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Gojon, Romain
Publications (7 of 7) Show all publications
Gojon, R., Bogey, C. & Mihaescu, M. (2019). Large Eddy Simulation of Highly Compressible Jets with Tripped Boundary Layers (vol. 25ed.). In: Salvetti M., Armenio V., Fröhlich J., Geurts B., Kuerten H. (Ed.), Direct and Large-Eddy Simulation XI. ERCOFTAC Series.: (pp. 333-339). Springer
Open this publication in new window or tab >>Large Eddy Simulation of Highly Compressible Jets with Tripped Boundary Layers
2019 (English)In: Direct and Large-Eddy Simulation XI. ERCOFTAC Series. / [ed] Salvetti M., Armenio V., Fröhlich J., Geurts B., Kuerten H., Springer, 2019, vol. 25, p. 333-339Chapter in book (Refereed)
Abstract [en]

In high-speed aircraft, supersonic jets used for propulsion can lead to very intense aerodynamically generated acoustic noise. Thus, there is a need to study the aerodynamic and aeroacoustic properties of highly compressible jets. In previous studies (Gojon et al, Temperature effects on the aerodynamic and acoustic fields of a rectangular supersonic jet, 2017, [1], Gojon et al, On the response of a rectangular supersonic jet to a near-field located parallel flat plate, 2017, [2]), several simulations of supersonic jets have been conducted. Unfortunately, the turbulence intensity at the nozzle exit was dependent on the internal geometry of the nozzle and could not be tuned. This is a pity given that, as shown experimentally (Zaman, AIAA J, 50(8):1784–1795, 2012, [3]) and numerically (Bogey et al, J Fluid Mech, 701:352–385, 2012, [4], Brés et al, Nozzle wall modeling in unstructured large eddy simulations for hot supersonic jet predictions, 2013, [5]) for subsonic and supersonic jets, the boundary layer state of the jet affects the jet flow and noise.

Place, publisher, year, edition, pages
Springer, 2019 Edition: vol. 25
Series
ERCOFTAC Series
National Category
Fluid Mechanics and Acoustics Aerospace Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-250022 (URN)10.1007/978-3-030-04915-7_44 (DOI)2-s2.0-85061316093 (Scopus ID)978-3-030-04914-0 (ISBN)978-3-030-04915-7 (ISBN)
Note

QC 20190617

Available from: 2019-04-25 Created: 2019-04-25 Last updated: 2019-06-17Bibliographically approved
Gojon, R. & Bogey, C. (2018). Flow Features near Plate Impinged by Ideally Expanded and Underexpanded Round Jets. AIAA Journal, 56(2), 445-457
Open this publication in new window or tab >>Flow Features near Plate Impinged by Ideally Expanded and Underexpanded Round Jets
2018 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 2, p. 445-457Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
AMER INST AERONAUTICS ASTRONAUTICS, 2018
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-222405 (URN)10.2514/1.J056421 (DOI)000423512500001 ()2-s2.0-85041444306 (Scopus ID)
Note

QC 20180228

Available from: 2018-02-28 Created: 2018-02-28 Last updated: 2018-02-28Bibliographically approved
Chen, S., Gojon, R. & Mihaescu, M. (2018). High-Temperature Effects on Aerodynamic and Acoustic Characteristics of a Rectangular Supersonic Jet. In: AIAA (Ed.), AIAA/CEAS Aeroacoustics Conference, AIAA AVIATION Forum, 2018: . Paper presented at AIAA/CEAS Aeroacoustics Conference, AIAA AVIATION Forum, (AIAA 2018-3303). , Article ID 3303.
Open this publication in new window or tab >>High-Temperature Effects on Aerodynamic and Acoustic Characteristics of a Rectangular Supersonic Jet
2018 (English)In: AIAA/CEAS Aeroacoustics Conference, AIAA AVIATION Forum, 2018 / [ed] AIAA, 2018, article id 3303Conference paper, Published 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.

Keywords
Large Eddy Simulation, Supersonic rectangular jets, Aeroacoustics, Temperature effects
National Category
Fluid Mechanics and Acoustics Aerospace Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-235799 (URN)10.2514/6.2018-3303 (DOI)2-s2.0-85051292277 (Scopus ID)978-1-62410-560-9 (ISBN)
Conference
AIAA/CEAS Aeroacoustics Conference, AIAA AVIATION Forum, (AIAA 2018-3303)
Note

QC 20181010

QC 20181017

Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2018-10-17Bibliographically approved
Ceci, A., Gojon, R. & Mihaescu, M. (2018). Large Eddy Simulations for Indirect Combustion Noise Assessment in a Nozzle Guide Vane Passage. Flow Turbulence and Combustion, 1-13
Open this publication in new window or tab >>Large Eddy Simulations for Indirect Combustion Noise Assessment in a Nozzle Guide Vane Passage
2018 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Flow, Turbulence and Combustion, ISSN 1386-6184, p. 1-13Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Large Eddy Simulation, Aeroacoustics, Indirect combustion noise, Entropy noise
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-235797 (URN)10.1007/s10494-018-9964-9 (DOI)2-s2.0-85052594186 (Scopus ID)
Note

QC 20181008

Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2018-10-08Bibliographically approved
Gojon, R., Bogey, C. & Mihaescu, M. (2018). Oscillation Modes in Screeching Jets. AIAA Journal, 56(7), 2918-2924
Open this publication in new window or tab >>Oscillation Modes in Screeching Jets
2018 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 7, p. 2918-2924Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics, 2018
Keywords
Supersonic jets; screech noise; origin of the oscillation modes; vortex sheet model; Large Eddy Simulations
National Category
Aerospace Engineering Fluid Mechanics and Acoustics
Research subject
Aerospace Engineering; Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-228596 (URN)10.2514/1.J056936 (DOI)000436234200035 ()2-s2.0-85049147886 (Scopus ID)
Note

QC 20180717

Available from: 2018-05-28 Created: 2018-05-28 Last updated: 2018-07-17Bibliographically approved
Bogey, C. & Gojon, R. (2017). Feedback loop and upwind-propagating waves in ideally expanded supersonic impinging round jets. Journal of Fluid Mechanics, 823, 562-591
Open this publication in new window or tab >>Feedback loop and upwind-propagating waves in ideally expanded supersonic impinging round jets
2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 823, p. 562-591Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2017
Keywords
acoustics, aeroacoustics, jet noise
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-211380 (URN)10.1017/jfm.2017.334 (DOI)000404893100006 ()2-s2.0-85021098312 (Scopus ID)
Note

QC 20170809

Available from: 2017-08-09 Created: 2017-08-09 Last updated: 2017-08-09Bibliographically approved
Gojon, R. & Bogey, C. (2017). Flow Structure Oscillations and Tone Production in Underexpanded Impinging Round Jets. AIAA Journal, 55(6), 1792-1805
Open this publication in new window or tab >>Flow Structure Oscillations and Tone Production in Underexpanded Impinging Round Jets
2017 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 55, no 6, p. 1792-1805Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics, 2017
National Category
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-210482 (URN)10.2514/1.J055618 (DOI)000402522200003 ()2-s2.0-85020260550 (Scopus ID)
Note

QC 20170705

Available from: 2017-07-05 Created: 2017-07-05 Last updated: 2017-07-05Bibliographically approved
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