Feedback loop and upwind-propagating waves in ideally expanded supersonic impinging round jets

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 r0, the jet is ideally expanded, and has a Mach number of 1.5 and a Reynolds number of 60000. Four distances between the nozzle exit and the flat plate, equal to 6r0, 8r0, 10r0 and 12r0, 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

Flow Features near Plate Impinged by Ideally Expanded and Underexpanded Round Jets

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 6 × 104. 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 non-ideally 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

Flow Structure Oscillations and Tone Production in Underexpanded Impinging Round Jets

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 60000. Four distances between the nozzle and the plate 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

Numerical study of the flow and the near acoustic fields of an underexpanded round free jet generating two screech tones

The flow and near acoustic fields of a supersonic round free jet are explored using a compressible large eddy simulation. At the exit of a straight pipe nozzle, the jet is underexpanded, and is characterized by a Nozzle Pressure Ratio of 4.03 and a Temperature Ratio of 1. It has a fully expanded Mach number of 1.56, an exit Mach number of 1, and a Reynolds number of 60000. Flow snapshots, mean flow fields and convection velocity in the jet shear layers are consistent with experimental data and theoretical results. Furthermore, two screech tones are found to emerge in the pressure spectrum calculated close to the nozzle. Using a Fourier decomposition of the pressure fields, the two screech tones are found to be associated with anticlockwise helical oscillation modes. Besides, the frequencies of the screech tones and the associated oscillation modes both agree with theoretical predictions and measurements. Moreover, pressure fields filtered at the screech frequencies reveal the presence of hydrodynamic-acoustic standing waves. In those waves, the regions of highest amplitude in the jet are located in the fifth and the sixth cells of the shock cell structure. The two screech tones therefore seem to be linked to two different loops established between the nozzle and the fifth and sixth shock cells, respectively. In the pressure fields, three other acoustic components, namely the low-frequency mixing noise, the high-frequency mixing noise and the broadband shock-associated noise, are noted. The directivity and frequency of the mixing noise are in line with numerical and experimental studies. A production mechanism of the mixing noise consisting of sudden intrusions of turbulent structures into the potential core is discussed. Then, the broadband shock-associated noise is studied. This noise component is due to the interactions between the turbulent structure in the shear layers and the shocks in the jet. By analyzing the near pressure fields, this noise component is found to be produced mainly in the sixth shock cell. Finally, using the size of this shock cell in the classical theoretical model of this noise component, a good agreement is found with the simulation results

Investigation of tone generation in ideally expanded supersonic planar impinging jets using large-eddy simulation

The generation of tones in a supersonic planar jet impinging on a flat plate normally has been investigated by performing compressible large-eddy simulations using low-dissipation and low-dispersion finite differences. At the exit of a straight nozzle of height $h$ , the jet is ideally expanded, and has a Mach number of 1.28 and a Reynolds number of 50000. Four distances between the nozzle and the plate between 3.94h and 9.1h have been considered. Flow snapshots and mean velocity fields are first presented. The variations of turbulence intensities and of the convection velocity in the jet shear layers are then examined. The properties of the jet near fields are subsequently described, in particular by applying Fourier decomposition to the pressure fields. Several coexisting tones appear to be generated by aeroacoustic feedback loops establishing between the nozzle lip and the flat plate, which also lead to the presence of hydrodynamic–acoustic standing waves. The tone frequencies are consistent with those given by the aeroacoustic feedback model and with measurements for high-aspect-ratio rectangular jets. The jet oscillation modes at these frequencies are characterized, and found to agree with experimental data. Their symmetric or antisymmetric natures are shown to be well predicted by a wave analysis carried out using a vortex sheet model of the jet, providing the allowable frequency ranges for the upstream-propagating acoustic waves. Thus, it is possible, for an ideally expanded impinging planar jet to predict both the frequencies of the tones and the symmetric or antisymmetric nature of the corresponding oscillation modes by combining the aeroacoustic feedback model and the wave analysis

Investigation of the feedback mechanism in ideally expanded round impinging jets using large-eddy simulation

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 × 10^4. 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

Effects of the angle of impact on the aeroacoustic feedback mechanism in supersonic impinging planar jets

Three planar impinging supersonic of infinite extent are simulated using compressible large-eddy simulations in order to study the effects of the angle of impact on the flow and acoustic fields of the jet. At the exit of a nozzle of height h, they are ideally expanded and have an exit velocity uj, yielding a Mach number of 1.28, and a Reynolds number of 5 × 10^4. They impinge on a flat plate at a distance 5.5h from the nozzle lips with angles of 60, 75, and 90 degrees between the jet direction and the plate. Mean velocity flows and snapshots of density, pressure and vorticity are first shown. The mean convection velocity of the turbulent structures in the jet shear-layers is then determined. The sound pressure levels are computed and several tones due to the presence of a feedback mechanism are found to establish between the nozzle lips and the flat plate. They agree well with the corresponding measurements and with the classical model of the feedback mechanism. Moreover, when the angle of impact deviates from 90 degrees to 75 degrees, a jump from the third to the fourth mode of the feedback mechanism and a reduction in intensity are noted. By applying a Fourier decomposition to the near pressure fields, hydrodynamic-acoustic standing waves are found for each dominant tone frequency. Moreover, as suggested by amplitude fields and velocity spectra in the jet shear layers, the feedback mechanism seems to establish mainly along the lip that is farther away from the plate when the impact angle is not normal. This jump from the third to the fourth mode is similar to the jump observed experimentally for an angle of impact of 90 degrees when the nozzle-to-plate distance increases from 5.5h to 5.85h. Finally, for an angle of impact of 60 degrees, it is seen that none of the modes of the feedback persists in time, but that several modes randomly establish during short periods of time. These rapid switches between different modes lead to several tones that are less energetic on average and centered around St = 0.25

Simulation numérique du rayonnement acoustique de jets ronds supersoniques impactant une paroi

Des simulations numériques de quatre jets rond supersoniques sous-détendus ont été réalisées. Les quatre jets impactent une paroi avec un angle normal, située à une distance comprise entre L = 4.16r0 et L = 9.32r0 des lèvres de la buse, où r0 est le rayon de la buse. Des champs moyens et instantanés de vitesse, densité et pression sont présentés, et les propriétés aéroacoustiques des jets sont étudiées

Large Eddy Simulations for Indirect Combustion Noise Assessment in a Nozzle Guide Vane Passage

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

Computational Analysis of the Indirect Combustion Noise Generation Mechanism in a Nozzle Guided Vane in Transonic Operating Conditions

The combustion noise in modern engines is mainly originating from two types of mechanisms. First, chemical reactions in the combustion chamber leads to an unsteady heat release which is responsible of the direct combustion noise. Second, hot and cold blobs of air coming from the combustion chamber are advected and accelerated through turbine stages, giving rise to entropy noise (or indirect combustion noise). In the present work, numerical characterization of indirect combustion noise of a Nozzle Guide Vane passage was assessed using three-dimensional Large Eddy Simulations. The present work offers an overview to the analytical, computational and experimental studies of the topic. Numerical simulations are conducted to reproduce the effects of incoming planar entropy waves from the combustion chamber and to characterize the generated acoustic power. The dynamic features of the flow are addressed by the means of frequency domain and modal analyses techniques such as Fourier Decomposition and Proper Orthogonal Decomposition. Finally, the predicted entropy noise from numerical calculations is compared with the analytical results of an actuator disk model for a stator stage. The present paper proves that the generated indirect combustion noise can be significant for transonic operating conditions. The blade acoustic response is characterized by the excitation of a latent dynamics at the forcing frequency of the planar entropy waves, and it increases as the amplitude of the incoming disturbances increases