Hossam S. Aly

Improving convergence in smoothed particle hydrodynamics simulations without pairing instability

The numerical convergence of smoothed particle hydrodynamics (SPH) can be severely restricted by random force errors induced by particle disorder, especially in shear flows, which are ubiquitous in astrophysics. The increase in the number NH of neighbours when switching to more extended smoothing kernels at fixed resolution (using an appropriate definition for the SPH resolution scale) is insufficient to combat these errors. Consequently, trading resolution for better convergence is necessary, but for traditional smoothing kernels this option is limited by the pairing (or clumping) instability. Therefore, we investigate the suitability of the Wendland functions as smoothing kernels and compare them with the traditional B-splines. Linear stability analysis in three dimensions and test simulations demonstrate that the Wendland kernels avoid the pairing instability for all NH, despite having vanishing derivative at the origin (disproving traditional ideas about the origin of this instability/ instead, we uncover a relation with the kernel Fourier transform and give an explanation in terms of the SPH density estimator). The Wendland kernels are computationally more convenient than the higher-order B-splines, allowing large NH and hence better numerical convergence (note that computational costs rise sub-linear with NH). Our analysis also shows that at low NH the quartic spline kernel with NH ~= 60 obtains much better convergence then the standard cubic spline.

CFD ANALYSIS OF INCOMPRESSIBLE TURBULENT SWIRLING FLOW THROUGH ZANKER PLATE

Abstract Zanker plate is a widely used flow straightener to eliminate turbulence originating from pipe fittings in experimental fluid flow applications. In this paper, steady state, incompressible, swirling turbulent flow through Zanker plate has been studied. The solution and the analysis were carried out using finite volume CFD solver FLUENT 6.2. Five turbulence models were used in the numerical investigation and their results were compared with the pressure drop correlation of BS EN ISO 5167–2:2003. The turbulence models investigated here are the standard k-ε, realizable k-ε, the Reynolds Stress Model (RSM), the large Eddy Simulation (LES), and the Detached Eddy Simulation (DES). The results showed that the DES model gave the best agreement with the ISO pressure drop correlation, therefore, the model was used further to predict streamline patterns, vortex formations and separated flow regions. The effects of Zanker plate thickness and Reynolds number on the flow characteristics have been investigated as well.

On The Secular Evolution of GG Tau A Circumbinary Disc: A Msialigned Disc Scenario

The binary system GG Tau A is observed to have a circumbinary disc with a dust ring located further out than expected, assuming a co-planar disc and a corresponding semi-major axis of 34 AU. Given the binary separation, this large cavity can be explained by relaxing the assumption of a co-planar disc and instead fit the observations with a mis-aligned circumbinary disc around an eccentric binary with a wider semi-major axis of 60 AU, consistent with fitting the proper motion data for the system. We run SPH simulations to check this possibility and indeed we find that a misalignment angle of 30 degrees and a binary eccentricity of 0.45 fit both the astrometric data and the disc cavity. However, such configuration could in principle be unstable to polar, rather that planar alignment. We investigate the secular evolution of this configuration and show that it is indeed stable throughout the disc lifetime.

Mathematical Modelling of Droplet Atomization Using the Population Balance Equation

This study investigates numerically the atomization process occurring in a plain jet airblast atomizer. The population balance equation is solved for the dispersed phase coupled with a CFD Eulerian multi-fluid model. The Sauter mean diameter values obtained numerically compare favorably with previous experimental data only at certain flow conditions. Finally, this study proposes some enhancements on using the numerical model which were revealed from the model formulation and the results obtained.