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  • 1. Bogey, Christophe
    et al.
    Gojon, Romain
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Feedback loop and upwind-propagating waves in ideally expanded supersonic impinging round jets2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 823, p. 562-591Article in journal (Refereed)
    Abstract [en]

    The aeroacoustic feedback loop establishing in a supersonic round jet impinging on a flat plate normally has been investigated by combining compressible large-eddy simulations and modelling of that loop. At the exit of a straight pipe nozzle of radius r(0), the jet is ideally expanded, and has a Mach number of 1.5 and a Reynolds number of 6 x 10(4). Four distances between the nozzle exit and the flat plate, equal to 6r(0), 8r(0), 10r(0) and 12r(0), have been considered. In this way, the variations of the convection velocity of the shear-layer turbulent structures according to the nozzle-to-plate distance are shown. In the spectra obtained inside and outside of the flow near the nozzle, several tones emerge at Strouhal numbers in agreement with measurements in the literature. At these frequencies, by applying Fourier decomposition to the pressure fields, hydrodynamic-acoustic standing waves containing a whole number of cells between the nozzle and the plate and axisymmetric or helical jet oscillations are found. The tone frequencies and the mode numbers inferred from the standing-wave patterns are in line with the classical feedback-loop model, in which the loop is closed by acoustic waves outside the jet. The axisymmetric or helical nature of the jet oscillations at the tone frequencies is also consistent with a wave analysis using a jet vortex-sheet model, providing the allowable frequency ranges for the upstream-propagating acoustic wave modes of the jet. In particular, the tones are located on the part of the dispersion relations of the modes where these waves have phase and group velocities close to the ambient speed of sound. Based on the observation of the pressure fields and on frequency-wavenumber spectra on the jet axis and in the shear layers, such waves are identified inside the present jets, for the first time to the best of our knowledge, for a supersonic jet flow. This study thus suggests that the feedback loop in ideally expanded impinging jets is completed by these waves.

  • 2.
    Ceci, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Gojon, Romain
    KTH, School of Engineering Sciences (SCI), Mechanics. ISAE-SUPAERO, Toulouse, France.
    Mihaescu, Mihai
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Large Eddy Simulations for Indirect Combustion Noise Assessment in a Nozzle Guide Vane Passage2018In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Flow, Turbulence and Combustion, ISSN 1386-6184, p. 1-13Article in journal (Refereed)
    Abstract [en]

    The combustion noise in aero-engines is known to originate from two different sources. First, the unsteady heat release in the combustion chamber generates the direct combustion noise. Second, hot and cold spots of air generated by the combustion process are convected and accelerated by the turbine stages and give rise to the so-called indirect combustion noise. The present work targets, by using a numerical approach, the generation mechanism of indirect combustion noise for a simplified geometry of a turbine stator passage. Periodic temperature fluctuations are imposed at the inlet, permitting to simulate hot and cold packets of air coming from the unsteady combustion. Three-dimensional Large Eddy Simulation (LES) calculations are conducted for transonic operating conditions to evaluate the blade acoustic response to the forced temperature perturbations at the inlet plane. Transonic conditions are characterized by trailing edge expansion waves and shocks. It is notably shown that their movement can be excited if disturbances with a particular frequency are injected in the domain.

  • 3.
    Chandramouli, Sathyanarayanan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gojon, Romain
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fridh, Jens
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical characterization of entropy noise with a density based solver2017In: 12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017, KTH Royal Institute of Technology, 2017Conference paper (Refereed)
    Abstract [en]

    In this work, dbnsTurbFoam, a new coupled density based solver, written in the framework of FOAM-EXTEND, is considered. The solver is first assessed on two canonical compressible flow scenarios, namely the Sod's shock tube and the ONERA S8 transonic channel. Results are compared with analytical formulations and experiments, respectively. 2-D Unsteady Reynolds Averaged Navier-Stokes simulations and 3-D Large Eddy Simulations of the flow within the passages of a geometrically simplified High Pressure Turbine Nozzle Guide Vane are then performed. Results are compared against experimental data in order to justify the geometrical simplifications made. Finally, the case of a sinusoidal temperature forcing at the inlet is considered in order to study the phenomenon of indirect combustion noise. Notably, the impact of the forcing on the vortex shedding dynamics and on the losses is discussed.

  • 4.
    Chen, Song
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Gojon, Romain
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics.
    High-Temperature Effects on Aerodynamic and Acoustic Characteristics of a Rectangular Supersonic Jet2018In: AIAA/CEAS Aeroacoustics Conference, AIAA AVIATION Forum, 2018 / [ed] AIAA, 2018, article id 3303Conference paper (Refereed)
    Abstract [en]

    Implicit large-eddy simulations (LES) are performed in this work to study the flow field and acous-tic characteristics of a rectangular supersonic jet. The focus is to investigate the high-temperatureeffects, i.e. when the jet total temperature is as high as 2100 K. Four cases with a jet temperatureratio(TR) of 1.0, 2.0, 4.0 and 7.0 are investigated. The rectangular nozzle selected for this study hasan aspect ratio of 2. The jets are overexpanded, with a series of shock cells in the jet core region.An artificial dissipation mechanism is used to damp the numerical oscillation and to represent theeffect of small-scale turbulence. The temperature-dependent thermal properties of air within thehigh-temperature regime are also considered by using the chemical equilibrium assumption. Thenumerical results show that the high temperature significantly increases the jet velocity and acousticMach number, although the jet Mach number is maintained roughly the same. Meanwhile, the lengthof the jet core region of the hot jet (TR = 7.0) is found to be reduced by around 30 %, compared tothe cold jet. The convection velocity and acoustic convection Mach number in the shear layer are alsoobserved to be increased when the jet temperature is high. The elevated acoustic convection Machnumber directly leads to a strong Mach wave radiation, and the crackle noise component has beenidentified by the pressure skewness and kurtosis factors. The Strouhal number of the screech tone isfound to be decreased slightly, and good agreements between the numerical results and the theoreticalanalysis are observed. Moreover, the sound pressure levels (SPL) associated with turbulent mixing,screech, Mach wave radiation, and Broadband shock associated noise are all found to be amplified indifferent levels for the hot jets. In the far field, the SPL is strongly increased by the high-temperatureeffect. Higher SPL is notably observed in the Mach wave radiation directions.

  • 5.
    Gojon, Romain
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Baier, Florian
    Univ. of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Temperature effects on the aerodynamic and acoustic fields of a rectangular supersonic jet2017In: Proceedings of the 55th AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics, 2017, p. 19-, article id AIAA2017-0002Conference paper (Refereed)
    Abstract [en]

    In the first part of the paper, a modified artificial dissipation mechanism permitting to perform Large-Eddy Simulations of highly compressible flows is proposed. This dissipation mechanism is evaluated using one linear 2-D test case and two non-linear 2-D test cases. In the second part, the flow and acoustic near-field of rectangular supersonic jets are explored using compressible LES based on this modified artificial dissipation mechanism. At the exit of a converging diverging rectangular nozzle of aspect ratio 2 and of design Mach number 1.5, the jets are overexpanded. Four simulations with four different temperature ratios ranging from 1 to 3 are performed in order to characterize the effect of the temperature on the aerodynamic and aeroacoustic fields of the jets. The geometry of the nozzle and the exit conditions are chosen in order to match those in the experimental study conducted at the University of Cincinnati. It is shown that the total number of cells in the shock cell structure decreases with the increase of the temperature ratio. However, the temperature does not influence the size of the first shock cell and the linear decrease of the shock cell size in the downstream direction. The Overall Sound Pressure Levels are then plotted along the minor and major axis. It is seen that the intensity of the screech feedback mechanism increases with the Temperature Ratio. Moreover, for JetTR2.5 and JetTR3, the strong flapping motion of the jet along the minor axis due to the screech feedback mechanism seems to yield to an asymmetric organization of the Mach wave radiation. The convection velocity of the turbulent structures in the jet shear layers along the minor axis is then studied. Once normalized by the jet exit velocity, the convection velocity is shown to decrease with the jet temperature ratio. In the last part of the paper, the near- and far-field acoustic are studied. In the near-field, screech tones which frequencies are consistent with both experimental data and a theoretical model are observed. In the far-field, four acoustic components typical of non-ideally supersonic jets are observed, namely the screech noise, the broadband shock-associated noise, the mixing noise and the Mach wave noise. Their directivities and frequencies are in agreement with experimental results and models.

  • 6.
    Gojon, Romain
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Ecole Centrale de Lyon, France.
    Bogey, C.
    Azimuthal organisation of turbulent structures in underexpanded impinging round jets2016In: 22nd AIAA/CEAS Aeroacoustics Conference, 2016, American Institute of Aeronautics and Astronautics, 2016Conference paper (Refereed)
    Abstract [en]

    The azimuthal organisation of the turbulent structures in underexpanded round jets impinging on a flat plate have been investigated using compressible large eddy simulation. The jet shear-layer region, as well as the region of the wall jets created on the plate after the impact are considered. The jets are characterized by a Nozzle Pressure Ratio of 4.03, a fully expanded Mach number of 1.56, and a Reynolds number of 6 × 104. The distance between the nozzle and the plate varies from 4.16r0 to 9.32r0. The jets generate acoustic tones due to a feedback mechanism. In this paper, the near pressure and density fields of the jets are analysed using Fast Fourier Transform on the nozzle exit plane, the plate, and an azimuthal plane. The amplitude and the phase fields on these sections at the tone frequencies are represented. Similar organisations of the turbulent structures are found in the jet shear layers and the wall jets. Thus, axisymmetric and helical arrangements of the structures in the shear layers lead to spiral and concentric distributions of the structures on the plate, respectively. In particular, for one of the 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. Moreover, the convection velocity of the turbulent structures on the plate is evaluated from the phase fields. Several diameters away from the jet axis, the velocities found for the present jets compare well with those found experimentally in the phase-averaged distributions of fluctuating pressure for impinging ideally expanded jets using fast-response Pressure-Sensitive Paint. Near the jet axis, given that the present jets are underexpanded, differences are observed, due to the presence of shock cell structures in the jets. Finally, the convection velocity of the turbulent structures on the wall are estimated from cross-correlations of radial velocity. The values obtained compare well with those determined from the phase fields.

  • 7.
    Gojon, Romain
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bogey, C.
    Investigation of the feedback mechanism in ideally expanded round impinging jets using large-eddy simulation2016In: 22nd AIAA/CEAS Aeroacoustics Conference, 2016, 2016Conference paper (Refereed)
    Abstract [en]

    Large-eddy simulations (LES) have been performed in order to study the tone generation mechanism in four supersonic ideally expanded round impinging jets. The jets have a Mach number of 1.5, and a Reynolds number of 6 × 104. They impinge normally on a flat plate located at a distance from the nozzle exit varying from 6r0 up to 12r0 where r0 is the jet nozzle radius. The aerodynamic properties of the jets are first investigated. In particular, the convection velocity of the turbulent structures in the jet shear layers is computed. In the spectra of pressure fluctuations in the vicinity of the nozzle exit, intense tones emerge. Their associated Strouhal numbers are in agreement with measurements available for round impinging jets with similar exit conditions. The tone frequencies also correspond well to the frequencies predicted by the classical model of the aeroacoustic feedback establishing between the nozzle lips and the flat plate. A study of the feedback mechanism is then proposed by applying Fourier decomposition to the near pressure fields. The feedback mechanism is found to lead to the formation of hydrodynamic-acoustic standing waves. Moreover, for each tone frequency, the corresponding axisymmetric or helical oscillation mode of the jet is examined. Finally, an analysis is conducted using a vortex sheet model of the jet in order to determine the allowable frequency ranges of the upstream-propagating neutral acoustic wave modes. The tone frequencies obtained in the LES fall within these ranges, depending on their axisymmetric or helical nature. © American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

  • 8.
    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.

  • 9.
    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.

  • 10.
    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.

  • 11.
    Gojon, Romain
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gutmark, E.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the response of a rectangular supersonic jet to a near-field located parallel flat plate2017In: 23rd AIAA/CEAS Aeroacoustics Conference, 2017, American Institute of Aeronautics and Astronautics, 2017Conference paper (Refereed)
    Abstract [en]

    In this paper, the flow and acoustic fields of a rectangular over-expanded supersonic jet interacting with a parallel plate are investigated using compressible LES. The jet exits from a converging diverging rectangular nozzle of aspect ratio 2 and of design Mach number 1.5. Four simulations with four different distances between the lower inner lip of the minor axis of the rectangular jet and the plate ranging from 0 to 3 equivalent diameters are performed. The geometry of the nozzle, the positions of the plate, and the exit conditions are chosen in order to match those in an experimental study conducted at the University of Cincinnati. Snapshots and mean velocity fields are first presented. A good agreement with the PIV experimental measurements is found. The Overall Sound Pressure Levels are then plotted along the minor and major axis. In a previous paper, the corresponding free jet has been found to undergo a strong flapping motion along the minor axis due to the screech feedback mechanism. In the present study, it is seen that the intensity of the screech feedback mechanism increases for some distances and decreases for some others compared to the one in the corresponding free jet. A study of the jets shear-layers is then proposed first by looking at two points space-time cross correlation of the axial velocity. The convection of the turbulent is thus studied. Then, two points space-time cross correlation of the pressure along the jets shear-layers are proposed and an amplification of the aeroacoustic feedback mechanism leading to screech noise is observed in the lower jet shear-layers for two cases. It is also observed that the screech feedback mechanism establishes mainly between the nozzle lips and the end of the tenth shock cell. The acoustic loading on the plate is finally studied. As pointed out in a previous study, the flapping motion of the jet at the screech frequency seems to yield to an asymmetric organization of the Mach wave radiation also at the screech frequency. Those organized Mach waves impinge in the plate and propagate back to the jet, exciting the shear-layer at the screech frequency. This will amplify the screech mechanism in the lower jet shear-layer. However, this amplification happens only for some nozzle-to-plate distances. Indeed, the screech mechanism leads to the formation of a standing wave pattern in terms of pressure loading at the screech frequency on the plate. There are regions with high amplitude, meaning the acoustic loading is organized mainly at the screech frequency, and regions with low amplitude, which means the acoustic loading is not organized mainly at the screech frequency. The amplification then depends on the location of the standing wave compared to the overall acoustic loading on the plate. If a region of high amplitude of the standing wave pattern coincide with the region of maximal acoustic loading, there is amplification of the screech mechanism.

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