| 1. |
- Malla, Bhupatindra, et al.
(författare)
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Proper Orthogonal Decomposition on LES and PIV Data Sets from a Mach 0.9 Jet.
- 2012
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Ingår i: 50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition. - : AIAA. ; , s. 12-
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Konferensbidrag (refereegranskat)abstract
- This paper contains the complementary analysis of the results obtained through computational and experimental methods using the technique of Proper Orthogonal Decomposition (POD). A detailed study of the experimentally obtained flow field data using the method of POD has already been accomplished. The experimental data consists of the 2D visualization of the velocity field obtained using Particle Image Velocimetry (PIV). The computational process is initialized with the simulation of the 3D flow field using a steady state RANS flow solver. RANS is used as a platform to proceed towards capturing the unsteadiness of the flow field which is accomplished using the Large Eddy Simulation (LES). A significant match in the characteristics of the mode shapes has been observed, in both the axial and radial components of the velocity field, however, there is difference in the distribution of the energies over the modes. The LES predicts the energies to be higher at the lowest modes and the energy drop off rate is higher. On the contrary, PIV predicts lower energies at the first few modes, in comparison to the LES results, however the energy drop off rate is smaller.
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| 2. |
- Stoddard, William, et al.
(författare)
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Simulations of Detonations Based Cycles in Tubes with Multiple Openings
- 2010
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Ingår i: 46<sup>th</sup> AIAA / ASME / SAE / ASEE Joint Propulsion Conference and Exhibit. - 9781600867477
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Konferensbidrag (refereegranskat)abstract
- Computational fluid dynamics (CFD) analysis on detonations, and the resulting exhaust dynamics are studied. Specifically the analysis seeks to quantify the relative ability of the cycles to refill with unburned air that can be mixed with fuel for pulsed detonation. The relative magnitude of the effects of ignition location, fuel location, and basic tube geometry on this self-aspiration are first investigated in smaller, more time-efficient simulations. From these, additional variables, such as ejectors, and different inlet shapes are tested with the most promising configuration of fuel ignition and overall pipe geometry. These further tests are carried out at scales more comparable to the experimental setups. The results obtained are compared with the previous pulse-jet configurations and known pulse detonation data. Steps are taken toward optimizing a static thrust self-aspirating valveless pulse detonation engine (PDE).
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| 3. |
- Golliard, Thomas, et al.
(författare)
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Temperature Impact on an Aerospike Nozzle Jet, a Computational Aeroacoustics Approach
- 2024
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Ingår i: AIAA SciTech Forum and Exposition, 2024. - : American Institute of Aeronautics and Astronautics (AIAA).
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Konferensbidrag (refereegranskat)abstract
- In this paper, the flow and acoustic characteristics of an aerospike nozzle jet at a Nozzle Pressure Ratio (NPR) = 3 and three different Temperature Ratios (TR) = 1, 3, 7 are presented. Implicit Large Eddy Simulations (ILES) are deployed to simulate the flow of the aerospike nozzle. The LES calculations are completed by aeroacoustic computations based on the Ffowcs Williams-Hawkings (FWH) equation. In the supersonic jet exhausting an aerospike nozzle, two shock-cell structures are observed: an annular and a circular one. In the direct vicinity of the annular nozzle, the annular jet is non-attached and reattaches further downstream at increasing distance with increasing jet Temperature Ratio (TR). The observed shock cells in the non-attached annular jet structure are longer for TR3 and TR7 compared to TR1. The shock strength in the annular jet is increasing with increasing jet TR. In the meantime, the total number of shock cells in the circular part of the jet decreases with increasing jet TR. Pressure spectra in the near-field show the presence of a strong tonal noise at upstream angles corresponding to screech tones. Two-point cross-correlations of pressure data acquired in monitoring points located along axial lines in the circular shear layer are computed to quantify the upstream propagating waves associated to this tonal component. Power spectral density of the radial velocity at several monitoring points located at the shock cells as well as at the separation bubble, highlights the main oscillation modes of the annular shock-cell structure. Supersonic convection velocities of turbulent structures are detected with increasing jet temperature by means of two-point cross-correlation. This confirms the presence of Mach waves observed in the instantaneous snapshots. The Mach waves radiation angles are in agreement with existing models. In the far-field spectra, the highest Sound Pressure Levels (SPL) are associated to those Mach waves at angles around 140° (TR3) and 120° (TR7). High skewness and kurtosis in the pressure signals indicate crackle noise at higher jet temperatures. Additionally, the shock-cell length is used to predict the central frequency of Broadband Shock-Associated Noise (BBSAN) as a function of observation angles. The far-field spectra display mixing noise as well as BBSAN, related to the interaction between the convected vortices in the shear layers and the shock-cell structure. With increasing jet temperature, higher SPL are detected in agreement with the BBSAN central frequencies which were computed using the annular and circular shock-cell length.
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| 4. |
- Semlitsch, Bernhard, et al.
(författare)
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The generation mechanism of higher screech tone harmonics in supersonic jets
- 2020
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 893:A9
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Tidskriftsartikel (refereegranskat)abstract
- The generation mechanism of screech harmonics in supersonic exhausts is revealed using shadowgraph imaging and acoustic far-field measurements for a rectangular, high aspect-ratio nozzle. The coherent information associated with screech and its harmonics, i.e. flow structures and acoustic radiation pattern, is extracted from the time-resolved shadowgraph images. We show that, for large lateral distortions of the jet plume, the passage of screech associated flow structures triggers the screech-cyclic formation of shocks, which travel downstream and merge with the original shocks. The interaction of the shock waves with the flow structures associated with screech alters the appearance of the perturbations in the mixing layer, which constitute the higher harmonics of screech. Visualisations of the acoustic radiation pattern expose that the third and higher screech tone harmonics originate from these interaction locations. Further, the occurrence of mode resonance between the screech and its harmonics is demonstrated, where the mode resonance location coincides with the screech tone origin
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| 5. |
- Ahn, Myeonghwan, et al.
(författare)
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A numerical study on near-field pressure fluctuations of symmetrical and anti-symmetrical flapping modes of twin-jet using a high-resolution shock-capturing scheme
- 2021
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Ingår i: Aerospace Science and Technology. - : Elsevier. - 1270-9638 .- 1626-3219. ; , s. 107147-107147
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Tidskriftsartikel (refereegranskat)abstract
- Screeching supersonic jets appears at off-design operating conditions and is perceived as an intense tonal noise. In a twin nozzle configuration, mutual interactions between the two jet plumes may occur with various coupling modes developing depending on the operating conditions and lateral distance between the jets. The investigation of the detailed flow behaviors and near-field pressure fluctuations with relevance to the twin jets system, the analysis of the developed instabilities, will enhance understanding of fundamental features associated with jets located close to each other.In the present study, the single jet is considered first to assess the large eddy simulation (LES) approach used and the near-field pressure fluctuation predictions. Based on the validated solver, twin jets are simulated. Two different twin-nozzle configurations having different separation distance or nozzle-to-nozzle centerline spacing are scrutinized for the same Mach number of 1.358. Notably, the twin jets are screeching by the coupling mode for both set-ups; however, the case of closer inter-nozzle distance presents a symmetrical dominant flapping mode, while the other case shows an anti-symmetrical flapping mode. The strength of the pressure fluctuation at the fundamental frequency changes depending on the location of the observer point (upstream or downstream) and the reference plane (twin-jet and normal to the twin-jet plane). The screech tones of the two cases, observable in the upstream region, are significantly different in the normal to the twin-jet plane direction because of the phase difference of fluctuating pressure. However, the first harmonic component remains strong, regardless of the flapping mode. It is also observed that, at the fundamental frequency, the amplitude of the pressure fluctuation at downstream locations is found to be strong in the normal to the twin-jet plane when the symmetrical flapping mode occurs. This feature is also observed in the twin-jet plane in the case of the opposite mode. By analyzing the developed vertical structures and performing correlation analyses of pressure fluctuations along jet shear layers, the periodicity of the flow in the downstream region with relevance to the fundamental frequency is revealed.
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| 6. |
- Ahn, MyeongHwan, et al.
(författare)
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Effects of Temperature on the Characteristics of Twin Square Jets by Large Eddy Simulations
- 2022
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Ingår i: AIAA Science and Technology Forum and Exposition, AIAA SciTech Forum 2022. - Reston, Virginia : American Institute of Aeronautics and Astronautics (AIAA).
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Konferensbidrag (refereegranskat)abstract
- In this study, we investigate the effects of temperature on the aerodynamic and aeroacoustics characteristics of twin square jets. Implicit Large Eddy Simulations (ILES) are performed for twin jets with a fixed nozzle pressure ratio (NPR) of 3.0 and temperature ratios (TR) of 1.0, 2.0, 4.0, and 7.0. A second-order central scheme is used to resolve acoustic waves, and an artificial dissipation model is applied to capture shock waves and to suppress non-physical oscillations. In addition, the variation of a specific heat ratio as function of temperature is considered under the chemical equilibrium assumption. The numerical results show that the length of potential core is reduced with the increase of temperature due to the enhanced mixing in jet shear layers which can be estimated by turbulent kinetic energy (TKE). Meanwhile, the fluctuations of the transverse velocity show different trends between the cases within the corresponding potential core length, which can be associated with the screeching phenomena of the twin-jet. As temperature increases, the convection Mach number in the jet shear layers is also increased so that the Mach wave is generated for TR of 2.0, 4.0, and 7.0. However, a crackle noise is only observed for TR of 4.0 and 7.0, whose generation is identified by the skewness and kurtosis factors. Relatively low temperature jets (TR of 1.0 and 2.0) are screeching so that peaks are observed in the spectra obtained upstream. On the other hand, broadband component is gradually increased when the jets are heated, and the largest increase is observed at the location exposed to the Mach wave radiation.
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| 7. |
- Ahn, MyeongHwan, et al.
(författare)
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Large-eddy simulations of flow and aeroacoustics of twin square jets including turbulence tripping
- 2023
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:6
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we investigate the flow and aeroacoustics of twin square (i.e., aspect ratio of 1.0) jets by implicit large-eddy simulations (LESs) under a nozzle pressure ratio of 3.0 and a temperature ratio of 1.0 conditions. A second-order central scheme coupled with a modified Jameson's artificial dissipation is used to resolve acoustics as well as to capture discontinuous solutions, e.g., shock waves. The flow boundary layer inside of the nozzle is tripped, using a small step in the convergent section of the nozzle. The time-averaged axial velocity and turbulent kinetic energy of LES with boundary layer tripping approaches better to particle image velocimetry experimental data than the LES without turbulence tripping case. A two-point space–time cross-correlation analysis suggests that the twin jets are screeching and are coupled to each other in a symmetrical flapping mode. Intense pressure fluctuations and standing waves are observed between the jets. Spectral proper orthogonal decomposition (SPOD) confirms the determined mode and the relevant wave propagation. The upstream propagating mode associated with the shock-cell structures is confined inside jets. Far-field noise obtained by solving Ffowcs Williams and Hawkings equation is in good agreement with the measured acoustic data. The symmetrical flapping mode of twin jets yields different levels of the screech tone depending on observation planes. The tonalities—the fundamental tone, second and third harmonics—appear clearly in the far-field, showing different contributions at angles corresponding to the directivities revealed by SPOD.
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| 8. |
- Ceci, Alessandro, et al.
(författare)
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Computational analysis of the indirect combustion noise generation mechanism in a nozzle guided vane in transonic operating conditions
- 2021
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Ingår i: Journal of Sound and Vibration. - : Elsevier. - 0022-460X .- 1095-8568. ; 496
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Chen, Song, et al.
(författare)
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Effect of an Adjacent Flat Plate on a Highly-Heated Rectangular Supersonic Jet
- 2020
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Ingår i: AIAA Scitech 2020 Forum. - AIAA : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- Solid surfaces located in the vicinity of a supersonic jet may affect its flow dynamics and greatly change the aeroacoustic characteristics. Large-eddy simulations (LES) are performed to investigate the plate effects on a highly-heated rectangular supersonic jet. The rectangular nozzle has an aspect ratio of 2.0 and is operated at the over-expanded condition with a nozzle pressure ratio of 3.0 and a nozzle temperature ratio of 7.0. Four cases, JetL0 to JetL3 with a plate-to-nozzle distance ranging from 0 to 3 times of the jet equivalent nozzle diameter are investigated. The large-scale implicit LES computations are performed by a well-validated in-house finite-volume based CFD code, which uses an artificial dissipation mechanism to represent the effect of small-scale turbulence and to damp the numerical oscillation near shocks. The temperature-dependent thermal properties of air in the highly-heated jets are considered by the chemical equilibrium assumption. Numerical results show that among the four cases, JetL0 with the plate directly attached at the nozzle lip shows significant different flow and acoustic fields from the others. It exhibits a longer jet potential core length but without forming a series of well-structured shock diamonds. The other cases show similar shock/expansion wave structures as observed in the free jet but their jet plumes bend towards the plate. This bending of jet leads to JetL1 scrubs over the plate in the downstream. The scrubbing effect, together with the unaffected shock-shear layer interactions and high plate pressure loading, makes JetL1 have a stronger OASPL in the near acoustic fields than the other cases. The spectrum analysis in the nozzle upstream direction shows that the plate removes or mitigates the screech tone observed in the free jet and slightly amplifies the acoustic amplitudes in the low-frequency range.
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| 10. |
- Chen, Song, et al.
(författare)
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Flow and aeroacoustic attributes of highly-heated transitional rectangular supersonic jets
- 2021
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638 .- 1626-3219. ; 114:106747
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Tidskriftsartikel (refereegranskat)abstract
- Heated transitional supersonic jets exhausting from a rectangular nozzle at over-expanded conditions are investigated by Large Eddy Simulations and Ffowcs-Williams and Hawkings acoustic analogy. Four cases with a fixed nozzle pressure ratio but different temperature ratios (TR) of 1.0, 2.0, 4.0, and 7.0 are analyzed. Numerical results show that with the increasing temperature the jet velocity significantly increases, whereas its Reynolds number decreases by about one order of magnitude, which leads to a 30% decrease in the jet potential core length and reduction in the number of shock cells. The increasing temperatures also result in supersonic shear layer convection Mach numbers and consequently Mach wave radiations in the acoustic fields. Pressure skewness and kurtosis factors indicate crackle noise and non-linear propagation effects in high temperatures. For the most heated jet TR 7.0, the Mach wave radiation is identified radiating noise at about 120 degrees, while the large turbulence structure noise at about 150 degrees. Furthermore, the vortex sheet model analysis and the LES data detect the existence of upstream-propagating neutral waves inside jet TR 7.0. The observed screech frequency falls within the range of antisymmetric mode indicating that the highly-heated jet is characterized by an antisymmetric oscillation mode at the screech frequency.
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