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

Instructor’s Presence in Student-Centered Learning

Education, like almost all social spheres, is changed drastically by the Internet. Distance and e-learning nowadays are synonymous, absorbing and modifying practically the whole didactic experience. Modern learning management systems (LMS) are widely used for adding additional value to the traditional learning process. At New Bulgarian University both the full-time and distance forms of education are enhanced by e-learning. Though e-learning mainly related to distance education it also influences face-to-face teaching. The Moodle NBU platform provides access to e-learning content for each course and supports active communications among instructors and students in both forms of study. The shift from traditional classroom to virtual platform may represent a real challenge for instructors and learners. It is up to the instructor to give the students a perception of community. At the same time students are given opportunities to lead learning activities, participate in discussions, and explore topics that interest them, i.e. a student-centered learning is achieved. Students have the flexibility to learn “anytime and anywhere”. The role of interactivity in establishing the instructor’s presence and live engagement in the course activities is vital to the effectiveness of student-centered learning. Teacher’s visibility and immediacy brings sense of reality and seriousness and at the same time proves their personal identity. This investigation employs survey research to assess the influence of instructor’s presence and immediacy over students' performance in e-learning classes. The analysis is based on our experience in delivering courses via Moodle both for distance and regular courses. In order to enhance understanding of pedagogical processes we developed a proper questionnaire to measure teacher’s presence and immediacy. Data were collected over 200 students across different bachelor programs at New Bulgarian University both in full-time and distance forms of studies. Our results and student's attitude to instructor’s presence in student-centered learning are shown in this paper

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

Dynamic modelling and validation of the metal/flux interface in continuous casting

This paper concerns the development and validation (using an oil/water system) of a finite volume computer model of the continuous casting process for steel flat products. The emphasis is on hydrodynamic aspects and in particular the dynamic behaviour of the metal/slag interface due to the momentum imparted by the submerged entry nozzle (SEN) jets that bring the metal into the mould. Instability and wave action encourage the entrainment of inclusions into the melt affecting product quality. To track the interface between oil and water a new implicit algorithm was developed, called the Counter Diffusion Method. To prevent numerical damping of low frequency waves, a time-filtered version of the k- model was incorporated in the model. The physics of the interface dynamics are affected by density stratification and surface interfacial tension, with additional source terms introduced in the turbulence model to account for the former effect. Gas bubbles were modelled using a Lagrangian tracking method. The model was validated against experimental measurements obtained in a water model apparatus at Arcelor Research. Silicon oil was used to represent the lighter layer whilst air was pumped through the SEN to represent the argon

Numerical modelling of silicon melt purification in induction directional solidification system

Solar grade silicon production is an energy intensive and harmful to the environment process. Yet 40% of this valuable product material is lost into sawdust (kerf loss) during wafering. The kerf waste from Fixed Abrasive Sawing of PV silicon wafers is pelletized and then remelted in an induction furnace. The furnace has a square cross-section quartz crucible, surrounded by graphite susceptors and heated by an induction coil that enables directional solidification of the new ingot. Top and bottom 'pancake' coils provide additional temperature control. Once melted, silicon becomes electrically conductive and subject to stirring by induction. To recycle the silicon, particulate impurities (due to the sawing, condensed silicon oxides or carbides) need to be removed. Flow control and the electromagnetic Leenov-Kolin force are used to expel particulates, through a novel dual frequency induction scheme. Three-dimensional, multi-physics numerical modelling captures the electromagnetic, fluid-flow and heat-transfer effects in this process. The presented results show it is possible to retain the impurity particles on the sides of the solidified ingot where they can be sliced off and removed

Pharmacoeconomic analysis of medical abortion in Bulgaria

The drug treatment for interrupting early pregnancy begins with the identification of prostaglandins. In the 1970s, many developed countries legalized abortion, which led to the development and active application of this healthcare technology worldwide.The cost-effectiveness method was used in the pharmacoeconomic analysis by comparing the costs of abortion of two of the most popular protocols and dosage regimens with the medicines available at the time of the study on the Bulgarian market and the costs of classic abortion. The eventual complications were also noted. With equivalent efficiency of the two procedures, the application of cost-minimum analysis (CMA) is an appropriate and logical choice.The analysis shows that the drug termination of pregnancy under the EMA protocol is (600 mg Mifepristone + 400 mcg Misoprostol) a slightly higher cost per patient compared to the surgical abortion if the procedure is performed during the first trimester of pregnancy (∆Costs = +10,61 BGN) and leads to cost savings for the patient during the second trimester of pregnancy (∆Costs = -90,96 BGN). Medication termination of pregnancy under WHO protocol results in a cost-saving per patient compared to surgical abortion, regardless of the period of pregnancy during which the procedure is performed (∆Cost = -50,43 BGN in the first trimester and ∆Cost = -156,60 BGN in the second trimester).Drug termination is non-invasive, highly effective, and safe, resembling the natural mechanism of spontaneous abortion. In most cases, medical abortion is cost-saving and can be considered as a reasonable alternative to surgical abortion

Pharmacoeconomic analysis of medical abortion in Bulgaria

The drug treatment for interrupting early pregnancy begins with the identification of prostaglandins. In the 1970s, many developed countries legalized abortion, which led to the development and active application of this healthcare technology worldwide.The cost-effectiveness method was used in the pharmacoeconomic analysis by comparing the costs of abortion of two of the most popular protocols and dosage regimens with the medicines available at the time of the study on the Bulgarian market and the costs of classic abortion. The eventual complications were also noted. Because of the equivalent efficiency of the two procedures, cost-effectiveness analysis is simplified to cost-minimization analysis (CMA).The analysis shows that the drug termination of pregnancy under the EMA protocol is with a slightly higher cost per patient compared to the surgical abortion if the procedure is performed during the first trimester of pregnancy (∆ Costs = + 10,61 BGN) and leads to cost savings for the patient during the second trimester of pregnancy (∆ Costs = - 90,96 BGN).Medication termination of pregnancy under the WHO protocol results in cost-savings per patient compared to surgical abortion, regardless of the period of pregnancy during which the procedure is performed (∆ Cost = - 50,43 BGN in the first trimester and ∆ Cost = - 156,60 BGN in the second trimester).Drug termination is non-invasive, highly effective, and safe, resembling the natural mechanism of spontaneous abortion. In most cases, medical abortion is saving costs and can be considered as a reasonable alternative to surgical abortion

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

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