Abstract: In this paper, the quality of the estimation of the convection velocity in jet shear layers using schlieren pictures is investigated. The aim is to discuss whether the convection velocity is likely to be biased if determined from schlieren images obtained at a high framerate, as in previous experiments using the phase shift method. For this, a numerical procedure is developed in order to generate schlieren-like images on the basis of simulation data, and applied to the results provided by the large-eddy simulation (LES) of an under-expanded round jet at an ideally expanded Mach number of 1.56. The results obtained from the schlieren pictures are compared with those obtained directly from the LES density fields. It is notably found that the location of the maximum of gray level fluctuations in the schlieren pictures corresponds well to that of the maximum of density fluctuations, and that the convection velocity estimated for low frequencies using schlieren pictures is underestimated for small separation distances between the two points used for the phase shift calculation.
Abstract: 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.
Abstract: Dans ce travail de recherche, on presente des developpements specifiques de diagnostiques optiques et leur application a l’etude aeroacoustique des jets rapides a haut nombre de Reynolds. Les resultats experimentaux presentes ici resultent de visualisation par strioscopie et, de maniere preponderante dans ce manuscrit, de mesure de masse volumique par diffusion Rayleigh. Ces methodes de caracterisation d’ecoulement, appliquees aux jets subsoniques ou supersoniques, ont ete associees a des mesures de bruit en champ lointain. La mesure par diffusion Rayleigh, qui repose sur la lumiere diffusee par les molecules constituantes du gaz, et n’est donc pas intrusive. Des difficultes apparaissent neanmoins pour exploiter les resultats lorsque le milieu diffusant contient des poussieres. Bien que l’air des ecoulements obtenu en soufflerie soit filtre, la quantite residuelle de poussieres a rendu necessaire le developpement d’une methode de nettoyage du signal en post-traitement. Le niveau des signaux obtenus par diffusion Rayleigh est tres faible, et domine par du bruit appele shot noise. Un gain significatif sur le niveau de ce bruit a ete obtenu en optimisant la chaine d’acquisition apres analyse des systemes existants. De plus une methode de traitement du signal derivee d’une methode existante a permis de calculer des spectres de masse volumique malgre le shot noise avec un seul capteur, la ou il en fallait deux auparavant. Les profils de p obtenus par cette technique ont montre qu’il existe une loi de similarite permettant de superposer les profils mesures a differentes positions axiales. Cette loi est identique pour les jets issus de trois tuyeres aux geometries differentes, et a des nombres de Mach de 0.7 et 0.9. Une loi de similarite est egalement observee pour p’rms si les profils sont mesures suffisamment loin de la tuyere. L’etude des spectres dans la couche de melange a mis en evidence un maximum faiblement marque autour d’une frequence centrale comprise entre St = 0:2 et St = 2 dans les regions mesurees, plus marque que dans les spectres de vitesse, et dont le comportement differe selon l’etat initialement laminaire ou turbulent du jet. L’evolution de la forme des spectres en fonction de la difference de masse volumique entre le jet et le milieu ambiant, ainsi qu’en fonction du nombre de Mach, a egalement ete etudiee. Une loi permettant de superposer les spectres a ete definie empiriquement sur la plage de variation des differents parametres. Des mesures simultanees entre l’acoustique en champ lointain et la masse volumique dans l’ecoulement ont ete realisees pour un jet a Mj = 0:9 et un jet a Mj = 1:32. Ces resultats ont permis l’estimation de coherences spectrales et de moyennes conditionnelles. Les resultats obtenus mettent en evidence la presence de structures liees au rayonnement acoustique dans une region situee proche de l’axe du jet en aval du cone potentiel. Pour finir, une etude a ete realisee sur le screech dans les jets supersoniques sous-detendus. Elle a permis d’identifier la position de la source de la retroaction acoustique pour les modes A1, A2, et B, ainsi que la structure du cycle de la boucle qui determine les changements de frequences observes aux sauts de modes.
Abstract: The primary objective of the present study was to develop a quantitative schlieren-imaging technique that can be used to study the dynamics of instability waves. The technique was initially validated by optically capturing a controlled acoustic wave generated by a compression driver and excellent agreement was obtained with microphone measurements. An underexpanded jet was considered as an ideal test case due to the complexity and multitude of instability mechanism. Further analysis of the underexpanded jet demonstrated that this technique can be used to capture the very high frequency mode related to the phenomenon of screech.
Cites background from "Estimation of convection speed in u..."
..., Panda and Seasholtz , to cite a few)....
Abstract: Explicit numerical methods for spatial derivation, filtering and time integration are proposed. They are developed with the aim of computing flow and noise with high accuracy and fidelity. All the methods are constructed in the same way by minimizing the dispersion and the dissipation errors in the wavenumber space up to kΔx = π/2 corresponding to four points per wavelength. They are shown to be more accurate, and also more efficient numerically, than most of the standard explicit high-order methods, for uniform and slowly non-uniform grids. Two problems involving long-range sound propagation are resolved to illustrate their respective precisions. Remarks about their practical applications are then made, especially about the connection with the boundary conditions. Finally, their relevance for the simulation of turbulent flows is emphasized.
TL;DR: A shock-capturing methodology is developed for non-linear computations using low-dissipation schemes and centered finite differences that allows in particular to distinguish shocks from linear waves, and from vortices when it is performed from dilatation rather than from pressure.
Abstract: A shock-capturing methodology is developed for non-linear computations using low-dissipation schemes and centered finite differences. It consists in applying an adaptative second-order filtering to handle discontinuities in combination with a background selective filtering to remove grid-to-grid oscillations. The shock-capturing filtering is written in its conservative form, and its magnitude is determined dynamically from the flow solutions. A shock-detection procedure based on a Jameson-like shock sensor is derived so as to apply the shock-capturing filtering only around shocks. A second-order filter with reduced errors in the Fourier space with respect to the standard second-order filter is also designed. Linear and non-linear 1D and 2D problems are solved to show that the methodology is capable of capturing shocks without providing dissipation outside shocks. The shock detection allows in particular to distinguish shocks from linear waves, and from vortices when it is performed from dilatation rather than from pressure. Finally the methodology is simple to implement and reasonable in terms of computational cost.
Abstract: Under certain conditions, shock-containing jets produce an intense tone referred to as screech. Screech was discovered about half a century ago by Alan Powell in England. Here I recount developments in supersonic jet screech — from Powell's first observation in 1951 to now. During this period more than 200 papers have been published — many offering only incremental advances. This paper provides a concise screech resource including a historical perspective, a summary of recent developments and a critical assessment of the state of the art. Topics include modulation of instability waves by shocks, shock-cell models and screech frequency prediction models, unsteady shock motions and clues about their role in shock noise generation. Also, detailed nearfield measurements and computer simulation methods now available are discussed. However, despite the advances, screech amplitude prediction remains an elusive but increasingly important goal not only due to concerns about sonic fatigue failure of aircraft structures but because knowledge gained by the study of screech can be applied to a variety of resonant flow situations, including jet impingement, cavity resonance, and closed-loop active flow control.