نشریه علوم کاربردی و محاسباتی در مکانیک
سال بیست و پنجم شماره 1 (پاییز و زمستان 1392)

In this research, the steady-state flow filed around a 3D body and rotor configuration of the ROBIN helicopter is simulated. The rotor is modeled by an actuator disk. For this purpose, an annular computational domain replaces the rotor and some modeled source terms are added to the momentum equations. This modeling of rotor considerably speed up computational time. The flow field over this helicopter fuselage is simulated for six cases of different advance ratios and thrust coefficients. The accuracy of using uniform, linear and BET distributions for the rotor in the source term is investigated. With the BET and some corrections to pitch angles, pressure coefficients and variation of thrust and power of rotor are predicted in agreement with experimental data. For the nominal thrust coefficient of 0.06, different loading methods on rotor using experimental results are compared for two advance ratio of 0.05 and 0.23. In the advance ratio of 0.23 for the upper surface of helicopter, no considerable difference among the different employed methods is observed. However, for the advance ratio of 0.05 and for the side that helicopter blades get closer to the body (Starboard side), the results with the modified methods are better than BET method. The presence of strut in the computational domain has important effects in high speeds and causes reduction of pressure coefficients over body especially in downstream of the strut.

Forming limit curve (FLC) is an important tool for evaluation of formability of sheet metals subjected to different loading. Free bulge of aluminume tubes has been investigated numerically to obtain FLC. To verify the numerical results one experimental setup has been manufactured which has the ability of controlling the internal pressure and axial feed. In this study، time and zone of bursting in tube hydroforming is predicted using seven ductile fracture criteria. For calibration of these criteria، the uniaxial tension is simulated in Abaqus/Explicit. Different loading curves are applied on tube to obtain the different points of FLC. Predicted FLCs by ductile fracture criteria have been compared with experimental results. The FLC which is predicted by Ayada'' s criterion has best match to experimental one. Results show that Prediction of all criteria are the same in strain position which is similar to uniaxial tension. Based on the results، criteria which apply the effect of average stress in prediction of dutile fracture are more appropriate for tube hydroforming process.

Forming limit curve (FLC) is an important tool for evaluation of formability of sheet metals subjected to different loading. Free bulge of aluminume tubes has been investigated numerically to obtain FLC. To verify the numerical results one experimental setup has been manufactured which has the ability of controlling the internal pressure and axial feed. In this study، time and zone of bursting in tube hydroforming is predicted using seven ductile fracture criteria. For calibration of these criteria، the uniaxial tension is simulated in Abaqus/Explicit. Different loading curves are applied on tube to obtain the different points of FLC. Predicted FLCs by ductile fracture criteria have been compared with experimental results. The FLC which is predicted by Ayada'' s criterion has best match to experimental one. Results show that Prediction of all criteria are the same in strain position which is similar to uniaxial tension. Based on the results، criteria which apply the effect of average stress in prediction of dutile fracture are more appropriate for tube hydroforming process.

In the present study، turbulent flow in channels of a plate heat exchanger with corrugated Chevron plates has been simulated numerically using the commercial CFD package، FLUENT. Unstructured mesh is used for discretizing the computational domain and SIMPLE algorithm is utilized to solve the pressure and velocity equations. There is a wide selection of different plate heat exchangers within the industry; therefore، 3D models of common corrugated Chevron plates with Chevron angles of 45-45 and 60-60 have been simulated to obtain and analyze temperature، pressure، and velocity fields of flow. The results show that geometrical parameters of the plates such as Chevron angle have indisputable effects on flow pattern and heat transfer of the heat exchanger. Finally، the effects of Chevron angle and Reynolds number (4000≤Re≤20000) on friction factor and Nusselt number of the flow were studied and compared with former experimental results. The numerical results show a good agreement with the experimental data. In addition، it was seen that increasing the Chevron angles would lead to increasing the friction factor and Nusselt number simultaneously.

In this study، the effect of suction and the parameters such as suction amplitude، suction coefficient and suction jet length on NACA0012 was evaluated. The turbulence employed model was the RSM model. Suction jet is located in 10% of the chord length. The range of suction jet entrance velocity was selected from 0. 1 to 0. 5 of freestream velocity. The maximum increase in lift to drag ratio was seen at suction amplitude of 0. 5. In addition، lift to drag ratio elevated when suction jet length increased and reached to its maximum value at 2. 5% of the chord length.

In the present study the effect of curvature ratio and coil pitch for TiO2/water nanofluid laminar flow on heat transfer behavior and pressure drop through helical coils with different geometries was investigated experimentally. The TiO2/water nanofluids at 0. 25% to 2% particle volume concentrations have been prepared by using a two-step method. The experiments were performed for coils with curvature ratio of 10 and 20 and coil pitch of 24 and 42. Based on the experimental data، it is found that the Nusselt number as well as Reynolds number improves while increasing particle volume concentration. This enhancement is due to the higher effective thermal conductivity of nanofluid and also intensification of nanoparticles. Besides، increase in density and viscosity of nanofluid compared to the base fluid leads to a pressure drop increment for flow through helical coiled tube. Furthermore، the heat transfer rate improves with the increase of pitch coils and decrease of curvature ratio. Additionally، curvature ratio shows more significant effect on Nusselt number and pressure drop than pitch spacing. 42. 1% enhancement on Nusselt number is obtained for the nanofluid with 2% volume concentarion in comparison to the base fluid. Finally، it is shown that the experimental results and predicted results of Nusselt number and pressure drop for nanofluid flow through helical coils hold reasonable agreement.

One of the cooling systems that has received considerable attentions in recent years is the adsorption chiller، which works based on the adsorption and desorption of a working fluid in a solid porous bed. Different parameters such as fin height directly affect the bed heat and mass transfer processes، and therefore، the performance of the adsorption chillers. In the present study a numerical model has been introduced to optimize the fin height in a RD silica gel bed with water as the working fluid. A control volume approach has been employed to determine the temperature distributions in the heat transfer fluid، metal tube، and fins، while the simultaneous solutions of the heat and mass transfer equations should be carried out in a three dimensional coordinate system for the adsorption bed. It was found that the coefficient of performance increases and specific cooling power decreases as the fin height of the bed increases.

In according to the simplicity of the lattice Boltzmann method’s (LBM) algorithm and its benefits، it has been used as a successful method in computational fluid dynamics in the last decades. In this paper، LBM was used to simulate the flow over a cylinder. To analyze the application of LBM in simulation curved surface، different methods to compute drag coefficient were used. These methods are: momentum exchange، momentum exchange on the wall، stress integration، computing stress by velocity components. The result shows the ability of Lattice Boltzmann method to simulate curved surface.

As the science of Mechanics has been improved in recent years، different standards have been created in designing structures such as pressure vessels. Most of these standards have been developed by means of experience and examination. Today، most of researches، proceed the numerical analysis of the different elements of the pressure vessels and somehow they have put some questions on the previous standards. In this paper، offering an applicable model، it is shown that heuristic optimization methods such as genetic algorithm، particle swarm optimization and especially imperialist competitive algorithm can be used as a simple and fast method for designing the vessels without breaking any of the engineering conditions and criteria. Due to the present results of this research، these methods are caused a noticeable decrease of the vessel’s weight، with a high convergence speed، in a short time، compare to the design carried out in handbooks. You may observe that the Imperialist Competitive Algorithm has a better performance and convergence than the two other methods.