نشریه علوم کاربردی و محاسباتی در مکانیک
سال بیست و یکم شماره 2 (بهار و تابستان 1389)

An algorithm based on the Boundary Element Method (BEM) is employed to evaluate the hydrodynamic performance of the marine propeller used in an underwater vehicle (UV). Since UVs are operating at two different working conditions (surface and submerged conditions), thus the drag force and propeller performance are different.In this paper the analysis of the hydrodynamic performance at two submerged and surface conditions is presented. From the propulsor designer view point, marine propeller should provide the highest possible thrust, minimum torque and the highest efficiency. A High Skew Propeller (HSP) of 5-bladed is chosen for running the underwater vehicle. This propeller is operating at the stern of the hull, where the inflow velocity to the propeller is non-uniform. The main dimensions of the propeller are predicted based on the given required thrust and the defined operating condition. The calculation results show that the pressure and the hydrodynamic characteristics of the HSP at open water are in a good agreement with the experimental data.It is concluded that the presented method is a practical numerical tool to obtain the hydrodynamic characteristics of the propeller for the underwater vehicles.

In this study a numerical model was developed to determine the tidal regime and its power in any location of the seas and the oceans. The methodology employed is firstly to use the measured water level data for a limited period and then to use the least square method and low pass FIR filter with a suitable cut frequency, to calculate components and phases of tides. Next by using the calculated amplitudes of tidal components the tide form number of the study area is calculated. In this way the type of tide of the area and its efficient component and astronomical period is determined. Finally using these information as well as gravity acceleration and water density the tidal power is estimated. In a case study the change of water level was measured in Bandar Abbas Coast of Persian Gulf for 29 days. Then using the developed model the tidal power in the study area was estimated. Considering this power it can be decided if devices for absorbing tidal energy can be economical to install in this area.

Successful development, evaluation, and qualification of propulsion systems such as turbojet and turbofan engines and rocket motors require the measurement of the performance and reliability of these systems. Thrust stands or testers are instruments for solid and liquid motors test. In this paper, a method for calculation of solid motors random vibrations effects on a thrust stand structure is developed. At first some Power Spectral Density (PSD) of three kinds of solid motors has been calculated by MATLAB codes. Then a typical thrust stand which has been designed based on special dynamics principles, has been modeled in Solidworks and transferred to ANSYS. In ANSYS, the modal and spectral analysis of the structure has been done. Then each motor's PSD has been applied on structure separately and maximum stress has been determined by ANSYS codes. Followed by structure fatigue life has been estimated by a formula derived from compound of Miner's cumulative fatigue damage ratio equation and Gaussian distribution curve. The results show that the amount of PSD at first frequencies has significant effect on structure fatigue life and it is not possible to reach to a conclusion about the vibrations effect, just based on maximum motor thrust.

Seamless tubular components with various cross sections, such as cylindrical, conical, and square cross section stepped tubes, are mainly produced in a tube hydroforming process. In forming those parts with this process, achieving shapes with sharp corners, requires an increase in the internal pressure. This may cause local thinning and bursting at the tube corners. The authors propose a new hydroforming die set in which the die corner filling is improved for cylindrical stepped tubes. In the new die, the forming of tube consists of two steps of bulging and final forming. In this paper, the effect of pressure path on thickness distribution and geometry of stepped tubes are studied by finite element simulation and experiment. The results in this paper illustrate that the bulging pressure and the pressure-axial feeding of the final forming stage influence the desired final geometry and that the proper selection of these parameters affects on the deformed part geometry. It is also shown that in the new die set it is possible to produce a sound part with uniform thickness distribution along the wall side.

The fuller die, which is used for elongated components, is a type of perform die employed in the closed-die forging process. In this research, using the finite-element method and the model test technique, both the required load and material flow are studied for the four-blow fullering operation. The experiments are conducted using lead as the model material and steel tools. Comparison of the numerical predictions and experimental results show a good agreement between them. For the extension of various process variables, FEM simulations are performed using ANSYS commercial software. Then, by means of linear regression and based on the finite-element results, some useful relationships are proposed for prediction of elongation, sideways spread and the required load for the four-blow fullering process. The designers can use the results of this investigation for prediction of elongation, sideways spread and fullering load instead of time consuming and expensive experiments

In this paper, Oseen differential equation and continuity equations are solved for viscous flow around a stationary sphere using Lamb relations for velocity and pressure components. It is assumed that the tangential stress is proportional to the slip velocity on the surface. Slip considerably affects the tangential and radial velocity profiles such that increasing slip at a given Reynolds number delays flow separation and inflect point creation in velocity profiles. Slip effects on total drag coefficient at lower Reynolds number are relatively limited. Increasing slip on the surface decreases the shear drag coefficient, while the radial drag coefficient increases. At full-slip conditions, shear drag coefficient is basically zero and radial drag coefficient reaches its maximum value. Therefore, it is concluded that flow around the spherical nano particle at full-slip condition is not equal to potential flow around the spherical particle.

In this paper, an analytical solution is presented to consider the effect of electromechanical coupling on the interlaminar stresses at the free-edge of an angle-ply piezoelectric laminated plate. To this end, a variational approach is used to investigate the free-edge effect in a long laminated piezoelectric plate under uniform axial strain. A comparison of coupled and uncoupled piezoelectric analysis is performed using the second-order shear deformation plate theory. The analytical solutions are discussed and compared with numerical results available in the literature. It is found that the interlaminar stresses at the free-edge are significantly higher in the coupled case for cross-ply laminates, whereas the coupling effect for the symmetric angle-ply laminate is of minor significance

In this paper, Fluent software is used to study the flow behavior of methane and air in a CNG-air mixer. Analyses were carried out on various designs to investigate the effect of engine speed on the λ. The design was optimized for throat size, number and size of the holes at the throat circumference. The prototypes were manufactured based on optimized dimensions that were obtained from CFD analyses. The experiments have been done on the prototypes in order to verify the results obtained from CFD analyses. The results show that the performance of the optimized design was good and the decrease of engine power was about 10% and the pollutants such as CO and HC in the gas mode in comparison with the gasoline mode were decreased 100% and 79% respectively.