مجله مکانیک سیالات و آیرودینامیک
سال یازدهم شماره 1 (بهار و تابستان 1401)

In this research, the split drag system at different AOA for a flying wing aircraft has been simulated and optimized by a numerical method. The split drag system provides vertical axis control by creating asymmetric drag between the right and left wings. The aircraft under study is a lambda shape aircraft with a swept-back angle of 56. The split drag control system is made up of two surfaces on top of each other, by opening in the opposite direction on one side of the aircraft, it creates the drag necessary to produce yawing moment. Their installation position is at the tip of the wings and on the trailing edge. When using split drag, in addition to the yawing moment, a disturbing rolling moment is created, which is caused by the drag difference between the upper and lower surface of this system, and the reason for this is the change in the AOA of the aircraft. Asymmetric opening of the surfaces can reduce the induced roll to zero and in some cases to a minimum. The test was carried out in AOA of 0 to 12ᵒ for drag split opening angles of 10, 20, and 30ᵒ. Calculations based on equations (RANS) are discretized with the finite volume method. The obtained results show how much to add to the angle of the split drag surfaces depending on the AOA in order to find the most optimal mode to neutralize the roll, and finally, the optimized diagrams of this system are obtained.

One of the novel strategies to improve the performance and emissions of diesel engines is the use of alternative fuels as well as suitable additives such as nanoparticles to diesel fuel. Nanofuels play an important role in the optimization of combustion processes, fuel consumption, and emissions. In this paper, the effect of adding different nanoparticles (cerium, aluminum, and copper oxide nanoparticles) at a concentration of 100 particles per million (ppm) to diesel fuel on the combustion process and emissions of diesel engines has been investigated by using FIRE computational fluid dynamics code. For validation, the in-cylinder pressure variations, the experimental peak pressure, and the angle of occurrence are compared with the numerical results. In addition, the experimental data of NOx, soot, power as well as brake specific fuel consumption were evaluated with numerical values. The results show that nanoparticles increase the amount of heat transfer to the fuel and decrease the ignition delay. Also, better mixing of fuel and air in cerium oxide nanoparticles compared to other nanoparticles improved the fuel ignition mechanism, which leads to more complete combustion and a 14.5% increase in power and also a 6% and 34% reduction in fuel consumption and soot compared to diesel fuel respectively. The only downside is the 31% increase in NOx, which can be reduced by catalytic converters.

The interior design of archeological museums should be such that the least damage is done to the works. Meanwhile, the temperature difference and the lack of uniform air ventilation in the interior of the galleries is one of the influential factors in the destruction of museums. Therefore, the main goal of this study was to standardize the ventilation process in all the interiors of the galleries so that all points have adequate ventilation and the temperature difference is minimized. All tests were performed in the Museum of Antiquities of Shush as a test sample. Due to its nature, this research is considered interdisciplinary, and its research method combines experimental, simulation, and case study strategies. First, periodic tests were performed experimentally in the Museum of Antiquities of Shush (case study). The temperature, speed, and direction of indoor air flow (dependent variable) were measured and recorded during the test period by precision digital devices. Simulations were performed by computational fluid dynamics (CFD) method with the help of Gambit and Fluent software after proving validity and reliability and architectural interventions were made. Then the data were analyzed, and test results were extracted. Based on the interventions that took place in the architecture and ventilation systems, with only the relocation of the blowers and vacuum cleaners and small changes in the composition of the interior architecture of the galleries, the effective flow of indoor air was established in all interior parts of the galleries. Its interior was improved by 85%.

One of the important and main tools for studying flow and evaluation criteria of other methods such as numerical methods is experimental data of the fluid mechanics. Thus, the appropriate quality of the data, that are measured in the laboratory, is important. One of the important information of any flow is fluid velocity field, that is measured by different instruments. One of those tools is the SPIV tool. This tool provides sheet information from the flow velocity components. Generally, the data extracted from this tool will have big errors in some points of the velocity field, for various reasons and laboratory conditions, and the values obtained in these points will be eliminated, which are called gappy points. Therefore, in order to reconstruct the velocity field at these gapps, methods are needed. In this regard, in the present study, we will use artificial neural networks, such as MLP and CNN. The optimization of the number of neurons in the MLP network was performed by the mean error of the test data and the matching of the images. The final error was obtained for each of the methods, which due to the errors and accommodation between the reconstructed velocity field and experimental data, it was obtained that for both velocity components, the CNN neural network had the best performance.

Estimation of the hydrodynamic coefficients of an autonomous underwater vehicle is the purpose of this paper. The Kalman filter method has been used for estimating the AUV hydrodynamic coefficients.  To estimate hydrodynamic coefficients without knowing their initial values, spatial and temporal information of the AUV are needed. This information can be collected through different methods including experimental methods which gather the required information by the sensors installed on the AUV. In this paper, outputs related to the time and location information are collected using robot maneuver simulation in CFX software, with a movable grid being used to simulate the robot maneuver. In movable grid method, when the mesh quality has been reduced by a certain level, the meshing around the robot is summoned to the WB environment and after performing the new meshing, it is returned to the CFX environment to continue the simulation. To derive the hydrodynamic coefficients of the autonomous underwater vehicle, a sinusoidal maneuver at three degrees of freedom is selected for simulation in CFX software. The collected results from the sine maneuver simulation are applied as input to the Kalman filter estimator code. The hydrodynamic coefficients whose extraction is desired, are defined as unknown parameters in the robot control equations. In this maneuver the hydrodynamic coefficients have been extracted with good accuracy. Also to improve the Matlab code and increase the accuracy of extracting hydrodynamic coefficients, the control equation are written in the matrix form. Thus, the number of extracted coefficients are decreased but the coefficients are extracted with higher accuracy.
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In this research, using the dynamic modes decomposition and using the basic concepts of the dynamical system, a data driven-physics informed reduced-order model has been developed for the viscous Burgers equation. Accordingly, based on the projection of the governing equation into the modal space, a reduced model is obtained according to the characteristics of the dominant modes. This model when used to simulate the field dynamics over a long time period, may become unstable. Therefore, an approach based on the concept of eddy viscosity closure has been used to stabilize the model behavior. This modification allows the reduced-order model to be a surrogate model of the original equation and predict the time evolution of the dynamical system with relative good accuracy. Comparing the results of the present reduced order model with the data obtained from the direct numerical simulations shows a high computational accuracy.

Today, the parabolic solar collector set is one of the most effective technologies for generating electricity. The performance of a solar cell changes under the accumulation of dust particles on its surface. In this research a three-dimensional flow model and the Eulerian approach with SST k-w based turbulence modeling are used to study the flow field around a set of solar cells, and the Lagrangian approach is used to model the displacement of dust particles. The numerical method used in this simulation is the SIMPLE Algorithm, whilst the upstream second-order method and the Trapezoidal method are applied for the particle motion equation integration. The results show an increase in the drag coefficient with the pitch angle. Also, the lift coefficient has an absolute maximum value at an angle of 30 degrees and approaches zero at the angles of zero and 90 degrees. In addition, the pitch torque at the center of the cell rotation has a maximum value at 30 and 45 degree pitch angles and the lateral torque has its maximum value at 90 ° angle. The particle settling behavior indicates that compared to other cells, the cell that faces the prevailing wind flow has more particle accumulation on it. Also, at low-speed airflow and high pitch angle, the particle deposition increases. The research outcomes show less efficiency reduction with increasing speed, and approximately the same efficiency at the pitch angle of 45, 60, and 75 degrees.

In recent years, fluidized bed reactors have attracted much attention due to features such as uniform temperature distribution, better mixing phases, and high heat transfer rates. The high heat transfer rate in the fluidized bed depends on the hydrodynamic factors of bubbling bed. Therefore, in this study, the effects of particle diameter change, inlet air velocity change and drag model change on Geldart Group B particle performance in a bubbling fluidized bed by studying the mean particles axial velocity distribution and the mean particles volume fraction distribution in the bubbling Fluidized bed. Therefore, in this research, the Eulerian multiphases flow approach and the kinetic theory of granular flow in the fluidized bed have been used. Particles with diameters (500,530,570,600 micrometers) are considered. As a result of this study, by increasing the solid particles diameter from 500 to 600 μm, the solid particles average velocity around the bed core decreases about 45%. By increasing the particle diameter from 500 to 600 μm, the solid particles accumulation in the bed bottom increases by 14%. The Syamlal-O'Brien model predicts the lowest downward velocity (negative velocity) near the walls and the lowest upward velocity (positive velocity) near the bed core compared to other drag models. The Syamlal-O'Brien model also predicts the highest average solid particle volume fraction compared to other drag models. With increasing velocity from 0.550 m/s to 0.587 m/s, the average particle velocity around the bed core increases by about 40%.

The movement of bubbles on inclined surfaces, such as inclined channels, has numerous scientific and industrial applications. In the present study, a three-dimensional study of the lateral motion of a bubble within an inclined channel due to the pressure gradient (Poiseuille flow) in the presence of gravity force is investigated. The Navier-Stokes equations are solved numerically using the finite difference/front tracking method. This method is a combination of drop capture and tracking methods. The results demonstrate that the dimensionless velocity in the flow direction is enhanced with the capillary number. Also, as the channel inclination angle increases, the amount of gravitational force in the direction of the flow ( ) increases and the amount of gravitational force in the direction perpendicular to the flow ( ) decreases, and the bubble becomes closer to the channel center. It is found that the axial velocity of the bubble increases with the channel inclination angle.

.To achieve clean and sustainable energy for power generation and displacement, the engine designers demand a high performance and powerful propulsion. The compressor blades have the task of increasing the loss coefficient and should be considered in the design to prevent the destructive phenomena such as the flow separation. If the reverse pressure on the vane could be engineered in such a way as to prevent the flow separation and control the vortices, a higher loss coefficient would be achieved. A reliable way to achieve this goal is to use a tandem, which is obtained by placing a small secondary blade behind the main blade. In the present numerical analysis, a tandem rotor and stage designed and tested at NASA's Lewis Research Center are studied. The desired geometry is extracted from the mentioned source and a high-quality network with about 896 thousand nodes is applied to it, and then considering the  SST turbulence model, it is analyzed by the CFX commercial software. The rotor and its stage are studied in 5 rounds and therefore 5 different pressure ratios, and the resulting vortices are also subject to investigation and interpretation. Finally, it can be seen that at 2105 revolutions (half of the nominal revolution), both in the rotor and in the tandem compressor stage, the vortices are not fully restrained and occupy a very small area of the pressure diagram in terms of chords, thus demonstrating inappropriate performance. At a pressure ratio of 0.9, we also see a lot of turbulence after the vane, which is not suitable for the operation of the compressor. On the other hand, at 4210 rpm (nominal rpm), the vortices are well restrained and a good reduction in the pressure is observed. It also occupies a lot of space in the graph of pressure by chord. Also, at the pressure ratio of 1.1038, we see a proper formation and control of vortices to reduce the turbulence.

An efficient approach to control the engine of a commercial aircraft is the Min-Max multi-loop structure. In this paper, a Min-Max controller with a switching structure containing output feedback regulators and a saturation function on the fuel flow rate is designed for a turbofan engine. In addition to desirable performance, the stability analysis is an important issue in the process of controller design for aero-engines. Because of the switching behavior of the Min-Max approach, the stability of each single loop by itself does not ensure the stability of the whole system. Therefore, a procedure is provided to analyze the stability of the closed loop system. For this purpose, the Min and Max operators and the saturation function are replaced by their nonlinear equivalents and the structure of the control system is converted to the canonical configuration of the Lure’s system. Then, the conditions for asymptotic stability are extracted and using the presented approach, an asymptotic stability proof is achieved for the closed loop system. In a simulation study with the nonlinear model of a turbofan engine, the performance of the designed Min-Max controller in the thrust attainment and limitation management is compared with the Min-Max/SMC technique.

In this research, the natural convection heat transfer for a square enclosure containing two Nano-fluids, water-alumina and water-copper, which is affected by a diagonal magnetic field, is simulated numerically and the effects of some parameters such as Rayleigh number ( ), Hartmann number ( ), magnetic field angle ( ), nanoparticle volume fraction ( ), type of heat source (linear or sine), length of the heat source ( ) and the non-uniformity parameter of the source ( ) have been studied on the flow and temperature fields. The results show that with increasing Rayleigh  number and Hartmann number, the amount of heat transfer increases and decreases, respectively. Also, the thermal performance of the enclosure is improved by increasing the angle of the magnetic field (from 0 to 90 degrees) and by adding solid nanoparticles to the base fluid, it means a relative increment in enclosure heat transfer is observed. In both high and low Rayleigh numbers, the maximum heat transfer is related to the constant temperature source. After that, for high Rayleigh numbers a sinusoidal source (λ = 1) and for low Rayleigh numbers a linear source (λ =0.5), where the conduction heat transfer dominates on enclosure, have the highest average Nusselt number, respectively. The results also indicate that when the length of heat sources increases, the rate of heat transfer increases too.