Change search
Refine search result
1 - 27 of 27
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the 'Create feeds' function.
  • 1. Agarwal, Anurag
    et al.
    Dowling, Ann P.
    Shin, Ho-Chul
    Graham, Will
    Sefi, Sandy
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis and Computer Science, NADA.
    Ray-tracing approach to calculate acoustic shielding by a flying wing airframe2007In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 45, no 5, p. 1080-1090Article in journal (Refereed)
    Abstract [en]

    The "silent aircraft" is in the form of a flying wing with a large wing planform and a propulsion system that is embedded in the rear of the airframe with intakes on the upper surface of the wing. Thus a large part of the forward-propagating noise from the intake ducts is expected to be shielded from observers on the ground by the wing. Acoustic shielding effects can be calculated by solving an external acoustic scattering problem for a moving aircraft. In this paper, acoustic shielding effects of the silent aircraft airframe are quantified by a ray-tracing method. The dominant frequencies from the noise spectrum of the engines are sufficiently high for ray theory to yield accurate results. It is shown that, for low-Mach number homentropic flows, a condition satisfied approximately during takeoff and approach, the acoustic rays propagate in straight lines. Thus, from Fermat's principle it is clear that classical geometrical optics and geometrical theory of diffraction solutions are applicable to this moving-body problem as well. The total amount of acoustic shielding at an observer located in the shadow region is calculated by adding the contributions from all the diffracted rays (edge-diffracted and creeping rays) and then subtrading the result from the incident field without the airframe. The three-dimensional ray-tracing solver is validated by comparing the numerical solutions with analytical high-frequency asymptotic solutions for canonical shapes. Experiments on a model-scale geometry have been conducted in an anechoic chamber to test the applicability of the ray-tracing technique. The results confirm the accuracy of the approach, which is then applied to a CAD representation of a prototype silent aircraft design. As expected, the flying wing configuration provides very significant ground shielding (in excess of 10 dB at all locations) of a source above the airframe.

  • 2. Amoignon, Olivier
    et al.
    Pralits, Jan O.
    Hanifi, Ardeshir
    Swedish Defence Research Agency.
    Berggren, M.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Shape optimization for delay of laminar-turbulent transition2006In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 44, no 5, p. 1009-1024Article in journal (Refereed)
    Abstract [en]

    A method using gradient-based optimization is introduced for the design of wing profiles with the aim of natural laminar How, as well as minimum wave drag. The Euler equations of gasdynamics, the laminar boundary-layer equations for compressible flows on infinite swept wings, and the linear parabolized stability equations (PSE) are solved to analyze the evolution of convectively unstable disturbances. Laminar-turbulent transition is assumed to be delayed by minimizing a measure of the disturbance kinetic energy of a chosen disturbance, which is computed using the PSE. The shape gradients of the disturbance kinetic energy are computed based on the solutions of the adjoints of the state equations just named. Numerical tests are carried out to optimize the RAE 2822 airfoil with the aim to delay simultaneously the transition, reduce the pressure drag coefficient, and maintain the coefficients of lift and pitch moments. Constraints are also applied on the geometry. Results show a reduction of the total amplification of a large number of disturbances, which is assumed to represent a delay of the transition in the boundary layer. Because delay of the transition implies reduction of the viscous drag, the present method enables shape optimization to perform viscous drag reduction.

  • 3.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Matrix-free methods for the stability and control of boundary layers2009In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 47, no 5, p. 1057-1068Article in journal (Refereed)
    Abstract [en]

    This paper presents matrix-free methods for the stability analysis and control design of high-dimensional systems arising from the discretized linearized Navier-Stokes equations. The methods are applied to the two-dimensional spatially developing Blasius boundary-layer. A critical step in the process of systematically investigating stability properties and designing feedback controllers is solving very large eigenvalue problems by storing only velocity fields at different times instead of large matrices. For stability analysis, where the entire dynamics of perturbations in space and time is of interest, iterative and adjoint-based optimization techniques are employed to compute the global eigenmodes and the optimal initial conditions. The latter are the initial conditions yielding the largest possible energy growth over a finite time interval. The leading global eigenmodes take the shape of Tollmien-Schlichting wavepackets located far downstream in streamwise direction, whereas the leading optimal disturbances are tilted structures located far upstream in the boundary layer. For control design on the other hand, the input-output behavior of the system is of interest and the snapshot-method is employed to compute balanced modes that correctly capture this behavior. The inputs are external disturbances and wall actuation and the outputs are sensors that extract wall shear stress. A low-dimensional model that capture the input-output behavior is constructed by projection onto balanced modes. The reduced-order model is then used to design a feedback control strategy such that the growth of disturbances are damped as they propagate downstream.

  • 4.
    Borglund, Dan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Robust Eigenvalue Analysis Using the Structured Singular Value: The mu-p Flutter Method2008In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 46, no 11, p. 2806-2813Article in journal (Refereed)
    Abstract [en]

    This paper introduces a new technique for robust aeroelastic analysis that extends standard linear flutter analysis to take deterministic uncertainty and variation into account. The basic principle of the proposed mu-p method is to exploit structured-singular-value (or mu) analysis to investigate if the system uncertainties can make the flutter determinant zero for a given flutter eigenvalue p. This makes it possible to compute regions of feasible eigenvalues in the complex plane as well as extreme eigenvalues that can be used to predict damping bounds and perform robust flutter analysis. The capability to predict damping bounds at subcritical flight conditions is a very attractive feature of the new method, as flight testing is rarely taken to the flutter point. The A-p formulation also opens up new possibilities to bound the magnitude of the system uncertainties based on frequency and/or damping estimates from flight testing. In the final part of the paper, the mu-p framework is successfully applied to perform robust aeroelastic analysis of a low-speed wind-tunnel model.

  • 5. Caraeni, Mirela
    et al.
    Fuchs, Laszlo
    Investigation of nonreflective boundary conditions for computational aeroacoustics2006In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 44, no 9, p. 1932-1940Article in journal (Refereed)
    Abstract [en]

    Direct aeroacoustic computations require nonreflective boundary conditions that allow disturbances to leave the domain freely without anomalous reflections. In the present paper we analyze a series of nonreflective boundary conditions already published in the literature and propose an improved outflow nonreflective boundary condition with reflection characteristic that is reduced greatly. For the solution of the linearized Euler equations, a sixth-order compact finite-difference algorithm is used together with a sixth-order explicit digital filter that suppresses the high-frequency spurious oscillations in the solution. A number of representative test cases are presented. The new outflow boundary condition is recommended for the simulation of sound produced by turbulence.

  • 6.
    Dadfar, Reza
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Control of instabilities in an unswept wing boundary layer2018In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 5, p. 1750-1759Article in journal (Refereed)
    Abstract [en]

    Linear control theory is used to construct an output feedback controller to attenuate the amplitude of the Tollmien–Schlichting waves inside the boundary layer developing over an unswept wing. The analysis is based on direct numerical simulations. The studied scenario includes the impulse response of the system to a generic disturbance in the freestream, which triggers a Tollmien–Schlichting wave packet inside the boundary layer. The performance of a linear quadratic Gaussian controller is analyzed to suppress the amplitude of the Tollmien–Schlichting wave packet using a row of sensors and plasma actuators localized at the wall. The target of the controller is chosen as a subset of proper orthogonal decomposition modes describing the dynamics of the unstable disturbances. The plasma actuators are implemented as volume forcing. To account for the limitations of the plasma actuators concerning a unidirectional forcing, several strategies are implemented in the linear quadratic Gaussian framework. Their performances are compared with that for classical linear quadratic Gaussian controller. These controllers successfully reduced the amplitude of the wave packet.

  • 7.
    Dadfar, Reza
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Semeraro, Onofrio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Hanifi, Anfreshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Output feedback control of flow on a flat plate past a leading edge using plasma actuatorsIn: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385XArticle in journal (Refereed)
    Abstract [en]

    The evolution and control of a two dimensional (2D) wavepacket developing on a flat plate with a leading edge is investigated by means of direct numerical simulation (DNS).

    The aim is to identify and suppress the wavepackets generated by freestream perturbations. A sensor is placed close to the wall in order to detect the upcoming wavepacket, while an actuator is placed further downstream to control it. A plasma actuator is modelled as an external forcing on the flow using a model based and validated on experimental investigations. A Linear Quadratic Gaussian (LQG) controller is designed and an output projection is used to build the objective function. Moreover, by appropriate selection of the Proper Orthogonal Decomposition (POD) modes, we identify the disturbances to be damped. A reduced-order model of the input-output system is constructed by using system identification via the Eigensystem Realization Algorithm (ERA) algorithm.

    A limitation of the plasma actuators is the uni-directional forcing of the generated wall jet, which is predetermined by the electrodes location. In this paper, we address this limitation by proposing and comparing two different solutions: i) by introducing an offset in the control signal such that the resulting total forcing is oriented along one direction; ii) by using two plasma actuators acting in opposite directions. The results are compared with the ideal case where constraints are not accounted for the control design. We show that the resulting controllers based on plasma actuators can successfully attenuate the amplitude of the wavepacket developing inside the boundary layer.

  • 8.
    Dadfar, Reza
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Semeraro, Onofrio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Output Feedback Control of Blasius Flow with Leading Edge Using Plasma Actuator2013In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 51, no 9, p. 2192-2207Article in journal (Refereed)
    Abstract [en]

    The evolution and control of a two-dimensional wave packet developing on a flat plate with a leading edge is investigated by means of direct numerical simulation. The aim is to identify and suppress the wave packets generated by freestream perturbations. A sensor is placed close to the wall to detect the upcoming wave packet, while an actuator is placed further downstream to control it. A plasma actuator is modeled as an external forcing on the flow using a model based and validated on experimental investigations. A linear quadratic Gaussian controller is designed, and an output projection is used to build the objective function. Moreover, by appropriate selection of the proper orthogonal decomposition modes, we identify the disturbances to be damped. A reduced-order model of the input-output system is constructed by using system identification via the eigensystem realization algorithm. A limitation of the plasma actuators is the unidirectional forcing of the generated wall jet, which is predetermined by the electrodes' location. In this paper, we address this limitation by proposing and comparing two different solutions: 1) introducing an offset in the control signal such that the resulting total forcing is oriented along one direction, and 2) using two plasma actuators acting in opposite directions. The results are compared with the ideal case where constraints are not accounted for the control design. We show that the resulting controllers based on plasma actuators can successfully attenuate the amplitude of the wave packet developing inside the boundary layer.

  • 9.
    Downs, R. S. , I I I
    et al.
    KTH.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Martensson, H.
    Miniature vortex generators for flow control in falkner-skan boundary layers2017In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 55, no 2, p. 352-364Article in journal (Refereed)
    Abstract [en]

    Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can suppress the growth of unsteady disturbances that would otherwise incite laminar-to-turbulent transition. Previous experimental and numerical works demonstrated the versatility of these miniature vortex generators in zero-pressure-gradient boundary layers. In this work, mean-flow disturbances developing from miniature vortex generators in adverse and favorable pressure-gradient boundary layers are measured systemically to assess the possibility of extending miniature vortex generator-based flow control to these scenarios. Boundary-layer streak amplitudes are measured across a range of Falkner-Skan m values, and an empirical scaling is found based on existing results. The effect of streaks on transition in an adverse pressure-gradient boundary layer is also tested, and moderate increases to laminar flow extents are observed.

  • 10. Duwig, C.
    et al.
    Fuchs, Laszlo
    Griebel, P.
    Siewert, P.
    Boschek, E.
    Study of a confined turbulent jet: Influence of combustion and pressure2007In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 45, no 3, p. 624-639Article in journal (Refereed)
    Abstract [en]

    Today, environmental issues play an important role in the viability of modern low-emission power plants. As a consequence, gas turbine combustors need to be operated at lean premixed conditions. However, successful designs require a detailed knowledge of the combustion process under realistic operating conditions. The present study focuses on nonreacting and reacting jets at operating pressures from 1 to 14 bar. First, an isothermal confined jet has been studied experimentally and numerically using particle image velocimetry and large eddy simulation. The flow is highly turbulent and includes large-scale unsteady structures. The comparison of the numerical results and the experimental velocity data showed an excellent agreement that was generally below the numerical or experimental uncertainty. Second, the influences of combustion and operating pressure on the flowfield were investigated. The large eddy simulation results showed that the jet core was lengthened, due to the density jump across the flame. The effect of pressure on the flame was studied using planar laser-induced fluorescence and large eddy simulation at a constant Mach number. The flame brush and the velocity fields were found to be relatively insensitive to an increase of pressure from 1 to 14 bar (and, correspondingly, to an increase of Reynolds and Karlovitz numbers). The numerical results suggest that increasing pressure decreases the laminar flame speed and increases the flame-front wrinkling, causing the turbulent flame speed to be less sensitive to pressure.

  • 11. Duwig, C.
    et al.
    Salewski, M.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Simulations of a turbulent flow past a sudden expansion: A sensitivity analysis2008In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 46, no 2, p. 408-419Conference paper (Refereed)
    Abstract [en]

    Large eddy simulation is used to study the flow behind a pair of symmetric backward-facing steps. As reported in the literature, the flow exhibits an asymmetric pattern characterized by the deflection of the jet toward one of the walls. The large eddy simulation results are compared with laser Doppler anemometry measurements showing the ability of the present numerical tool to capture the complex features of the flow. Furthermore, a sensitivity study is conducted to assess the influence of the grid resolution, the inflow boundary, the channel width, and the step size on the flowfield. The flow was found to be only weakly sensitive to the grid, assuring the quality of the simulation results. The inflow boundary influences the mean results only marginally unless low-frequency fluctuations are applied. In this case, the flowfield recovers a mean symmetry with suppression of the jet bending. The jet mean bending has also been shown to increase with the step size h and to decrease with increasing channel width.

  • 12. Elnady, T.
    et al.
    Bodén, Hans
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Elhadidi, B.
    Validation of an Inverse Semi-Analytical Technique to Educe Liner Impedance2009In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 47, no 12, p. 2836-2844Article in journal (Refereed)
    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.

  • 13.
    Ferro, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Downs, Robert S., III
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stagnation Line Adjustment in Flat-Plate Experiments via Test-Section Venting2015In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 53, no 4, p. 1112-1116Article in journal (Refereed)
  • 14.
    Gojon, Romain
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Bogey, Christophe
    Flow Features near Plate Impinged by Ideally Expanded and Underexpanded Round Jets2018In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 2, p. 445-457Article in journal (Refereed)
    Abstract [en]

    The properties of the flow near the plate and in the wall jets have been investigated from large-eddy simulation data of round impinging jets. Four jets are underexpanded and four jets are ideally expanded, which allowed examination of the influence of the presence of shock-cell structures. The underexpanded jets are characterized by a fully expanded Mach number of 1.56 and an exit Mach number of 1. The ideally expanded jets have a Mach number of 1.5. The Reynolds number of the eight jets is equal to 6x104. The jets impinge normally on a flat plate located from 4.16r0 to 12r0 downstream of the nozzle and generate acoustic tones due to an aeroacoustic feedback mechanism. In this paper, the near pressure and density fields of the jets are characterized using Fourier transform on the nozzle exit plane, the plate, and an azimuthal plane. First, mean and rms radial velocities of the wall jets are examined. The impact of the shock-cell structure on the wall jet is discussed. The pressure spectra on the plate are then shown as a function of the radial coordinate. The tone frequencies are all visible where the jet shear layers impinge the plate, but only some of them emerge in the wall jet created after the impact. For the ideally expanded jets, the temporal organization of the wall jet along the frequencies of the feedback mechanism decreases with the nozzle-to-plate distance, but for the nonideally expanded jets, this organization is linked to the oscillation of the Mach disk located just upstream of the plate. Consecutively, the amplitude and the phase fields at the tone frequencies are represented on the three planes mentioned earlier. Similar spatial organizations of the turbulent structures are found in the jet shear layers and in the wall jets. Thus, axisymmetric and helical arrangements of the structures in the jet shear layers lead to concentric and spiral distributions of the structures on the plate, respectively. In particular, for one of the underexpanded jets, a spiral shape and concentric rings, associated with two tone frequencies generated simultaneously, are observed on the flat plate in the pressure and density phase fields. Finally, the convection velocity of the turbulent structures at the tone frequencies in the wall jets are evaluated based on phase fields, and the mean convection velocity is computed using cross correlations of radial velocity. The results are in good agreement with those from a recent experimental study of ideally expanded impinging jets.

  • 15.
    Gojon, Romain
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. Univ Lyon, France.
    Bogey, Christophe
    Flow Structure Oscillations and Tone Production in Underexpanded Impinging Round Jets2017In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 55, no 6, p. 1792-1805Article in journal (Refereed)
    Abstract [en]

    Flow structure oscillations and tone generation mechanisms in an underexpanded round jet impinging on a flat plate normally have been investigated using compressible large-eddy simulations. At the exit of a pipe nozzle of diameter D, the jet is characterized by a nozzle pressure ratio of 4.03, an exit Mach number of 1, a fully expanded Mach number of 1.56, and a Reynolds number of 6 x 10(4). Four distances between the nozzle and the plate of 2.08D, 2.80D, 3.65D, and 4.66D are considered. Snapshots of vorticity, density, pressure, and mean velocity flowfields are first presented. The latter results compare well with data of the literature. In three cases, in particular, a Mach disk appears to form just upstream from the plate. The convection velocity of flow structures between the nozzle and the plate, and its dependence on the nozzle-to-plate distance, are then examined. The properties of the jet near pressure fields are subsequently described using Fourier analysis. Tones emerge in the spectra at frequencies consistent with those expected for an aeroacoustic feedback loop between the nozzle and the plate as well as with measurements. Their amplitudes are particularly high in the presence of a near-wall Mach disk. The axisymmetric or helical natures of the jet oscillations at the tone frequencies are determined. The motions of the Mach disk found just upstream from the plate for certain nozzle-to-plate distances are then explored. As noted for the jet oscillations, axially pulsing and helical motions are observed, in agreement with experiments. Finally, the intermittency of the tone intensities is studied. They significantly vary in time, except for the two cases where the near-wall Mach disk has a nearly periodic motion at the dominant tone frequency.

  • 16.
    Gojon, Romain
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. Univ Toulouse, DAEP, ISAE SUPAERO, 10 Ave Edouard Belin, F-31400 Toulouse, France.
    Bogey, Christophe
    École Centrale de Lyon, 69134 Ecully Cedex, France.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Oscillation Modes in Screeching Jets2018In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 7, p. 2918-2924Article in journal (Refereed)
    Abstract [en]

    Nonideally expanded supersonic jets generate three basic noise components, namely, the turbulent mixing noise, the broadband shock-associated noise, and the screech noise. The mixing noise, obtained for both subsonic and supersonic jets, is most intense in the downstream direction; and it occurs at Strouhal numbers of around 0.15. The broadband shock-associated noise is radiated mainly in the radial direction, and it has a central frequency varying with the emission angle. The screech noise consists of tones measured in the upstream direction. These tones are due to an aeroacoustic feedback mechanism establishing between turbulent structures propagating downstream and acoustic waves propagating upstream.

  • 17. Jirásek, Adam
    Mass flow boundary conditions for subsonic inflow and outflow boundary2006In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 44, no 5, p. 939-947Article in journal (Refereed)
    Abstract [en]

    The development and verification of an inflow and outflow mass flow boundary condition are described. In addition, an outflow Mach number boundary condition was implemented and tested. The main motivation behind the development of a mass flow boundary condition is a need to bridge the gap between the requirement of direct setting of mass flow as one of the most important parameters, and the difficulty of indirect control of mass flow using static pressure outflow and total states inflow boundary conditions. The mass flow and Mach number boundary conditions were verified in low-speed flow through a two-dimensional channel with constant area and in high-speed flow through the Royal Airforce Establishment M2129 S-duct with large separation. Special attention was paid to the behavior of the mass flow boundary conditions in choked flow.

  • 18.
    Monokrousos, Antonios
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Feedback Control of Boundary-Layer Bypass Transition: Comparison of Simulations with Experiments2010In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 48, no 8, p. 1848-1851Article in journal (Refereed)
  • 19.
    Munday, David
    et al.
    Aerospace Engineering, University of Cincinnati.
    Mihaescu, Mihai
    Aerospace Engineering, University of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Experimental and Numerical Study of Jets from Elliptic Nozzles with Conic Plug2011In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 49, no 3, p. 554-564Article in journal (Refereed)
    Abstract [en]

    The study presents results concerning jets exhausting from elliptic nozzles with conic centerbody plugs. A nozzle with 3:1 ratio of exit heights is surveyed experimentally using particle image velocimetry for jet Mach numbers from 0.24 to 1.0, heated and unheated. At the exit, the nozzle inner surface has a slope of zero in the major axis plane, while in the minor axis plane the slope is steeper than that for a corresponding round nozzle. The conic plug causes splitting of the initial elliptic jet into two jets with their centerlines in the major-axis plane of the elliptic nozzle. Increasing Mj or total temperature causes the bifurcated potential cores of the individual jets to be slightly smaller and to diverge from one to another at a slightly greater angle from the nozzle centerline. Large Eddy Simulations of the experimental nozzle and two alternate nozzles are analyzed to find how the flow features are influenced by not forcing the jet towards the conic plug in the minor axis plane and by changing the ratio of exit heights to 2:1. Changing nozzle shape caused reduction in the spreading rate in the major axis and increase in spreading in the minor axis. Bifurcation occurs for all nozzles studied.

  • 20.
    O'Reilly, Ciarán J.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aeroacoustics.
    Rice, Henry J.
    Trinity College, Dublin, Ireland.
    Investigation of a jet-noise-shielding methodology2015In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 53, no 11, p. 3286-3296Article in journal (Refereed)
    Abstract [en]

    Ongoing research toward the reduction of environmental noise from aircraft is investigating the possible shielding of engine-noise sources by novel airframe configurations. To assess the noise-reduction benefits attainable from such configurations, it is necessary to develop appropriate acoustic evaluation tools. In this paper, a jet-noise-shielding- prediction methodology is described. The Tam–Auriault (“Jet Mixing Noise from Fine-Scale Turbulence,” AIAA Journal, Vol. 37, No. 2, 1999, pp. 145–153) jet-noise model with a Reynolds-averaged Navier–Stokes solution input, together with a Fresnel–Kirchhoff diffraction method (Fundamentals of Physical Acoustics, Wiley-Interscience, New York, 2000, pp. 472–494), is used to make isolated and shielded far-field jet-noise predictions. This methodology is employed as a sensitivity-analysis tool to establish the relative importance of the source location, spatial extent, and directivity in jet-noise-shielding predictions. Predictions have been made for a shielded single-stream Mach 0.9 jet, and compared with experimental data. Good qualitative agreement is observed, and the disagreement in the shielding levels is most likely due to underestimation of the source axial extent by the jet-noise model. 

  • 21. Shur, M.
    et al.
    Strelets, M.
    Travin, A.
    Christophe, J.
    Kucukcoskun, K.
    Schram, C.
    Sack, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Experimental/numerical study of ducted-fan noise: Effect of duct inlet shape2018In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 3, p. 979-996Article in journal (Refereed)
    Abstract [en]

    The paper presents an investigation of flow and noise produced by the generic fan of an environmental control system installed in a circular duct, with a focus on the evaluation of the effect of mean flow distortions and elevated turbulence levels upstream of the fan, which are here created by replacing a baseline smooth bell-mouth duct inlet by inlets with a rectangular-to-circular transition of the duct cross section and with a T junction formed by two circular pipes. The study includes both experiments, which are aimed primarily at supplying data for a validation of simulation approaches and numerical simulations based on a hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation approach. An acoustic modal decomposition of the measured and computed unsteady pressure fields is then carried out, which allows extracting "pure" (with filtered out hydrodynamic/turbulent fluctuations) acoustic modes propagating upstream and downstream under reflection-free conditions. A comparison with the experiment is performed for both the "raw" pressure spectra on the duct walls and the extracted individual acoustic modes. It is shown that, in terms of acoustics, a replacement of the clean inlet by the rectangular/circular one is almost neutral, whereas the T-junction inlet causes a strong noise penalty.

  • 22. Stolz, Steffen
    et al.
    Schlatter, Philipp
    Kleiser, Leonhard
    Large-eddy simulations of subharmonic transition in a supersonic boundary layer2007In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 45, no 5, p. 1019-1027Article in journal (Refereed)
    Abstract [en]

    We investigate the performance of two recently developed subgrid-scale models, the approximate deconvolution model and the high-pass filtered Smagorinsky model, in large-eddy simulations of laminar-turbulent transition in a supersonic boundary layer. Subharmonic transition in a boundary layer at a freestream Mach number of 4.5 and a Reynolds number (based on initial displacement thickness) of 10,000 is considered, which has been studied previously in detail by direct numerical simulations. For computational efficiency, the temporal simulation approach has been adopted. The discretization is based on Fourier collocation and various high-order finite difference schemes in the wall-parallel and wall-normal directions, respectively. Large-eddy simulations results are assessed by comparing statistical and instantaneous quantities during transition with data obtained from a sufficiently resolved simulations accurately reproduce the direct direct numerical simulation. The results show that the large-eddy numerical simulations data from the slightly disturbed laminar flow through transition into the turbulent stage, with a computational effort of two orders of magnitude less than the direct numerical simulations. Both subgrid-scale models are formulated locally in space and in a fully three-dimensional manner and do not need an ad hoc adaptation to nonturbulent or near-wall regions.

  • 23. Vinuesa, Ricardo
    et al.
    Rozier, Paul H.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nagib, Hassan M.
    Experiments and Computations of Localized Pressure Gradients with Different History Effects2014In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 52, no 2, p. 368-384Article in journal (Refereed)
    Abstract [en]

    The study of high-Reynolds-number wall-bounded turbulent flows has become a very active area of research in the past decade, where several recent results have challenged current understanding. In this study, four different localized pressure gradient configurations are characterized by computing them using four Reynolds-averaged Navier-Stokes turbulence models (Spalart-Allmaras, k-epsilon, shear stress transport, and the Reynolds stress model) and comparing their predictions with experimental measurements of mean flow quantities and wall shear stress. The pressure gradients were imposed on high-Reynolds-number, two-dimensional turbulent boundary layers developing on a flat plate by changing the ceiling geometry of the test section. The computations showed that the shear stress transport model produced the best agreement with the experiments. It was found that what is called "numerical transition" (a procedure by which the laminar boundary conditions are transformed into inflow conditions to characterize the initial turbulent profile) causes the major differences between the various models, thereby highlighting the need for models representative of true transition in computational codes. Also, both experiments and computations confirm the nonuniversality of the von Karman coefficient kappa. Finally, a procedure is demonstrated for simpler two-dimensional computations that can be representative of flows with some mild three-dimensional geometries.

  • 24.
    von Stillfried, Florian
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Wallin, Stefan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Evaluation of a vortex generator model in adverse pressure gradient boundary layers2011In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 49, no 5, p. 982-993Article in journal (Refereed)
    Abstract [en]

    The use of a two-dimensional statistical passive vortex generator model, applied to an adverse pressure gradient boundary-layer flow, is evaluated qualitatively against experimental and fully resolved vortex generator computations. The modeling approach taken here has the advantage of substantially reducing the complexity of including such flow separation control devices in a computational mesh, thus giving the opportunity to carry out faster parametric studies. Additional stresses, originating from the vortex generator model approach, are added as additional turbulent stresses to the mean governing equations instead of resolving vortex structures in the computational domain. The vortex generator model has been applied to allow direct comparison with prior experiments carried out at the Royal Institute of Technology Stockholm. Variations of the vortex generator streamwise position and tests of different vortex generator setups, such as co- and counter-rotational settings, are presented. Distributions of wall-pressure and skin-friction coefficients are used to evaluate the vortex generator model against fully resolved vortex generator data. It is shown that the vortex generator model successfully predicts attached and separated flow states. Moreover, the results illustrate the vortex generator model's capability to predict flow control sensitivity with respect to the streamwise position.

  • 25.
    von Stillfried, Florian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Wallin, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Statistical Vortex-Generator-Jet Model for Turbulent Flow Separation Control2013In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 51, no 5, p. 1119-1129Article in journal (Refereed)
    Abstract [en]

    This contribution describes the development and evaluation of a new statistical modeling approach for active vortex generator jets. Previous experiments from the Technische Universitat Braunschweig and their subsequent evaluation by the present authors showed that the induced flowfield can be reasonably well represented by two-dimensional Lamb-Oseen vortices. Based on that, an analytical expression for the Lamb-Oseen-vortex-model maximum circulation Gamma(max) was derived in terms of the freestream velocity U-infinity, the jet-to-freestream velocity ratio lambda, and the jet skew angle beta, as well as the actuator diameter Phi(VGJ). Based on the parameterized results, universal values for the Lamb-Oseen-vortex-model parameters at the actuator position were determined for the development of the statistical vortex-generator-jet model. The idea behind the statistical modeling approach is that the vortices are represented by their spanwise-averaged velocity correlations, or vortex stresses, that are added to the turbulence stresses in a Reynolds stress turbulence model. The spanwise-averaged vortex stresses are derived by computing the spanwise-averaged second-order statistics of the vortex flowfleld. These vortex-generator-jet model results were compared to the spanwise-averaged vortex stresses from experiments and from fully resolved computational fluid dynamics investigations, and reasonable qualitative as well as quantitative agreement was found.

  • 26.
    von Stillfried, Florian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wallin, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vortex-Generator Models for Zero- and Adverse-Pressure-Gradient Flows2012In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 50, no 4, p. 855-866Article in journal (Refereed)
    Abstract [en]

    A computational fluid-dynamics investigation, including passive vortex generators (VGs) that generate streamwise counter-rotating vortex structures, usually requires a grid with fully resolved VG geometries and vortex structures with a corresponding large number of grid points to obtain an accurate solution. An efficient way to avoid such a setup and time-consuming process in turbulent shear-layer flows is to introduce statistics-based vortex-generator modeling. The second-order statistics of the initial vortices are computed by using a vortex model in combination with the lifting-line theory. The statistics are added as additional turbulence stress terms to the equations within a differential Reynolds stress-turbulence model. In this investigation, results from statistical VG model computations for zero- and adverse-pressure-gradient flat-plate boundary-layer flows, as well as for the flow in a plane asymmetric diffuser, are evaluated against results from fully resolved VG computations and experiments. It could be shown that the initial near-field forcing is too weak for the proposed VG model. An improved VG model description removes some drawbacks by adding additional statistical forcing terms. Results become more comparable:, resulting in improved predictions when compared to experiments and fully resolved computations.

  • 27.
    von Stillfried, Florian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wallin, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Casper, Marcus
    Ortmanns, Jens
    Evaluation and Parameterization of Round Vortex Generator Jet Experiments for Flow Control2012In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 50, no 11, p. 2508-2524Article in journal (Refereed)
    Abstract [en]

    Particle image velocimetry experiments on the application of vortex generator jets (VGJs) with single and pair configurations in a zero-pressure gradient flat plate boundary-layer flow have been carried out at the Technische Universitat Braunschweig. The experiments were analyzed at the Royal Institute of Technology by means of a velocity triple decomposition that enables the extraction of vortex velocity fields. The overall aim of this contribution is to identify and classify the influence of the different VGJ parameters on the generated vortex structures so that conclusions may be drawn for efficient flow separation control configurations. The spanwise-averaged second-order statistics (ViVj) over bar (y) of the vortex velocity field V-i(y, z) are computed in order to evaluate the influence of the VGJ parameters. It could be shown that the velocity ratio lambda and the skew angle beta have a major influence. Furthermore, the chosen single VGJ data are compared to corresponding VGJ pair results of a similar actuator setup. It was found that the VGJ pairs could be statistically well described by means of single VGJ vortices. Moreover, VGJ arrays should give very similar results for the configurations examined, facilitating VGJ modeling based on a statistical ansatz.

1 - 27 of 27
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf