مجله مکانیک سیالات و آیرودینامیک
سال دوم شماره 3 (پاییز 1392)

There are different approaches for simulating two-phase flows. In this Paper a Non Conservative force field is used to model bubble’s effect on the fluid flow. To reach this goal، firstly bubble-fluid interaction model for simulating bubble effect on fluid flow is presented in addition to bubble dynamic analysis in the fluid flow. According to the main idea in this research، two phase flow analysis is reduced to single phase flow. This model makes it possible to use the powerful LES method for simulating this phenomenon. In this present work، two concentric cylinders which inner one rotating with constant velocity (Taylor-Couette flow) is solved by LES method which is the best method in turbulence modeling. Thus، this problem is simulated for different Reynolds numbers. Moreover، the results are compared with DNS method. Furthermore، the effects of micro bubble injection on the velocity profile and turbulence intensity are discussed. On the other hand، the effects of micro bubble number on drag reduction are investigated.

In this study we investigate three different boundary conditions which are introduced to model solid curved boundary conditions in Lattice Boltzmann method، in terms of accuracy and stability، these boundary conditions are FH Boundary condition، Mass conserving FH boundary condition and OSIF boundary condition. All of these three boundary condtions are based on standard bounce back boundary condition. In standard bounce back boundary condition those distribution functions which escape from flow domain in one time step return to the flow domain exactly with the same value in the next time step. Those boundary conditions which claim that they have optimized the bounce back boundary condition try to modify those distribution functions which return to the flow domain in order to maintain conservation of mass and momemtum. Conservation of momentum for moving boundary conditions is very essential since exactly the same momemtum of the solid body should be transferred to the fluid during movement. in this study it is shown that despite the Mass Conserving FH boundary condition and OSIF boundary condition are able to fullfil conservation of mass for non-moving solid curved boundaries but they are not able to meet conservation of momentum for moving solid curved boundaries. The results from simulating flow over a two-dimensional circular cylinder in a channel and also simulating flow over and oscillating cylinder with longitudinal oscillation show that FH boundary condition predict accurate and acceptable results compared to the results form literature. Therefore to simulate moving curved solid boundary in applicational problems، FH boundary condition can be used with confidence and the results obtained can be easily trusted.

In this investigation، a numerical method based on potential flow for aerodynamic analysis of morphing MAVs has been developed. At first، results of this method are validated with experimental data and then، effect of taper ratio in range 0. 3 to 3. 0 on aerodynamic characteristic have been studied. Results show generally increasing taper ratio، whether is the range of zero to one or bigger one، cause increasing lift coefficient and improving performance of the MAVs. The only disadvantage of taper ratio of bigger one is appearing large pitching moments. Of course، in MAVs is negligible due to the small size. In this study، also a mechanism for creation of different taper ratios has been discussed.

In this paper a computer program is developed in order to investigate the effect of non equilibrium evaporation and condensation of water vapor at the bubble wall and that of chemical reactions of gases and vapor inside a bubble. Bubble instantaneous radius was obtained using Gilmore equation which considered the compressibility of the liquid. Pressure inside the bubble is calculated using the Van der Waals equation. The Hertz-Knudsen model is employed for modeling the evaporation and condensation of water molecules at the bubble wall. Numerical analysis is done for bubble in acoustic cavitation condition. With change the bubble inner gas type and decreasing bubble oscillation frequency، results have been developed for hydrodynamic cavitation. Numerical results show that for high driving pressure frequencies، the modeling phase change does not a significant effect on the bubble behavior. But by decreasing driving frequency the maximum bubble temperature decreases. It is also clarified that most of all the vapor molecules undergo chemical reactions at the strong collapse.

DBD plasma actuators are devices for active flow control, which recently have been widely of interest for aerodynamicists. Due to their advantages, there is a great focus on plasma actuators with the ability of unsteady actuation. In this paper, using a plasma power supply with the ability of unsteady actuation, the effective parameters on the performance of DBD plasma actuators in two conditions, i.e. steady and unsteady actuations, was experimentally measured and compared. In this regard, the effects of the applied voltage, carrier frequency, excitation frequency and duty cycle on the induced velocity, electric current and power consumption of the actuator was investigated. For the steady actuation, it was revealed that an increase in the applied voltage and carrier frequency leads to firstly an increase in the induced flow velocity, but afterwards, when streaky glow discharge regime starts (non-uniform plasma formation), induced velocity decreases. On the other hand, it wasfound that for the unsteady actuation, the variation of the excitation frequency does not significantly affect the electrical current and the induced velocity, but these values increase almost linearly with increasing the duty cycle. Generally, it was observed that using the unsteady actuation in comparison to the steady one, can cause considerable reduction in the electrical power consumption and also can have a better performance in the induced flow control.

In the following paper, the accuracy of full turbulent K-e and transitional K-w models in external aerodynamic capturing of NACA0012 and sensitivity analysis have been comprehensively investigated. Among these investigations, some sorts of sensitivity analysis, including changes in free-stream Mach number, free-stream Reynolds number, free-stream angle of attack and generated-grid density have been done. Also the effects of transition inception point and the magnitude of the conformity between numerical results and theoretical concepts about transition and related effective parameters on the magnitude of lift, drag, lift-slope, pressure distribution, and pressure/friction lift-and-drag coefficients have been investigated in detail. To confirm the validity and the accuracy of the results, the numerical results have been compared with some published references.

This paper discusses the application of the homogeneous nucleation equation and the droplet growth equation for prediction of condensation in steam nozzles. In some papers on the low condensation pressure (condensation pressure less than 0.5 bar) were showed which calculated droplet radius and pressure distribution had deviations from experimental results. In this study, for simulation of spontaneous condensation in low pressure, empirical function of classical theory of nucleation has been combined with droplet growth equation derived by Young. Finally, calculation results of pressure distribution and droplet radius has been compared with the experimental data. It is observed good agreement with experiments.

In this research, toroidal and clustered linear aero spike nozzles were considered numerically with and without side walls to see the effects on flow field of base and plug and also on thrust coefficient. Considering flow pattern and analyzing results show that, the effect of 3d flow on linear aero spike nozzles could not be neglected. However, side truncation reduces weight, although is effective on overall thrust coefficient, adding wall authors to improve thrust coefficient, but because of more vice versa viscose force on longer walls, it could be commodious if absolute summation of forces increase thrust coefficient.Results of this research show adding side walls in the two nozzle type can be effective in improving thrust coefficient 12 to 31 percent (with attention to nozzle kind and width).