A computational fluid dynamics (CFD) method has been employed to compute vortical flows around slender wing/body configurations. The emphasis of the paper is on the effectiveness of an adaptive grid procedure in "capturing" concentrated vortices generated at sharp edges or flow separation lines of lifting surfaces flying at high angles of attack. The method is based on a tetrahedral unstructured grid technology developed at the NASA Langley Research Center. Two steady-state, subsonic, inviscid and Navier-Stokes flow test cases are presented to demonstrate the applicability of the method for solving vortical flow problems. The first test case concerns vortex flow over a simple 65 delta wing with different values of leading-edge radius. Although the geometry is quite simple, it poses a challenging problem for computing vortices originating from blunt leading edges. The second case is that of a more complex fighter configuration. The superiority of the adapted solutions in capturing the vortex flow structure over the conventional unadapted results is demonstrated by comparisons with the wind-tunnel experimental data. The study shows that numerical prediction of vortical flows is highly sensitive to the local grid resolution and that the implementation of grid adaptation is essential when applying CFD methods to such complicated flow problems.

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Vortical Flow Prediction Using an Adaptive Unstructured Grid Method

This paper studied a two dimensional fluid flow and heat transfer in a finned open cavity filled by a water/Al2O3 nanofluid. Discrete ordinates (DO) method was used to investigate the effects of nanoparticle absorption and dispersion coefficients on the volumetric radiation rays passing a nanofluid. The employed nanofluid was water (base fluid) and alumina nanoparticles. A relatively, low volume fraction of the nanoparticles was needed for the preparation of the semitransparent medium. The finite volume technique of SIMPLE algorithm was coupled to DO to solve the continuous, momentum, and energy equations. The enclosure inner walls are the gray diffuse emitters and reflectors. As a result, a new dimensionless energy equation was presented in which could involve both effects of mixed convection and thermal radiation of DO based on Reynolds, Prandtl and Planck numbers. The results of the present study are different from previous studies, which neglected the effects of radiation or even considered them with inappropriate radiation method of Rosseland’s approximation for the nanofluid medium.

Discrete ordinates thermal radiation with mixed convection to involve nanoparticles absorption, scattering and dispersion along radiation beams through the nanofluid

Microstructure and strength of ultrasonic-assisted brazed joints of Si3N4/6061Al composites

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Progress and perspectives of in-situ optical monitoring in laser beam welding: Sensing, characterization and modeling

A validated CFD model coupling the multiple phases and multiple physics was developed for the dynamic simulation of pulse wave laser welding (PWLBW) process of 1.2 mm-thick Ti-6Al-4V alloy sheets assembled in butt joint with a reserved air gap. A laser spot diameter of 700 μm was used to compensate for the energy loss with the clearance applied at 0.2 mm and therefore improve the gap bridging ability. The distributions and evolutions of temperature, velocity, phase interface and weld pool dimensions were characterized and discussed to reveal the joining mechanism and the heat and flow behaviors during welding. The results show that the liquid bridges firstly occur in the middle height of sheets as the welding begins, and rapidly extend to the whole thickness with the increasing heat input. The reduction on laser power causes the rebounding of keyhole surface and the backfilling of molten metal. The keyhole closes at root surface and rebounds completely in 2 microseconds, leading to the merging of liquid bridges, bottom-up moving of high temperature area, and temporarily enlargement on weld pool size at root surface. During the pulse interval, the weld pool volume shrinks significantly and the molten metal slows down from Marangoni vortexes to thermal buoyance flows. The proposed process parameters allow an air gap accounting for 16.67% of specimen thickness and result in a well-formed and defect-free weld bead.

Weld pool dynamics and joining mechanism in pulse wave laser beam welding of Ti-6Al-4V titanium alloy sheets assembled in butt joint with an air gap

Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420

Investigating the effect of process parameters on the mechanical properties and temperature distribution in fiber laser welding of AISI304 and AISI 420 sheet using response surface methodology

The purpose of this study is simulation of of polymer electrolyte membrane fuel cell. Proton-exchange membrane fuel cells are promising power sources for use in power plants and vehicles. These fuel cells provide a high level of energy efficiency at low temperature without any pollution. The convection inside the cell plays a key role in the electrochemical reactions and the performance of the cell. Accordingly, the transport processes in these cells have been investigated thoroughly in previous studies that also carried out functional modeling.,A multi-phase model was used to study the limitations of the reactions and their impact on the performance of the cell. The governing equations (conservation of mass, momentum and particle transport) were solved by computational fluid dynamics (CFD) (ANSYS fluent) using appropriate source terms. The two-phase flow in the fuel cell was simulated three-dimensionally under steady-state conditions. The flow of water inside the cell was also simulated at high-current density.,The simulation results suggested that the porosity of the gas diffusion layer (GDL) is one of the most important design parameters with a significant impact on the current density limitation and, consequently, on the cell performance.,This study was mainly focused on the two-phase analysis of the steady flow in the fuel cell and on investigating the impacts of a two-phase flow on the performance of the cell and also on the flow in the GDL, the membrane and the catalyst layer using the CFD.

Ahmad Soleimani

Papers

Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420

Investigating the effect of process parameters on the mechanical properties and temperature distribution in fiber laser welding of AISI304 and AISI 420 sheet using response surface methodology

Three dimensional numerical simulation of polymer electrolyte membrane fuel cell