مجله مکانیک سیالات و آیرودینامیک
سال نهم شماره 1 (بهار و تابستان 1399)

Lattice fin is an aerodynamic control surface with an outer frame and square or diamond-shaped grid structure of secant septum. The purpose of grid fins is to provide a level stabilizer or control level of the missile while reducing the hinge moment at speeds in which many forces enter the fins. In this research, with computational fluid dynamics method, the septum and density effect of lattice fins on the aerodynamic coefficients of the missile in steady state and supersonic flow regime has been studied. Simulation of lattice fins in this paper, at first have two types of 3D sharp diamond and flat plates, then compared with the near experiment results that were available to them and after that selecting the 3D sharp diamond septum. At last variant of missile aerodynamic coefficients with 50% increase and decrease of septum density fin (unchanged in the overall dimensions of the fin frame) was compared to the reference state. Diagram of axial and vertical force coefficients, as well as the pitch moment and center of pressure in the assumed missile, are given for various angle attacks in Mach 3. Numerical flow simulation results around two types of lattice fins indicate the accuracy of numerical solution in calculating the flow complexities on lattice-shaped geometries and also matching of 3D sharp diamond septum results with the experimental results of wind tunnel. Also, calculations show that by increasing and decreasing the 50% density of septum in a fixed frame, the position of center of pressure can be changed by about one diameter to the front or rear of missile.

When the wing of the plane is placed in the air flow direction, due to surface effects, a boundary layer is created near its surfaces.  The boundary layer phenomenon affects Airfoil’s performance and has significant effects on the lift and drag coefficients, this phenomenon leads to restrictions that prevent the increase in wing performance. Therefore, in order to achieve optimal conditions, it is necessary to control the formed boundary layer by several techniques. In this paper, by the creation a groove perpendicular to the outstanding edge of the airfoil’s NACA0012, suction flow controlled and the airplane wing performance is investigated in two-dimensional and three-dimensional models and under attack angles and different flow rates. For this purpose, the necessary simulation was carried out using the fluent software, using the K-Kl-ω three-equation turbulence model. The width of the jet (created groove) is %2.5 of the length of the airfoil chord (%2.5C), the location of the groove is %10 of the length of the chord (from the leading edge of the airfoil) and the speed of suction is considered to be half the free speed. The results show that increasing the coefficient of lift can be achieved by creating the suction flow at the wing level and reduced the drag coefficient, which causes a delay the separation of the flow Subsequently, the angles of attack and the appropriate free flow speed rate can be selected to improve the flight of the Unmanned plan.

A numerical method is presented for the simulation of the two-way interaction of a rigid body with an incompressible Newtonian fluid in a two-dimensional configuration. In this method, the moving boundaries which are modeled by the radial base functions, are implemented on a Fourier pseudo-spectral solver of the Navier-Stokes equations in the vorticity-velocity formulation. At the beginning of each time step, the conservative velocity fields, satisfying the immersed boundary conditions along with a modified vorticity field, are implemented to the pseudo-spectral solver. The Poisson equations are solved in parallel, because the velocity boundary conditions obtain independently. The dynamics of the solid body is followed by a second-order method in which the forces are obtained from a vorticity-based integral method. The time integration is performed using a third-order parallel Runge-Kutta method. Employing a parallel fast Fourier pseudo-spectral solver along with parallelization of the time integrations, in combination with modeling of the solid boundaries using the radial base functions result in a very fast and efficient solver that makes it possible real-time simulations (which requires at least 21 flow snapshots per second). The accuracy and efficiency of the method is demonstrated by solving some sample problems.

One of the throats in designing hypersonic wind tunnel is the design of heater. The purpose of this study is designing a heater with ceramic cored, in 1800K temperature for preventing air condensation in wind tunnel. In initial part of design the fluid flow and heat transfer equation have been solved through an inviscid engineering code. This code estimates the temperature of transient outlet flow with static physical and thermodynamic properties of fluid and solid, geometrical dimensions of bed and wall distribution temperature. The final part of design has been solved with computational methods and using of Navir Stocks and energy equation, with K-ω SST turbulence model. The solver is 2nd order and pressure base. The numerical results are in acceptable agreement with experimental. By parameter analysis, various design for obtaining a suitable heater have been evaluated. The evaluated parameters are height, diameter, porosity of the bed and the diameter of the hole and diameter of the bed. The results show that the diameter of the hole, the diameter of the bed, the height and prosity the most important parameters in the design of the heating bed and have a great impact on the temperature of the exhaust air and the duration the test respectively. 14% increase or decrease of each parameter increase 61, 51, 28, and 8% of execution time and 6, 3, 2 and 1.8 % of the output air temperature respectively. The hole diameter and prosity of the heater bed are two important parameters in determining the dimension of the heater bed. Finally with using of the results the final design has been modified with bed diameter of 0.45 meter, height of 1.7 meter, hole diameter of 3 millimeter and prosity 0.2512.

In the present research, the effects of buoyancy force and radiative parameters on the hydrodynamic and thermal behaviors of mixed convection flow of a radiating gas in an inclined two-dimensional duct with a trapezoidal recess are studied and investigated. This recess is created inside the duct by two inclined backward and forward facing steps. For modeling the inclined surfaces of this recess in the Cartesian coordinates, the Blocked region method is used. To obtain the velocity and temperature fields, the dimensionless forms of the governing equations are solved using the finite volume method and by applying the Simple algorithm. The discrete-ordinates method is used to calculate the divergence of the radiative heat flux in the energy equation. The results of the numerical solution show that with increasing the Grashof number and duct inclination angle, the values of the mean bulk temperature along the recess and also the values of the friction coefficient and heat transfer rates on the bottom wall of recess increase. Besides, an increase in the magnitudes of radiation-conduction parameter and optical thickness results in an enhancement in the values of the mean bulk temperature and total Nusselt number along the bottom wall of recess; while these values decrease by increasing the magnitudes of albedo coefficient.

In this study, the effectiveness of three types of winglets such as Blended , Multi-tip, and Raked on a specific wing,is investigated by using a numerical method based on finite volume and pressure-based algorithms. In thisnumerical method, the Spalart-Allmaras turbulence model is used. In this simulation Reynolds number is 1.5×105and SD7032 airfoil is used for wing section by 4.6 aspect ratio. The stresses on the wing surface are calculated bythe wall functions, and the convective fluxes are computed by the second-order upwind accuracy. In this research,the specific airfoil is used for wing and winglet and the effectiveness on the aerodynamic performance of arectangular wing has been investigated. Evaluation of aerodynamic coefficients and flow physics have shown thatusing the same airfoil for wing and winglet with different angles of attack, It has increased the performance of theMulti-tip winglet by 3.1 percent compared to the case of using two airfoil and also for Blended and Multi-tipwinglets compared to the wing without winglet will increase an average aerodynamic performance by 10.5 and 10percent. But for Raked winglet, it has only a small effect on reducing the power of the vortex core.

In this paper for the first time, the effect of direction of wall movement on mixed convection in circle quarter porous enclosure with heat absorption/generation is investigated by LBM. The magnetic field is applied to the enclosure in uniform and periodic forms. Mixed convection is caused by the movement of walls at different angles. The results show that increasing the Richardson number, Hartmann number, heat absorption/generation coefficient and decrease the porosity coefficient reduce the average Nusselt number. With fixed all parameters the maximum value of the average Nusselt number is related to the 90 ° velocity angle, in which case the average Nusselt number is about 25% higher.  Increasing the Richardson number reduces the effect of the magnetic field. Periodic applied of a magnetic field results in about 30% more than uniform applied. Increasing the porosity coefficient also increases the effect of the Hartmann number and the angle of wall movement. It is observed that the simultaneous increase of the heat absorption/generation coefficient and the Hartmann number lead to a further decrease of the average Nusselt number.

In this research, interaction influences of magnetic field and solid nanoparticles on three-dimensional forced convection flow in a horizontal duct with an inclined backward-facing step are studied. The nanofluid is considered as the working fluid in duct. The Blocked region method is applied to simulate the inclined backwardfacing step in Cartesian coordinates. To obtain the velocity and temperature fields, the Navier-Stokes and energy equations are solved using the Finite volume method and SIMPLE algorithm. Influences of Hartmann number ( ) and concentrations of nanoparticles ( 𝜙 ) on the velocity and temperature fields in duct and distributions of fiction coefficient and Nusselt number along the bottom wall of duct are analyzed with full details. Results of this research clearly show that the hydrodynamics and thermal behaviors of flow are considerably dependent on the magnetic field strength and volume fractions of nanoparticles. In fact, the highest values of fiction coefficients and heat transfer rates on the bottom wall are related to the case of and 𝜙 .

In internal combustion engines, the fuel system is one of the most accurate and sensitive parts of the engine. In this study, the effect of fuel filter lifetime on engine characteristics was examined experimentally. The main parameters examined in this study were fuel pressure and ignition advance. Then, using the response surface method (RSM), the interaction effect of the variables on the engine ignition advance was examined and a smart control system was designed to determine the appropriate time to change the fuel filter in an engine and inform the user. The maximum inlet pressure of the gasoline filter with a filter lifetime of 50,000 km was measured as 4.5 bar. Also, the lowest amount of inlet pressure, if using a new filter, was measured as 3.5 bar. The results of this study showed that the fuel filter lifetime also has a significant effect on the main parameters of the engine, such as the weight ratio of the fuel-to-air mixture and the ignition advance. Studies in Euro2 and Euro4 engines have shown that the ignition advance at 1,000 rpm engine speed with a new fuel filter was seven degrees (minimum value), while the maximum ignition advance at 5,000 rpm engine speed and with a 50,000 km filter lifetime was measured at 37 degrees. The results showed that gasoline filter clogging has a significant effect on fuel pressure, fuel-to-air mixing ratio, and ignition advance, which in turn will have a direct impact on combustion quality and engine efficiency.

In this research, the combustion chamber of the liquid propellant engine has been simulated numerically by the computational fluid dynamics (CFD) method. After the simulation, several methods have been proposed to improve the combustion. These include using baffle inside the combustion chamber, increasing the equilibrium ratio and the nano catalyst. In each of these methods, the combustion temperature, mass fraction of fuel and oxidizer, mass fraction of combustion products and mass fraction of pollutants were calculated and compared with a simple chamber. The results showed that using these methods increases the combustion heat by 28.36%, reduces fuel mass fraction by 27.91%, increases mass fraction of complete combustion products including water and nitrogen, and reduces NOx pollutant mass fraction by 26.85 as it is known that NOx pollutant is a product of incomplete combustion and generally reducing it results in better combustion.

Dynamic stall is a phenomenon which appears due to the vortex shedding on the oscillating wing section at high angles of attack. Occurrence of the dynamic stall causes a severe decrease in the lift force and huge increase in the drag force. The Co-Flow Jet (CFJ) is one of the active flow controls to prevent this phenomenon. In this paper, the effect of this active flow control on the NACA 0025 airfoil for different Reynolds numbers is investigated. For numerical solution of the fluid flow, the Reynolds-averaged Navier-Stokes equations in two-dimensional, incompressible, and unsteady form with the SST-k-ω turbulence model is solved using an in-house computer code. The developed code is validated with the previous experiment data, and a fairly good agreement is observed. In order to investigate the effects of the CFJ, three different momentum coefficients of 0.05, 0.07 and 0.08 and five Reynolds numbers of 5×104, 7.5×104, 105, 1.5×105, and 3×105 are studied. It is found in the examined cases that the baseline airfoil in the Reynolds numbers of 105 and lower has different behavior compared to the higher Reynolds numbers; while in order to eliminate the dynamic stall, it requires more jet momentum of 0.08, while for the higher investigated Reynolds numbers, by applying the jet momentum of 0.07, the dynamic stall is completely eliminated.

In the present study, using numerical solution to predict the flow field and aerodynamic forces of three types of missile with the different designs of the fin and also to comparison of the ordinary and lattice fin. One of the most important parameters in the stability and performance of a missile is the fins. The design and optimization of these fins can have significant effects on the aerodynamic efficiency and maneuverability of the missile. The results are presented in the range of Mach numbers 0.5 to 3 and the angle of attack 0 to 10 degrees. In this study, parameters such as drag force, lift force, flow field, and Mach number distribution were analyzed. The results showed that missile with lattice fin in the tail position; it produced the greatest drag force. Also, the lift force for the missile with lattice fin in the tail position is higher than the other two missiles (ordinary and lattice canard) and increases with increasing angle of attack. The momentum forces for fins located in the canard position shows less value for the lattice fin than the flat fin. The results show that the drag force also increased from subsonic Mach numbers to supersonic Mach numbers, and then decreased with rising Mach numbers. In addition, with the increase of Mach number to the hypersonic Mach numbers, normal shock waves are formed in front of the fin, causing a sharp increase in drag force.