On the coupling of Navier–Stokes and linearised Euler equations for aeroacoustic simulation

Aerodynamic generation of sound is governed by the Navier–Stokes equations while acoustic propagation in a non-uniform medium is effectively described by the linearised Euler equations. Different numerical schemes are required for the efficient solution of these two sets of equations, and therefore, coupling techniques become an essential issue. Two types of one-way coupling between the flow solver and the acoustic solver are discussed: (a) for aerodynamic sound generated at solid surfaces, and (b) in the free stream. Test results indicate how the coupling achieves the necessary accuracy so that Computational Fluid Dynamics codes can be used in aeroacoustic simulations.

[Lecture review] 4. 'Three great mathematicians: Euler', 6 March 2002

Tony Mann provides a review of the lecture: 'Three great mathematicians: Euler' held on 6 March 2002 as the fourth and last lecture of the Gresham College History of Mathematics season.

Investigation of wall-slip effect on paste release characteristic in flip-chip stencil printing process

As the trend toward further miniaturisation of pocket and handheld consumer electronic products continues apace, the requirements for even smaller solder joints will continue. With further reductions in the size of solder joints, the reliability of solder joints will become more and more critical to the long-term performance of electronic products. Solder joints play an important role in electronics packaging, serving both as electrical interconnections between the components and the board, and as mechanical support for components. With world-wide legislation for the removal/reduction of lead and other hazardous materials from electrical and electronic products, the electronics manufacturing industry has been faced with an urgent search for new lead-free solder alloy systems and other solder alternatives. In order to achieve high volume, low cost production, the stencil printing process and subsequent wafer bumping of solder paste has become indispensable. There is wide agreement in industry that the paste printing process accounts for the majority of assembly defects, and most defects originate from poor understanding of the effect of printing process parameters on printing performance. The printing of ICAs and lead-free solder pastes through the very small stencil apertures required for flip chip applications was expected to result in increased stencil clogging and incomplete transfer of paste to the printed circuit pads. Paste release from the stencil apertures is dependent on the interaction between the solder paste, surface pad and aperture wall; including its shape. At these very narrow aperture sizes the paste rheology becomes crucial for consistent paste withdrawal because for smaller paste volumes surface tension effects become dominant over viscous flow. Successful aperture filling and release will greatly depend on the rheology of the paste material. Wall-slip plays an important role in characterising the flow behaviour of solder paste materials. The wall- slip arises due to the various attractive and repulsive forces acting between the solder particles and the walls of the measuring geometry. These interactions could lead to the presence of a thin solvent layer adjacent to the wall, which gives rise to slippage. The wall slip effect can play an important role in ensuring successful paste release after the printing process. The aim of this study was to investigate the influence of the paste microstructure on slip formation for the paste materials (lead-free solder paste and isotropic conductive adhesives). The effect of surface roughness on the paste viscosity was investigated. It was also found that altering the surface roughness of the parallel plate measuring geometry did not significantly eliminate wall slip as was expected. But results indicate that the use of a relatively rough surface helps to increase paste adhesion to the plates, inducing structural breakdown of the paste. Most importantly, the study also demonstrated on how the wall slip formation in the paste material could be utilised for understanding of the paste microstructure and its flow behaviour

On the modelling of fire atmosphere/sprinkler spray interacting using an Euler-Lagrange framework

An Euler-Lagrange particle tracking model, developed for simulating fire atmosphere/sprinkler spray interactions, is described. Full details of the model along with the approximations made and restrictions applying are presented. Errors commonly found in previous formulations of the source terms used in this two-phase approach are described and corrected. In order to demonstrate the capabilities of the model it is applied to the simulation of a fire in a long corridor containing a sprinkler. The simulation presented is three-dimensional and transient and considers mass, momentum and energy transfer between the gaseous atmosphere and injected liquid droplets.