FLUID MECHANICS AND AERODYNAMICS JOURNAL
Year:2020 | Volume:8 | Issue:2 (24)|Papers Count:12

Investigation of the hydrodynamic parameters of the moving objects under the free surface in order to study the effects of velocity, curvature, thickness and angle of attack of hydrofoils has been initiated since many years. The computational fluid dynamics software, FLUENT, provides the solution of the governing equations with VOF method for two phases flows. The RSM turbulence model is used in this article which is more accurate generally. In the present study, the effects of free surface and finite depth on the hydrofoils characteristics are investigated simultaneously and compared with those derived from the BEM method and validated with the existing and experimental values. The results show that for the hydrofoils in the more depth, the lift and drag coefficients and lift to drag fraction increases. Furthermore, in shallow water flows, the hydrofoils with higher camber and lower angle of attack have better performance and the downstream waves damp earlier.

In this paper, the effects of the atmospheric boundary layer as well as the inflow turbulence on the performance of a large-scale wind turbine are investigated. The reference wind turbine is the NREL 5 megawatts with a rotor diameter of 126 meters. Wind shear modeling is carried out using the mixing length theory. Due to the importance of turbulence analysis, the Sandia method is applied. According to the reference level, the roughness coefficients of 0. 01, 0. 2 and 0. 5 and disturbances in the roughness of 0. 5 with the intensity of 1, 5 and 15 percent are studied. The aerodynamic forces of the rotor are calculated based on the modified blade element momentum theory. The results show that in the case of maximum roughness the averaged output power is reduced to 160 kW. Moreover, at 15% turbulence intensity, 50 kN and 25 kN are added to the maximum/minimum thrust force value, respectively.

In this study a heat transfer phenomena in a gun tube using Differential Quadrature Method was examined. The phenomena was dealt in a two dimensional transient manner. In order to apply a general condition to the problem, the internal volume of the cylinder was divided into two distinct zones. One zone was exposed to a fluid with high temperature and high pressure conditions due to the burning of the propellant. The other zone was exposed to a fluid in environmental condition. As the time passes, the bullet moves toward the front zone. Therefore the volume of the rear zone increases, while the volume of the front zone decreases. The boundary conditions of the internal boundary nodes are determined due to the axial position of the bullet. The boundary condition of the nodes located in the rear zone are set to the high temperature and high pressure fluid, while the boundary condition of the front nodes is set to standstill fluid conditions. According to the mentioned conditions, general transient heat conduction equation in cylindrical coordinate was utilized and temperature distribution of the gun tube was extracted for numerous rounds. Finally an experimental study was examined to validate the proposed method. A comparison between the simulation results and experimental results shows a high consistency between them. The results of this study can be used for stress analysis of gun tube and estimation of its erosion and lifetime.

The aim of inverse design problems is achieving a geometry corresponding to the wall target pressure distribution. One of the newest inverse design methods, was Elastic Surface Algorithm (ESA) in which the airfoil wall was modeled as a flexible curved beam and the difference between target and current pressure distributions in each iteration was the deformation factor of the airfoil wall. The first version of ESA used for the inverse design of sharp leading edge blade of axial flow compressors, is subject to oscillation, instability and divergence due to high pressure gradients on the blade leading edge. Therefore, it cannot be used for a sharp leading edge blade. The purpose of this research is to develop ESA for axial-flow compressor cascade with sharp leading edge blades. The main basis of this Improvement is paying attention to the deflection curve of the beam, which is continuous and differentiable in all points. The physical property of Timoshenko's beam in large deformations is used in the upgraded version without applying any geometric filtration to eliminate the fractures of the intermediate profiles. In order to increase the displacement of the beam at each iteration, an optimal relationship between the beam characteristics including the elasticity modulus, the thickness and width of the beam is used. Finally, the upgraded version has been validated in a few cases for subsonic inviscid flow regime. The results indicate the robustness, flexibility and high convergence rate of the upgraded ESA in the design of axial-flow compressor blades.

One of the best methods for improving quality and clarity of the ultrasound imaging is the use of coated bubbles. In this paper, the dynamics of an encapsulated bubble near the wall is simulated with different elasticity for the ultrasound imaging. For this purpose, a computer program has been developed in MATLAB software, in which the modified Rayleight-Plesset differential equation is solved numerically using by the fourth-order Runge Kutta method. Initially, the results were compared with experimental data, then the radial behavior of the encapsulated bubble (UCA) was investigated for two adjacent states with rigid and elastic wall and the scattered pressure from the bubble was simulated. In addition, the effect of the shell viscosity (Ks), the initial radius of the bubble and the distance between the bubble and the wall on the dynamic behavior of the bubble and the amount of scattered pressure therefrom are investigated. The results are presented in the form of table and graphs. The results show that the bubble shell's viscosity and the initial bubble radius have a significant effect on the bubble dynamics. Finally, the frequency response of the encapsulated bubble has been investigated and the effect of the initial radius of the bubble, the shell material and the elastic modulus of the wall on the strength of the fundamental spectrum has been presented. The results show that by increasing the elastic modulus of the wall, the strength of the fundamental spectrum reach the limit state and does not change.

The main goal of this research is to study of interaction between the propulsion system and the WIG Craft main parts. Therefore, the effects of some parameters such as the propeller rotation direction and the propulsion system location on the aerodynamic quality of the WIG craft have been studied deeply. Using ANSYS-CFX Software, The SIMPLE algorithm has been utilized to consider the pressure-velocity coupling, additionally the k-ω SST model has been applied as a turbulence model to take the account of the flow separation. The numerical approach is verified by comparing its results with experimental data of a three-blade aerial propeller. These comparisons indicated that there was a good agreement between present numerical results and experimental ones. The results revealed that the propeller rotation direction has no significant effect on the aerodynamic quality of the WIG craft and at α =3o, the highest aerodynamic quality is achieved. Moreover, as the propulsion system approached closer to the WIG craft center line, the drag and lift forces on the horizontal tail are respectively enhanced and decreased, subsequently in this situation the longitudinal stability of the vehicle is decreased; whereas by changing the vertical position of the propulsion system, the drag and lift forces on the horizontal tail are correspondingly reduced and increased, Thus the longitudinal stability of the WIG craft is increased.

In recent years, research on metal oxide gas micro-sensors has been rapidly developed. These sensors are small in size, low cost in fabrication and consume little power. The purpose of the current study is to numerically investigate converge micro-channel on gas inlet temperature under the influence of thermal creeping. The governing nonlinear differential equations, mass, momentum, energy, and species, are coupled and solved by a commercial code. Since the Knudsen number is between 0. 01 and 0. 1, the slip boundary condition, Maxwell equation, is utilized. The result shows that flow velocity and temperature increases from the micro-channel inlet to the heat source and decreases from the heat source to the micro-channel outlet. Also as the inlet height and convergence increases, at the first flow velocity increases then decreases. This trend for temperature is reverse of the trend for flow velocity.

Cavitation phenomenon has destructive effects on the turbomachines performance but the occurrence of this phenomenon inside the injector nozzle has significant effect on the fuel spray hydrodynamic behavior. The nozzle geometry, needle lift profile and fuel type are effective parameters in creation of this phenomenon. The main goal and novelty of the present study is to investigate the effect of simultaneous change of the injector nozzle geometry and the needle lift profile on the creation of cavitation and diesel fuel spray hydrodynamic behavior. Thus in the first part, the liquid flow and spray characteristics of the cylindrical and converged conical nozzles with the same needle lift profile are investigated numerically using AVL-Fire CFD code. In the second step the converged conical nozzles with different needle lift profiles are simulated. Numerical results of the second part of this study show that the converged conical nozzles with different needle lift profiles have a longer spray penetration length and smaller Sauter-mean diameter than nozzles simulated in the first part of this study.

In the present study, the sensitivity of the behavior of an Upper-Convected-Maxwell polymer and an Oldroyd-B fluid are compared to a Newtonian fluid. The governing equations are the Navier-Stokes equations and viscoelastic fluid equations. These equations are non-dimensionalized and it is found that Reynolds and Mach numbers are only the dimensionless parameters that exists in all three mentioned fluids. The viscosity of fluids as a friction factor causes that Reynolds number and its changes play an important role in the oscillations and also the attenuation of the fluid transient during Fluid hammer phenomenon. The numerical method used is a two-step variant of the Lax-Friedrichs (LxF) method. The results show that the viscoelastic properties of fluids, such as the non-linear relationship of stress and strain, relaxation time and. . ., reduce their variability compared to Newtonian fluid. Finally, the maximum sensitivity to Reynolds variations in Newtonian fluid and the minimum amount is observed in the Upper-Convected-Maxwell polymer that there are strong viscoelastic properties in its equations.

In the present study, large eddy simulation of unsteady, three-dimensional and turbulent flow over non-confined backward facing step (BFS) is numerically conducted. Due to the capability of the compact differential scheme in high-order solution of the compressible flow, this method is used to solve the filtered Navier-Stokes equations in the generalized curvilinear coordinate using a multi-block structured grid. To study the influence of the sub-grid scale (SGS) stress model, the Smagorinsky model, the MKEV model, the dynamic Smagorinsky model (DSM), and the WALE model are considered. The numerical results include the general characteristics of the flow over BFS such as the reattachment length, friction and pressure coefficients, the mean velocity and the Reynolds stresses. Moreover, the present LES results are compared with the available experimental data and DNS, LES and RANS results and a very good agreement is achieved. Moreover, the obtained results using the DSM and WALE models give a better agreement with the DNS and experimental data than those by Smagorinsky and MKEV models.

In the aerodynamic design of missiles, it is necessary to consider a wide design space. The complexity of a design space depends on the number of input variables. Missile diameter, nozzle, body length, the number of rows of fin, the number of fins for each row, the size and shape of each ffin, and their cross-section can be provided as examples of design variables. In this study, the aerodynamic characteristics of drag force, lift force, rotational torque, maneuverability, lift-drag force ratio as well as compressive force at Mach numbers and different attack angles were investigated. The results showed that aerodynamic coefficients increased with the addition of a fin in front of the missile. It is also observed that the stability of the missile improves when the fin is near the end of the missile but the ratio of lift to drag and maneuverability is reduced. It was also found that by increasing the length and height of the fin lifting force to the drag force, vertical force and the stability of missile increases. Also, it was observed that lift-drag force ratio increases with the increase in the number of fin rows.

In this article, investigating fin type geometry effects with numerical analysis on aerodynamics coefficients in steady state and supersonic flow. In fact the Grid Fin is an aerodynamic control surface with a grid structure (square or rhomboid) of thin septum that has many advantages and provide high levels of stabilizer or control surface for the missile. The simulated lattice fins in this paper have rhomboid partitions and investigating the variance of aerodynamic coefficients of the missile with an increase of 25% & 50% in the span, an increase of 50% & 100% in chords and an increase of 1. 5 times the width with the constant fin thickness assumption (no change in general dimensions of the fin frame). Initially, the axial and vertical force coefficients as well as the pitch moment and center of pressure on missile were calculated at various angles of flight on Mach 3 in main grid fin and compared with numerical and experimental results in reference articles. The results of the comparison show the numerical resolution accuracy on calculating the flow complexities in the missile latticework with the experimental results of the wind tunnel. Further, by increasing the span, chord and lattice width, the aerodynamic coefficients variant of the missile were compared to the reference missile. Calculations show that by increasing the span and chord, all the aerodynamic coefficients will increase to the extent that they will be mentioned, and with increasing width with constant fin thickness, Drag coefficient will be reduced to a small extent and the remaining aerodynamic coefficients will increase.