The magnetohydrodynamics model is used to investigate the spatial damping in a Hall thruster beam plasma under the effect of thermal motion and electron beam density. Plasma dispersion equation is derived analytically using the normal mode analysis of the small oscillations. The dispersion equation is solved numerically to find out the damping modes in a Hall thrusters plasma. The damping length shows dependence on the radial magnetic field, beam density, ion density, electron drift velocity, collision frequency, electron and ion temperature. We investigated that the damping length increases with ion density and electron-ion temperature, whereas it decreases with the radial magnetic field, beam density, electron drift velocity and collision frequency.

Biological methods are applied to protect river banks against Waves. There are several biological plants grown nearby banks naturally and artificially that reed is the most common. Plants can damp the Wave energy and as a result, they stabilize the river banks and slopes. In the present paper based on the experimental studies and dimensional analysis, a parametric model is derived for the Wave damping by reed and transmission. To evaluate parameters of the developed model, an experimental study has been carried out. A flume with 8 meters length, 50 cm width and the height of 80 cm was constructed at the Hydraulic Laboratory of Shahid Chamran University (SCU), Iran. Along the flume, a suitable small area was considered for the plant with different populations. More than 80 experiments were conducted and in each experiment Waves with 1m, 1.5m, 2m, 3m and 3.5m Wave length were produced by and appropriate means. Based on results of the dimensional analysis and the experimental study a model is presented model is in a very good agreement with observed data. This study shows that when the bed length of the biological (reed) protection is increase the transmission coefficient will decrease nonlinealy.

This paper presents a numerical method to evaluate the system conditions in transient mode for a standing Wave thermo-acoustic engine (STAE). This method focuses on increasing the accuracy of predictions and creating a better eyesight of the operating conditions of these devices. Combining the numerical solution of transient mode with the electrical circuit analogy of standing Wave TAE, is the main idea of this work. Electrical circuit analogy of STAE is done by assuming each component of a STAE as a lumped element. The result of Combining the time-dependent numerical solution and the electrical circuit analogy is a code, which determines the condition of startup, steady periodic, and damping regime of the spontaneous oscillations in the STAE. Accordingly, a nonlinear temperature distribution is obtained along the hot core. The developed numerical approach is in line with the experimental results of a STAE. Based on this method, the temperature variations along the stack and the impact of stack material on startup time and onset temperature are numerically investigated. Additionally, the onset temperature profiles are calculated and presented comparatively for the numerical and experimental results. The startup time of spontaneous oscillations is calculated for the standing Wave system. Consequently, the best geometry that quickly reaches the sustained oscillations can be selected using this model. Examining the temperature profile during the startup and damping process highlights a temperature difference between these two processes. Observations show that using a material with lower thermal conductivity in the stack section can reduce the onset-damping temperature difference. Also, with the help of this method the startup moment of spontaneous oscillations was calculated as second in the system. The data obtained from the experimental tests and the temperature profiles resulting from the numerical solution method, showed a good agreement with each other for the onset and damping process in the system. The novel approach described here shows potential in capturing the onset-damping behavior of thermoacoustic systems efficiently.

Local site conditions have a strong effect on ground response during earthquakes. Two important soil parameters that control the amplification effects of seismic motions by a soil column are the soil hysteretic damping ratio and shear Wave velocity. This paper presents the results of in situ damping ratio measurements performed using continuous surface Wave attenuation data at a site in Semnan University campus and analysis used to obtain the near surface soils damping ratio profile. Once the frequency dependent attenuation coefficients are determined, the shear damping ratio profile is calculated using an algorithm based on constrained inversion analysis. A computer code is developed to calculate the shear damping ratio in each soil layer. Comparisons of the in situ shear damping ratio profile determined from continuous surface Wave with cross hole independent test measurements are also presented. Values of shear damping ratio, obtained using continuous surface Wave measurements, were less than the measured using cross hole tests, possibly because of the higher frequencies used in cross hole tests.

A triaxial testing system is described for measuring the Wave velocity, stiffness, damping and stress-strain behavior of layered granular materials and rock specimens under impact, cyclic and monotonic loading. The system is equipped with high-frequency GAPSENSORs (GSs) in front of target plates connected on the side of a specimen surface to measure Wave velocity, axial and radial deformations locally, LDTs to measure axial and radial deformations locally, and a load cell. Based on the first arrival of compressional Wave of the deformation time-histories under impact loading, the pulse velocity is evaluated. In addition, using a reduced deformation amplitude technique at different elevations of the specimen, damping ratio is calculated. Results indicate that measurement of the Wave velocity and damping ratio using low noise level GSs is a simple, nondestructive and reproducible method. Comparing the results of small strain shear modulus and damping ratio using local axial and radial strains measured by LDTs and GSs in the cyclic loading, with those of the proposed method, the high precision of the used innovative method is confirmed. Using this system, a continuous stress-strain relation for a strain range of 0.0001% to several % can be obtained from a single test using a single specimen.

Perforated breakwaters are newer generation of vertical breakwaters in which some how the problems of high Wave reflection and forces exerted to the structure have been solved. In this paper the effect of hole shapes of vertical perforated breakwaters on reflection of solitary Waves was investigated using numerical method. A single and double walls vertical breakwater with four different hole shapes and four different front wall porosity were considered. The breakwater was modeled in 3D using FVM method of FLOW-3D software. The free surface was modeled using VOF method while RNG-k-e was used to model turbulent effect. The results show that in single wall breakwater as porosity increases the Wave reflection increases as well so that a 40% porous wall reflects 90% of incident Waves. Furthermore the reflection coefficient has a fluctuating nature as non- dimensional ratio of B/L increases (B is width of caisson and L is incident Wave length). Moreover double wall breakwater is able to damp Wave energy up to 20 to 25% greater than that of the single wall.

In this paper, we consider a nonautonomous viscoelastic Wave equation with linear damping and delayed terms. Under some appropriate assumptions, we prove the global existence using the semi-group theory. Furthermore, for a small enough coefficient of delay, we obtained a stability result via a suitable Lyapunov function where the kernel function decays exponentially.