Subsonic Free Jet Research Articles

Turbulent jet simulation using high-order DG methods for aeroacoustic analysis

This work deals with the application of a high-order discontinuous Galerkin (DG) method to predict the noise generated by a subsonic free jet at high Reynolds number ReD=106, based on the nozzle diameter. The large-eddy simulation (LES) technique is used to compute the turbulent flow dynamics and acoustics in the near field. The estimation of the far-field noise is made by means of the Ffowcs Williams and Hawkings surface integral formulation. A fourth-order DG simulation of this jet configuration is presented in this paper. The DG simulation is based on the Smagorinsky model and the computational grid employed is fully tetrahedral. This DG simulation is compared to an LES computation performed in previous work by means of a second-order finite-volume (FV) solver using a similar type of grid and SGS model. The results from the DG and FV computations are also compared to the available experimental data. It appears from this study that the DG computation is able to resolve higher-frequency components close to the nozzle exit, which is related to the better match found with the experimental data in the far field.

Experimental study of turbulent-jet wave packets and their acoustic efficiency

This paper details the statistical and time-resolved analysis of the relationship between the near-field pressure fluctuations of unforced, subsonic free jets (0.4 ≤ M ≤ 0.6) and their far-field sound emissions. Near-field and far-field microphone measurements were taken on a conical array close to the jets and an azimuthal ring at 20∘ to the jet axis, respectively. Recent velocity and pressure measurements indicate the presence of linear wave packets in the near field by closely matching predictions from the linear homogenous parabolized stability equations, but the agreement breaks down both beyond the end of the potential core and when considering higher order statistical moments, such as the two-point coherence. Proper orthogonal decomposition (POD), interpreted in terms of inhomogeneous linear models using the resolvent framework allows us to understand these discrepancies. A new technique is developed for projecting time-domain pressure measurements onto a statistically obtained POD basis, yielding the time-resolved activity of each POD mode and its correlation with the far field. A single POD mode, interpreted as an optimal high-gain structure that arises due to turbulent forcing, captures the salient near-field–far-field correlation signature; further, the signatures of the next two modes, understood as suboptimally forced structures, suggest that these POD modes represent higher order, acoustically important near-field behavior. An existing Green's-function-based technique is used to make far-field predictions, and results are interpreted in terms of POD/resolvent modes, indicating the acoustic importance of this higher order behavior. The technique is extended to provide time-domain far-field predictions.

An experimental study on the near flow field characteristics of non-circular jets

Subsonic turbulent free jet, issuing from a lobed contoured nozzle in to quiescent air is investigated experimentally. Results are compared with a cir ular jet from a nozzle of the same exit area and same contraction profile. Mean flow characteristics, turbulence intensities and Reynolds shear stresses in the near field region are investigated by using Hot-wire Anemometry. An overall decrease in turbulence intensities and enhanced flow entrainment in the near field of lobed nozzle are observed.

Computational study of horizontal subsonic free jets of hydrogen: Validation and classical similarity analysis

Computational fluid dynamic simulations were performed using the commercial program Fluent in order to study the hydrogen concentration decay along the centerline of free horizontal subsonic jets. The hydrogen jets were assumed to be steady and turbulent. To explore the buoyancy effect on the hydrogen dispersion processes, the Froude numbers were varied from 250 to 1000. Zero gravity jets were also simulated and analyzed in order to determine the jet momentum dominated limit. Classical similarity analysis was performed and our results were compared to the results of vertical axisymmetric free jets from the literature.

Noise, Turbulence, and Thrust of Subsonic Freejets from Lobed Nozzles

A study of noise benefit, vis-a-vis thrust penalty, and its correlation to turbulence intensities was conducted for free jets issuing from lobed nozzles. Four convergent nozzles with constant exit area were used in the experiments. Three of these were of rectangular lobed configuration having six, ten and fourteen lobes; the fourth was a circular nozzle. Increasing the number of lobes resulted in a progressive reduction in the turbulence intensities as well as in the overall radiated noise. The noise reduction was pronounced at the low frequency end of the spectrum. However, there was an increase in the high frequency noise that rendered the overall benefit less attractive when compared on a scaled-up A-weighted basis. A reduction in noise was accompanied by a commensurate reduction in the turbulent kinetic energy in the flow field. As expected, increasing the number of lobes involved progressive reduction in the thrust coefficient. Among the cases studied, the six-lobed nozzle had the optimum reduction in turbulence and noise with the least thrust penalty.

Near field dynamics of subsonic free square jets. A computational and experimental study

Results of a combined numerical and experimental investigation of the near field of low-subsonic air square jets are presented. The study focuses on examining the role of initial conditions and other features of the jet dynamics in determining the nature and frequency of occurrence of axis switching and the related mechanisms which enhance entrainment, mixing, and turbulence production. Three different experimental square jet facilities were utilized, including orifice jets with low and high initial turbulence level, and pipe jets. Unsteady, spatially developing jets were investigated computationally using direct and monotonically-integrated large-eddy simulation approaches, and appropriate inflow/outflow boundary conditions. Insight on the axis-switching process was obtained using the detailed database from the simulations to investigate how the unsteady vorticity dynamics reflects on the time-averaged properties of the jet cross sections. The different experimental jets were chosen such that important parameters affecting the initial conditions could be tested. Depending on the particular initial conditions of the subsonic jets studied, several or no axis switchings were observed in the first few diameters of jet development. Observed trends reported, include the effects of initial conditions such as, ratio of equivalent diameter to characteristic-momentum-thickness, turbulence level, nonuniform azimuthal momentum-thickness distributions, and Reynolds number.

Analysis of the two-phase flow in a de laval spray nozzle and exit plume

One method used in spray forming and coating technology involves transonic/supersonic gas-droplet two- phase flows through a de Laval nozzle and subsequent subsonic freejet flow from the nozzle to the sprayed surface. To the first- order approximation, this complex phenomenon can be treated in a quasione-dimensional manner to simulate the entire converging- diverging nozzle flow field (with particle injection at the throat) as well as the plume (freejet) region. The basic numerical technique and computer model solve the steady gas field equations through a conservative variable approach and treat the droplet phase in a Lagrangian manner, with full aerodynamic and energetic coupling between the droplets and the transport gas handled via source terms. These analyses are simple and economical to execute. The one- dimensional models are valuable in constructing algorithms for automated process control. Finally, these one- dimensional models give direction to two- and three- dimensional simulations and serve as a test bed for models based on particle dynamics and energetics.

Nonlinear dynamics of forced transitional jets: periodic and chaotic attractors

Conclusive experimental evidence is presented for the existence of a low-dimensional temporal dynamical system in an open flow, namely the near field of an axisymmetric, subsonic free jet. An initially laminar jet (4 cm air jet in the Reynolds number range 1.1 × 104[Lt ]ReD× 9.1 × 104) with a top-hat profile was studied using single-frequency, longitudinal, bulk excitation. Two non-dimensional control parameters – forcing frequencyStD(≡fexD/Ue, wherefezis the excitation frequency,Dis the jet exit diameter andUeis the exit velocity) and forcing amplitudeaf(≡u’f/Ue, whereu’fis the jet exit r.m.s. longitudinal velocity fluctuation at the excitation frequency) – were varied over the ranges 10-4<af< 0.3 and 0.3 <StD< 3.0 in order to construct a phase diagram. Periodic and chaotic states were found over large domains of the parameter space. The periodic attractors correspond to stable pairing (SP) and stable double pairing (SDP) of rolled-up vortices. One chaotic attractor, near SP in the parameter space, results from nearly periodic modulations of pairing (NPMP) of vortices. At large scales (i.e. approximately the size of the attractor) in phase space, NPMP exhibits approximately quasi-periodic behaviour, including modulation sidebands around ½fexinu-spectra, large closed loops in its Poincaré sections, correlation dimensionv∼ 2 and largest Lyapunov exponent λ1∼ 0. But investigations at smaller scales (i.e. distances greater than, but of the order of, trajectory separation) in phase space reveal chaos, as shown byv> 2 and λ1> 0. The other chaotic attractor, near SDP, results from nearly periodic modulations of the first vortex pairing but chaotic modulations of the second pairing and has a broadband spectrum, a dimension 2.5 [Lt ]v[Lt ] 3 and the largest Lyapunov exponent 0.2 [Lt ] λ1[Lt ] 0.7 bits per orbit (depending on measurement locations in physical and parameter spaces).A definition that distinguishes between physically and dynamically open flows is proposed and justified by our experimental results. The most important conclusion of this study is that a physically open flow, even one that is apparently dynamically open due to convective instability, can exhibit dynamically closed behaviour as a result of feedback. A conceptual model for transitional jets is proposed based on twodimensional instabilities, subharmonic resonance and feedback from downstream vortical structures to the nozzle lip. Feedback was quantified and shown to affect the exit fundamental–subharmonic phase difference ϕ – a crucial variable in subharmonic resonance and, hence, vortex pairing. The effect of feedback, the sensitivity of pairings to ϕ, the phase diagram, and the documented periodic and chaotic attractors demonstrate the validity of the proposed conceptual model.

Une approche stochastique pour la modélisation du bruit des écoulements turbulents

A new approach to noise modeling for free turbulent flows is presented. A set of equations is derived from the acoustic variables. In the left-hand side of this system, one finds the linearized Euler equations, whereas the right-hand side exibits source terms. These source terms are modeled in using a stochastic space-time description of the three-dimensionnal turbulent motion. The linearized Euler equations including a stochastic description of noise sources are solved numerically. The Stochastic and Noise Generation (SNGR) model is applied to a subsonic free jet. The weak interaction between the stationnary mean flow and the acoustic field is taken account. The limitations of the SNGR model lie in the statistical properties of the synthetic turbulent field and in the use of an axisymmetric modeling of the acoustic propagation

Numerical Analysis of Gas-Particle Two-Phase Subsonic Freejets

This paper describes a numerical analysis of gas-droplet two-phase subsonic free jets in the axisymmetric system. Thermal coupling through heat transfer to droplets, as well as momentum coupling through aerodynamic drag responsible for droplet motion, is taken into account in the present numerical model. The Navier-Stokes equations for a gas-phase interacting with particle phase are solved by a time-dependent difference technique and the particle-phase is solved by a discrete particle cloud model. The jet flow structures of mixture composed of air and water-droplets with 1 μm, 5 μm, and 30 μm, respectively, in diameter are calculated with a single particle size. Some of significant characteristics for the two-phase subsonic free jets are pointed out, in particular, focusing upon the effect of particle size on the flow structure.

Temperature measurements on an air plasma jet in an induction plasmatron at reduced pressures

Temperature measurements are reported for a subsonic free air-plasma jet derived from a VGU-2 plasmatron in the pressure range 5.103 ⩽ P ⩽105 Pa.

Observation of laser light scattering by a turbulent gas without seeding

An experiment of laser light scattering by a turbulent highly subsonic free jet of air is presented. The originality of the method lies in the fact that no particle seeding is needed. The experimental scattering cross section, which can be interpreted from the three-dimensional spectral density of temperature fluctuations, is compared to the theoretical one obtained by turbulent mixing theories. The scattered field time correlations measured in the heterodyne mode are interpreted by considering the Doppler effect on moving turbulent eddies and compared with theory. The intensity time correlations measured in the homodyne mode are interpreted in terms of local properties of the turbulence.

Coherent structures in the mixing zone of A subsonic hot free jet

The existence of a large scale structure of turbulence in a free hot jet is emphasized by measurements carried out on the initial assumption of a fine scale structure. Second order analysis allows one to split the signals picked up in the jet and in the acoustic field into coherent and non-coherent components. The validity of this decomposition with regard to the physical picture of the phenomenon, i.e. vortices convected in the mixing zone, is tested by means of conditional sampling analysis, applied for several diagnostics.

Probe Blockage Effects in Free Jets and Closed Tunnels

The effect of probe blockage on the free-stream pressure and Mach number has been studied for the two limiting cases of open free jets and closed tunnels. Cylinder probe calibrations carried out in subsonic free jets show that the blockage effect is much greater than previous analytical solutions predict. The blockage effect in free jets was found to be of approximately the same magnitude as in closed tunnels. Generalized blockage corrections are derived which indicate the importance of minimizing blockage effects both when calibrating a probe and when using it to make pressure measurements, especially in the transonic flow regime.

Flow near the stagnation point of an obstacle washed by a two-dimensional subsonic stream

An investigation is made of two-dimensional steady flow of a viscous incompressible fluid in the vicinity of the stagnation point of an infinite step, in flow of a subsonic free jet. The velocity profile of the undisturbed flow far from the step takes the form of a power series in the coordinate along the step surface. Results of numerical solution are presented.

Pressure Cross Correlations in the Investigation of Aerodynamic Noise from Jets

The employment of pressure cross correlations to investigate the noise-generating regions of jets is described. The use of this technique makes it possible to quantitatively map the noise-generating regions of a jet for a particular farfield angular position. The results of experiments, utilizing this method, on a subsonic free jet are presented. These results indicate that the turbulent volume mainly responsible for the noise generated is confined to a rather limited cylindrical volume centered about the jet axis in the vicinity of the end of the potential core region with a length of approximately six jet diameters. In addition, for the higher farfield angles the shear mixing region becomes an important factor. As the jet velocity was reduced, this volume showed a tendency to contract toward the jet nozzle with the shear mixing region showing increase contribution to the sound pressure at all farfield angles. A comparison of these data is made with the results of a similar experiment conducted on a General Electric T-58 gas generator modified to a turbojet configuration.

Predicted air entrainment by subsonic free round jets.

Predicted air entrainment by subsonic free round jets.

Jet Noise : 1st Report, Far Noise Field of Subsonic Free Air Jet

It is the purpose of the present investigation to theoretically indicate the relation between the far-noise-field of subsonic-free-air-jet and the flow-field and make clear the characteristics of the acoustic field of jets on the basis of experiment. The noise intensity depends upon the turbulent velocity correlations and its derivative correlations. The noise emitted in the jet-axis direction is mainly contributed to fluctuations parallel to the axis with lower frequency and that radiated in the direction perpendicular to the axis is determined by those perpendicular, with higher frequency. The eddy convection effect is to make spectra move towards the lower frequency side, which is proportional to ω-4 in higher frequency and to ω2 in lower. The acoustic intensity of higher frequency increases proportionally to y1-3 in the mixing region and to y1-10 in the turbulent region and of lower, to y1+3 and to y1-4 respectively, that is, the higher frequency noise is generated mainly near the nozzle exit and the lower, in the transition region.

On Energy Separation by Aerodynamic Processes

Expressions for Damping Matrices in Linear Vibration Problems P. LANCASTER 256 F RECENT CONTRIBUTIONS to the Readers' Forum have given further consideration to the heating effects in the Hartmann generator which were first investigated by Sprenger. Interest seems now to be concentrated on certain unsteady-flow phenomena associated with supersonic exciting jets: alternating swallowing and spilling of the jet accompanied by the movement of pressure pulses out of and into the resonator. Recent attempts to analyze the heating process' 4 have used models based on these unsteady phenomena and have hinged on the question of whether or not the pressure pulses steepen into shocks. I t seems judicious to recall at this time that Sprenger found very large heating effects for subsonic jets also—provided that turbulence was introduced into the jet. (He used a fine nylon thread across the nozzle to accomplish this.) This subsonic heating was also observed by Sibulkin and Vrebalovich. I t seems quite impossible to explain it using the shock-tube theories presently being formulated. Sprenger found too that the heating produced within a resonance tube by supersonic jets was increased (by a factor as great as four) by the introduction of turbulence. Another aerodynamic-heating effect was reported in the Readers' Forum by Glassman and John. They discovered that the total temperature increased along the center line of both supersonic and subsonic free jets, the ambient and jet total temperatures being equal. The rise in temperature began only at the end of the potential core of the jet. The nozzle used in the supercritical case was designed to produce a shockless jet.