مجله مکانیک سیالات و آیرودینامیک
سال پنجم شماره 2 (پاییز و زمستان 1395)

Multiphase flows, combined with porous media, with application in both nature and industry are greatly important. The main objective of this research is to analyze drop interaction with porous media. Multiphase flow through a porous medium, including circular obstacle in non-Darcian regime at different pressure gradients was investigated. The porous medium, wherein a liquid flow was present, a drop with different fluid properties was forced to pass. The drop impact with porous medium leads to different patterns: break-up, trap, and coalescence. Lattice Boltzmann method (LBM), which has shown high capability and flexibility in relation with multiphase flows combined with porous media, have been implemented. The effective dimensionless numbers related to this flow (such as, Ohnesorge, Capillary number, and density ratio of the two phases) were thoroughly analyzed. The values of exerted dimensionless pressure were 0.000108, 0.000144, and 0.000180 and the range of Ohnesorge was 0.19-0.76. Our results show that there are high complexity and many effective factors, which necessitate their concurrent examinations. The effects of factors connected to drop and secondary phase properties (such as, surface tension and density ratio), along with flow characteristics (such as pressure gradient), are considerable. Consequently, the simulations of this study, using certain assumptions and methods, show excellent capability to predict drop behavior in porous media, which is the valuable feature of this study.

Turbine blades always face inevitable fault and Tolerances, which cause the final blade geometry differs from the designed one. On the other hand, blade geometry has important effects on turbine performance causing blade dimensional faults influence turbine major characters, like efficiency. The mentioned situations cause turbine performances vary from its optimum condition. So, obtaining allowable geometrical tolerances having effect on turbine efficiency is necessary. The most influential factors leading to these errors are various fabrication methods and geometrical changes during turbine operation. The most important of these can be pointed to maximum thickness, chord length, and blade height tolerances. In this research, through one-dimensional modeling, Hanover axial flow turbine efficiency changes from its design point due to dimensional tolerances is studied. Ainely-Mathieson, Dunham-Came, and Aungier loss models were applied in this modeling.

This paper introduces a new type of flame holder. Flow field is investigated, using PIV measurements in cold flow regime. The experiments on two types of V-shape flame holder: one with smooth edges and the other with wavy edges. The instantaneous velocity fields, Strouhal number, average velocity, vorticity, and recirculation zone length were among the parameters affected by the flame holder geometry. In this work, we examined the influence of these characteristics. Complex structure and unsteadiness of the flow was obtained with high accuracy. The results indicate an increase in length and width of the recirculation zone when using the wavy edges instead of smooth edges. For both geometries, a symmetrical flow structure was observed behind the flame holder. Also, higher static pressure drop was detected behind the flame holder for geometry with wavy edges.

In this research, the behavior and characteristics of the wake of the flow around an elliptic cylinder at zero angle of attack in the presence of a tripping wire of 1 mm diameter were investigated experimentally. An aluminum elliptic cylinder with major axis, minor axis, and height of 42.4 mm, 21.2mm, and 390 mm, respectively, was used for this purpose. The cylinder model was examined in the test section of a blower type wind tunnel. The Reynolds numbers of the experiment, based on major axis were 25700 and 51400 for 10 m/s and 20 m/s speeds, respectively. Tripping wire placed symmetrically at both sides of the cylinder, and each were tested at angles of zero, 23.7, and 40.9 degrees with respect to the stagnation point. The drag coefficient of the smooth cylinder for both of the Reynolds numbers was about 0.6. The results indicate that in the best possible case, the drag coefficient for the 1 mm wire reduces by 56.5% at angle of the 23.7 degree. In the best case, it also reduces by 8.6% for angle of the zero degree, while in the worst case, the drag coefficient for angle of 40. 9 degree increase by 20.7%.

In this paper, the influences of regional pre-evacuation of an altitude test facility on starting time of a second throat supersonic exhaust diffuser are numerically investigated. Detailed numerical studies have been carried out to evaluate the physics of the flow and starting time of the diffuser for different pre-evacuation zones along the diffuser and the test chamber. Unsteady axisymmetric compressible Navier–Stokes equations, incorporated with the two equation kω-SST turbulence model are solved, with density-based solver to extract the current flow features. The numerical method is properly validated with the measured data available in the literature. Our investigations show that the amount of pre-evacuation volume has strong effects on starting time of the diffuser. As we extend pre-evacuation zone along the diffuser, the smaller starting time of diffuser is resulted. However, the increasing of pre-evacuated test chamber size causes the increasing of starting time of the diffuser.

In this paper, the full two-dimensional Navier–Stokes equations in the external turbulent flow were numerically modeled. Thus, in order to calculate the fluxes, the Ausm method was utilized and for modeling turbulent flows, the single-equation Spalart-Allmaras model was used. The employed Spalart-Allmaras model is a modification of this turbulence model, which prevents the formation of negative dynamic viscosity, a point that has gone generally unnoticed. The suggested modification was conducted on the distributing and the source terms in the single equation of turbulence model. For validation and assessment of accuracy and stability of the turbulence model and the calculation method of fluxes, the totally turbulent flow with Reynolds number of 9106 and Mach number of 0.799 and angle of attack 2.26 and also flow with Reynolds numbers of 6106 and and Mach number of 0.15 and angles of attack zero,10 around NACA 0012 airfoil are investigate. The results of this study show that computational framework designed have a good agreement with experimental data .furthermore the number of iterations required for convergence is reduced.