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  • 301.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Centre for Mathematical Sciences, Cambridge, England.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    A numerical study of the unstratified and stratified Ekman layer2014Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, s. 672-704Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the turbulent Ekman layer at moderately high Reynolds number, 1600 < Re = delta(E)G/v < 3000, using direct numerical simulations (DNS). Here, delta(E) = root 2v/f is the laminar Ekman layer thickness, G the geostrophic wind, v the kinematic viscosity and f is the Coriolis parameter. We present results for both neutrally, moderately and strongly stably stratified conditions. For unstratified cases, large-scale roll-like structures extending from the outer region down to the wall are observed. These structures have a clear dominant frequency and could be related to periodic oscillations or instabilities developing near the low-level jet. We discuss the effect of stratification and Re on one-point and two-point statistics. In the strongly stratified Ekman layer we observe stable co-existing large-scale laminar and turbulent patches appearing in the form of inclined bands, similar to other wall-bounded flows. For weaker stratification, continuously sustained turbulence strongly affected by buoyancy is produced. We discuss the scaling of turbulent length scales, height of the Ekman layer, friction velocity, veering angle at the wall and heat flux. The boundary-layer thickness, the friction velocity and the veering angle depend on Lf/u(tau), where u(tau) is the friction velocity and L the Obukhov length scale, whereas the heat fluxes appear to scale with L+ = Lu-tau/v.

  • 302.
    Dickson, Crispin
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    A few aspects of aircraft noise2007Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    A few aspects of aircraft noise were evaluated. These were (i) methods of subjective evaluations, (ii) effects of equalization and (iii) the effects of cognitive aspects.

    In the first paper, sorting algorithms were used instead of conventional paired comparison method in order to reduce the number of pairs in the evaluation of subjective judgments. The quick sorting algorithm method revealed more than 99% correlation coefficient with paired comparison method although the method used N*log(N) evaluations instead of N(N-1)/2.

    In the second paper, equalization effects on perception were evaluated in two steps, first with stationary aircraft sounds and second with non-stationary aircraft sounds.

    The first experiment examined the effects of stationary sound segments respect to three different angle positions of the aircrafts relative to the observer (78.7°, 90° & 101.3°), two different SNR conditions (sounds having original broadband plus tonal components versus control broadband sounds having no tonal components) and two different flight conditions (arrival and take-off). Subjects were asked to scale five perceptual attributes (loudness, annoyance, hardness, power and pitch) using Borg CR100 scale. The angle condition showed highly significant effects on annoyance and hardness. Maximal effects were found at an angle of 78.7°. The SNR revealed a significant impact on loudness, power and pitch.

    The second experiment analyzed the effects of tonal components and the problem of appropriate equalization. The spectrum of the signals was modified in two steps (buzz-saw, isolated BPF tone). Further EPNL-equalization, A-, B-, C-, D- and spectral broadband equalizations were applied to the synthesized sounds. Annoyance, loudness, hardness and pitch in the isolated tone conditions showed significantly stronger effects than the buzz-conditions on the perceived judgments. The EPNL-equalization led to a lower degree of differentiation between the spectral conditions compared to B- and C-level equalization.

    In the third paper, the effects of aircraft sounds on children’s cognitive performance were investigated. Impact of aircraft noise on children cognition was found significantly higher in reading comprehensions than in basic mathematics and problem solving tests. It seems children are very sensitive to the modifications in the aircraft noise but further studies are necessary to compliment such a finding.

  • 303.
    Dickson, Crispin
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    A method for time-varying annoyance rating of aircraft noise2009Inngår i: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 126, nr 1, s. 1-3Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The method of continuous judgment by category is used and evaluated to measure time-varying attributes in aircraft flyover sounds. The results are also used to estimate preference between the different experimental sounds. Jurors were asked to rate perceived annoyance on a Borg CR 100 scale continuously during the playback of 11 flyover sequences and the results showed differences in perception in the time segment where the sound had been modified. The method can be used to evaluate maximum perceived annoyance, threshold levels, duration of perceptual presence temporal integration in perception, and perceptual mixtures over time.

  • 304.
    Dickson, Crispin
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    A quicker method of using paired comparisons for the sound quality evaluation2006Inngår i: EURONOISE 2006 - The 6th European Conference on Noise Control: Advanced Solutions for Noise Control, 2006Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Paired Comparison Method (PCM) is widely used by the researchers for evaluating subjective judgments of various stimuli. The conventional PCM is usually used only for a limited number of stimuli (N) since the number of pairs to judge becomes N(N-1)/2 and therefore the number of pairs increases with the number of stimuli. This makes the paired comparisons method very time consuming and more demanding if the numbers of stimuli increase. A novel method is therefore developed which gives similar results as PCM but requires significantly less number of pairs for the comparisons. As a result, this new method could be used to judge a larger number of stimuli. To this end a sorting algorithm such as mergesort is used to obtain the matrix of preference, resulting in about O(N log N) pairs of comparisons. A sorting algorithm reduces the number of pair comparisons using the transitivity of earlier responses given by the subject. Such pair comparisons provide the opportunity to include a larger number of stimuli to include for the evaluation. 12 aircraft sounds were presented to the subjects using headphones. The conventional PCM gave 66 pairs of sounds to be judged by the subjects whereas the mergesort produced only 31 pairs of sounds to be judged. The study reveals the comparisons between the results of conventional PCM and the mergesort.

  • 305.
    Dickson, Crispin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Khan, Shafiquzzaman
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Effect of aircraft noise on children’s cognitive performanceInngår i: Journal of Environmental Psychology, ISSN 0272-4944, E-ISSN 1522-9610Artikkel i tidsskrift (Annet vitenskapelig)
  • 306.
    Dickson, Crispin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Khan, Shafiquzzaman
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Bisping, Rudolf
    Quality of stationary and non-stationary aircraft sounds: the effect of tonal components and level equalizationInngår i: Acta AcusticaArtikkel i tidsskrift (Annet vitenskapelig)
  • 307.
    Dinh, Truc Nam
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnkraftssäkerhet.
    Nourgaliev, R. R.
    Theofanous, T. G.
    On the numerical simulation of acceleration-driven multi-fluid mixing2006Inngår i: Multiphase Science and Technology, ISSN 0276-1459, E-ISSN 1943-6181, Vol. 18, nr 3, s. 199-230Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper is concerned with computational prediction of acceleration-induced multi-fluid mixing phenomena. Premises and performance of existing approaches are reviewed and analyzed with focus on a late phase behavior. We introduce a new framework whose central idea is to use an interfacial area transport equation (IATE) and a subgrid scale model (SGS) of multi-fluid turbulence to provide a natural transition from DNS-based simulation toward an effective-field model (EFM) and deeply into well-mixed states with continuous refinement of length scale. We present new results and important insights derived from our work on four platform technologies: DNS, EFM, IATE and SGS. We discuss the approach to ensure that developments in different areas effectively emerge and function seamlessly in an overall computational platform for multi-fluid mixing.

  • 308.
    Dogan, Eda
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hearst, R. Jason
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England.;Norwegian Univ Sci & Technol, Dept Energy & Proc Engn, NO-7491 Trondheim, Norway..
    Hanson, Ronald E.
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England.;York Univ, Dept Mech Engn, Toronto, ON M3J 1P3, Canada..
    Ganapathisubramani, Bharathram
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England..
    Spatial characteristics of a zero-pressure-gradient turbulent boundary layer in the presence of free-stream turbulence2019Inngår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 8, artikkel-id 084601Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Particle image velocimetry (PIV) measurements are performed to examine the structural organization inside a turbulent boundary layer under the influence of free-stream turbulence (FST). In particular, streamwise-wall-normal plane PIV measurements are presented for two cases at two different turbulent intensity levels (about 13% and 8%). The free-stream turbulence is generated using an active grid in a wind tunnel. The statistical information of the flow regarding the wall-normal velocity and Reynolds shear stress are presented. The effect of increasing the turbulence level in the free stream for these flows has been found to have similarities with increasing Reynolds number for high-Reynolds-number canonical flows. Quadrant analysis is performed to determine the contributions of different Reynolds-stress-producing events. In this regard, the distribution of momentum transport events shows some similarity with channel flows, which can be justified by comparison of similar intermittency characteristics of both flows. In addition, the coherent structures found inside the boundary layer have inclined features that are consistent with the previous studies for canonical flows. The fact that the external disturbance, such as FST in this study, does not alter the organization of the structures inside the boundary layer supports the growing evidence for a universal structure for wall-bounded flows.

  • 309.
    Dogan, Eda
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Gatti, Davide
    Karlsruhe Inst Technol, Inst Fluid Mech, Kaiserstr 10, D-76131 Karlsruhe, Germany..
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Quantification of amplitude modulation in wall-bounded turbulence2019Inngår i: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 51, nr 1, artikkel-id 011408Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Many recent investigations on the scale interactions in wall-bounded turbulent flows focus on describing so-called amplitude modulation, the phenomenon that deals with the influence of large scales in the outer region on the amplitude of the small-scale fluctuations in the near-wall region. The present study revisits this phenomenon regarding two aspects, namely the method for decomposing the scales and the quantification of the modulation. First, the paper presents a summary of the literature that has dealt with either or both aspects. Second, for decomposing the scales, different spectral filters (temporal, spatial or both) and empirical mode decomposition (EMD) are evaluated and compared. The common data set is a well-resolved large-eddy simulation that offers a wide range of Reynolds numbers spanning Re-theta = 880-8200. The quantification of the amplitude modulation is discussed for the resulting scale components. Particular focus is given to evaluate the efficacy of the various filters to separate scales for the range of Reynolds numbers of interest. Different to previous studies, the different methods have been evaluated using the same data set, thereby allowing a fair comparison between the various approaches. It is observed that using a spectral filter in the spanwise direction is an effective approach to separate the small and large scales in the flow, even at comparably low Reynolds numbers, whereas filtering in time should be approached with caution in the low-to-moderate Re range. Additionally, using filters in both spanwise and time directions, which would separate both wide and long-living structures from the small and fast scales, gives a cleaner image for the small-scales although the contribution to the scales interaction from that filter implementation has been found negligible. Applying EMD to decompose the scales gives similar results to Fourier filters for the energy content of the scales and thereby for the quantification of the amplitude modulation using the decomposed scales. No direct advantage of EMD over classical Fourier filters could be seen. Potential issues regarding different decomposition methods and different definitions of the amplitude modulation are also discussed.

  • 310.
    Do-Quang, Minh
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik.
    Carlson, A
    Amberg, G
    The impact of ink-jet droplets on a paper-like structure2011Inngår i: Fluid Dynamics and Materials Processing, ISSN 1555-2578, Vol. 7, nr 4, s. 389-402Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Inkjet technology has been recognized as one of the most successful and promising micro-system technologies. The wide application areas of printer heads and the increasing demand of high quality prints are making ink consumption and print see-through important topics in the inkjet technology. In the present study we investigate numerically the impact of ink droplets onto a porous material that mimics the paper structure. The mathematical framework is based on a free energy formulation, coupling the Cahn-Hilliard and Navier Stokes equations, for the modelling of the two-phase flow. The case studied here consists of a multiphase flow of air-liquid along with the interaction between a solid structure and an interface. In order to characterize the multiphase flow characteristics, we investigate the effects of surface tension and surface wettability on the penetration depth and spreading into the paper-like structure.

  • 311.
    Do-Quang, Minh
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik.
    Geyl, Laurent
    KTH, Skolan för elektro- och systemteknik (EES), Mikrosystemteknik.
    Stemme, Göran
    KTH, Skolan för elektro- och systemteknik (EES), Mikrosystemteknik.
    van der Wijngaart, Wouter
    KTH, Skolan för elektro- och systemteknik (EES), Mikrosystemteknik.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik.
    Fluid dynamic behavior of dispensing small droplets through a thin liquid film2010Inngår i: Microfluidics and Nanofluidics, ISSN 1613-4982, Vol. 9, nr 2-3, s. 303-311Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper presents a technology for dispensing droplets through thin liquid layers. The system consists of a free liquid film, which is suspended in a frame and positioned in front of a piezoelectric printhead. A droplet, generated by the printhead, merges with the film, but due to its momentum, passes through and forms a droplet that separates on the other side and continues its flight. The technology allows the dispensing, mixing and ejecting of picolitre liquid samples in a single step. This paper overviews the concept, potential applications, experiments, results and a numerical model. The experimental work includes studying the flight of ink droplets, which ejected from an inkjet print head, fly through a free ink film, suspended in a frame and positioned in front of the printhead. We experimentally observed that the minimum velocity required for the 80 pl droplets to fly through the 75 ± 24 lm thick ink film was of 6.6 m s-1. We also present a numerical simulation of the passage of liquid droplets through a liquid film. The numerical results for different initial speeds of droplets and their shapes are taken into account. We observed that during the droplet-film interaction, the surface energy is partially converted to kinetic energy, and this, together with the impact time, helps the droplets penetrate the film. The model includes the Navier- Stokes equations with continuum-surface-tension force derived from the phase-field/Cahn-Hilliard equation. This system allows us to simulate the motion of a free surface in the presence of surface tension during merging, mixing and ejection of droplets. The influence of dispensing conditions was studied and it was found that the residual velocity of droplets after their passage through the thin liquid film well matches the measured velocity from the experiment.

  • 312.
    Drioli, Carlo
    KTH, Skolan för datavetenskap och kommunikation (CSC), Tal, musik och hörsel, TMH.
    A flow waveform-matched low-dimensional glottal model based on physical knowledge2005Inngår i: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 117, nr 5, s. 3184-3195Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The purpose of this study is to explore the possibility for physically based mathematical models of the voice source to accurately reproduce inverse filtered glottal volume-velocity waveforms. A low-dimensional, self-oscillating model of the glottal source with waveform-matching properties is proposed. The model relies on a lumped mechano-aerodynamic scheme loosely inspired by the one-and multimass lumped models. The vocal folds are represented by a single mechanical resonator and a propagation line which takes into account the vertical phase differences. The vocal-fold displacement is coupled to the glottal flow by means of an aerodynamic driving block which includes a general parametric nonlinear component. The principal characteristics of the flow-induced oscillations are retained, and the overall model is able to match inverse-filtered glottal flow signals. The method offers in principle the possibility of performing transformations of the glottal flow by acting on the physiologically based parameters of the model. This is a desirable property, e.g., for speech synthesis applications. The model was tested on a data set which included inverse-filtered glottal flow waveforms of different characteristics. The results demonstrate the possibility of reproducing natural speech waveforms with high accuracy, and of controlling important characteristics of the synthesis such as pitch.

  • 313.
    Du, Lin
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Karlsson, Mikael
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Knutsson, M.
    Modelling of Acoustic Resonators Using the Linearized Navier Stokes Equations2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

    To tune the acoustics of intake systems resonators are often used. A problem with this solution is that the performance of these resonators can be affected a lot by flow. First, for low frequencies (Strouhal-numbers) the acoustic induced vorticity across a resonator inlet opening will create damping, which can reduce the efficiency. Secondly, the vorticity across the opening can also change the end-correction (added mass) for the resonator, which can modify the resonance frequency. However, the largest problem that can occur is whistling. This happens since the vortex-sound interaction across a resonator opening for certain Strouhal-numbers will amplify incoming sound waves. A whistling can then be created if this amplified sound forms a feedback loop, e.g., via reflections from system boundaries or the resonator. To analyse this kind of problem it is necessary to have a model that allows for both sound and vorticity and their interaction. This means using a convected wave equation type of model is not sufficient. A better approach is to apply the linearized Navier Stokes equations, which will give a full model of the vortex-sound effects. In this paper an effort to apply this approach on a set of generic resonators is described. Besides the numerical results comparisons with experiments are also presented.

  • 314. Ducoin, A.
    et al.
    Loiseau, Jean-Christophe
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik.
    Robinet, J. -C
    Numerical investigation of the interaction between laminar to turbulent transition and the wake of an airfoil2016Inngår i: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 57, s. 231-248Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The objective of this work is to investigate numerically the different physical mechanisms of the transition to turbulence of a separated boundary-layer flow over an airfoil at low angle of attack. In this study, the spectral elements code Nek5000 is used to simulate the flow over a SD7003 wing section at an angle of attack of α=4(ring operator). Several laminar cases are first studied from Re=2000 to Re=10000, and a gradual increase of the Reynolds number is then performed in order to investigate one transitional case at Re=20000. Computations are compared with measurements where the instability mechanisms in the separated zone and near wake zone have been analyzed. The mechanism of transition is investigated, where the DMD (Dynamic Mode Decomposition) is used in order to extract the main physical modes of the flow and to highlight the interaction between the transition and the wake flow. The results suggest that the transition process appears to be physically independent of the wake flow, while the LSB shedding process is locked-in with the von Kármán instability and acts as a sub-harmonic.

  • 315.
    Ducret, Fabrice
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg.
    Studies of sound generation and propagation in flow ducts2006Licentiatavhandling, monografi (Annet vitenskapelig)
    Abstract [en]

    This thesis contains three papers investigating problems of interest for noise control in ducts.

    The first part of this thesis treats the sound propagation in rectangular ducts with flexible walls. Various experimental techniques are performed to measure the internal sound propagation and radiation to the surrounding. An analytical model is derived to calculate the coupled propagation wavenumber and radiated sound power. The two-port formalism is used.

    The second part starts with the sound propagation in open ended circular straight pipe with airflow (a tailpipe). Various aspects such as: acoustic damping, reflection and transmission at the open termination are investigated. Sound absorption due to vorticity shed at the opening is also treated. The geometry of the opening is then modified (oblique cuts, diffusers) and comparisons with the reference straight pipe is made for the sound transmission and flow induced noise generation. The effect of an upstream bend close to the opening is also investigated.

    In the third part the acoustic impedance of perforated plates are investigated. In particular the application to small perforation ratios ( ≈ 1% ) and holes or slits with apertures of sub-millimetre size, so called micro-perforated plates, are of interest. Linear and non-linear regimes are investigated. A model is derived to calculate the linear acoustic impedance of perforated elements.

  • 316.
    Ducret, Fabrice
    et al.
    Ørsted, DTU, Acoustic Technology, Technical University of Denmark.
    Jacobsen, F.
    Åbom, Mats
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    Development of micro-perforated acoustic elements for vehicle applications2005Inngår i: 12th International Congress on Sound and Vibration 2005: ICSV 2005, 2005, s. 2343-2350Konferansepaper (Fagfellevurdert)
    Abstract [en]

    There is a growing interest in the automotive industry on micro-perforated acoustic elements for reducing noise radiation from engine systems. The traditional means of acoustic attenuation through porous materials has some serious drawbacks (poor efficiency at low frequencies, low capability to withstand flow and thermal conditions commonly encountered in engine systems). Novel elements consisting of holes/slits of sizes of about 10-1 mm are currently investigated. An indicator of paramount importance when designing such elements is the acoustic flow impedance (pressure drop across the sample/acoustic particle velocity). A detailed description of the impedance of these elements is necessary for evaluating the efficiency of reflection of acoustic waves. This is required for reducing the propagation of noise further downstream. A concise fundamental theoretical investigation for optimal design is not possible due to highly complex geometries attributed to micro-perforated patterns. Moreover, mean flow, thermal and non-linear effects come into play for vehicle applications. This paper describes preliminary results from a project investigating the above effects.

  • 317.
    Duguet, Yohann
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philip
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Formation of turbulent patterns near the onset of transition in plane Couette flow2010Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 650, s. 119-129Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The formation of turbulent patterns in plane Couette flow is investigated near the onset of transition, using numerical simulation in a very large domain of size 800 h x 2h x 356 h. Based on a maximum observation time of 20 000 inertial units, the threshold for the appearance of sustained turbulent motion is Re-c = 324 +/- 1. For Re-c < Re <= 380, turbulent-banded patterns form, irrespective of whether the initial perturbation is a noise or localized disturbance. Measurements of the turbulent fraction versus Re show evidence for a discontinuous phase transition scenario where turbulent spots play the role of the nuclei. Using a smaller computational box, the angle selection of the turbulent bands in the early stages of their development is shown to be related to the amplitude of the initial perturbation.

  • 318.
    Duguet, Yohann
    et al.
    Universit´e Paris-Sud, Orsay, France.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Stochastic motion of a laminar/turbulent interface in a shear flow2011Inngår i: / [ed] K. Bajer, 2011Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Plane Couette flow is a classical prototype of a shear flow where transition toturbulence is subcritical, i.e. happens despite linear stability of the base flow. In this studywe are interested in the spatio-temporal competition between the (active) turbulent phase andthe (absorbing) laminar. Our three-dimensional numerical simulations show that the delimitinginterface, when parallel to the streamwise direction, moves in a stochastic manner which wemodel as a continuous-time random walk. Statistical analysis suggests a Gaussian diffusionprocess and allows us to determine the average speed of this interface as a function of theReynolds number Re, as well as the threshold in Re above which turbulence contaminatesthe whole domain. For the lowest value of Re, this stochastic motion competes with anotherdeterministic regime of growth of the localised perturbations. The latter, a rather unexpectedregime, is shown to be linked to the recently found localised snaking solutions of the Navier-Stokes equations. An extension of this thinking to more general orientations of the interfaceswill be proposed.

  • 319. Duguet, Yohann
    et al.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Eckhardt, Bruno
    Self-Sustained Localized Structures in a Boundary-Layer Flow2012Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, nr 4, s. 044501-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    When a boundary layer starts to develop spatially over a flat plate, only disturbances of sufficiently large amplitude survive and trigger turbulence subcritically. Direct numerical simulation of the Blasius boundary-layer flow is carried out to track the dynamics in the region of phase space separating transitional from relaminarizing trajectories. In this intermediate regime, the corresponding disturbance is fully localized and spreads slowly in space. This structure is dominated by a robust pair of low-speed streaks, whose convective instabilities spawn hairpin vortices evolving downstream into transient disturbances. A quasicyclic mechanism for the generation of offspring is unfolded using dynamical rescaling with the local boundary-layer thickness.

  • 320.
    Dulac, Adrien
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Kraft- och värmeteknologi.
    A CFD Analysis towards Flow Characteristics of three Pre-swirler Designs2012Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    Although pre-swirlers play a determinant role in the transport of air from stationary parts to rotating holes, knowledge about their actual performance is limited. Therefore, this paper aims to relate how the pre-swirler pressure drop affects the performance of different pre-swirlers in terms of discharge coefficient, adiabatic pre-swirl effectiveness, and swirl ratio. The results are extracted from numerical simulations carried out on three different designs, two guide vanes, and a nozzle. When available, the results are compared to experimental data.

    The guide vanes have shown similar responses to the pressure drop variations. Their discharge coefficients remain relatively insensitive with an average value of 97%. The swirl ratio range from 0.704 to 1.013 and 0.703 to 1.023 respectively for a pressure drop varying from 3 to 7 bars. The adiabatic pre-swirl effectiveness is of 96% and 94%, respectively, under steady state operation.The nozzle design has shown inferior performance as compared to the guide vane designs. Its discharge coefficient remains around 91% and the swirl ratio varies between 0.678 and 1.121 for a pressure drop ranging from 3 to 10 bars. Under steady state operation, the adiabatic pre-swirl effectiveness is 1.22.

    The influence of through-flows on the aforementioned parameters was also analyzed. It was observed that the through-flow deteriorates the performance of the pre-swirlers, whether in terms of dimensionless pre-swirl effectiveness, or swirl ratio. The discharge coefficient was however not affected.

  • 321.
    Dumitrescu, O.
    et al.
    Hungary.
    Stanciuc, R.
    Hungary.
    Gherman, G. B.
    Hungary.
    Dragan, Valeriu
    Losses and blade tip clearance for a centrifugal compressor2018Inngår i: INCAS Bulletin, ISSN 2066-8201, E-ISSN 2247-4528, Vol. 10, nr 2, s. 27-37Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present paper presents the numerical analysis for a transonic centrifugal compressor using steady state CFD. The blade tip clearance effect over the position of shock waves, tip losses and the performances of the impeller are studied. Numerical simulations have been performed using RANS modelling, with the k-omega SST turbulence model (Shear Stress Transport). Eight cases were taken into consideration for the impeller with the following blade tip clearances values: 0 mm, 0.1 mm, 0.3 mm, 0.4 mm, 0.5mm, 0.7 mm, 1 mm, 2 mm, at the same operating conditions. For the entire stage only seven cases were studied, without the value for 0.1 mm because of its abnormal behaviour, as can be seen in the case of the impeller simulations. Results showed that the position of the shock wave does not change with the increase of the tip clearance. Aerodynamic losses due to shock wave, secondary flow and turbulence can be seen in the polytropic efficiency of the centrifugal impeller and the difference between the two extreme cases is about 3.2 %.

  • 322. Durup, E
    et al.
    Jansson, Erik Valter
    KTH, Skolan för datavetenskap och kommunikation (CSC), Tal, musik och hörsel, TMH.
    The quest of the violin Bridge-Hill2005Inngår i: Acta Acoustica united with Acustica, ISSN 1610-1928, E-ISSN 1861-9959, Vol. 91, nr 2, s. 206-213Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Good violins have a Bridge-Hill, i.e. a hump between 2 and 3 kHz in their frequency responses, both in radiated sound and in bridge mobility. Experiments have proved that the Hill is not confined to the bridge only. Present experiments show that the Hill can be modelled by a plate-bridge and a rectangular spruce plate with f-holes shaped as up-side-down Us. The flaps, the wings cut free by the up-side-down Us and especially their areas determine the peak frequency of the Hill. By tuning the bridge and the wings of a complete violin the Hill is obtained and can be tuned within a wide frequency range.

  • 323.
    Duwig, Christophe
    et al.
    Energy Sciences, Lund University.
    Gherman, Bogdan
    Energy Sciences, Lund University.
    Mihaescu, Mihai
    Energy Sciences, Lund University.
    Salewski, Mirko
    Energy Sciences, Lund University.
    Fuchs, Laszlo
    Energy Sciences, Lund University.
    Numerical Study of Thermo-acoustic Waves Generation by a Swirling Flame Using a New Approach Based on large eddy simulation2005Inngår i: Volume 2: Turbo Expo 2005: Power for Land, Sea, and Air (GT2005), 2005, s. 67-75Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The new challenge of the Gas Turbine industry is to develop new technologies for meeting electricity demand growth and reducing harmful emissions. Thus, a better understanding of the combustion phenomenon and an improvement in simulation capabilities are needed. In this paper, we present a new technique that is computationally efficient, for capturing the thermo-acoustic waves in low Mach number combustors. The idea is to utilize the fact that the acoustic related pressure fluctuations are small as compared to the dynamic pressure. Semi-compressible LES of reacting flow is performed and while the pressure perturbations related to the acoustic-wave propagation are handled separately. The equations are solved individually while allowing interaction between the two systems. The technique has been applied to the study of the generation and propagation of thermo-acoustic waves in a combustion chamber. A sensitivity analysis is presented and most important instability modes are identified.

  • 324.
    Duyar, Serkan
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Kraft- och värmeteknologi.
    Modeling diesel combustion in heavy duty engine using detailed chemistry approach and CFD2014Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    Emission and fuel consumption are among the key parameters when designing a combustion system. Combustion CFD can assist in this task only if good enough accuracy is achieved regarding combustion and emission predictions.

    The aim of this master thesis is to evaluate the use of detailed reaction mechanisms (as a substitute for standard combustion model) in terms of computational time and result accuracy. Several mechanisms for n-heptane are tested. Lund University optical engine experimental case is used for this evaluation.Results showed that detailed chemistry can predict ignition accurately but differences are observed in the peak cylinder pressure. The computational time also increased significantly as size and complexity of the mechanism increased.

    Recommendations are given to improve predictions using detailed chemistry approach which is found to be an interesting approach especially for lift-off length predictions.

  • 325. Dörr, P. C.
    et al.
    Kloker, M. J.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Effect of upstream flow deformation using plasma actuators on crossflow transition induced by unsteady vortical free-stream disturbances2017Inngår i: 47th AIAA Fluid Dynamics Conference, 2017, American Institute of Aeronautics and Astronautics, 2017Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Upstream flow deformation (UFD) has been shown to be an effective technique to delay roughness induced laminar-turbulent transition at low free-stream turbulence level in three-dimensional boundary-layer flows. Beneficial steady crossflow vortex (CFV) control modes are excited and the resulting nonlinear CFVs induce a useful mean-flow distortion. We recently showed by direct numerical simulations that plasma actuators, modeled by localized steady volume forcing, can be employed to excite the UFD control modes. In the current work we investigate the same actuator set-ups to control transition caused by traveling CFVs that are excited by single unsteady vortical free-stream disturbances (FSDs) impinging on the boundary layer. FSDs of various wavenumbers and frequencies are imposed either upstream or downstream, or at the position of the actuators to also scrutinize if the volume forcing has a direct unfavorable effect on the receptivity to the FSDs that adds to the stabilization by the UFD. For all investigated cases we show that a significant transition delay is achieved.

  • 326. Echternach, Matthias
    et al.
    Sundberg, Johan
    KTH, Skolan för datavetenskap och kommunikation (CSC), Tal, musik och hörsel, TMH.
    Baumann, Tobias
    Markl, Michael
    Richter, Bernhard
    Vocal tract area functions and formant frequencies in opera tenors' modal and falsetto registers2011Inngår i: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 129, nr 6, s. 3955-3963Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    According to recent model investigations, vocal tract resonance is relevant to vocal registers. However, no experimental corroboration of this claim has been published so far. In the present investigation, ten professional tenors' vocal tract configurations were analyzed using MRI volumetry. All subjects produced a sustained tone on the pitch F4 (349 Hz) on the vowel /a/(1) in modal and (2) in falsetto register. The area functions were estimated from the MRI data and their associated formant frequencies were calculated. In a second condition the same subjects repeated the same tasks in a sound treated room and their formant frequencies were estimated by means of inverse filtering. In both recordings similar formant frequencies were observed. Vocal tract shapes differed between modal and falsetto register. In modal as compared to falsetto the lip opening and the oral cavity were wider and the first formant frequency was higher. In this sense the presented results are in agreement with the claim that the formant frequencies differ between registers.

  • 327.
    Edlund, Jens
    KTH, Skolan för datavetenskap och kommunikation (CSC), Tal, musik och hörsel, TMH, Tal-kommunikation.
    In search for the conversational homunculus: serving to understand spoken human face-to-face interaction2011Doktoravhandling, monografi (Annet vitenskapelig)
    Abstract [en]

    In the group of people with whom I have worked most closely, we recently attempted to dress our visionary goal in words: “to learn enough about human face-to-face interaction that we are able to create an artificial conversational partner that is humanlike”. The “conversational homunculus” figuring in the title of this book represents this “artificial conversational partner”. The vision is motivated by an urge to test computationally our understandings of how human-human interaction functions, and the bulk of my work leads towards the conversational homunculus in one way or another. This book compiles and summarises that work: it sets out with a presenting and providing background and motivation for the long term research goal of creating a humanlike spoken dialogue system, and continues along the lines of an initial iteration of an iterative research process towards that goal, beginning with the planning and collection of human-human interaction corpora, continuing with the analysis and modelling of the human-human corpora, and ending in the implementation of, experimentation with and evaluation of humanlike components for in human-machine interaction. The studies presented have a clear focus on interactive phenomena at the expense of propositional content and syntactic constructs, and typically investigate the regulation of dialogue flow and feedback, or the establishment of mutual understanding and grounding.

  • 328.
    Efraimsson, Gunilla
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aeroakustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Forsberg, Nicolas
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aeroakustik.
    Nordström, Jan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aeroakustik.
    Simulations of Acoustic Waves in a Turbo-Fan Engine Air Intake2010Inngår i: 16th AIAA/CEAS Aeroacoustics Conference (31st AIAA Aeroacoustics Conference), 2010Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    In this work, propagation of acoustic waves in the air intake of an turbo-fan engine are simulated using a commercial Navier-Stokes solver. Three different acoustic modes are studied, the first and second radial mode with no azimuthal variation and the first radial mode with four lobes in the azimuthal direction. From the results it can be concluded that the propagation of sound waves in a curved intake can indeed be simulated using a commercial CFD solver. Also, the acoustic source, when given as a boundary condition, should be set at the fan plane. A strong influence of the flow or the curved geometry is identified, yielding a focusing of sound waves to the middle part of the duct. A transmission loss of the acoustic power from the fan plane to the inlet plane of around 5 dB is identified for the first radial modes for acoustic powers in the interval [128 db, 158 dB]. Non-linear effects are identified for powers of 148 dB and higher, which seems reasonable. Finally, a shielding effect of supersonic regions is identified.

  • 329.
    Efraimsson, Gunilla
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aeroakustik.
    Pieper, Timm
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aeroakustik.
    Åbom, Mats
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strömningsakustik.
    Boij, Susann
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strömningsakustik.
    Simulation of Wave Scattering at an Orifice by using a Navier-Stokes Solver2007Inngår i: 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference) , Rome, Italy, May 21-23, 2007, 2007Konferansepaper (Annet vitenskapelig)
  • 330. Einarsson, J.
    et al.
    Candelier, F.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Angilella, J. R.
    Mehlig, B.
    Effect of weak fluid inertia upon Jeffery orbits2015Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 91, nr 4, artikkel-id 041002Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We consider the rotation of small neutrally buoyant axisymmetric particles in a viscous steady shear flow. When inertial effects are negligible the problem exhibits infinitely many periodic solutions, the "Jeffery orbits." We compute how inertial effects lift their degeneracy by perturbatively solving the coupled particle-flow equations. We obtain an equation of motion valid at small shear Reynolds numbers, for spheroidal particles with arbitrary aspect ratios. We analyze how the linear stability of the "log-rolling" orbit depends on particle shape and find it to be unstable for prolate spheroids. This resolves a puzzle in the interpretation of direct numerical simulations of the problem. In general, both unsteady and nonlinear terms in the Navier-Stokes equations are important.

  • 331. Einarsson, J.
    et al.
    Candelier, F.
    Lundell, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Angilella, J. R.
    Mehlig, B.
    Rotation of a spheroid in a simple shear at small Reynolds number2015Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, nr 6, artikkel-id 063301Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We derive an effective equation of motion for the orientational dynamics of a neutrally buoyant spheroid suspended in a simple shear flow, valid for arbitrary particle aspect ratios and to linear order in the shear Reynolds number. We show how inertial effects lift the degeneracy of the Jeffery orbits and determine the stabilities of the log-rolling and tumbling orbits at infinitesimal shear Reynolds numbers. For prolate spheroids, we find stable tumbling in the shear plane and log-rolling is unstable. For oblate spheroids, by contrast, log-rolling is stable and tumbling is unstable provided that the particle is not too disk-like (moderate asphericity). For very flat oblate spheroids, both log-rolling and tumbling are stable, separated by an unstable limit cycle.

  • 332. Eitel-Amor, G
    et al.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Turbulent boundary layers in long computational domains2015Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Wall-bounded turbulence emerges e.g. along the surface of moving ships and airplanes or in pipelines. The prediction of skin friction and drag is directly related to fuel consumption or the power needed to transport gases through pipelines, thereby emphasizing the practical relevance of wall turbulence

  • 333.
    Eitel-Amor, Georg
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Simulation and validation of a spatially evolving turbulent boundary layer up to Reθ = 83002014Inngår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 47, s. 57-69Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Results of a finely resolved large-eddy simulation (LES) of a spatially developing zero-pressure-gradient turbulent boundary layer up to a Reynolds number of Reθ = 8300 are presented. The very long computational domain provides substantial assessment for suggested high Reynolds number (Re) trends. Statistics, integral quantities and spectral data are validated using high quality direct numerical simulation (DNS) ranging up to Reθ = 4300 and hot-wire measurements covering the remaining Re-range. The mean velocity, turbulent fluctuations, skin friction, and shape factor show excellent agreement with the reference data. Through utilisation of filtered DNS, subtle differences between the LES and DNS could to a large extent be explained by the reduced spanwise resolution of the LES. Spectra and correlations for the streamwise velocity and the wall-shear stress evidence a clear scale-separation and a footprint of large outer scales on the near-wall small scales. While the inner peak decreases in importance and reduces to 4% of the total energy at the end of the domain, the energy of the outer peak scales in outer units. In the near-wall region a clear k - 1 region emerges. Consideration of the two-dimensional spectra in time and spanwise space reveals that an outer time scale λt ≈ 10δ99 / U∞, with the boundary layer thickness δ99 and free-stream velocity U∞, is the correct scale throughout the boundary layer rather than the transformed streamwise wavelength multiplied by a (scale independent) convection velocity. Maps for the covariance of small scale energy and large scale motions exhibit a stronger linear Re dependence for the amplitude of the off-diagonal peak compared to the diagonal one, thereby indicating that the strength of the amplitude modulation can only qualitatively be assessed through the diagonal peak. In addition, the magnitude of the wall-pressure fluctuations confirms mixed scaling, and pressure spectra at the highest Re give a first indication of a -7/3 wave number dependence. © 2014 Elsevier Inc.

  • 334. El Bardisi, M.
    et al.
    Bodén, Hans
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strömningsakustik.
    Sources of noise in greater Cairo: example East Cairo2005Inngår i: 12th International Congress on Sound and Vibration 2005: ICSV 2005, 2005, s. 3481-3488Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Noise in big cities such as Cairo Egypt is an important pollutant in the modern society. Noise coming from airports, road traffic, rail traffic, power plants as well as commercial activities in the city are important contributors to the overall noise level. A noise study including measurements and predictions for noise in East Cairo coming from airport, road traffic, metro and railways was performed and analyzed. A comparison of the different sources was made and the number of affected people in the mentioned area was estimated. The dominating sources and the best mitigation to minimize the noise level was discussed and evaluated. In the analysis, some new projects was discussed individually to find its effect on noise pollution, also expectation for the coming years with the new projects and proposals where discussed. The proposed new projects include the new terminal building and runway at Cairo international airport and the proposed Heliopolis Metro Line-4 Ramsis Square - Nasr City. The analysis showed the dominating noise sources and evaluated the mentioned projects from the noise point of view. Noise out from air traffic was the dominating source of noise in East Cairo.

  • 335.
    Eliasson, Veronica
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    On focusing of shock waves2007Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Both experimental and numerical investigations of converging shock waves have been performed. In the experiments, a shock tube was used to create and study converging shock waves of various geometrical shapes. Two methods were used to create polygonally shaped shocks. In the first method, the geometry of the outer boundary of the test section of the shock tube was varied. Four different exchangeable shapes of the outer boundary were considered: a circle, a smooth pentagon, a heptagon, and an octagon. In the second method, an initially cylindrical shock wave was perturbed by metal cylinders placed in various patterns and positions inside the test section. For three or more regularly spaced cylinders, the resulting diffracted shock fronts formed polygonal shaped patterns near the point of focus. Regular reflection was observed for the case with three cylinders and Mach refection was observed for cases with four or more cylinders. When the shock wave is close to the center of convergence, light emission is observed. An experimental investigation of the light emission was conducted and results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape.

    The shock wave focusing was also studied numerically using the Euler equations for a gas obeying the ideal gas law with constant specific heats. Two problems were analyzed; an axisymmetric model of the shock tube used in the experiments and a cylindrical shock wave diffracted by cylinders in a two dimensional test section. The results showed good agreement with the experiments. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there.

    As a practical example of shock wave focusing, converging shocks in an electrohydraulic lithotripter were simulated. The maximum radius of a gas bubble subjected to the pressure field obtained from the lithotripter was calculated and compared for various geometrical shapes and materials of the reflector. Results showed that the shape had a large impact while the material did not influence the maximum radius of the gas bubble.

  • 336.
    Eliasson, Veronica
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    On focusing of strong shock waves2005Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Focusing of strong shock waves in a gas-filled thin test section with various forms of the reflector boundary is investigated. The test section is mounted at the end of the horizontal co-axial shock tube. Two different methods to produce shock waves of various forms are implemented. In the first method the reflector boundary of the test section is exchangeable and four different reflectors are used: a circle, a smooth pentagon, a heptagon and an octagon. It is shown that the form of the converging shock wave is influenced both by the shape of the reflector boundary and by the nonlinear dynamic interaction between the shape of the shock and the propagation velocity of the shock front. Further, the reflected outgoing shock wave is affected by the shape of the reflector through the flow ahead of the shock front. In the second method cylindrical obstacles are placed in the test section at various positions and in various patterns, to create disturbances in the flow that will shape the shock wave. It is shown that it is possible to shape the shock wave in a desired way by means of obstacles. The influence of the supports of the inner body of the co-axial shock tube on the form of the shock is also investigated. A square shaped shock wave is observed close to the center of convergence for the circular and octagonal reflector boundaries but not in any other setups. This square-like shape is believed to be caused by the supports for the inner body. The production of light, as a result of shock convergence, has been preliminary investigated. Flashes of light have been observed during the focusing and reflection process.

  • 337.
    Eliasson, Veronica
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    The Production of Converging Polygonal Shock Waves by Means of Reflectors and Cylindrical Obstacles2006Inngår i: Flow Dynamics / [ed] Tokuyama, M; Maruyama, S, 2006, Vol. 832, s. 445-449Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Converging and reflecting strong shock waves are investigated experimentally in a horizontal co-axial shock tube. The shock tube has a test section mounted at the end of the tube. Two different methods to produce various geometrical shapes of shock waves are tested. In the first method the reflector boundary of the test section is exchangeable and four different reflectors are used: a circle, a smooth pentagon, a heptagon and an octagon. It is shown that the form of the converging shock wave is influenced both by the shape of the reflector boundary and by the nonlinear dynamics between the shape of the shock and the velocity of the shock front. Further, the reflected outgoing shock wave is affected by the shape of the reflector through the flow ahead of the shock front. In the second method we use cylindrical obstacles, placed in the test section at various positions and patterns, to create disturbances in the flow that will shape the shock wave. It is shown that it is possible to shape the shock wave in a desired way with these obstacles. The influence of the supports of the inner body of the co-axial shock tube is also investigated. A square shaped shock wave is observed close to the center of convergence for the circular and octagonal reflectors but not in any other setups. This square-like shape is believed to be caused by the supports for the inner body.

  • 338.
    Eliasson, Veronica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Controlling the form of strong converging shocks by means of disturbances.2007Inngår i: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 17, nr 1-2, s. 29-42Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The influence of artificial disturbances on the behavior of strong converging cylindrical shocks is investigated experimentally and numerically. Ring-shaped shocks, generated in an annular cross sectional shock tube are transformed to converging cylindrical shocks in a thin cylindrical test section, mounted at the rear end of the shock tube. The converging cylindrical shocks are perturbed by small cylinders placed at different locations and in various patterns in the test section. Their influence on the shock convergence and reflection process is investigated. It is found that disturbances arranged in a symmetrical pattern will produce a symmetrical deformation of the converging shockfront. For example, a square formation produces a square-like shock and an octagon formation a shock with an octagonal front. This introduces an alternative way of tailoring the form of a converging shock, instead of using a specific form of a reflector boundary. The influence of disturbances arranged in non-symmetric patterns on the shape of the shockfront is also investigated.

  • 339.
    Eliasson, Veronica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lesser, Martin
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Focusing of strong shocks in an annular shock tube2007Inngår i: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 15, nr 3-4, s. 205-217Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Focusing of strong shock waves in a gas-filled thin convergence chamber with various forms of the reflector boundary is investigated experimentally and numerically. The convergence chamber is mounted at the end of the horizontal co-axial shock tube. The construction of the convergence chamber allows the assembly of the outer chamber boundaries of various shapes. Boundaries with three different shapes have been used in the present investigation-a circle, an octagon and a smooth pentagon. The shock tube in the current study was able to produce annular shocks with the initial Mach number in the range M-s = 2.3-3.6. The influence of the shape of the boundary on the shape and properties of the converging and reflected shock waves in the chamber has then been investigated both experimentally and numerically. It was found that the form of the converging shock is initially governed by the shape of the reflector and the nonlinear interaction between the shape of the shock and velocity of shock propagation. Very close to the center of convergence the shock obtains a square-like form in case of a circular and octagonal reflector boundary. This is believed to stem from the instability of the converging shock front triggered by the disturbances in the flow field. The outgoing, reflected shocks were also observed to be influenced by the shape of the boundary through the flow ahead as created by the converging shocks.

  • 340.
    Eliasson, Veronica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Henshaw, William
    Department of Mechanical Engineering, University of California, Berkeley.
    Appelö, Daniel
    Lawrence Livermore National Laboratory, Livermore.
    On cylindrically converging shock waves shaped by obstacles2008Inngår i: Physica D: Non-linear phenomena, ISSN 0167-2789, E-ISSN 1872-8022, Vol. 237, nr 14-17, s. 2203-2209Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Motivated by recent experiments, numerical simulations of cylindrically converging shock waves were performed. The converging shocks impinged upon a set of 0-16 regularly space obstacles. For more than two obstacles the resulting diffracted shock fronts formed polygonal shaped patterns near the point of focus. The maximum pressure and temperature as a function of the number of obstacles were studied. The self-similar behavior of cylindrical, triangular and square-shaped shocks was also investigated.

  • 341.
    Eliasson, Veronica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Kjellander, Malte
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Regular versus Mach reflection for converging polygonal shocks2007Inngår i: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 17, nr 1-2, s. 43-50Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The onset of Mach reflection or regular reflection at the vertices of a converging polygonal shock wave was investigated experimentally in a horizontal annular shock tube. The converging shock waves were visualized by schlieren optics. Two different types of polygonal shock convergence patterns were observed. We compared the behavior during the focusing process for triangular and square-shaped shocks. It is shown that once a triangular shaped shock is formed, the corners in the converging shock will undergo regular reflection and consequently the shape will remain unaltered during the focusing process. A square-shaped shock suffers Mach reflections at the corners and hence a reconfiguring process takes place; the converging shock wave alternates between a square and an octagon formation during the focusing process.

  • 342.
    Eliasson, Veronica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    Szeri, Andrew
    University of California, Department of Mechanical Engineering, Berkeley.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Light emission during shock wave focusing in air and argon2007Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, nr 10, s. 106106-1-106106-17Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The light emission from a converging shock wave was investigated experimentally. Results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape. The light emission appears as the shock wave collapses. The full width at half maximum of the light pulse is about 200 ns for all geometrical shapes. It was also found that argon as a test gas produces more light than air. Numerical simulations showed good agreement with experimental results regarding the shape of the shock and the flow field behind the shock. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there.

  • 343.
    Elnady, T.
    et al.
    KTH, Tidigare Institutioner, Farkostteknik.
    Bodén, H.
    KTH, Tidigare Institutioner, Farkostteknik.
    Glav, R.
    KTH, Tidigare Institutioner, Farkostteknik.
    Application of the point matching method to model circumferentially segmented non-locally reacting liners2001Inngår i: 7th AIAA/CEAS Aeroacoustics Conference and Exhibit, 2001Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Acoustic liners are widely used to attenuate sound waves inside the aircraft jet engines. Previous research has proved that segmenting the liner and the positioning of the liner segments affect the attenuation characteristics of the liner. The combined effect of circumferentially segmented and non-locally reacting liners received little attention. The aim of this work is to investigate these effects, and to compare the properties of circumferentially segmented duct liners with those of uniform liners, in order to identify any potential benefits of circumferentially segmented liners. A new technique is proposed here; the point-matching method. Briefly, it is a straightforward numerical method based on a closed form ansatz, which fulfils the governing equations and is matched to the boundary conditions point-wise. A code, previously developed for automobile applications, is used to obtain the wave numbers of the different modes, from which the transmission loss for each mode can be calculated at the desired range of frequencies. An infinite cylindrical duct of diameter 40 cms was chosen to apply different non-locally (bulk) reacting liner configurations on. It was found that the existence of hard surfaces in a lined duct and their arrangement greatly affect the behavior of each mode and the energy distribution among them.

  • 344. Elnady, T.
    et al.
    Bodén, Hans
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strömningsakustik.
    Elhadidi, B.
    Validation of an Inverse Semi-Analytical Technique to Educe Liner Impedance2009Inngår i: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 47, nr 12, s. 2836-2844Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, the acoustic impedance of a liner is educed by a novel semi-analytical inverse technique. The liner sample is placed flush with the solid walls in a rectangular duct with grazing flow. The technique uses complex acoustic pressure measured at four positions at the wall of the duct, upstream and downstream of the lined section, and educes the impedance with a mode-matching method. Previous studies neglected grazing flow nonuniformity and the pressure discontinuity that appears at the liner-wall boundary caused by the discontinuity of the acoustic particle velocity into the wall. In the present paper, the mode-matching formulation is rederived in terms of pressure instead of velocity potential which is found to be more numerically stable. Moreover, the proposed methodology is validated with benchmark data from an experiment performed by NASA. First, the ability of the code to reproduce the pressure field for a given impedance is tested. Second, the ability to educe the correct impedance for a given pressure distribution is tested. The results of the mode-matching code are in very good agreement with the experimental data. The effect of shear flow is investigated and it can be concluded that the assumption of uniform flow is appropriate for the chosen liner, duct size, and frequency range of interest.

  • 345. Elnady, T.
    et al.
    Elsaadany, S.
    Åbom, Mats
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    Flow and pressure drop calculation using two-ports2008Inngår i: Int. Congr. Sound Vib., ICSV, 2008, s. 743-750Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Exhaust systems should be carefully designed for different applications. The main objective of an exhaust system is to reduce the engine noise. Maximum noise reduction is usually desired to the limit of a certain back pressure which is set by the engine manufacturer in order not to deteriorate the engine efficiency. Therefore, a parallel calculation of the flow and pressure drop must be performed. The amount of flow flowing through each element will also affect its acoustic properties. Usually, acoustic and flow calculations are done separately on two different software. This paper describes a new technique that enables both calculations to be done using the same input data on the same platform. Acoustic calculations are usually performed in the frequency domain in the plane wave region using the two-port theory and then the acoustic pressure in the system is solved for using well-known algorithms to handle arbitrary connected two-ports. The stagnation pressure and volume flow can also be calculated using the same algorithm by deriving a flow two-port for each element using the stagnation pressure and the volume flow velocity as the state variables. The proposed theory is first discussed listing the flow matrices for common elements in exhaust elements, and then different systems are analyzed and compared with the measurements.

  • 346. Elnady, T.
    et al.
    Åbom, Mats
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strömningsakustik.
    SIDLAB: New 1D sound propagation simulation software for complex duct networks2006Inngår i: 13th International Congress on Sound and Vibration 2006, 2006, s. 4262-4269Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Low frequency sound generation and propagation in duct and pipe systems is important in many applications. The examples range from ventilation systems to IC-engine exhaust and inlet systems. Plane wave two-port theory can be used to describe the sound transmission along the system. Based on a long experience of different applications, a new user-friendly software (SIDLAB) has been developed to meet today's engineering needs. The new software is MATLAB based and allows access to the source code. This gives the user a flexibility to do other calculations than those already defined and further post-process the data. SIDLAB includes typical two-port elements for many applications but also unique elements, e.g., for modelling devices for exhaust gas cleaning. It is also unique that the two-port elements are allowed to be active, i.e., contain sources. There is no limitation to the number of elements and they can be connected together in any arbitrary scheme. By introducing one-port elements, the network can handle several inlets and outlets. SIDLAB can simulate passive results, such as transmission loss and transfer matrices between arbitrary nodes (points) in the network. It can also simulate active results, such as pressure and sound power at each node in the network. In this paper an overview of the new software is presented together with an example of the analysis of a complex muffler.

  • 347. Elnady, T.
    et al.
    Åbom, Mats
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strömningsakustik.
    Allam, Sabry
    Modeling perforates in mufflers using two-ports2010Inngår i: Journal of Vibration and Acoustics-Transactions of the ASME, ISSN 1048-9002, E-ISSN 1528-8927, Vol. 132, nr 6, s. 061010-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    One of the main sources of noise of a vehicle is the engine where its noise is usually damped by means of acoustic mufflers A very common problem in the modeling of automotive mufflers is that of two flow ducts coupled through a perforate A new segmentation approach is developed here based on two port analysis techniques in order to model perforated pipes using general two port codes which are widely available Examples are given for simple muffler configurations and the convergence of the technique is investigated based on the number of segments used The results are compared with closed form solutions form the literature Finally an analysts of a complicated multi chamber perforated muffler system is presented The two port simulation results show good agreement with both the measurements and the simulations using the classical four port elements [DOI 10 1115/1 4001510]

  • 348. Elnady, Tamer
    et al.
    Elsaadany, Sara
    Ain Shams University, Egypt.
    Herrin, D.W.
    Investigation of the Acoustic Performance of After Treatment Devices2011Inngår i: SAE International Journal of Passenger Cars - Mechanical Systems, ISSN 1946-3995, Vol. 4, nr 2, s. 1068-1075Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Diesel engines produce harmful exhaust emissions and high exhaust noise levels. One way of mitigating both exhaust emissions and noise is via the use of after treatment devices such as Catalytic Converters (CC), Selective Catalytic Reducers (SCR), Diesel Oxidation Catalysts (DOC), and Diesel Particulate Filters (DPF). The objective of this investigation is to characterize and simulate the acoustic performance of different types of filters so that maximum benefit can be achieved. A number of after treatment device configurations for trucks were selected and measured. A measurement campaign was conducted to characterize the two-port transfer matrix of these devices. The simulation was performed using the two-port theory where the two-port models are limited to the plane wave range in the filter cavity. These models are implemented in SIDLAB Software for the simulation of low frequency sound propagation in ducts, and SIDLAB was used to predict the transfer matrix of the tested configurations. This paper presents guidelines for dividing these complicated systems into a number of simple 1D elements. Specifically, strategies for modeling the side inlet and outlet end caps are documented. The model takes about 15 minutes to set-up and 15 seconds to solve which demonstrates the power of using two-port techniques in modeling exhaust systems. The comparisons show good agreement between the measured and simulated transmission loss in the plane wave region.

  • 349. Elnady, Tamer
    et al.
    Elsaadany, Sara
    Åbom, Mats
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strömningsakustik.
    Investigation into modeling of multi-perforated mufflers2009Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    There is a strong competition among automotive manufacturers to reduce the radiated noise levels. One main source is the engine where the main noise control strategy is by using efficient mufflers. Resistive mufflers are now widely used to attenuate IC-Engine noise due to its better performance over the reactive ones. Resistive damping can be achieved either by using absorbing material or perforates in the form of tubes or sheets. Perforates mufflers have an increased performance with flow when the acoustic impedance is increased by introducing flow through the perforate holes. On the other hand, perforates can deteriorate the engine performance, if badly designed, by increasing the flow back pressure. Modeling of perforated mufflers started in the seventies when simple geometries were used. There were two approaches to analyze two tubes connected with a perforate (i.e. four-port), segmentation and distributed. Both approaches were limited to a few specified geometries. Recently, the authors published a new technique based on the segmentation approach where four ports can be replaced by a number of two-ports so that it can be used in general two-port codes. This paper investigates the use of these techniques in modeling complex perforated muffler geometries. Fifteen different configurations were modeled and compared to measurements. There are some limitations to the use of these models in some configurations because of strong 3D effects that limits the validity of these models to almost half the plane wave region. These configurations are mainly the double plug flow muffler and the parallel tube mufflers.

  • 350.
    Elnady, Tamer
    et al.
    Sound and Vibration Lab., Faculty of Engineering, Ain Shams University.
    Åbom, Mats
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    Modelling perforates in mufflers using two-ports2005Inngår i: 12th International Congress on Sound and Vibration 2005: ICSV 2005, 2005, s. 2294-2301Konferansepaper (Fagfellevurdert)
    Abstract [en]

    One of the main sources of noise of a vehicle is the engine where its noise is usually damped by means of acoustic mufflers. A very common problem in the modeling of automotive mufflers is that of two flow ducts coupled through a perforate. A new segmentation approach is developed here based on two-port analysis techniques, in order to model perforated pipes using general two-port codes which are widely available. Examples are given for simple muffler configurations and the convergence of the technique is investigated based on the number of segments used. The results are compared with closed form solutions form the literature. Finally, an analysis of a complicated multi-chamber perforated muffler system is presented. The two-port simulation results show good agreement with both the measurements, and the simulations using the classical four-port elements.

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