Scientia Iranica
Year:2013 | Volume:20 | Issue:3 (TRANSACTIONS B: MECHANICAL ENGINEERING)|Papers Count:9

This paper deals with the influence of Hall and ion slip effects on the magneto-hydrodynamic flow of a micropolar fluid past a non-conducting wedge. The analysis has been made by assuming that the fluid is viscous, incompressible and electrically conducting. The partial differential equations governing the flow and heat transfer are converted into highly non-linear ordinary differential equations by using the similarity transformations. These equations are then solved numerically. The effects of various parameters involved in the problem have been studied with the help of graphs and numerical values of skin friction coefficients and Nusselt number are presented in tabular form. Favorable comparison with previously published work on various special cases of the problem has been made. Results show that the local skin friction coefficient due to translational motion increases with the angle of the wedge and Hall effect parameter, hence heat transfer rate increases with these parameters. The result gets reversed with a increase in material, ion slip and magnetic field parameters.

Using the new modified three-fluid model, the effect of variation of inlet pressure on predictions of pressure drop in the downward condensing annular flow of steam inside vertical pipes is studied. To achieve this, using the new modified three-fluid model and Stevanovic et al.'s correlation for the steam-liquid film interfacial friction coefficient, pressure drop is calculated in two new inlet pressures (i.e., 1.5 and 2.5 (MPa)), for which there is no available experimental data of pressure drop. The pressure drop predictions of the new modified three-fluid model and those of the Stevanovic et al.'s correlation are compared.

In the present paper, a two-dimensional compressible oscillating flow in the tube section of a pulse tube refrigerator system is modeled, based on the successive approximation method. In this respect, the variables are expanded up to second-order terms. For pulse tubes with an outer adiabatic surface, the conservation equations of mass, momentum, energy, and the equation of state for the ideal gas are applied. The effects of operating frequency and taper angle on the temperature distribution, heat transfer behavior, and time-averaged enthalpy flow during a cycle are investigated. Increasing the frequency leads to a higher heat transfer rate in the pulse tube. The enthalpy flow, as the cooling performance representative of the pulse tube, reaches maximum for an optimum convergent taper angle. The temperature distribution and heat transfer process along the axial direction, as well as the phase behavior of the heat transfer coefficient, are also studied. It is shown that, moving from the cold to the hot end of the pulse tube, the temperature variation domain and heat transfer rate decrease.

The voxel based finite element (FE) method used to obtain primary data for non-invasive imaging techniques has emerged as a major focus of interest in several disciplines such as medicine, mechanics and material engineering for solving micro and nano-scale problems. Owing to the fact that, voxel based FE models are directly affected by parameters of imaging techniques such as Computed Tomography (CT) and Magnetic Resonance Imaging (MRI), the consequences of these effects on the natural vibration analysis of structures having complex geometry in micro scale, are investigated in this study. In this context, voxel based FE models are obtained using Micro-CT imaging data that has three different resolutions of vertebral trabecular bone tissue. Furthermore, resolutions of image data sets are artificially increased and equalized for evaluating voxel based FE models that are free from FE size effects. Natural vibration characteristics of voxel based FE models are investigated not only numerically but also including associated mode shapes. Unpredictable vibrational behavior for various voxel sizes, is, thus, revealed. Element size effects of voxel based FE models are considerably different from the effects on structural components with regular prismatic shapes. Obtained results show that, investigated parameters have a crucial influence on the natural vibration behavior of trabecular bone tissue which is selected as an example of complex geometries. Modal behaviors that are effective in micro local regions, but less in the whole body, where there are possibilities for working with approximate geometry without considering the micro structure have been observed. Moreover, the results are new from a theoretical point of view, and they also represent the importance of quality in imaging data, which, in practical applications must be taken into consideration.

An existing operational trisonic wind tunnel is upgraded to improve its performance criterion in the transonic regime. In this research, the test section is modified according to the operational requirements of the various existing transonic wind tunnels. Several perforated walls are designed, manufactured, and installed on the top and bottom sides of the test section. The flow in the test section of the wind tunnel is surveyed for the empty condition prior to testing models. Once satisfactory results regarding the flow quality requirements in the test section under various conditions were achieved, a 2D model, NACA 0012, and a 3D standard model for the transonic wind tunnels, AGARD-B, are manufactured and tested under various conditions for the purpose of integral calibration and validation of the tunnel data. Surface pressure distribution as well as the force and moment data compare well with the existing data from other tunnels for similar models tested under the same conditions.

A series of experiments were conducted to study the effect of leading-edge roughness on the state of the boundary layer of a wind turbine blade section using multiple hot-film sensors. The experiments involved static and dynamic tests, where airfoil motion was of plunging type oscillation. The application of surface grit roughness simulates surface irregularities that occur on the wind turbine blades. The measurements showed that increasing the angle of attack results in movement of transition locations toward the leading edge. Surface roughness moved the transition point toward the leading edge and caused early trailing edge turbulent separation, which resulted in reducing the effectiveness of the airfoil. Boundary layer instability frequencies were dominated through the transition.

A theoretical formulation, of Navier solutions of rectangular plates based on a new higher order shear deformation model is presented for the static response of functionally graded plates (FGPs). This theory enforces traction free boundary conditions at plate surfaces. Shear correction factors are not required because a correct representation of transverse shearing strain is given. Unlike any other theory, the number of unknown functions involved is only four, as against five in cases of other shear deformation theories. The mechanical properties of the plate are assumed to vary continuously in the thickness direction by a simple power-law distribution in terms of the volume fractions of the constituents. Numerical illustrations concern the flexural behavior of FG plates with metal-ceramic composition. Parametric studies are performed for varying ceramic volume fractions, volume fraction profiles, aspect ratios and length to thickness ratios. Results are verified with available results in the literature. It can be concluded that the proposed theory is accurate and simple in solving the static bending behavior of functionally graded plates.

Wind tunnel experiments were conducted to evaluate surface pressure distribution over a semi span swept wing with a sweep angle of 33o. The wing section has a laminar flow airfoil similar to that of the NACA 6-series. The tests were conducted at speeds ranging from 50 to 70 m/s with and without surface roughness. Surface static pressure was measured on the wing upper surface at three different chordwise rows located at the inboard, middle, and outboard stations. The differences between pressure distributions on the three sections of the wing were studied and the experimental results showed that roughness elements do not influence the pressure distribution significantly, except at the inboard station. On the other hand, spectral analysis of the pressuretime signals acquired from the pressure orifices over the wing upper surface showed that roughness had significantly affected the zero frequency amplitude. In this study, the zero frequency amplitude and its variations with roughness elements was investigated at three different chordwise positions, inboard, middle, and outboard stations. Results showed that the 3-D roughness elements amplified zero frequency amplitude over the wing surface. Zero frequency distribution at the inboard station, closer to the wing root, in comparison with the middle station, was reduced after an initial amplification along the chord. Moreover, the effect of roughness on the zero frequency instability at the first section was negligible due to the narrow instability amplification region. On the other hand, at the outboard station, closer to the wing tip, the instabilities were amplified over a larger region, with respect to the middle station.

Reduction in ship resistance, in order to decrease fuel consumption and also achieve higher speeds, has been the topic of major research over the last three decades. One of the most attractive ideas in this field is micro bubble drag reduction, which attempts to obtain optimum injection flow rate based on ship specifications. The model test results of a 70 cm catamaran model was used to quantify the effect of air injection rate on drag reduction, and to estimate a simple formulation for calculating an efficient injection rate by considering the main parameters of the ship, such as: length, width and speed. The test results show that excessive air injection decreases the drag reduction effect, while suitable injection reduces total drag by about 5%-8%