Scientia Iranica
Volume:28 Issue: 2, Mar-Apr 2021

In this paper, the effect of inlet pressure on the performance of vortex tube device has been investigated using 3D simulation and CFD technique by Fluent software. The flow inside the device is considered as compressible and turbulent. In order to understand and investigate the effect of inlet pressure, different inlet pressures are entered into the device and the results are extracted and analyzed. The main goal is to achieve the minimum cold exit temperature and maximum swirl velocity in the vortex tube. This paper indicates that inlet pressure of 4.8 bars is an optimal inlet pressure which is justifiable economically and also in terms of the amount of produced cooling. The CFD results show that increase in inlet pressure, increases the entropy production and subsequently the system disorder. Finally, the existing gaps in the previous studies will be filled by examining the inlet and exit exergies in the vortex tube device. Inlet exergy has not considerable changes in terms of α and have a constant value and at α=0.3691 the minimum exergy efficiency is occurred according to the calculations.

The current study investigates the optimization of solar stills (SSs) productivity under different levels of solar radiation intensity, film cooling flow rate and water depth. Taguchi method is applied to perform a minimum number of experiments and to find the optimal water depth and cooling rate that maximize the productivity of the SS. An orthogonal array, signal-to-noise ratio and analysis of variance (ANOVA) are used to investigate the influence of the operating parameters on the solar still productivity. Taguchi method and ANOVA result several that water depth is the most influencing parameter. Furthermore, the results show that increasing the solar radiation and water film cooling improve the productivity. However, increasing the flow rate more than 4 kg/hr has a slight effect on the productivity. On the other hand, decreasing the water depth leads to a great enhancement in the productivity due to the faster evaporation and condensation rates. The levels of the operating parameters under-investigation are selected as follows: solar radiation intensity (5587,5673 and 5741 W/m2/day), film cooling flow rate (2, 4 and 6 kg/hr) and water depth (0.5, 1 and 1.5 cm). The improvement in the productivity with film cooling was about 6.05% at the water depth (0.5 cm)

Two macroscopic turbulent models, P-dL and N-K, have been proposed in recent years for simulating turbulent unidirectional flow in porous media. In this paper a modification on N-K model has been proposed for turbulent oscillating flow in porous media. To this purpose, Turbulent oscillating flow in porous media has been simulated in microscale employing a periodic array. The k-ε model was applied to solve turbulent oscillating flow in periodic array. Control volume approach has been used to discretize Navier-Stokes and k-ε equations and the well-established SIMPLE method has been conducted to deal with pressure and velocity coupling. To modify N-K model the effect of different parameters such as frequency and Reynolds number has been investigated and the constants in source terms of turbulent kinetic energy and its dissipation rate has been modified versus Re according to microscale results. In order to validate the new modified constants, the modified N-K model was applied to turbulent oscillating flow in porous media and results were compared to original N-K macroscopic model.

In this paper, a gain-scheduled three loop autopilot is designed for a pursuit system which can guarantee the mixed H_2/H_∞ performance and time domain constraints. The gain-scheduled autopilot problem is converted into a static state feedback control for a Linear Parameter Varying (LPV) system and then a control method is proposed by following the Linear Matrix Inequalities (LMIs) approach to satisfy the mixed H_2/H_∞ performance with regional pole placement constraints without any constraints on system matrices. The final gain-scheduled controller is obtained by the interpolation of the finite number of fixed controllers in every vertex, guaranteeing the stability and performance of the LPV system. The simulation results demonstrate the efficiency of the proposed method for the three loop autopilot design.

In this study the hemodynamic analysis of complete coronary bypass graft with elastic walls and different percentages of stenosis are investigated numerically. Blood flow is considered Newtonian and unsteady. The objective of this study is to deal with the influence of the wall elasticity, flow pulsatility and stenosis percentage on the flow configuration, Wall Shear Stress (WSS) and rotational flows. By comparing the rigid and elastic wall results of WSS, it is concluded that WSS obtains lower values in toe, heel and bed of the host vessel under the bifurcation in the elastic mode, which is closer to reality. Also it is concluded that with increasing the stenosis percentage, the possibility of occurring rotational flow will increase. The maximum and minimum values of WSS are observed in the stenosis of 70%. From the pulsatility of flow, it is observed that unsteady flow shows more accurate results also velocity and WSS have lower values compared with the steady state results.

A viscoelastic microcantilever beam is analytically analyzed based on the modified strain gradient theory. The Kelvin Voigt scheme is used to model the beam viscoelasticity. Applying Bernoulli-Euler inextensibility of the centerline condition via Hamilton’s principle, the nonlinear equation of motion and related boundary conditions are derived based on shortening effect theory and discretized by Galerkin method. Inner damping, nonlinear curvature effect, and nonlinear inertia terms are applied. The generalized derived formulation in this article, allows modeling of any nonlinearity combinations such as nonlinear terms arises due to inertia, damping, and stiffness, as well as modeling the size effect via considering modified coupled stress or modified strain gradient theories. First mode nonlinear frequency and time response of the viscoelastic microcantilever beam are analytically evaluated utilizing multiple time scale method and validated by numerical findings. Results indicate that the nonlinear terms have an appreciable effect on natural frequency and time response of a viscoelastic microcantilever. Furthermore, the investigation reveals that due to the size effects, natural frequency enhances drastically, especially when the thickness of the beam and the length scale parameter are comparable. Outcomes clarify the importance of size effects in analyzing of the mechanical behavior of small scale structures.

This paper proposes the use of a novel electrode containing modified design explicitly intended to promote gas-assisted tool rotation-induced debris removal. The proposed tool was observed to be efficient in dispensing the accumulation of eroded materials from the discharge gaps. In this work, the influence of process parameters like discharge current, tool speed, gas pressure, pulse duration, and duty cycle has been investigated on output responses: material removal rate (MRR), electrode wear ratio (EWR), and surface roughness (SR). A comparative study of the output responses was made with a solid rotary tool and the gas-aided multi-hole slotted tool. The outcome reveals that the application of the multi-hole slotted tool increased the MRR in AAEDD by 40-80%. Besides this, the EWR decreased in AAEDD by 17–25% compared to REDD. Moreover, the SR of the AAEDD process was comparatively higher (9-15%) than that of the REDD process. It has been found less surface crack, micro pores and recast layers on specimens machined by the AAEDD process in comparison to the REDD process. The present work proposed a novel method for improving the machining performance by improving the flushing efficiency of the machining gap on the way to improve the material removal mechanism.

The finite element solutions of static deflection and stress values are obtained in this article for the functionally graded structure considering variable grading patterns (power-law, sigmoid and exponential) including the porosity effect. The unknown values are obtained computationally via a customized computer code with the help of cubic-order displacement functions considering the varied distribution of porosity (even and uneven) through the panel thickness. Also, the values are simulated through design software (ANSYS) to establish the present numerical solution accuracy. The comparison as well as the element sensitivity behaviour of the present numerical model verified by solving different kinds of numerical examples available in the published domain. Lastly, the effect of several influential geometry relevant parameters (aspect ratio, curvature ratio, thickness ratio, porosity index, type of porosity, power-law exponent, geometrical configuration and support conditions) affecting the structural stiffness and the corresponding outcomes (deflection and stress) of the FG structure are computed through the presently derived numerical model.

Collagen network is one of the articular cartilage (AC) vital components, which contributes to the depth-dependent and anisotropic response of the tissue. As it is computationally expensive to simulate all the structural details of the AC network, they were typically simplified in numerical analysis. In particular, the so-called arcade-like structure, which has been widely used in the previous complex simulations, does not capture the rotations of the fibrillar bundles. In this study, we investigate the role of such possible rotations in the AC mechanical response by a set of advanced, biphasic, and parametric finite element (FE) simulations of indentation tests. Our results unveil the influence of fibrillar rotation (FR) on the mechanical response by increasing the fibrillar stress while regionally affecting the stress in the upper layers of the AC tissue. On the contrary, the FR did not significantly alter the tissue elasticity, and consequently might be ignored safely in pure contact mechanical problems. It is concluded that the excessive FR might regionally increase the stress, which can have a degenerative effect on the collagen constituent, and therefore, should not be neglected in the corresponding future studies, in which the upper AC layers resist high permanent shear strains.

The present work deals with the mathematical inspection of Stoneley wave propagation through the corrugated irregular common interface of two dissimilar magneto-elastic transversely isotropic (MTI) half-space media under the impression of hydrostatic stresses. For the enumeration of the Lorentz’s force besmeared in the structure, generalized Ohm’s law and Maxwell’s equation have been considered. The interior deformations are calculated analytically to obtain the wave frequency equation using prescribed boundary conditions. To investigate the impacts of irregularity and various affecting parameters such as magnetic couplings and hydrostatic stresses on the wave propagation, frequency curves are framed-up for the phase velocity of the wave.

In underground U-tube heat exchangers (boreholes) it is important to predict its heating performance to design and select the proper parameters such as length, diameter, material etc. to have an optimized borehole from the point of view of heat capacity and economical aspects. For this reason, having trusty equations is vital to foresee borehole heating performance and applying it in design issues. In this study a single vertical U-tube borehole with constant wall temperature is considered and analytical equations for temperature distribution in the surrounding ground around the borehole is evaluated based on one and two dimensional heat conduction respectively. The analytical equation is compared to experimental data for a borehole with 50 m depth in which warm water of 40 C is pumped into it a time period of 120 hours and the heat transfer rate per unit length is recorded. The comparison between analytical expression and experimental data shows a good agreement between them. Also the borehole entropy generation number is studied and the optimized parameters are evaluated to minimize it. It is concluded that for the considered borehole, entropy generation number is decreased by increasing its length and by decreasing the borehole radius and pipe outer radius.

Transitional boundary layer over a pitching airfoil at low Reynolds number (Re = 2.7×10^5) is experimentally investigated through the space-frequency and time-frequency analyses of hot-film signals. Boundary layer events are visualized based on the space-frequency and time-frequency plots. The precursor phenomena for turbulent as well as fully separated flows are presented based on the time-frequency analysis. A new technique based on the time-frequency analysis of hot-film signals is presented to measure the transition onset as well as the relaminarization locations. This technique is based on the analysis of high-frequency disturbances of the measured data. Special attention is focused on the spatial/temporal progression of the transition onset and the relaminarization points, compared to the static values, for different oscillation frequencies and amplitudes. Investigations are performed prior to, within and beyond the static stall angle of attack conditions. The results obtained by the new technique will be discussed and compared with the observations from the previous investigators.