lock-in

The present work investigates the nonlinear vibration of a cantilever cylindrical micro-beam subjected to voltage and fluid flow as a micro vortex generator. As the microbeam is subjected to the fluid with given velocity, in addition to the load due to fluid added mass, the lift and drag forces as the two basic flow-induced factors affecting the dynamics of the micro-beam are modelled using the Van der pol equation. The Euler-Bernoulli beam theory is used to model the cross fluid motion of the beam under nonlinear electrostatic force as result of applied voltage. The Galerkin method is used to convert the partial differential equation to regular differential equations as well as to solve the coupled nonlinear equations governing the micro-beam motion and the wake oscillation to evaluate the response of the coupled structure to a combined applied voltage and fluid flow. The effect of fluid flow on Reynolds number and fluid vortex frequency as two main parameters in creation of Lock-in phenomenon is studied. In addition to effect of different fluid velocities, response of the micro-beam to different input voltages in the presence of fluid flow is investigated and it is shown that for a given flowing fluid, applied voltage can be used to control the lock-in regime.

An important types of density currents is called turbidity currents, where the density difference is due to the presence of suspended solid particles. In this paper, 3D numerical simulation of lock-exchange particle laden density current is presented in order to investigate the hydrodynamic of the current in density stratified environment. Simulations are carried out using Large Eddy Simulation method, three-dimensional Box filter, dynamic Smagorinsky method and Van driest damping function. The obtained results in stratified case are in good agreement with experimental data. Also, the presence of stratified environment significantly reduces the current velocity, but does not have any significant effect on the sedimentation pattern. In addition, the results show that increasing the slope leads to increase in sedimentation. It was also observed that increasing the particle diameter reduces the current momentum and increased sedimentation. For more accurate representation of the particle interaction, the particle settling velocity also varies with concentration. The analysis show that assuming the variable settling velocity in the early stages of the current leads to insignificant change in the front velocity, but when the flow advances more, the faster front velocity will be predicted.

The application of computational fluid dynamics is being developed in recent years in order to evaluate the numerical impact of wind damage on high-rise buildings due to the increasing computing power of computers. With regard to the turbulent downturns around flexible, slender and long-winded buildings with relatively high Reynolds numbers, the study of aeroelastic behavior of tall buildings is essential. In this paper, the turbulent wind flow is simulated numerically with four different velocities around the high standard CAARC building. Large Eddy Simulation has been used to solve the turbulence effect in solving fluid flow equations and the response of tall buildings to wind forces is determined by solving the differential equation of motion. A two-way coupling method is used to transfer data between two areas of fluid and structural solution in each step of time. According to the results of the numerical simulation, the pressure coefficients, streamlines and instantaneous pressure field around the tall building are in good agreement with the common characteristics of the flow around the airborne objects. The critical speed corresponding to the lock in phenomenon in this problem is calculated using a Strouhal number equal to 100m/s. Also, the history of displacement of the roof of the building in the direction of the wind and perpendicular to its length have been extracted for different wind velocities and the mean and their standard deviations respectively have been calculated. The continuous increase in the range of the fluctuations of the building under the wind blowing at 100m/s is observed. This point indicates the efficiency and capability of the numerical process in detecting aeroelastic instability with a predicted speed.

Kinematic trajectory optimization of the dual-arm cam-lock parallel robot in the different lock configuration has been done in this paper. A different path has been considered for each of lock configuration. The optimal trajectory of each joint has been calculated by minimizing an objective function in whole trajectory. According to the number of redundancy in the different configurations, an initial guess of the variables have been considered. Then the initial guesses have been modified and optimum results have been obtained by using Pontryagin’s minimum principles and determining the governing initial condition on the system. According to the optimal joint variable, optimal trajectory has been obtained for each of the joints. In all of the configurations, optimal performance index has been achieved. Also the direct kinematic equations have been considered as the constraints of the system.