نشریه علوم کاربردی و محاسباتی در مکانیک
سال سی و پنجم شماره 1 (بهار 1402)

A pitching airfoil, the time-spectral method can be used, which is a Fourier series-based spectral method with a suitable convergence speed. The disadvantage of the time spectral method is that in the entire computational domain, the number of time intervals is constant, which unnecessarily increasing the amount of computer memory and CPU time (The dependent variables). By using the adaptive time spectral method, this weakness of the time spectral method can be eliminated by the optimal distribution of time intervals (independent variable) in the computational domain (proportional to the flow gradient). In the present research, the adaptive time spectral method was added to an inviscid flow solver. The results of this method were compared with the results of the standard (non-adaptive) time spectral method and experimental data. Also, two components of computer memory and CPU time were studied. The results obtained for the three cases (Case1, Case2, and Case5) with Mach numbers 0.6, 0.6, and 0.755 respectively of the NACA0012 pitching airfoil showed that while having an acceptable solution accuracy, the amount of computer memory and CPU time is significantly reduced compared to the standard time spectral method. The CPU time for Case2 and Case5 for 4-time intervals has been reduced by 21 and 24 percent, respectively. And for Case1 for 4, 8, 1,0 and 12-time intervals, it has been reduced by 16, 38, ,31 and 29 percent respectively.

The present article deals with the optimal design of the contour or profile of the divergent part of an over-expanded supersonic nozzle in order to achieve the maximum possible thrust while maintaining the length and the exit to throat area ratio of the nozzle. To do so, a reliable and robust tool has been developed by combining computational fluid dynamics (CFD) and artificial intelligence. At first, the original profile is modeled by an innovative method using a third-order B-spline, and then by changing the breakpoints of the profile, a set of possible profiles is produced. This set of profiles has been analyzed by CFD. The geometry of the nozzle along with the thrust force obtained from the profiles have been used to train the artificial neural network. In the next step, the optimal profile was obtained by applying the genetic algorithm. Finally, the prediction of the artificial intelligence for the optimal nozzle thrust is compared with the value obtained from the CFD, which shows the validity of the present approach. The comparison between the original profile and the optimal profile for the nozzle pressure ratio of 14 shows a 36% increase in thrust and a 138% increase in the total pressure recovery factor. The comparison of nozzles for off-design conditions shows that the performance of the optimal nozzle is better than the original nozzle up to a pressure ratio of 30. If the nozzle pressure ratio exceeds 30, using the optimal nozzle instead of the original nozzle will not have priority.

The aim of this research is to improve the aerodynamic performance of a NACA-23012 airfoil equipped with a High lift device by changing its geometric parameters. In this research, Navier-Stokes equations are solved in turbulent and incompressible flow conditions using Fluent software. After the airfoil and flap modeling process, at different flap angles (5 to 30 degrees), unstructured meshing was produced in Gambit software and the improvement of aerodynamic performance due to changes in geometric parameters was investigated. The flow is assumed to be steady, turbulent and incompressible, and the algorithm for solving the equations is also selected as pressure-based. The flow Reynolds range is 3.6*106 and the turbulence model used is realizable k-epsilon. Comparison of the results and aerodynamic characteristics of the airfoil equipped with a flap after making changes in the geometric parameters in the Fluent software, shows that the aerodynamic coefficients are improved significantly (about 15%) and also the flow separation is shifted towards the end of the flap. Also, the investigation of the pressure and velocity gradients at different stages show that the changes are very effective and better distributed compared to the reference article.

Pressing molds are widely used in the industry due to the high speed of parts production. Due to the very low clearance between the mandrel and the matrix and having complex shapes, the production of these molds is very expensive and costly.  The hardness of the mold and subsequently their fatigue life should be high. One of the processes that has the ability to create surfaces with high dimensional accuracy and complex shapes is the wirecut process. In this research, in order to improve the performance and increase the life of cutting dies, the hardness of Mo40 steel is optimized. For this purpose, based on the method of the response procedure, the design of the experiment has been designed for the three parameters of wire feed speed, wire tension and generator power. Next, the obtained data are clustered and then fuzzy rules with three inputs (wire speed, tension and power) and output (hardness) are extracted. The obtained rules were entered in the toolbox of fuzzy inference systems of MATLAB software. Based on the defined fuzzy inference system, it is possible to predict the hardness based on the parameters of wire speed, wire tension and generator power. In the next phase, based on this system and within the range of available variables, the optimal value of hardness and corresponding variables were extracted. In the final phase, these values ​​were experimentally tested and their agreement with the obtained value was observed.

In this research, numerical studies have been performed by Reynolds averaged Navier Stokes (RANS) and large eddy simulations (LES) to investigate the effect of a novel film cooling design multi-holes on the film cooling effectiveness over a flat surface. A single cylindrical hole with 11.1 mm diameter and multi-holes (14 holes with a diameter of 2.97 mm) with fan configuration are considered for simulations. Numerical simulations are performed at a fixed density ratio of 1.6, length-to-diameter of 4, an inclined angle of 35o and blowing ratio 1.25. The results of the present study show that replacing a single hole with multi-holes results in a considerable increase in film cooling effectiveness in both axial and lateral directions, so that leads to an increase of about 94 percent in the area-averaged film cooling effectiveness. In the fan-shaped configuration, the scale of hairpin vortices is smaller than the single cylindrical hole, and forming of the vortices occurs in farther regions downstream of the hole. A pair of anti-jet vortices is formed in the structure of the multi-holes and the rotation direction of these vortices is opposite to the CRVP direction.

The present study numerically evaluates vortex-induced vibration (VIV) for an electrically conductive fluid around a cylinder on an elastic support. The flow is subjected to a uniform magnetic field in the z-direction to evaluate the VIV attenuation performance of this open-loop active control method. To analyze this fluid-structure interaction (FSI) problem, the finite volume method (FVM) was employed based on the SIMPLE algorithm to solve the governing continuity and momentum equations of the fluid flow and an explicit integration method to solve the motion equations of the rigid structure. Simulations were carried out at reduced velocities of 2-9 and different Stuart numbers. The simulation results demonstrated that the magnetic field was significantly effective and wholly (100%) suppressed transverse and longitudinal VIVs. A rise in the magnetic field magnitude eliminated vortex shedding for all the reduced velocities, leading to a symmetric wake. The symmetric vortex even disappeared above the critical Stuart number.