sediment Transpornt in Littoral Zones) is developed for investigation of sediment transport process in littoral zones. The finite difference method is used for solving governing equations. The results show a good agreement between this model and the measurements available.
Keywords: Littoral transport, Wave, Shore, current, sediment transport
M. A. Lotfollahi Yaghin, K. Farzad and M. Naghipour,
Volume 23, Issue 1 (7-2004)
Abstract
Similar to random sea waves, forces on the offshore structures due to waves are random. These forces can be mainly divided into two components, namely, inline forces and transverse or lift forces. The random nature of lift forces is more complicated than that of inline forces and both should be combined for design purposes. In the present paper, two different approaches have been used to determine time series of lift forces. Along these lines, the determination of lift coefficients is discussed which have then been used to obtain transverse forces and compared with experimental data. The experimental data used in this study were collected at Delft Hydraulics Laboratory on a full-scale rough vertical cylinder.
M. Latifi, M. Amani, S. M. Etrati and A. H. Sadri,
Volume 23, Issue 1 (7-2004)
Abstract
Quality control of textile products is an important stage in textile industries. To this end, the conventional method in fault detection is human inspection. In the present work, Wavelet transform was applied on images of simple circular knitted fabrics to diagnose five regular defects. The results showed that the method applied was accurate and fast in addition to being capable of determining fault position and dimensions. Therefore, the Wavelet transform method is suitable for online fault detection
H. Saeedi, M. Modarres-Hashemi and S. Sadri,
Volume 24, Issue 1 (7-2005)
Abstract
With progress in radar systems, a number of methods have been developed for signal processing and detection in radars. A number of modern radar signal processing methods use time-frequency transforms, especially the wavelet transform (WT) which is a well-known linear transform. The interference canceling is one of the most important applications of the wavelet transform. In Ad-hoc detection methods, the interference is firstly canceled and then a simple detector, like an energy detector, is
used. Therefore, we have used wavelet-based approaches to cancel the interference and then an energy detector has been employed. In this paper, it is shown that in practical cases where the performance of matched filter or near-matched filter is degraded, wavelet-based methods are more efficient. Also, we have shown that for cases where targets with slow radial velocity or one close to blind velocity are removed by the MTI filter, wavelet-based denoising has a better performance.
M.r. Soltani, O.r. Keramati, M. Dehghan Manshadi, M. Aalaei, and S.a. Sin,
Volume 26, Issue 1 (7-2007)
Abstract
An intensive experimental investigation was conducted to study the effect of vertical tail, single and twin (with different cant angles) on the flow field and the corresponding aerodynamic forces and moments of a model of a fighter A/C. Aerodynamic forces under different flight conditions and different vertical tail settings were measured in a supersonic wind tunnel. Furthermore, effects of vertical tail on the model wake at subsonic speed were investigated. In addition to the force and pressure measurements, schlieren system was used to visualize the shock formation and movement oat various locations on the model. The results show existence of a pair of symmetric vortices for the model equipped with a 22 degree vertical tail cant angle. The vortices burst symmetrically at moderate angle of attack. The drag coefficient increases with increasing cant angle at low to moderate alpha and decreases when alpha is further increased.
Y. Mirbagheri, H. Nahvi, J. Parvizian,
Volume 34, Issue 2 (1-2016)
Abstract
Grid dispersion is one of the criteria of validating the finite element method (FEM) in simulating acoustic or elastic wave propagation. The difficulty usually arisen when using this method for simulation of wave propagation problems, roots in the discontinuous field which causes the magnitude and the direction of the wave speed vector, to vary from one element to the adjacent one. To solve this problem and improve the response accuracy, two approaches are usually suggested: changing the integration method and changing shape functions. The Finite Element iso-geometric analysis (IGA) is used in this research. In the IGA, the B-spline or non-uniform rational B-spline (NURBS) functions are used which improve the response accuracy, especially in one-dimensional structural dynamics problems. At the boundary of two adjacent elements, the degree of continuity of the shape functions used in IGA can be higher than zero. In this research, for the first time, a two dimensional grid dispersion analysis has been used for wave propagation in plane strain problems using B-spline FEM is presented. Results indicate that, for the same degree of freedom, the grid dispersion of B-spline FEM is about half of the grid dispersion of the classic FEM.
M. S. Eskandarjuy , A. Baghlani,
Volume 35, Issue 2 (2-2017)
Abstract
In this paper, wave propagation method was applied to detect damage of structures. Spectral Finite Element Method
(SFEM) was used to analyze wave propagation in structures. Two types of structures i.e. rod and Euler-Bernoulli beam were
modelled using spectral elements. The advantage of spectral finite element over conventional Finite Element Method (FEM), in
wave propagation problems, is its accuracy and lower computational time. Two examples of rod and Euler-Bernoulli beam with
embeded concentrated mass were presented to illustrate the superiority of SFEM to FEM. Finally, a cracked beam was modeled
and analyzed using spectral finite elements and the location of the crack was determined using time history response of beam
structure.
R. Ghiasi, M. R. Ghasemi, M. R. Sohrabi,
Volume 36, Issue 1 (9-2017)
Abstract
Utilizing surrogate models based on artificial intelligence methods for detecting structural damages has attracted the attention of many researchers in recent decades. In this study, a new kernel based on Littlewood-Paley Wavelet (LPW) is proposed for Extreme Learning Machine (ELM) algorithm to improve the accuracy of detecting multiple damages in structural systems. ELM is used as metamodel (surrogate model) of exact finite element analysis of structures in order to efficiently reduce the computational cost through updating process. In the proposed two-step method, first a damage index, based on Frequency Response Function (FRF) of the structure, is used to identify the location of damages. In the second step, the severity of damages in identified elements is detected using ELM. In order to evaluate the efficacy of ELM, the results obtained from the proposed kernel were compared with other kernels proposed for ELM as well as Least Square Support Vector Machine algorithm. The solved numerical problems indicated that ELM algorithm accuracy in detecting structural damages is increased drastically in case of using LPW kernel.
M. Moradi, M. Bagheri Nouri,
Volume 36, Issue 1 (9-2017)
Abstract
In order to obtain transmission spectra through a phononic crystal as well as its waveguide, a new algorithm is presented in this paper. By extracting displacement-based forms of elastic wave equations and their discretization, Displacement- Based Finite Difference Time Domain (DBFDTD) algorithm is presented. Two numerical examples are solvcd with this method and the results are compared with the conventional Finite Difference Time Domain (FDTD) method. In addition, the computational cost of the new approach has been compared with the conventional FDTD method. This comparison showed that the computation time of the DBFDTD method is 40 percent less than that of the conventional FDTD method.
Sh Rezaei, M Eskandari-Ghadi, M. Rahimian,
Volume 36, Issue 1 (9-2017)
Abstract
The acoustic wave velocity depends on elasticity and density at most materials, but because of anisotropy and especially piezoelectric coupling effect, the acoustic wave propagation at piezoelectric based crystalloacoustic materials, is an applied and challenging problem. In this paper, using modified Christoffel's equation based on group velocity concept, the effect of anisotropy and piezoelectric coupling at different wafers of lithium niobate crystalloacoustic (strong anisotropy) on acoustic wave velocity (semi-longitudinal, semi-vertical transverse wave and semi-horizontal transverse wave) is investigated, and validated by experimental data. Then, the acoustic wave velocity ranges that can be supported are determined. The result of this study can be essential at acoustic metamaterials design, Phononic crystal and piezoelectric based wave-guides.
M. Ettefagh, H. Mirab , R. Fathi,
Volume 36, Issue 2 (3-2018)
Abstract
One of the new methods for powering low-power electronic devices employed in the sea, is using of mechanical energies of sea waves. In this method, piezoelectric material is employed to convert the mechanical energy of sea waves into electrical energy. The advantage of this method is based on not implementing the battery charging system. Although, many studies have been done about energy harvesting from sea waves, energy harvesting with considering random JONWSAP wave theory is not fully studied up to now. The random JONSWAP wave model is a more realistic approximation of sea waves in comparison of Airy wave model. Therefore, in this paper a vertical beam with the piezoelectric patches, which is fixed to the seabed, is considered as energy harvester system. The energy harvesting system is simulated by MATLAB software, and then the vibration response of the beam and consequently the generated power is obtained considering the JONWSAP wave theory. In addition, the reliability of the system and the effect of piezoelectric patches uncertainties on the generated power are studied by statistical method. Furthermore, the failure possibility of harvester based on violation criteria is investigated.
F. Shirmohammadi, M. M. Saadatpour,
Volume 37, Issue 1 (9-2018)
Abstract
In this article spectral modal method is developed for studying wave propagation in thin plates with constant or variable thickness. Theses plates are subjected to the impact forces and different boundary conditions. Spectral modal method can be considered as the combination of Dynamic Stiffness Method (DSM), Fourier Analysis Method (FAM) and Finite Stripe Method (FSM). Using modeling of continuous distribution of mass and an exact stiffness causes solutions in frequency domain. Unlike the most numerical methods, in this method refining meshes is no longer necessary in which the cost and computational time is decreased. In this paper the important parameters of the method and their effects on results are studied through different examples.
S. M. Zandi, A. Rafizadeh,
Volume 37, Issue 1 (9-2018)
Abstract
In this article, a meshless method based on exponential basis functions (EBFs) is presented to simulate the harmonic waves with moving free-surfaces generated by the piston-type wave maker. Accordingly, velocity potential is adopted in a Mixed Eulerian-Lagrangian (MEL) approach. Boundary conditions are met through a point-wise collocation approach. In order to update the geometry in the simulation time, the free surface points are only moved vertically. To reduce the reflection in the wave flume, a damping zone is added at the far end opposite to the wave maker, where the velocity is modified by adding an artificial damping term. The results indicated the ability of this numerical method in simulating free surface flow problems like non-linear waves with a good accuracy, as well as suitable performances and the least run time calculation.
R. Ghiasi , M. R. Ghasemi ,
Volume 39, Issue 1 (8-2020)
Abstract
This paper focuses on the processing of structural health monitoring (SHM) big data. Extracted features of a structure are reduced using an optimization algorithm to find a minimal subset of salient features by removing noisy, irrelevant and redundant data. The PSO-Harmony algorithm is introduced for feature selection to enhance the capability of the proposed method for processing the measured big data, which have been collected from sensors of the structure and uncertainties associated with this process. Structural response signals under ambient vibration are preprocessed according to wavelet packet decomposition (WPD) and statistical characteristics for feature extraction. It optimizes feature vectors to be used as inputs to surrogate models based on the wavelet weighted support vector machine (WWLS-SVM) and radial basis function neural network (RBFNN). Two illustrative test examples are considered, the benchmark dataset from IASC-ASCE SHM group and a 120-bar dome truss. The results indicate that the features acquired by WPT from vibrational signal have higher sensitivity to the damage of the structure. Furthermore, the proposed PSO-Harmony is compared with four well-known metaheuristic optimization algorithms. The obtaind results show that the proposed method has a better performance and convergence rate. Finally, the proposed feature subset selection method has the capability of 90% data reduction
F. Shabani, M. Saghafian, D. Saeidi, F. F. Momennasab ,
Volume 39, Issue 2 (2-2021)
Abstract
Particulate separation has many applications in medicine, biology and industry. In this research, the separation of polystyrene particles with a diameter of 10, 20 and 30 μm in the fluid flow of a microchannel is investigated. The microchannel consists of a spiral region and a straight region under the influence of acoustic waves. In the spiral region, the particles under hydrodynamic effects undergo the initial separation; then the particles enter the straight region of the microchannel, and the final separation of the particles is done by the force generated and exerted through the acoustic waves. The effects of acoustic frequency and the number of spiral region loops on separation are investigated. The results show that for measured dimensions and parameters, at 1 MHz acoustic wave, when the number of loops is 2 for the spiral region, the particles at the end of the path are in a suitable position for separation. In addition, the results show that the separation of particles with this hybrid system is better than that done by the simple methods, and the separation rate can be as high as 100%
