مجله مکانیک سیالات و آیرودینامیک
سال دوم شماره 1 (بهار 1392)

In recent years، many different methods have been developed to simulate free surface flows coupled with moving bodies. In this paper، for simulating ship maneuver، VOF method (CICSAM) and also for coupling velocity field and pressure a fractional step method are used. To increase simulation accuracy، the mesh size should be significantly fine. Unfortunately، fine grid causes increase in simulation time. So، multigrid method was applied for solving the Poisson equation obtained in the Navier-Stokes equations، when using fractional step method. In that case، we generated some coarse grids، using agglomeration method، which needs implemention of a fully unstructured grid. To evaluate the accuracy of the algorithm، some test cases، such as two sided cavity، dam break with obstacle، and barge towing problem، as well as a comparison between V and W cycles in multigrid method have been studied. The results show that implementation of multigrid method can improve the performance of the computer code up to 100%. Moreover، W cycle behaves better than V cycle in most cases.

Prediction of fluid-elastic instability is a great matter of importance in designing cross-flowtube bundles from the perspective of vibration. In the present paper، the threshold of fluid-elastic instability has been numerically predicted via simulation of incompressible، unsteady، and turbulent cross-flow through a tube bundle in a normal triangular arrangement. The conditions of simulation is the same as a previous experiment. A finite volume solver، based on Cartesian-staggered grid، has been implemented and the interactions between fluid and structure are solved fully coupled method of solving flow and structure equations in each time step. The fluid elastic instability was predicted and analyzed by presenting the structural responses، trajectory of flexible cylinders، and critical reduced velocities.

In this paper، direct numerical simulation of two-phase incompressible gas-liquid flow for simulation of bubble motion in a microtube is presented. Microtube radius is 10 μm. The interface is tracked using volume of fluid (VOF) methodwith continuous surface force (CSF) model. The flow is solved using a finite volume scheme، based on PISO algorithm. Numerical simulation is performed on a axisymmetric mesh with cyclic boundary condition for different values of pressure gradient، void fraction، and bubble period. The mean pressure gradient is fixed for each simulation and the superficial Reynolds numbers of gas and liquid are 0. 3 – 7 and 5 – 210، respectively. The numerical results are coincident with Serizawa regime map and there is a linear relation between void fraction and gas flow ratio. Pressure drop and liquid film thickness are compared with different empirical correlations. The computed liquid film thickness is closer to Irandoust correlation.

This paper deals with aerodynamic design of fan and compressor assembly for turbofan engines having arbitrary bypass ratios. Two different softwares were developed in this respect. First، based on general performance data at design point condition، a one-dimensional approach was activated. Initial flow passages of both fan and compressor، together with splitter position were developed utilizing row by row analysis along the meanline radius. Considering design restriction parameters، with diffusion factor as the most important one، the initial geometries of blades projection on the meridional plane and their numbers at each row was estimated. Then، these data were imported to the second software، developed for three-dimensional design purposes. This latter approach is based on the well-known streamline curvature method. Executing this computerized program، one can obtain the full geometries of fan and compressor blades with all gas dynamics data at different axial and radial positions. Different kinds of losses، including profile، shock wave and tip clearance losses were considered through the calculations، in order to evaluate entropy changes. To validate current design procedure، an existent two-stage fan was redesigned. Finally، a parametric study was carried out to investigate the effects of variations in some geometric and gas dynamics parameters on the general performances of either the fan or compressor modulus.

In this research، shock wave motion، shock/boundary layer interaction، and flow separation in a transonic flow over a NACA0012 airfoil were investigated experimentally. Thirteen Kulite sensors were utilized for measurement of pressure fluctuations over the airfoil. Experiments were performed for angles of attack between -4 to 4 degrees and flow Mach numbers of M= 0. 39 to 0. 82. The Kulite sensors were distributed on one side of the airfoil surface from 13% to 63% of the chord from the leading edge. Data acquisitioning were performed using a 10KHz sample rate and a resolution of 0. 05 KPa/bit. A Schlieren system was also used for visualization of shock lines. The results show that the transition from laminar to turbulent cause increase in amplitude of pressure fluctuations during the process. If boundary layer separation occurs due to interaction of boundary layer with shock waves، amplitude of pressure fluctuations reduces compared to upstream of the separated region. The results also show that as Mach number increases، the onset location of laminar to turbulent transition moves toward the leading edge and the shock lines move toward the trailing edge. Another significant phenomenon was occurrence of a phase difference between pressure signals of two positions namely: after shock wave and inside separation bubble.

In order to determine the stability of underwater robots، their governing differential equations of motion in vertical and horizontal planes should be investigated. For horizontal plane stability، using Routh stability criterion، four hydrodynamic coefficients، including derivatives of the lateral force and vertical moment with respect to sway and angular velocity are required. These coefficients are usually measured in a towing tank with rotating arm experimental facilities. In present work، these coefficients were calculated for an AUV، using computational fluid dynamics. Towing of the model، with various drift angles، in a towing tank and also rotation of the model in a rotating arm are simulated numerically، to obtain the required coefficients. The coefficients were then used to investigate the stability of the robot in horizontal plane and to determine the effect of control surfaces on the robot''s stability.

In this paper، inflow turbulence effects on the flow wake parameters around a NACA001 airfoil at zero incidence angle and Reynolds number of 38،700 were investigated. Various turbulence promoter networks were used to achieve turbulence intensity of 5% and 6%. Experiments have been conducted for three different turbulence intensities in which mean velocity parameters، turbulence intensity، frequency response، and Strouhal number were measured. Half of the wake width (b1/2) in all three cases of turbulence، was increased with increasing x/d and turbulence intensity. It was also found that vortex formation length is decreased with turbulence intensity. Besides، it became clear that Strouhal number is reduced with any increase in turbulence intensity or in the distance from the trailing edge of the airfoil.