مجله مکانیک سیالات و آیرودینامیک
سال سوم شماره 1 (بهار 1393)

Due to the importance of the impacts of spraying a liquid into FCC riser reactors، the hydrodynamic characteristics of risers have been numerically and experimentally investigated. Based on a 3D transient model with Eulerian approach، a cold simulation was carried out in conditions similar to flow in a FCC riser reactor. The governing equations for the phases and an equation based on the kinetic theory of granular flow (KTGF)، as well as the dispersed turbulence model were applied. The governing equations were numerically solved by FVM for all of phases. Besides volume fraction distribution of the phases، solid particle velocity was determined by utilizing a fast digital image capturing system and particle image velocimetry (PIV) technique. In addition، the particle velocity profile in each height of the riser was obtained via image processing and the cross-correlation algorithm. The results of two-phase simulations show that in most axial levels of the riser، there is a parabola-shaped velocity profile so that the particle velocity near the bed walls is lower than that in the central regions. The results of three-phase simulations revealed that the injection of the third phase into the bed leads to phenomenal changes of the flow pattern. Our results based on CFD are in relatively good agreements with the experimental data.

Collimate beam in aerodynamic lenses are used widely for aerosol measuring. With passage of aerosol from inside an aerodynamic lens system، collimated particles beam forms. In order to evaluate the performance of an aerodynamic lens and to determine the quality of particle focus with various size and density in slip and continuous flow regime، particle-gas numerical analysis is used. Initially، compressible gas flow within aerodynamic lens with slip boundary condition on the wall was numerically analyzed using FLUENT software. Boundary condition for slip have been forced with the possibility of user programming in above software. In this case، using a repeat method، the speed of gas flow on the walls proportional to shear stress was set and this was continued until complete convergence. After convergence of the above procedure and solving gas flow in slip regime، uniform distribution of particles was injected into aerodynamic lens system and then the particles motion was analyzed by a Lagrangian method. Also، it was assumed that the flow on particles be slip. In these conditions، one of the forces acted on the particles is Stokes drag with variable Cunningham coefficient. By this method، the effect of slip on the drag coefficient and Brownian force were modeled. Solution process was assumed one way and we ignored the effects of particles on each other and on fluid flow.

In this paper، thermodynamic design of an axial flow power turbine for converting an existing turbojet to a turbo-shaft engine is presented. The exhaust nozzle of the turbojet is replaced by an axial flow power turbine. The turbine model is developed، based on Ainley-Mathieson model. The results of Ainley-Mathieson model were verified، using a 3D simulation and existing experimental data. The turbine performance was investigated for several blade angles of attack. Results show that the turbine stage has a minimum loss and a maximum efficiency، when angles of attack are -3 degrees for stator and -8 degrees for rotor blades.

This article introduces a method for fast estimation of aerodynamic coefficients and flow field data reconstruction around a NACA0012 airfoil، based on proper orthogonal decomposition (both in subsonic and supersonic regimes). Also، the effects of snapshot numbers and their arrangements in reconstruction procedure have been investigated. In this method، a combined form of POD، cubic spline interpolation، and polynomial extrapolation method، has been used for estimation of gappy data of the flow field with respect to the variations of angle of attack and Mach number. The proposed method predicts the required data much faster than CFD with appropriate accuracy. Finally، the results were compared with related CFD ones، showing relatively close agreements.

Buffeting is a high frequency aeroelastic instability that occurs in transonic speeds. The main objective of this paper is to present a method for determining the loading caused by this phenomenon on the body of the vehicle. For this، vibration equations are transformed into frequency domain. Then، the vehicle’s geometry and the kind of flow around the vehicle have been identified and for each of these flows، formulations are presented to predict the power spectral density of the external loading due to the buffeting phenomena، using experimental and statistics methods. The applied aeroelastic moment on the vehicle was calculated by determining the buffeting loads around the vehicle in frequency domain. Aerodynamic damping، which is calculated using experimental relations and quasi-steady flow presumption، was used for this aim. The results show that shock wave can apply a huge torque on the body، which is more than that obtained by static calculations. Thus، investigation of buffeting phenomena is necessary to have a successful launch. The presented method could be used in a wide range of space vehicles without expensive experimental analysis. The results were verified by experimental data from other reliable resources.

In this paper، the effects of size and density on mixing of solid particles in a fluidized bed have been studied experimentally. For this purpose، a bed consisting of silica sands has been used. The mixing of particles in a two-layer bed has been studied. The lower layer consists of silica sands، while in the upper layer tracer sand particles with different shape، size، density، and color have been used. All events occurring on the bed surface are recorded by a camera above the bed and tracer particles are being tracked، using image processing technique. Particles being mixed in the bed are studied by studying the variation of the tracer particles concentration index with time. First، the mechanism of particle mixing in a two-layer fluidized bed was investigated. Then، the effect of the characteristics of mixing particles on mixing process was studied. The results show that increasing the tracer particle density (approaching the bed sands density)، enhances the penetration of tracer in the bed and reduces the tracer presence on the bed surface. Also، the size of sand particles has significant effect on mixing quality; it makes it possible to find a proper sand size to mix the particles with different densities.