نشریه علوم کاربردی و محاسباتی در مکانیک
سال بیست و یکم شماره 1 (پاییز و زمستان 1388)

Abstract Ornithopters، or mechanical birds، produce lift and thrust through flapping motion of their wings. Many factors affect this physical phenomenon، and here we investigate effect of flapping frequency on aerodynamic and inertial forces. A six component measurement balance is used here، which is calibrated using a standard loading procedure، which uses a least squares curve fitting. A stand is designed to hold the flapping wing system during the test. A data acquisition system is set up to record important data with acceptable sampling frequency. Aerodynamic performance of the vehicle for hovering (i. e. no wind) situation is investigated. Produced lift and thrust are measured for different flapping frequency، and shows the aerodynamic forces are affected by elastic deformation and inertial flapping forces. This information are being used for validation of unsteady flow simulations needed for engineering design of ornithopters، and will be used for design optimization، as well.

Light wells are extensively used in high-rise buildings as an integrated part of the building for daylighting. Since the solar illuminance is usually higher on horizontal surfaces than the vertical ones، roof skylights transmit more daylight than vertical windows. For proper design of daylighting، a parametric study on the effective parameters in daylighting is required. In this study، the effects of these parameters are investigated for a four-story building. Attenuation of the daylight in the floors، effects of reflectivity of the surfaces of the light well، as well as effects of the reflectivities of light well floor and room walls، effects of the dimensions of the skylight، and the slope of skylight to horizon have been studied،. The daylight factor for the three situations of the lightwell: four-sided، three-sided، and in the corner of the building have been obtained. The radiosity method or point by point algorithm has been used for daylighting calculations. Results of the present study can be used in improving the daylighting of the buildings.

Engine cooling system plays a major role in performance of vehicle engine، in which the radiator and the fan are the main components. In this article، the performance of two fans with different characteristics has been appraised within an engine cooling system. For this purpose، performance characteristics curves of both fans، including، static pressure and total pressure against flow rate have been generated. Then a test rig، similar to that of an engine''s cooling system، was set up for the fan and the radiator. Subsequently the air flow rate and Radiator''s inlet flow distribution have been measured precisely under working conditions using hot wire anemometer. also، the inlet airflow pattern was analyzed. Results reveal that intersection point of the system resistance curve and static pressure curve of the fans، do not represent the actual and true values of flow rate of the fans. However the actual flow rate is less than those anticipated. Then، some modifications were made to the entrance of one of the fans and it''s shroud، and similar tests have been performed. Although the test results after modification showed more uniformity in the flow towards the radiator، the increase of flow rate was minimal in the cooling system. Also، road tests of the vehicle did not represent much improvement on the performance of the cooling system.

In recent years، the hydroforming has attracted the attention of the automotive industry due to reduction in weight and increase in strength of automotive parts. Thus، the knowledge of the process to produce parts with desired quality and accuracy has been growing. One of the effective methods in improving the formability in the hydroforming is known as pulsating hydroforming. However، the reason of improvement of the formability in pulsating hydroforming is still unclear for the closed-die hydroforming of tubes. In this study، the pulsating hydroforming of tube with box die was simulated by the finite element method to examine the effect of pulsating pressure on improvement of formability. In addition، the calculated results are compared with experimental results and it is shown that the improvement of the formability occurs before contact of the tube and die in expansion zone of the tube. Thus، it can be obtained from the results، that the main reason of improvement of the formability is due to the deformation behavior before start of contact of the tube and die. Flat bulging appears for the pulsating pressure in the former stage، whereas round bulging and wrinkling occur for the high and low non-pulsating pressures، respectively.

The aim of this article is analyzing heat and mass transfer in counter current open cooling towers. After reviewing preceding researches and rewriting the governing equations، a method is proposed which is independent of Lewis number variations in solving the problem، has been introduced. Also، the effect of Lewis number variations has been compared with this method. This method has been presented in Design and Rating situations which is more comprehensive than the preceding researches. Finally، the results of present method have been analyzed and investigated using the performance data of an industrial cooling tower.

Exergy analysis is a method that can calculate thermodynamic losses by means of second law of thermodynamic. In this study، exergy analysis has been performed for the silencer of OM314 diesel engine. Irreversibility due to heat transfer from silencer to environment، silencer''s internal irreversibility and total irreversibility for different conditions of engine operation have been measured and calculated. For finding out the relation between irreversibility and exit sound intensity، simultaneously، exit sound intensity from the silencer has been measured. Results illustrate that by tripling of silencer irreversibility، exit sound intensity increases only 10 percent. Also the results shows that the irreversibility due to heat transfers have the most contribution in total irreversibility of silencer. So from the view point of second law of thermodynamics، it seems that cooling the silencer can be an effective method for sound reduction. Also results illustrate that، sound intensity of silencer is basically dependent to engine speed.

In this paper،variable confinement parameters were successfully developed for compressible vorticity confinement. Three variable confinement parameters that have velocity dimension were defined based on three artificial dissipation schemes. The resulting confinement parameters are functions of spectral radii of the Jacobian matrices and the Jacobian matrices themselves. Therefore، the confinement parameters implicitly contain the grid size and the other local fluid properties. The ability of the new confinement parameters for 3-D supersonic flows over a blunt-nose-cylinder at high angles of attack with massive separation in the presence of an adverse pressure gradient condition was considered. Effect of various dissipation schemes on the solution was studied. Results showed that the new confinement parameters allow the capture of vortical layers. Vorticity confinement prevents the diffusion of the secondary vortex at leeside plane on coarse grid. As the grid size increases، improvement of the results (especially، confinement results of CUSP confinement parameter) is seen. When variable confinement parameters are used، the tuning constant is equal to or larger than the equivalent value of the constant confinement [Hu et al.] [8]. This means that this value is closer to the unity than constant confinement، especially for CUSP confinement parameter.

Accurate prediction of springback is essential for the design of tools used in sheet metal forming. One of the major causes for the production of inconsistent part is springback، the elastic recovery in the material after the tooling is removed. In this paper، the numerical simulation of springback and side wall curl in U-bending has been performed. This operation presents a complex form of springback and side wall curl occurring in sheet metal forming since the sheet undergoes stretching، bending and unbending deformations. The effects of some parameters such as coefficient of friction and blank-holder force on the springback and side wall curl are studied. The results show that the role of friction and blank-holder force in the U-bending process is considerable. For instance، springback and side-wall curl initially increase with raising the coefficient of friction and blank-holder force but they decrease again when they go beyond certain values.