Numerical techniques for computational aeroacoustics

The problem of aerodynamic noise is considered following the Computational Aeroacoustics approach which is based on direct numerical simulation of the sound field. In the region of sound generation, the unsteady airflow is computed separately from the sound using Computational Fluid Dynamics (CFD) codes. Overlapping this region and extending further away is the acoustic domain where the linearised Euler equations governing the sound propagation in moving medium are solved numerically. After considering a finite volume technique of improved accuracy, preference is given to an optimised higher order finite difference scheme which is validated against analytical solutions of the governing equations. A coupling technique of two different CFD codes with the acoustic solver is demonstrated to capture the mechanism of sound generation by vortices hitting solid objects in the flow. Sub-grid turbulence and its effect 011sound generation has not been considered in this thesis. The contribution made to the knowledge of Computational Aeroacoustics can be summarised in the following: 1) Extending the order of accuracy of the staggered leap-frog method for the linearised Euler equations in both finite volume and finite difference formulations; 2) Heuristically determined optimal coefficients for the staggered dispersion relation preserving scheme; 3) A solution procedure for the linearised Euler equations involving mirroring at solid boundaries which combines the flexibility of the finite volume method with the higher accuracy of the finite difference schemes; 4) A method for identifying the sound sources in the CFD solution at solid walls and an expansion technique for sound sources inside the flow; 5) Better understanding of the three-level structure of the motions in air: mean flow, flow perturbations, and acoustic waves. It can be used, together with detailed simulation results, in the search for ways of reducing the aerodynamic noise generated by propellers, jets, wind turbines, tunnel exits, and wind-streamed buildings

Time-dependent numerical modelling of acoustic cavitation in liquid metal driven by electromagnetic induction

The numerically simulated method of using electromagnetic field from an alternating current is a patented method to create in liquid metal, under the conditions of resonance, acoustic waves of sufficient strength to cause cavitation and implosion of gas bubbles, leading to beneficial degassing and grain refinement. The modelling stages of electromagnetics are described below along with acoustics in liquids, bubble dynamics, and their interactions. Sample results are presented for a cylindrical container with liquid aluminium surrounded by an induction coil. The possibility of establishing acoustic resonance and sustaining the bubble oscillation at a useful level is demonstrated. Limitations of the time-dependent approach to this multi-physics modelling problem are also discussed

Vacuum arc remelting time dependent modelling

Vacuum arc remelting (VAR) aims at production of high quality, segregation-free alloys. The quality of the produced ingots depends on the operating conditions which could be monitored and analyzed using numerical modelling. The remelting process uniformity is controlled by critical medium scale time variations of the order 1-100 s, which are physically initiated by the droplet detachment and the large scale arc motion at the top of liquid pool [1,2]. The newly developed numerical modelling tools are addressing the 3-dimensional magnetohydrodynamic and thermal behaviour in the liquid zone and the adjacent ingot, electrode and crucible

Multiple timescale modelling of particle suspensions in metal melts subjected to external forces

Electro-magnetic (EM) fields are widely used in metallurgy in order to stir conducting metals without the risk of contamination or causing an instability or chemical reaction. During the manufacturing of metal matrix composites (MMC), ceramic micro- and nano-particles are added into the metal melt, and ultrasonic (US) processing and EM stirring are used to break the agglomerates and to enhance the dispersion of the particles. EM stirring can also be used to remove the unwanted particles from liquid metal by pushing them towards the walls of the cru-cible where they adhere and can be easily removed. A model has been developed to account for the complex interaction of the particles with each other, with the walls, as well as with the flow of the metal melt. Particles are modelled as elastic spheres with adhesion. Adhesion is incorporated in the model using the Johnson, Kendal, Robert (JKR) and Derjaguin, Muller, Toporov (DMT) theories. The case of the oblique impact of the particles is modelled according to the Thornton and Yin method based on the partial-slip theory developed by Mindlin & Deresievics. The developed particle model is then coupled with the magneto-hydrodynamics (MHD) code PHYSICA in order to demonstrate the effect of the EM stirring and vibration. Multiple time-scales are used which permits modelling the realistic time range of metal-processing and at the same time capture the individual collisions between particles with suffi-cient precision. Several methods of predicting the particle collisions are employed and their ef-ficiency is compared for the case of removing contaminating particles from liquid meta

Contactless ultrasound generation in a crucible

Ultrasound treatment is used in light alloys during solidification to refine microstructure, remove gas, or disperse immersed particles. A mechanical sonotrode immersed in the melt is most effective when probe tip vibrations lead to cavitation. Liquid contact with the probe can be problematic for high temperature or reactive melts leading to contamination. An alternative contactless method of generating ultrasonic waves is proposed, using electromagnetic (EM) induction. As a bonus, the EM force induces vigorous stirring distributing the effect to treat larger volumes of material. In a typical application, the induction coil surrounding the crucible— also used to melt the alloy—may be adopted for this purpose with suitable tuning. Alternatively, a top coil, immersed in the melt (but still contactless due to EM force repulsion) may be used. Numerical simulations of sound, flow, and EM fields suggest that large pressure amplitudes leading to cavitation may be achievable with this method

Modelling three-dimensional microstructure evolution influenced by concurrent structural mechanical mechanisms

The interdependence between structural mechanics and microstructure solidification is an inherently three-dimensional phenomenon, where the complex physical processes and mechanical interactions can lead to dendrites growing at orientations influenced by twisting and out of plane bending. These effects can have a significant impact on the formation of defects and the overall macroscopic material properties of the structure. However, all attempts to numerically model this process so far have been limited to two dimensional representations of the problem, which necessitates ignoring any potential behaviour that may arise from these more complex deformation events. For this reason, the two-dimensional numerical methods presented in previous papers, which couple a Finite Volume Structural Mechanics Solver to a Cellular Automata solidification solver, have been expanded so that problems may now be simulated in three dimensions. Results are presented which do not aim to predict any specific mechanism but rather highlight the new capabilities of this improved three-dimensional modelling framework

Contactless ultrasonic treatment in direct chill casting

Uniformity of composition and grain refinement are desirable traits in the direct chill (DC) casting of non-ferrous alloy ingots. Ultrasonic treatment (UST) is a proven method for achieving grain refinement, with uniformity of composition achieved with additional melt stirring. The immersed sonotrode technique has been employed for this purpose to treat alloys both within the launder prior to DC casting, and directly in the sump. In both cases mixing is weak, relying on buoyancy driven flow or in the latter case on acoustic streaming. In this work we consider an alternative electromagnetic (EM) technique used directly in the caster, inducing ultrasonic vibrations coupled to strong melt stirring. This ‘contactless sonotrode’ technique relies on a kilohertz frequency induction coil lowered towards the melt with the frequency tuned to reach acoustic resonance within the melt pool. The technique developed with a combination of numerical models and physical experiments has been successfully used in batch to refine the microstructure and degas aluminum in a crucible. In this work we extend the numerical model, coupling electromagnetics, fluid flow, gas cavitation, heat transfer and solidification to examine the feasibility of use in the DC process. Simulations show that a consistent resonant mode is obtainable within a vigorously mixed melt pool, with high pressure regions at the Blake threshold required for cavitation localized to the liquidus temperature. It is assumed extreme conditions in the mushy zone due to cavitation would promote dendrite fragmentation and that, coupled with strong stirring, would lead to fine equiaxed grains

A study of the complex dynamics of dendrite solidification coupled to structural mechanics

The impact of structural mechanics is often overlooked when modelling the solidification of dendritic microstructures, despite experimental observations that the interaction between these processes can be a factor leading to the development of crystal mosaicity throughout the microstructure which can itself lead to more serious defects. When considered at all, the structural mechanical behaviour of columnar dendrites is often considered as being analogous to a cantilever beam both in interpretations of experimental results and in existing numerical modelling. While this is not an unreasonable assumption when considering a dendrite in isolation, this is a scenario that infrequently occurs. In this paper a parametric study is presented using a Cellular Automata solidification solver coupled to a Finite Volume Structural Mechanics solver. These results highlight the complex non-linear behaviour that arises when considering dendrite interaction, demonstrating the significantly different microstructures that can be obtained by varying only the force experienced by the system

Modelling the deagglomeration of nanoparticle inclusions by ultrasonic melt processing

The aerospace and automotive industries are seeking advanced materials with low weight-to-strength ratio, such as light alloy-based metal matrix composites (MMC) with nanoparticle rein-forcements. However, van der Waals and adhesive forces between nanoparticles result in large agglomerates that compromise the final properties of MMCs. Ultrasonic melt processing is a potential technology for de-agglomerating these clusters and producing samples with improved properties via grain refinement. This paper considers two hypotheses of cluster de-agglomeration: the breakup of a cluster due to the growth of gas contained in the cluster and the stripping of nanoparticles by pulsating neighbouring bubbles