مجله مکانیک سیالات و آیرودینامیک
سال نهم شماره 2 (پاییز و زمستان 1399)

In this paper, a new portable plugin extension has been designed and optimized, which can be installed on dual-spin projectiles. The aerodynamic and dynamic stability effects of this plugin extension have been investigated in such a way that byincreasing its performance, it is possible to modify the projectile path by the optimal extension. This portable device with four canards mounted on the projectile's non-rotating part can increase the torque coefficient and correct the projectile path. Initially, to achieve the design parameters, various simulations were performed, and the effects of several parameters on the aerodynamics and stability of the projectile were studied. Adjusting to real conditions of Mach 2 to 3 and at the attack angles of 0 and 2 degrees, the projectile was simulated with the new add-on, and the results were compared with that of the projectile without any extensions. By defining the design variables and objective functions in terms of aerodynamics and stability, the optimization of this plugin extension with canard was investigated by the response surface method (RSM). In the first phase of optimization, dynamic and gyroscopic stabilities were mentioned as response functions, and rotational power was chosen in the next phase of the study. As a result, the projectile's stability was investigated with a new optimized extension, and the optimum add-on system displayed excellent aerodynamic capabilities. Although the new plugin device leads to a slight increase in the projectile instability, the stability conditions are still fully met, and the optimized plugin extension can be used in dual-spin projectiles.

In the process of forming a reflective shock wave, the separation of the flow in the reflection site is a common phenomenon. This phenomenon, known as the shock-induced flow separation, sometimes leads to undesirable effects such as reducing the volume and disrupting the flow of air into the ramjet and scrarmjet engines. Therefore, we try to reduce these undesirable effects by different flow control methods. Magnetohydrodynamics is one of the advanced methods for controlling the flow, which shrinks the bubble separation by accelerating to the boundary layer by Lorentz force. The purpose of this research is the numerical study of the effects of this method on the separation caused by the reflection shock wave. The simulations are performed in two-dimensional conditions for different Hartmann numbers and the effects on flow characteristics such as pressure, velocity and flow stream line are investigated. The results indicate that the application of the magnetohydrodynamic field can significantly reduce the adverse pressure gradient and the vortex flow, and it can also reduce the size of the separation bubble by 40% and increase the skin friction coefficient by increasing the wall shear stress. Moreover, the application of the magnetohydrodynamic field can change the shock angle, causing the separation bubble to move forward by about 2% of the channel length.

In this paper, the movement of an oil spill around the Khark Island has been studied, using the hydrographic information of the Persian Gulf region and the Mike21 software. To obtain the water flow flux, a model with an irregular triangular network containing 11325 nodes was assumed and the tidal level of Hengam Island in the Strait of Hormuz and the land boundary condition on the coast were considered. Then the tidal currents in the Khark region were simulated by considering 3 open boundaries and one land boundary (the Bushehr coast). In order to calibrate the information, the tidal level in Khark station was compared with the output of the model tidal level. Based on the results of the model, the amount of water level fluctuations caused by the tides from February 15-25, 2011 were calculated to be in the range of 0.33 to 2.27 meters and the speed of the tidal currents were calculated from 0 to 0.7 meters per second. Also, the amount of water level fluctuations caused by the tides from 10 to 20 August 2011 were 0.36 to 2.19 meters and the speed of tidal currents were 0 to 0.6 meters per second. Based on the value of current obtained from the model for a station, the tidal current time series was plotted and its time-space equation was obtained. To obtain the path taken by the oil spill, the time for the spill to reach full emulsion and thus stop, is obtained using Adios2 software. In 2011 the tidal currents obtained in February were faster than August and as a result, in February 2011, there were a lot of fluctuations in the movement of the oil spill.

This study is conducted on the effects of leading-edge slat and longitudinal slots in delaying the flow separation. The case study is a conventional wind turbine airfoil and four different types of slots have been investigated. The aerodynamic simulation is performed on the basis of a steady state air flow over the NREL S809 airfoil and the solution is obtained numerically using the structured grids. The results show that at Reynolds number of 1e+6 and an angle of attack equal to 16.22o, with the addition of leading-edge slat, the separation is delayed from x / c = 0.47 to x / c = 0.67 and the lift coefficient is increased by 64% (from 1.17 to 1.92). So, by adding several types of longitudinal slots, it is observed that a sinusoidal slot with 45o of phase lead, has the best performance.Through studying the values of the aperture, depth and location of the sinusoidal slot, the best values of these parameters were obtained as follows: 3% of the chord for the aperture value, 0.5% of the chord for the depth value and 0.85 for the x/c ratio. By completely removing the flow separation, at the mentioned Reynolds number with the same angle of attack, the lift coefficient has 117% increase, reaching the value of 2.54.

Cardiovascular diseases have been one of the main causes of death throughout the world in recent decades.One of the most common heart diseases is stenotic arteries, which usually appears with middle age. This disease, called atherosclerosis, causes an abnormal reduction in the inside diameter of blood vessels. In this study, the effect of a uniform magnetic field on blood flow and artery walls is investigated. The geometry of the problem is simulated in three dimensions. The governing equations, which include continuity, momentum, ohm’s law, stress-strain of linear elastic material, and fluid-solid interaction with moving mesh method, are defined, coupled, and solved with a finite element code in the Comsol software.The results indicate that the magnetic field has a significant effect on the behavior of blood flow and artery walls. For example, the Hartmann number is inversely related to the blood flow velocity and is directly related to the wall shear stress, the von Mises stress, and the artery wall displacement. It is also observed that the trend of changes with non-Newtonian viscosity model is greater than the Newtonian model.

In the present research, studies were conducted by a three-dimensional simulation of fluid flow passing around both an undamaged wing and a wing damaged by a bullet (namely a star-shaped damaged wing), using the Ansys-Fluent numerical software. We assumed a viscous, unsteady and incompressible flow to observe the effects of wing damage on aerodynamic performance and coefficients such as the lift and drag coefficients. In order to conduct the study, for the wing consisting of  airfoil, the Reynolds number  value was considered equal to . After meshing and gaining grid independency, the results were validated. Due to the turbulence of the flow regime, we have used the  turbulence model to properly investigate the problem near the wall and outer layers. As a novelty in the present study, in addition to the mentioned model, and turbulence models have also been used to simulate the problem and examine the differences resulting from their usage. The numerical results were validated with the valid results available, as a good agreement was observed. The numerical results show that a star-shaped damaged in the wing leads to reducing the lift force, increasing the drag forces and thus reducing the aerodynamic performance of the wing. Also, the results show that by increasing the angle of attack, a severe star-shaped damage on the wing reduces the lift forces and increases the drag forces.

In the present study, the effect of viscousdissipation on the flow and conjugate heat transfer of water-alumina nanofluid in a two-dimensional microchannel under the influence of a magnetic field is investigated, using the incompressible lattice Boltzmann method.The upper wall of the microchannel is insulated and uniform heat flux is imposed on the lower wall of the solid region. The investigation has been carried out at Reynolds numbers of 50, 75 and 100, for a nanofluid with 2% volume fraction. The nanoparticle diameters varied from 10 to 50 nm and variable Hartmann numbers ranging from 0 to 30 were considered. The results showed that in the case of ignoring viscous dissipations, using a magnetic field in conjugate heat transfer does not have a significant negative effect on the average Nusselt number, and despite the usual expectation, can increase the Nusselt number, especially at higher Reynolds numbers. It is also noted that the average Nusselt number when ignoring the viscous dissipations, is higher than when these dissipations are taken into account. Hence, the highest Nusselt number in this case, is observed at Reynolds and Hartmann number values of 100 and 30, respectively.

In the present article, the influence of alumina nanoparticles addition on the evaporation of diesel fuel droplets in the combustion chamber of the gas turbine model has been numerically investigated. The nano-fuel, resulting from adding the alumina nanoparticles in volume fractions of 0.5 and 1 percent, has been considered as a single-phase fluid. To model the two-phase flow, which is consisted of fuel droplets and inlet air, the Eulerian-Lagrangian approach is applied. In order to investigate the characteristics of the reactive spray flow, Reynolds Averaged Navier-Stokes turbulence approach, discrete ordinates radiation heat transfer model, and steady diffusion flamelet combustion model have been employed. Comparisons between the present results and the existing experimental data have been made to evaluate the obtained accuracy. The results show that, with the addition of alumina nanoparticles, the heat capacity and lifetime of fuel droplets are increased. Also, in the presence of nanoparticles, the penetration depth of the droplets increases, and droplets evaporate in further distances from the inlet boundary. This implies a reduction in the evaporation rate of fuel droplets in the presence of alumina nanoparticles.

In the present study, the ground effect on the dynamics stall of the flapping wing with the bending deflection angle is experimentally investigated in the forward flight.For this purpose, first, a bending deflection mechanism and the installed facilities for the wind tunnel have been designed and fabricated.Then the simple and bending flapping wings are experimentally examined for h/c=1,1.5, flapping frequency 3.5 Hz and different angles of attack from 00 to 22.50 with velocity 3 m/s. Also, to investigate the effect of flapping frequency and distance from surface on lift force, thrust force and loading power of the simple and bending flapping wings with no angle of attack tested for h/c=1,1.5,2, different flapping frequency from0 Hz to 5 Hz and velocity 3 m/s. Results indicated that decreasing the distance from the surface, dynamic stall of the simple flapping wing occurs at lower angles of attack compared to the bending flapping wing. More specifically, in the minimum distance from the surface (h/c = 1), the stall angle of the simple flapping wing and bending flapping wing (with bending deflection angle of 1070) takes place at 12.50 and 150, respectively. The performance of the bending flapping wing is generally better than the simple flapping wing. Besides, by enhancing flapping frequency and decreasing the distance from the surface, aerodynamic forces and loading power increase.

The aerodynamic coefficients of any flying object can be estimated with high accuracy, by aero-ballistic tests, monitored in aerodynamic laboratories. For test-running management, it is necessary to determine the number and type of estimated variables and the station placement of each test. For this purpose, the sensitivity of variables under measurement, in relation to the associated aerodynamic coefficients must be calculated and surveyed. As the trajectory path is a nonlinear equation with six degrees of freedom, in this article we estimate the aerodynamic coefficients and sensitivity of each output to the changes of aerodynamic coefficients using the least square method and fisher data matrix. In other word, if the test data such as the speed and pitch angle are to be measured in an aero-ballistic test, the results of this research can specify their accuracy and sensitivity to each aerodynamic coefficient and the relevant coefficient errors.

In this research three different models of rotational force are evaluated in the study of a hovering fruit fly inspired wing, with combined flapping and pitching motions, using a quasi-static simulation by the blade element method, and the results achieved thereby, are compared with previously published results. Then these models are compared with each other, and the model which has the lowest relative error is introduced. The traditional rotational force model, which depends on the wing's translational velocity, does not consider any rotational force at the beginning and end of the stroke. The new rotational force models which are designed to accurately predict this force also consider a second component for this force which is due solely to wing pitching. In this research, the instantaneous and mean coefficients in the blade element method are compared with published computational fluid dynamics and experimental results. The models are compared in terms of the error in predicting the maximum point on the instantaneous force coefficients curve. The root-mean-square error analysis shows that one of the rotational force models, which includes the two force components described by the Kutta-Joukowski theorem and the force due solely to wing pitching, has higher relative accuracy than other models and can be proposed for quasi-static simulations.

The propulsive force in an unmanned aerial vehicle is usually produced by propellers and the UAVs require maximum propulsive force and minimum drag force to perform various operations. The aircraft's performance is influenced by the spinner shape; therefore, accurate analysis of its aerodynamic shape can improve the performance. In this research, the aerodynamic design of the spinner and the geometric parameters effective on the UAV performance are mentioned. The ultimate goal is to use an optimized spinner for dual-propeller UAVs. Initially, the effect of spinner geometry on the aerodynamic features of the pusher propeller is investigated. Then, a numerical simulation based on computational fluid dynamics is done and the outcomes for both the presence and the absence of spinner are compared. In the numerical simulations, the flow field is considered as three-dimensional, unsteady and turbulent, and these results are compared with the published data for verification of the numerical procedure. The research's motivation is to improve five design variables in the design of pusher propeller spinner namely,  (the radius of reference plane),  (the reference length),  (the gap between the spinner and cowling),  (the reference slope angle), and  (the slope angle of spinner cap) and to provide solutions to select the appropriate spinner by an efficient method such as the Taguchi method. This process is followed in such a way as to achieve the desired propeller performance as an objective function. Eventually, the optimum spinner which achieves improved performance, increased propeller propulsion efficiency and declined fuselage drag force is obtained.