| 1. |
- Mihaescu, Mihai
(författare)
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Computational Aeroacoustics Based on Large Eddy Simulation and Acoustic Analogies
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis presents a numerical method developed by the author and its applications for computing the generated sound by an unsteady flow field and its propagation. The full equations of motion for compressible and unsteady flows describe both flow field and sound generation and propagation. It is assumed that the flow variables can be decomposed into semi-compressible / incompressible components and inviscid, irrotational acoustic components. The present method is based on Large Eddy Simulation (LES) to compute the turbulent flow and an approach based on an inhomogeneous wave equation to compute the radiated acoustic field. In this way one can avoid the necessity for a very large computational effort associated with direct simulation of the near- and specially far- field sound generated by a turbulent flow. The governing equations are written in the form of a non-homogeneous wave equation for the acoustic fluctuation with acoustic sources on the right-hand side. The thesis includes the details of the coupling between the flow solver and the acoustic one, as well as the results for test cases employed to validate the numerical algorithm and the implemented boundary conditions. The method has been successfully applied to compute the near- and far- acoustic fields generated by various unsteady flows such as a round hot turbulent jet ejected from a pipe close to a solid boundary, coaxial turbulent non-isothermal jets (separate exhaust system), or the flow around a wind-turbine.
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| 2. |
- Szász, Robert-Zoltán, et al.
(författare)
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Algorithm for automatic quantification of flashback and flash forward events from high-speed chemiluminescence recordings
- 2016
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Ingår i: Springer Proceedings in Physics. - Cham : Springer Science+Business Media B.V.. - 9783319306001 - 9783319306025 ; , s. 519-526
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Konferensbidrag (refereegranskat)abstract
- Three methods are employed to identify and quantify flashback and flash forward events based on chemiluminescence recordings of swirling flames. The approaches differ in the procedure to determine the instantaneous flame position. The results revealed that the most robust method is to determine a threshold relative to the instantaneous maximum intensity. Analysis of the complete dataset indicated that flashback events are significantly slower than flash forward events.
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| 3. |
- Mylavarapu, Goutham, et al.
(författare)
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Importance of paranasal sinuses in computational modeling of nasal airflow
- 2009
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Ingår i: 47th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition. - : AIAA. - 9781563479694 ; , s. 2009-0772-
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Konferensbidrag (refereegranskat)abstract
- This paper investigates the importance of including paranasal sinuses in the computational modeling of the nasal airflow. Three dimensional models of human nasal airway with and without including paranasal sinuses were reconstructed from Computed Tomography (CT) axial images of a subject with healthy nasal airway. The reconstruction process was performed using MIMICS® software program. The airway volume was discretized using TGRID® mesh generator. Steady Reynolds-Averaged Navier-Stokes (RANS) simulations were carried in both inspiratory and expiratory phases of respiratory cycle at a peak flow rate of 15 L/min in FLUENT®. The results show that the left and right nasal resistances change with less than 11% when paranasal sinuses are included in the computational model of the nasal airway. The flow into the sinuses is characterized by very low velocities during both inspiration and expiration conditions. The velocity distributions in the main nasal passage show small change predominantly in regions closer to the paranasal sinuses when compared to the model where sinuses were not included.
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| 4. |
- Mylavarapu, Goutham, et al.
(författare)
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Planning human upper airway surgery using Computational Fluid Dynamics
- 2013
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Ingår i: Journal of Biomechanics. - : Elsevier. - 0021-9290 .- 1873-2380. ; 46:12, s. 1979-1986
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Tidskriftsartikel (refereegranskat)abstract
- The study advances the idea of using computational fluid dynamics in the process of planning surgical treatment modalities for patients with obstructive airway disorders. It is hypothesized that the a priori knowledge of the functional outcome of surgical intervention on the flow and airway resistance can guide the surgeon in choosing an effective surgical strategy. Computed tomography images spanning the respiratory tract of an adult patient with a combined glottic and subglottic stenosis are used to reconstruct three-dimensional geometrical models of the airway. Computational fluid dynamics is used to obtain airway flow patterns during inspiration and expiration in these models. Numerical predictions about flow velocity, pressure distribution on the airway lumen, wall shear stress, and airway resistance are obtained so that the relevance of each individual stenotic level is quantified. Four different virtual surgeries in different combinations are assessed in order to remedy the constricted airway. The virtual surgery based airway models are evaluated by comparisons with the pre-treatment flow modeling results. The predicted numerical data revealed that the removal of the constriction at the level of the vocal folds will have the most significant effect on the airway resistance. The flow simulations offer a quantitative method of evaluating the airway resistance in patients with combined glottic and subglottic stenoses. Predictions of airway resistances and other numerical calculations from different virtual surgeries give additional inputs for the surgeon, in deciding the most appropriate surgery on a case-by-case basis.
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| 5. |
- Mylavarapu, Goutham, et al.
(författare)
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Virtual Surgery : a Computational Engineering Approach to a Medical Problem
- 2010
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Ingår i: proceedings of International Conference on Jets, Wakes and Separated Flows ICJWSF-2010.
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Konferensbidrag (refereegranskat)abstract
- The aim of the study is to utilize Computational Fluid Dynamics in the process of planning surgical treatment modalities for patients with obstructive airway disorders. It is hypothesized that the a-priori knowledge of the functional outcome of surgical intervention on the flow and airway resistance can guide the surgeon in choosing an effective surgical strategy. Computed Tomography images spanning the respiratory tract of an adult patient with a combined glottic and subglottic stenosis are used to reconstruct three-dimensional geometrical models of the airway. Computational Fluid Dynamics is used to obtain airway flow patterns during inspiration and expiration in these models. Numerical predictions about flow velocity, pressure distribution on the airway lumen, wall shear stress, and airway resistance are obtained so that the relevance of each individual stenotic level is quantified. Four different virtual surgeries in different combinations are assessed in order to remedy the constricted airway. The virtual surgery based airway models are evaluated by comparisons with the pre-treatment flow modeling results. The predicted numerical data revealed that the removal of the constriction at the level of the vocal folds will have the most significant effect on the airway resistance. The flow simulations offer a quantitative method of evaluating the airway resistance in patients with combined glottic and subglottic stenosis. Predictions of airway resistances and other numerical calculations from different virtual surgeries give additional inputs for the surgeon, in deciding the most appropriate surgery on a case-by-case basis.
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| 6. |
- Semlitsch, Bernhard, 1985-, et al.
(författare)
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Large Eddy Simulations of Microjets Impact on Supersonic Jet Exiting a C-D Conical Nozzle
- 2013
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The effect of multiple microjets on the acoustic noise production originating from a super-sonic jet exhausting a gas turbine engine is studied numerically using the Large Eddy Simulation (LES) approach. The nozzle exit design Mach-number is 1.56, while the total temperature ratio is kept to 1.27. The nozzle contour is a double cone converging- diverging nozzle. The emerging jet is slightly over-expanded. A double shock-diamond pattern develops in the supersonic flow. The study focuses on the changes in the flow pattern, the shock-associated noise and the radiated near-field acoustics when using fluidics as compared with a baseline, (i.e. without fluidics).Just downstream of the nozzle lip, twelve cylindrical microjets are placed circumferentially, with a 60 inclination angle towards the nozzle centerline axis, in the streamwise flow direction. The pressurized mass-flow feeding the microjets is assumed to be initially at ambient conditions. The amount of pressurization is given as an Injection Pressure Ratio (IPR) and represents the investigation parameter.Acoustic based experiments performed at University of Cincinnati (UC) exhibited acous- tic benefit when using the mentioned set-up for the microjets. However, the impact that injection had on the flow-field was dicult to be quantified. Thus, LES calculations have been performed to analyze the compressible flow-field, the shock-structure alteration and thrust evaluations associated with the fluidics.
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| 7. |
- Pourian, Bijan, 1962-
(författare)
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ANALYZING THE HYDRO DYNAMICS AND THE CHEMICAL REACTIONS IN PULP DIGESTER SYSTEMS USING CFD MODELLING
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this thesis is to use differential analysis and finite volume method (FVM) to model and analyze a continuous pulp digester in order to create a detailed picture of the flow behaviour and chemical reactions in the digester. This information will be used to optimize wood chip flow and reactions and to diagnose and avoid faults such as hang-ups and channelling. As digesters increase in size, the importance of control of the liquor flow in the wood chip bed also increases. Pulping reactors are often faced with production disturbances that cause reduced fibre quality, non-uniform chemical reactions and damaging channelling phenomena. The models that are proposed in this thesis aim to consider the fluid dynamics, thermodynamics and chemical reactions together in order to create a model with more detail than has been previously reported. The digester is of the continuous type and has a circular cross-section, with a height of 60 m and a diameter that increases stepwise from 6 m at the top to 7 m at the bottom. The geometry of the digester, including an internal pipe with three different diameters, three inlet flows at three levels of the digester, inlets and outlets at the top and bottom, and a number of peripheral circulation pipes connected to the screens at three levels, is represented in simulations by a 2D axisymmetric model constructed in Gambit. The interactions between the two phases - the wood chips and the alkaline liquor - are studied using the finite volume method (FVM). Eulerian and Mixture numerical methods are evaluated for solving a mass transfer multiphase model with a porous matrix, species transport and reaction system in FLUENT. Variable porosity is used to model channelling in the digester. Comparisons of the results of simulations with real digester data indicate that the multiphase model, with the species transport and mass transfer modules accurately predict the pulping chemical reactions inside the digester and are able to evaluate pulp quality. The species transport model with reaction scheme is able to predict the mass balance equation and stoichiometry of the pulping reaction. The porous media scheme complements this approach to give a more complete view of pulp production, including the effect of faults in the process. The approaches to the FVM described in this thesis enable an accurate view of the pulping process which can contribute to improved control and optimization of the process.
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| 8. |
- Baudoin, Eric, et al.
(författare)
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Effect of partial premixing on stabilization and local extinction of turbulent methane/air flames
- 2013
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Ingår i: Flow, Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1573-1987 .- 1386-6184. ; 90:2, s. 269-284
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Tidskriftsartikel (refereegranskat)abstract
- Abstract in UndeterminedThe stabilization characteristics and local extinction structures of partially premixed methane/air flames were studied using simultaneous OH-PLIF/PIV techniques, and large eddy simulations employing a two-scalar flamelet model. Partial premixing was made in a mixing chamber comprised of two concentric tubes, where the degree of partial premixing of fuel and air was controlled by varying the mixing length of the chamber. At the exit of the mixing chamber a cone was mounted to stabilize the flames at high turbulence intensities. The stability regime of flames was determined for different degree of partial premixing and Reynolds numbers. It was found that in general partially premixed flames at low Reynolds numbers become more stable when the level of partial premixing of air to the fuel stream decreases. At high Reynolds numbers, for the presently studied burner configuration there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime, significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes grow in size leading to eventual flame blowout. Local flame extinction was found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.
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| 9. |
- Baudoin, Eric, et al.
(författare)
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Effect of partial premixing on stabilization and local extinction of turbulent methane/air flames
- 2011
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Ingår i: Proceedings of the 17th Mediterranean Combustion symposium, MCS 7. - 9788888104126
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Konferensbidrag (refereegranskat)abstract
- The stabilization characteristics and local extinction structures of partially premixed methane/air flames were studied using simultaneous OH-PLIF/PIV techniques, and large eddy simulations employing a two-scalar flamelet model. Partial premixing was made in a mixing chamber comprised of two concentric tubes, where the degree of partial premixing of fuel and air was controlled by varying the mixing length of the chamber. At the exit of the mixing chamber a cone was mounted to stabilize the flames at high turbulence intensities. The stability regime of flames was determined for different degree of partial premixing and Reynolds numbers. It was found that in general partially premixed flames at low Reynolds numbers become more stable when the level of partial premixing of air to the fuel stream decreases. At high Reynolds numbers, for the presently studied burner configuration there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime,significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes grow in size leading to eventual flame blowout. Local flame extinction was found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.
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| 10. |
- Fuchs, Alexander, 1985-, et al.
(författare)
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Assessment of Rheological Models Applied to Blood Flow in Human Thoracic Aorta
- 2023
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Ingår i: Bioengineering. - : MDPI AG. - 2306-5354. ; 10:11
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: The purpose of this study is to assess the importance of non-Newtonian rheological models on blood flow in the human thoracic aorta. Methods: The pulsatile flow in the aorta is simulated using the models of Casson, Quemada and Walburn-Schneck in addition to a case of fixed (Newtonian) viscosity. The impact of the four rheological models (using constant hematocrit) was assessed with respect to (i) magnitude and deviation of the viscosity relative to a reference value (the Newtonian case); (ii) wall shear stress (WSS) and its time derivative; (iii) common WSS-related indicators, OSI, TAWSS and RRT; (iv) relative volume and surface-based retrograde flow; and (v) the impact of rheological models on the transport of small particles in the thoracic aorta. Results: The time-dependent flow in the thoracic aorta implies relatively large variations in the instantaneous WSS, due to variations in the instantaneous viscosity by as much as an order of magnitude. The largest effect was observed for low shear rates (tens s-1). The different viscosity models had a small impact in terms of time- and spaced-averaged quantities. The significance of the rheological models was clearly demonstrated in the instantaneous WSS, for the space-averaged WSS (about 10%) and the corresponding temporal derivative of WSS (up to 20%). The longer-term accumulated effect of the rheological model was observed for the transport of spherical particles of 2 mm and 2 mm in diameter (density of 1200 kg/m3). Large particles' total residence time in the brachiocephalic artery was 60% longer compared to the smaller particles. For the left common carotid artery, the opposite was observed: the smaller particles resided considerably longer than their larger counterparts. Conclusions: The dependence on the non-Newtonian properties of blood is mostly important at low shear regions (near walls, stagnation regions). Time- and space-averaging parameters of interest reduce the impact of the rheological model and may thereby lead to under-estimation of viscous effects. The rheological model affects the local WSS and its temporal derivative. In addition, the transport of small particles includes the accumulated effect of the blood rheological model as the several forces (e.g., drag, added mass and lift) acting on the particles are viscosity dependent. Mass transport is an essential factor for the development of pathologies in the arterial wall, implying that rheological models are important for assessing such risks.
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