Scientia Iranica
Volume:27 Issue: 3, May-Jun 2020

A series of experiments were conducted to study impacts of the free stream Mach number, back pressure and bleed on the stability of a supersonic intake. The flow stability is related to the buzz phenomenon; oscillation of all shock waves of the intake and it may further occur during the time when the intake mass flow rate is decreasing. The present intake is an axisymmetric intake for Mach number of 2.0. The results showed that the stability margin of the intake decreases when the freestream Mach number increases for both bleed off and bleed on cases. For the configuration without bleed, the frequency of buzz oscillation is increased when the freestream Mach number is decreased or when the back pressure is increased. By applying the bleed and consequently preventing the flow separation, the intake is more stable and the shocks oscillate with a smaller amplitude during the buzz phenomenon. When the bleed is applied, the buzz triggering mechanism varied from the Dailey criterion to that of the Ferri one, a phenomenon that changes the stability characteristics of the intake considerably.

In this study Electrical discharge machining (EDM) process, widely used in mold manufacturing, is modeled and optimized using artificial neural network and an optimization heuristic algorithm. Material removal rate (MRR), tool wear rate (TWR), and surface roughness (SR) are considered as performance characteristics of the EDM process. Optimization of process parameters in order to find a combination of process parameters to simultaneously minimize TWR and SR and maximize MRR is the objective of this study. In order to establish the relations between the input and the output process parameters, back propagation neural network (BPNN) used. In the last section of this research, particle swarm optimization (PSO) algorithm has been employed for optimization of the multiple response characteristics. A set of verification tests is also performed to verify the accuracy of optimization procedure in determination of the optimal levels of process parameters. Results demonstrate that propose modeling technique (BPNN) can precisely simulate actual EDM process with less than 1% error. Furthermore less than 4% error for PSO algorithm results is quite efficient in optimization procedure.

This paper aimed at developing an empirical correlation for heat transfer from a protruded surface in the presence of a cross-flow jet. Finite volume method has been used to solve the governing differential equations for mass, momentum, energy as well as turbulence by imposing appropriate boundary conditions. Extensive numerical computations have been carried out to vary each of the independent variables to collect data for area-weighted average Nusselt number. Both the duct and the nozzle Reynolds number are varied from 6,000-20,000. The volume fraction and Prandtl number are also varied in the range of and , respectively. The number of protrusion (n) is varied from 1 to 4. A nonlinear regression analysis has been executed using CFD data to develop an empirical correlation for the Nusselt number in terms of pertinent independent parameters. The volume fraction of the nanofluid is found to be the most significant parameter to influence heat transfer rate among all other parameters. It has been observed that the predicted Nusselt number matches well with the computed one. The variations of the Nusselt number as the function of the independent parameters has been demonstrated. The present numerical methodologies have been validated with some open literature.

The nonlinear dynamic response, Limit Cycle Oscillations (LCOs), of high aspect ratio wings using a novel indicial aerodynamics in subsonic flow is investigated. Using the nonlinear beam theory, the structural model is derived including the in-plane and out-of-plane bending and torsion motions, all nonlinearities up to cubic order arising from large deformation, mass distribution, and cross-sectional mass imbalance. Based on new approximations of the indicial functions, a comprehensive unsteady aerodynamic model is used. Such an indicial aerodynamics while being coupled to nonlinear structural equations can result in a unified nonlinear aeroelastic formulation in both the incompressible and subsonic compressible flow. The effect of flight conditions, wing tip initial disturbances, stiffness ratio between bending modes, and nonlinearity due to inertia and cross-sectional mass imbalance on the characteristics of LCO are discussed. The results show that the compressibility can affect the LCO boundary up to 12 percent which implies that an appropriate Mach-dependent aerodynamics is required to achieve a more reasonable and realistic description of dynamic behavior of the system. It is shown that the presence of LCO before the linear flutter speed depends on initial disturbances as well as wing characteristics.

In this study, Mg-Sn alloys were produced through the powder metallurgy (P/M) method by adding Sn in different ratios into Mg powder. A new mixing technique has been used in production to prevent the disadvantages of high reactivity that the Mg powders have. The prepared powder mixtures were turned into components by processing through hot pressing. The produced components were characterized by density measurements, microstructure examinations and mechanical tests. The density measurements were made according to the Archimedes principle. The microstructural characterization was performed by X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) investigations and energy dispersive spectrometry (EDS) analyses. The hardness measurements and the tensile tests were used for the determination of mechanical properties. Densities close to the theoretical density were obtained in the produced parts. XRD and SEM investigations have shown that the components produced are composed of α-Mg and Mg2Sn phases of the microstructure consisting of coaxial grains. The rising Sn content increased the amount of discrete Mg2Sn precipitates at the grain boundaries, thereby ensuring higher hardness and strength values.

The aim of this research was to provide a simple yet realistic model of the sino-nasal tissue as a major requirement for developing more efficient endoscopic neurosurgery simulation systems. Ex-vivo indention tests were performed on the orbital floor soft tissue of four sheep specimens. The resulting force-displacement data was incorporated into an inverse finite element model to obtain the hyperelastic mechanical properties of the tissue. Material characterization was performed for Polynomial, Yeoh, Mooney-Rivlin and Neo-Hookean hyperelastic models, using a Sequential Quadratic Programming algorithm. Experimental results indicated a relatively large elastic deformation, up to 6mm, during indentation test with a considerable nonlinearity in the force-displacement response. All hyperelastic models could satisfy the convergence criteria of the optimization procedure, with the highest convergence rate and a close fittings accuracy associated with the Yeoh hyperelastic model. The initial guess of the material constants was found to affect the number of iterations before converging, but not the optimization results. The normalized mean square errors of fitting between the model and experimental curves were obtained as 2.39%, 4.26% and 4.65% for three sheep samples, suggesting that the Yeoh model can adequately describe the typical hyperelastic mechanical behavior of the sino-nasal tissue for surgery simulation.

Surface pressure distributions and boundary layer profiles are measured over the nose surface of a submarine model in a wind tunnel. The tests are conducted for two different nose shapes in order to study the effects of nose shape on the flow field around the model. The influence of Reynolds numbers, which are 0.5×106, 0.8×106 and 106, and pitch angles, α = 0, 5, 10 and 15°, on the surface pressure distribution over the surface of two nose shapes are investigated. Furthermore, the effect of the longitudinal pressure gradient on the boundary layer velocity profiles and the probability of the separation in the plane of symmetry of the nose are studied. It is found that the Reynolds number does not have a significant influence on the nose surface pressure distribution at all pitch angles. The results show that the presence of the adverse pressure gradient in major portion of the blunter nose shape causes the non-dimensional velocity profiles of boundary layer in locations of 0.1≤X/L≤0.23 are deviated from the log layer profile. Therefore the separation on the blunter nose shape is more likely than the other nose at high pitch angle manoeuvres.

In the present paper, an exact mathematical solution has been obtained for nonlinear free transverse vibration of beams, for the first time. The nonlinear governing partial differential equation in un-deformed coordinates system has been converted in two coupled partial differential equations in deformed coordinates system. A mathematical explanation is obtained for nonlinear mode shapes as well as natural frequencies versus geometrical and material properties of beam. It is shown that as the s th mode of transverse vibration excited, the mode 2s th of in-plane vibration will be developed. The result of present work is compared with those obtained from Galerkin method and the observed agreement confirms the exact mathematical solution. It is shown that governing equation is linear in time domain. As a parameter, the amplitude to length ratio (Λ⁄l) has been proposed to show when the nonlinear terms become dominant in the behavior of structure

Insulator-based dielectrophoresis is a recently developed technique in which insulating posts are used to produce non-uniformity in the electric field in a microchannel. This study presents the effects of insulating posts geometry and arrangement on the trapping efficiency of red blood cells in an alternating current- Insulator-based dielectrophoresis system. Microchannels containing square, circular and diamond-shaped posts with particles under the influence of positive dielectrophoresis force and fluid flow were considered. Finite element method was used to compute the velocity of the flow and electric field. The numerical method was verified by comparing the numerical results with experimental data. Two distinct criteria for examining particle trapping for distinct shapes and arrangements of insulating posts were introduced. Particle tracing simulation was implemented to observe particle trapping and compare the trapping performance of systems with distinct posts. As shown in the results for the system with circular and square posts, insulators should be narrowed to improve particle trapping, while diamond post should be widened to increase the trapping efficiency. In addition, the particle tracing results showed that microchannel with square posts is more efficient in particle trapping.

This work systematically investigated the effects of process parameters on the technological responses, including the tensile force TF and average micro hardness AMH in the gas tungsten arc welding (GTAW) of titanium. Controlled parameters are the welding current I, gas flow rate F, and arc gap G. The objective of this work is to improve the tensile strength with respect to micro hardness constraints. A GTAW welding machine was adopted in conjunction with the Box-Behnken matrix to conduct experimental trails. The nonlinear relationships between welding parameters and responses were developed using response surface method (RSM). Subsequently, an optimization technique entitled desirability approach (DA) was used to solve the trade-off analysis between responses considered and find the optimal parameters. The conformity test was performed in order to evaluate the accuracy of optimizing values. The results showed that the welding current had the greatest influence on the outputs considered, compared to other factors. The measured improvements using optimal parameters of tensile force and average micro hardness are approximately 4.10% and 6.12% in comparison with initial settings. A hybrid approach comprising RSM and desirability approach can be considered as an effective method for parameter optimization and observation of reliable values in GTAW processes.

In this paper, a new methodology has been proposed to enhance the conformal mapping applications in the process of optimum trajectory planning in Terrain Following (TF) and Terrain Avoidance (TA) Flights. The new approach uses the conformal mapping concept as a flattener tool to transform the constrained trajectory-planning problem with flight altitude restrictions due to the presence of obstacles into a regenerated problem with no obstacle and minimal height constraints. In this regard, the Schwarz–Christoffel theorem has been utilized to incorporate the height constraints into the aircraft dynamic equations of motion. The regenerated optimal control problem then is solved employing a numerical method namely the direct Legendre-Gauss-Radau pseudospectral algorithm. A composite performance index of flight time, terrain masking, and aerodynamic control effort is optimized. Furthermore, to obtain realistic trajectories, the aircraft maximum climb and descent rates are imposed as inequality constraints in the solution algorithm. Several case studies for two-dimensional flight scenarios show the applicability of this approach in TF/TA trajectory-planning. Extensive simulations confirm the efficiency of the proposed approach and verify the feasibility of solutions satisfying all of the constraints underlying the problem

Cavitation Phenomenon in Centrifugal pumps is the main cause of failures in pump components, such as impeller and volute. To evaluate this phenomenon, firs of all the flow field in a BB2 API multistage centrifugal pump with and without cavitation situation is studied. Additionally, to improve impeller inlet condition and reduction of cavitation possibility, Stepannof and Dixon theory is used. This study mainly focuses on the concept of cavitation’s in pump, pump performance curve, system pump curve, and net positive suction head (NPSH). The ultimate goal of this project is to determine the best operating pump range. It is interesting to examine the system pump curve prediction to identify the inception cavitation zone. Therefore, a theoretical system pump curve was generated using Microsoft Excel 2010, in addition, Catia V5 R21 and ANSYS CFX 14. Were applied to create computational fluid dynamic model From simulation results, a decrease of NPSHa values produces the onset of cavitation. The major findings of this thesis present the theoretical and numerical results concerning the pump characteristics and performance breakdown at different flow conditions. Therefore the best operating pump range is identified a flow rate of 330 m3/hr to avoid the occurrence of cavitation in pump.