Georgy A. Faranosov

Instability wave control in turbulent jet by plasma actuators

Instability waves in the shear layer of turbulent jets are known to be a significant source of jet noise, which makes their suppression important for the aviation industry. In this study we apply plasma actuators in order to control instability waves in the shear layer of a turbulent air jet at atmospheric pressure. Three types of plasma actuators are studied: high-frequency dielectric barrier discharge, slipping surface discharge, and surface barrier corona discharge. Particle image velocimetry measurements of the shear layer demonstrate that the plasma actuators have control authority over instability waves and effectively suppress the instability waves artificially generated in the shear layer. It makes these actuators promising for application in active control systems for jet noise mitigation.

Correlations of jet noise azimuthal components and their role in source identification

In this paper, we study the subsonic cold jet noise using the azimuthal decomposition technique (ADT). The results of measurement of correlations for jet noise azimuthal components are reported. It is shown that the correlations for tone-excited jet strongly differ from those for unexcited jet; for example, an unexpectedly high value of correlation for tone-excited jet noise azimuthal components has been obtained for observation angles close to 90o to the jet axis. This behavior of the correlation renders it quite promising for the jet noise mechanism identification and source localization. An analytical model based on qudrupole source distribution is proposed for description of the correlations for unexited jet noise azimuthal components and its applicability is validated experimentally. I. Introduction t present the process of noise generation by turbulent subsonic cold jets is thought to be related with different mechanisms: fine-scale turbulence (Refs. 1 and 2), eigen-oscillations of large-scale vortex structures (Ref. 3), instability waves (Refs. 4 and 5) etc. Identification of these mechanisms in the process of jet noise generation and assessment of their contribution to the total noise is an important task. To achieve this on the basis of the measurements of only far-field total noise directivity is difficult, however. The azimuthal decomposition of the far field noise allows us to obtain more detailed characteristics of the sound sources; the previous studies (Refs. 3 and 6) on modeling the azimuthal components show that the observed experimental data for cold subsonic jet noise can be explained if both large-scale vortex structures (vortex rings) and fine-scale turbulence (modeled as moving point quadrupoles) are accounted for as sound sources, the vortex rings’ contribution to the total noise being about 40%. An excellent collapse of the modeling and experimental curves has been observed, which evinces that this is a plausible framework to model turbulent cold subsonic jet noise. To further validate this model and get new insights into the structure of the sound sources, the analysis has been expanded to include correlation characteristics of the azimuthal components. In Ref. 7 the first results of correlation measurements of azimuthal harmonics for unexcited cold subsonic jets (i.e. the correlation between the simultaneous measurements for the same azimuthal mode at two different points) have been reported that experimentally verify the absence of correlation between different modes, which is what should be expected from the theoretical considerations (azimuthal components are orthogonal). A The cross-correlation curve for tone excited jet has characteristic peculiarities, albeit it is generally similar in shape to the unexcited jet curve. These curves demonstrate that the large-scale structure sound radiation is concentrated in the region at the right angle to the jet, because in this region the cross-correlation curve for the tone excited jet is significantly higher than the curve for the unexcited jet. This property of the curve is somewhat unexpected from the general view that large-scale structures in jet radiate in the downstream direction, whereas at the right angle to the jet direction it is so-called fine scale turbulence that radiates. In Ref. 7 the spatial correlations of the azimuthal components have been obtained, but no attempt has been made to propose a theoretical model to explain the observed spatial correlation curves. Such an attempt is made in the present work. The sound sources are modeled as a distribution of moving point quadrupoles. The results of modeling are compared with the results of measurements of cross-correlation function for the far sound field of unexcited cold jet with velocity 120 m/s. The comparison is performed for the zeroth harmonics a0 in two frequency bands 600. II. Experimental Setup

Acoustic control of instability waves in a turbulent jet

The possibility of acoustic control of instability waves formed in the mixing layer of a jet is experimentally investigated. The feasibility of suppressing a hydrodynamic instability wave in a subsonic turbulent jet by an external acoustic action is demonstrated. This result can be used in designing active control systems for jet noise suppression.

Intensification and suppression of jet noise sources in the vicinity of lifting surfaces

Theoretical and an experimental investigations of jet noise modification are carried out for the jet issuing in the vicinity of the wing. The effect of interaction between instability waves in the jet and flap solid boundaries has been studied. Expermental investigation of jet noise from a dual flow nozzles near a wing provides assessment of several parameters (chevrons on primary and secondary nozzles, distance between the trailing edge of the flap and the nozzle, distance between the plane of the wing to the jet axis, presence of the pylon, etc.) on jet noise installation effect.

On the effect of flap deflection on jet flow for a jet-pylon-wing configuration: near-field and acoustic modelling results

A computational investigation of jet-pylon-wing-flap interaction based on a model installed jet geometry for a co-axial jet at bypass ratio (BPR) 5 is presented. For numerical modelling of jet installation effects, Monotonically Integrated Large Eddy Simulations (MILES) are conducted with the CABARET method. For accurate sound predictions at downstream jet radiation angles, a penetrable Ffowcs Williams-Hawkings (FWH) technique with multiple closing disks is applied. The axial and vertical distributions of the mean axial velocity and turbulent kinetic energy obtained from the CABARET calculations are compared with the reference RANS/ILES solution of Lyubimov (2013) and with the experiment data for the axisymmetric jet case and for the nozzle-with-pylon case. The farfield acoustic spectra predictions are obtained for the jet flow with penetrable (for jetaiframe configuration) and impenetrable (for wing-flap alone) FWH formulations. The sound spectra computed is compared with the acoustic results for jet-pylon-wing-flap configuration of similar geometrical parameters at BPR 10 from the literature with the scaling based on the mixed out jet core parameters in accordance with Lush (1971) method.

Adaptation of the Azimuthal Decomposition Technique to Jet Noise Measurements in Full-Scale Tests

To develop jet noise-reduction concepts, it is necessary to understand the physics underlying the noise-generation process. Decomposition of the sound field into azimuthal modes is one of the effective experimental methods that allows extracting subtle features of different types of noise sources. For turbulent flows, the interpretation of the noise analysis results using the azimuthal decomposition technique was developed by the authors in previous papers. In the present paper, a generalization of this method allowing the decomposition of the jet acoustic field into azimuthal modes by means of reduced number of microphones is developed. It is shown that jet noise measurements by only three microphones in each cross section allow reconstruction of three azimuthal mode directivities (axisymmetric, first, and second) for both low and moderate frequency bands. Furthermore, two-microphone measurements, provided the microphones are properly located, make it possible to reconstruct directivity of the axisymmetr...

Jet-wing interaction: computational modelling based on MILES CABARET and acoustic analogy

The computational model of a jet-wing configuration from the recent TsAGI experiment [1] that corresponds to a dual-stream co-axial subsonic jet under a swept lifting wing is considered. For the jet and wing calculations, Monotonically Integrated Large Eddy Simulations (MILES) are performed for which a modern parallel unstructured-grid computational code based on the high-resolution CABARET scheme [2],[3] is used. The computational domain includes the co-axial nozzle with a central body and a lifting wing section where round flowstoppers were installed on the wing tips to prevent downwash effects. For far-field noise predictions, the MILES CABARET solver is coupled with the Ffowcs Williams – Hawkings (FW-H) integral method. The FW-H solution corresponds to a large closed permeable control surface with multiple closing disks at the outlet side in accordance with the best practice. The results of the acoustic modelling are compared with the experiment. The differences, which can be associated with the unaccounted effects of sound refraction through a non-uniform free-stream flow are discussed.

Airfoil Flow and Noise Computation Using Monotonically Integrated LES and Acoustic Analogy

A new scalable CABARET MILES method has been applied for modelling of flow around NACA0012 airfoil. Results of computational modelling on several grids are compared with the experiment. The flow solver is coupled with the Ffowcs Williams – Hawking formulation for far field noise modelling. The CABARET-FWH results for a model problem of pulsating sphere are first demonstrated. For airfoil noise modelling, the computational results are compared with the experiment.

Two-dimensional model of the interaction of a plane acoustic wave with nozzle edge and wing trailing edge

The interaction of a plane acoustic wave with two-dimensional model of nozzle edge and trailing edge is investigated theoretically by means of the Wiener-Hopf technique. The nozzle edge and the trailing edge are simulated by two half-planes with offset edges. Shear layer behind the nozzle edge is represented by a vortex sheet supporting Kelvin-Helmholtz instability waves. The considered configuration combines two well-known models (nozzle edge and trailing edge), and reveals additional interesting physical aspects. To obtain the solution, the matrix Wiener-Hopf equation is solved in conjunction with a requirement that the full Kutta condition is imposed at the edges. Factorization of the kernel matrix is performed by the combination of Padé approximation and the pole removal technique. This procedure is used to obtain numerical results. The results indicate that the diffracted acoustic field may be significantly intensified due to scattering of hydrodynamic instability waves into sound waves provided that the trailing edge is close enough to the vortex sheet. Similar mechanism may be responsible for the intensification of jet noise near a wing.

Jet and jet–wing noise modelling based on the CABARET MILES flow solver and the Ffowcs Williams–Hawkings method:

A co-axial subsonic unheated jet with and without a swept lifting wing at free-stream conditions from a recent jet–wing TsAGI experiment is considered. For computational modelling, Monotonically Integrated Large Eddy Simulations (MILES) are conducted based on the CABARET scheme which is implemented in a modern parallel unstructured-grid compressible Navier–Stokes computational code. The computational domain of the installed configuration includes a part of the nozzle and a wing section with round flow-fences present in the experiment to preclude the downwash effects. For the isolated jet, the same size of the computational domain is applied and two grid resolutions are considered to investigate the sensitivity of the current far-field noise predictions to the computational grid. The meanflow velocity profiles predicted downstream of the nozzle are compared with the flow data available. For far-field acoustic predictions, the Ffowcs Williams-Hawkings (FW-H) integral method is used. The Ffowcs Williams-Hawkings solutions correspond to a large closed permeable control surface with multiple closing disks at the outlet side set-up in accordance with the best practice to avoid pseudo sound in the acoustic modelling. The comparison of the acoustic predictions with the far-field spectra measured in the experiment for 30° and 90° observer angles to the jet flow are presented and discussed. It is shown that the current modelling robustly captures the same relative trends of the spectra behavior as observed in the experiment: while the presence of the wing does not lead to any significant change of sound spectrum in comparison with the isolated jet for 30°, there is a 8 - 10 dB sound amplification due to the jet–wing interaction at 90° angle to the jet.