Sharif Digital Repository

The three dimensional resonant interaction between a surface wave and harmonic interfacial waves in a horizontally infinite two-layer fluid was investigated theoretically and experimentally. Initially, both layers assumed to be inviscid, but after obtaining the final equations of the interfacial wave's amplitude, with perturbation, the viscous damping effect, was incorporated in to the interaction equations. It is shown that, surface wave can trigger four opposite- traveling interfacial waves whose amplitudes grow with time. But because of viscosity damping effect, these amplitudes will become constant and won't change with time. The frequencies of the interfacial waves are approximately... 

In this thesis, implementing parametric resonance for the purpose of improvement in sensitivity of MEMS gyroscopes is studied. Based on a parametric study on effect of each factor on the sensor’s performance, the desired values for each parameter is determined. Stability of periodic orbits is studied using Floquet Theory. In addition, three expositions are defined and proved in order to make Floquet Theory applicable for stability analysis of origin. Based on this analysis, the relation between stabilities in the system and occurrence of parametric resonance is illustrated. Due to the complexity of dynamics of a parametrically excited MEMS gyroscope, bifurcations are observed in performance... 

In this research, the dynamic of flow, dispersion of contaminant and characteristic of wake in the presence of two flexible stems of polyurethane is investigated in a hydraulic flume. The stream velocity of 2.28, 4.47 and 7.58 cm/s are considered, which are equivalent to Reynolds number in stem diameter scale 173, 340 and 576. The spacing ratio of two stems are one, two and three in both transverse and longitudinal direction. Velocity fields are measured using Particle Image Velocimetry (PIV) technique. The size of wake area behind stems is determined using the flow field measurement. The results show that the spacing ratio l/d affects the wake area size and interactions of wakes. The... 

Surface plasmons are the result of a resonant interaction between the waves and the mobile electrons at the interface between negative and positive permittivity materials stimulated by incident light. Due to their specific quantum mechanical nature and zero electrical resistance, superconductors are capable of supporting low-loss plasma waves with extremely large propagation length. In this study, palsmon confinement in some superconducting waveguide structures has been measured by computing the effective index parameter. Superconductors are classified in to high-Tc an low-Tc categories. It is not possible for plasma waves to propagate in low-Tc superconductors, because the plasma energy... 

Vertical milling machine is one of the most common machine tools for precision parts production. The static deflection of the machine tool and thus the displacement of the tool, is due to the high machining forces is the most important factor in reducing the dimensional Precision of the work piece. Also, the overlap of the frequency range of operation with the natural frequencies causes an undesirable resonance phenomenon. Reducing the strain energy of the column and thus the reducing the displacement of the tool, and increasing the first natural frequency of the milling machine, given that the frequency range of operating is below the first frequency, is a desirable change that can be... 

One of the special regimes of a wind turbine transformer operation is its repetitive connection and disconnection from the wind turbine (or the grid) via a short length of cable. Energizing the transformer will stress the transformer in two ways: the passage of inrush currents, and the initiation of possible high frequency resonances in the windings. In this thesis, first the 3D-wound core transformer design is investigated as a mean for reduction of the transformer inrush currents drawn from the grid. It is shown that this desirable performance is due to H-balance equation in this type of core and therefore the transformers of this type preserve a suitable response under energization to be... 

Flexible vegetation has a significant impact on the small-scale hydrodynamics of aquatic environments, which governs the dispersion of dissolved and particulate material. Flexible canopies bend and oscillate in both the in-line and cross-flow directions due to periodic forcing associated with vortex shedding. The resultant plant motion affects the vegetation wake structure and, hence, the rate of lateral dispersion in these environments. Despite a recently-developed understanding of dispersion in rigid canopies, a framework that can predict the effect of plant-induced vibrations on mixing in flexible canopy environments is still lacking. Here, we investigate the role of the flexibility of... 

The theory of resonant wave interaction has found considerable applications in meteorology, oceanography and limnology. It is known that the mechanism serves as an effective means of energy transfer in the atmosphere, oceans and lakes. In this project, the three-dimensional resonant interaction of a surface wave with two pairs of interfacial waves in a stratified fluid is studied theoretically. Witham’s variational method is formulated in order to apply to weak resonant interaction among waves whose amplitudes and phase angles vary slowly with position and time. The method is applied to a surface-interfacial wave interaction in a horizontally infinite two layer fluid. Combination of two... 

Hydrodynamics of flow is affected by physical and biological interactions between flow and aquatic vegetation. This study represents the role of array distribution on lateral dispersion in emergent aquatic vegetation using two flexible and semi-rigid models. Experiments were carried out with three different arrangements, including random, staggered, and in-line. The Reynolds numbers at the stem scale ranged from 120 to 690, and the stem densities were 1% and 1.44%. Furthermore, the effects of stem flexibility and, specifically, stem resonance on lateral dispersion were examined. An image processing technique was used to measure plume width and calculate lateral dispersion. It was found that... 

This experimental study investigates the interaction between surface waves and flexible stems in aquatic environments, focusing on the case where the wave frequency is close to the natural frequency of the stems. Aquatic plants have significant structural and ecological effects on aquatic environments, inducing various changes in the flow passing through them. In this research, a polyurethane stem model is used, which is submerged and randomly placed in water. The oscillation amplitude and pollutant dispersion are investigated. The experiments were conducted at four frequencies of 0.7, 0.83, 1, and 1.125 Hz, two wave heights of 2 and 3 cm, and two densities of 0.8 and 1.2%. Image processing...