Subsonic Turbulent Jets Research Articles

Large Eddy Simulations of Sound Radiation from Subsonic Turbulent Jets

Large eddy simulations (LES) of subsonic turbulent jets, including the near-field flow and far-field sound radiation, were performed using two different subgrid-scale turbulence models. Two different cases were considered. As a validation case, predictions from a Mach 0.9, Reynolds number 3.6 × 10 3 randomly forced turbulent jet were performed. Both the near-field velocity statistics and far-field sound directivity were found to be in excellent agreement with previous experimental data and direct numerical simulation (DNS) results. The second case involved a Mach 0.4, Reynolds number 5 x 10 3 harmonically forced jet. Both axisymmetric and azimuthal inlet disturbances were imposed to facilitate detailed comparisons to recent DNS of axisymmetric jet sound, and to highlight the effects of small-scale turbulence. As part of this second case, the validity of Kirchhoff's method for far-field sound prediction was also assessed in conjunction with LES and the effect of the subgrid-scale turbulence model on sound radiation was examined. The sound source location and levels were different between the three-dimensional turbulent jets and similar two-dimensional axisymmetric laminar jets. Far-field sound radiation predictions, obtained using Kirchhoff's method, were in good agreement with the directly predicted LES results. LES predictions obtained with the dynamic Smagorinsky and the dynamic mixed subgrid-scale turbulence models were similar, although the mixed model resulted in higher turbulence and sound levels. The directivity was not uniform as predicted by Lighthill's theory for a fully turbulent jet, but exhibited a preferred radiation angle between 35 and 70 deg in agreement with reported experimental data from an acoustically excited jet.

Large eddy simulations of sound radiation from subsonic turbulent jets

Large eddy simulations (LES) of subsonic turbulent jets, including the near-field flow and far-field sound radiation, were performed using two different subgrid-scale turbulence models. Two different cases were considered. As a validation case, predictions from a Mach 0.9, Reynolds number 3.6 × 10 3 randomly forced turbulent jet were performed. Both the near-field velocity statistics and far-field sound directivity were found to be in excellent agreement with previous experimental data and direct numerical simulation (DNS) results. The second case involved a Mach 0.4, Reynolds number 5 x 10 3 harmonically forced jet. Both axisymmetric and azimuthal inlet disturbances were imposed to facilitate detailed comparisons to recent DNS of axisymmetric jet sound, and to highlight the effects of small-scale turbulence. As part of this second case, the validity of Kirchhoff's method for far-field sound prediction was also assessed in conjunction with LES and the effect of the subgrid-scale turbulence model on sound radiation was examined. The sound source location and levels were different between the three-dimensional turbulent jets and similar two-dimensional axisymmetric laminar jets. Far-field sound radiation predictions, obtained using Kirchhoff's method, were in good agreement with the directly predicted LES results. LES predictions obtained with the dynamic Smagorinsky and the dynamic mixed subgrid-scale turbulence models were similar, although the mixed model resulted in higher turbulence and sound levels. The directivity was not uniform as predicted by Lighthill's theory for a fully turbulent jet, but exhibited a preferred radiation angle between 35 and 70 deg in agreement with reported experimental data from an acoustically excited jet.

Sound generation by interacting vortices in a subsonic turbulent jet

The results of an experimental investigation of sound generation by the interacting toroidal and oblique vortices formed in a subsonic turbulent jet under the action of saw-tooth finite-amplitude sound waves with simultaneous longitudinal internal and transverse external excitation are presented (the sound pressure level on the jet edge is 165–175 dB). The direct schlieren method with an exposure time −3·10−7 s was employed. This made it possible to visualize not only the jet and the vortices formed in it but also the sound waves. It was confirmed experimentally that the vortex interaction may be accompanied by sound generation of fairly high intensity.

Numerical Investigation of the Noise Radiated from Hot Subsonic Turbulent Jets

Experimental studies have highlighted the existence of additional sources of sound due to temperature e uctuations in heated jets, in comparison with unheated jets. Whereas cold jets have been the subject of many numerical investigations, little research has been devoted to the topic of hot jets. Thus, a specie c model to investigate the acoustic radiation from these jets based on numerical predictions using a k‐ turbulence closure is attempted. First, a model for the additional source term is suggested, which is a function of mean axial velocity, mean temperature, and turbulent kinetic energy. Next a computation model of acoustic intensity spectrum is developed in which three different contributions appear: The e rst represents the contribution of the velocity e uctuations in the sound emission, the second that of the temperature e uctuations, and the third a mixed term issuing from the two preceding contributions. Then the model is applied to compute the acoustic radiation of hot jets. Results provided quite a full description of the acoustic features of a hot jet: spectrum shape, radiated acoustic intensity levels, and ine uence of jet temperature. Finally, the model seemed capable of predicting the trends in noise radiation of turbulent hot jets correctly.

Quantitative visualization of high-speed 3D turbulent flow structures using holographic interferometric tomography

Using holographic interferometry the three-dimensional structure of unsteady and large-scale motions within subsonic and transonic turbulent jet flows has been studied. The instantaneous 3D flow structure is obtained by tomographic reconstruction techniques from quantitative phase maps recorded using a rapid-switching, double reference beam, double pulse laser system. The reconstruction of the jets studied here reveal a three-dimensional nature of the flow. In particular an increasing complexity can be seen in the turbulence as the flow progresses from the jet nozzle. Furthermore, a coherent three-dimensional, possibly rotating, structure can be seen to exist within these jets. The type of flow features illustrated here are not just of fundamental importance for understanding the behavior of free jet flows, but are also common to a number of industrial applications, ranging from the combustion flow within an IC engine to the transonic flow through the stages of a gas turbine.

Numerical investigation of the noise radiated from hot subsonic turbulent jets

Numerical investigation of the noise radiated from hot subsonic turbulent jets

Disturbance generation in a subsonic turbulent jet under the influence of high-intensity sound

The effect of high-intensity sound (L=140–160 dB) on a subsonic turbulent air jet under transverse external irradiation was studied experimentally. It is shown that the onset and development of disturbances in the early stage does not depend on the excitation frequency, while the intensity of the jet mixing is determined in large measure by the distance between the sound-generated vortices and by the degree of interaction between them.

Formation of zinc particles in the 5 to 50 NM size range in a subsonic turbulent jet

Formation of zinc particles in the 5 to 50 NM size range in a subsonic turbulent jet

Physical characteristics of subsonic jets in a cross-stream

This paper deals with local flow characteristics of subsonic turbulent jets in the presence of a cross-flow. For the various types of jet considered (a cylindrical jet and coaxial jets) the experimental results concern the axes and the velocity profiles in the plane of symmetry of the flow. In the case of the cylindrical jet, the shape of the universal axial velocity profile is defined, as are the law of velocity decay along the axis and the laws of variation of the thicknesses of the jet. Finally, the existence of a link between the axis equation and the law of axial velocity decay in the zone of similarity of the velocity profiles is established.

Escape of a turbulent jet under the effect of acoustic perturbations

Results are presented of experimental investigations of the local effect of acoustic oscillations of different frequency and constant intensity on the root part of a nonisothermal axisymmetric subsonic turbulent jet escaping from a gas jet atomizer at a different velocity in the S = 0.053–3.84 range of Strouhaille numbers. Data have been obtained indicating the presence of unstable escape modes of a subsonic turbulent jet in an acoustic field; experimental dependences are presented of the relative aperture of the turbulent jet flowing in an acoustic field as a function of various parameters.

Noise intensity in the field of a subsonic turbulent jet

Results are shown of digital-computer calculations pertaining to the acoustic properties of subsonic jets, by a method proposed earlier in [1, 2]. The gasdynamic and turbulence parameters of a jet, needed for calculating the acoustic radiation, have been determined before-hand.

Acoustic effect on the root part of a turbulent jet

Results are presented of experimental investigations of the effect of low-frequency acoustic perturbations of different frequency and constant intensity on the root part of a nonisothermal subsonic turbulent jet escaping from a direct-jet injector with a conical diffusor in the 0.031–0.054 range of Strouhal numbers. Experimental dependences of the mean velocity and the longitudinal intensity of the turbulence are presented as a function of different parameters for both the unperturbed flow and for the flow in the acoustic field.

Space correlations of the fluctuating pressure in subsonic turbulent jets

The turbulence structure of a circular air jet was studied by correlating both full- and narrow-band signals from locally displaced microphone probes. A high degree of statistical interdependence was found for the pressure in planes normal to the axis, and in particular for frequency components at St = 0·5 where the bulk of the fluctuating near-field energy is concentrated. Thus the axisymmetric contents of the turbulence travels downstream in an almost deterministic wave-like manner. With the tuburlence thus having been found to be anything but isotropic, some of the assumptions common in jet-noise theory were re-examined and an alternative model is suggested in this paper. The continuing work was stimulated by, and may contribute to, the controversy about the degree of order in jet turbulence as disputed at the aerodynamic noise symposium at Loughborough University in 1970 (see reference [24]).

Measurement of pressure fluctuations within subsonic turbulent jets

The possibilities of and limitations on direct fluctuating pressure measurements in turbulent flows are discussed. Insertion of pressure probes into the flow gives rise to several error mechanisms, some of which are treated analytically here. Experiments with standard condenser microphones were confined here to the so-called potential core of circular air jets. In connection with earlier publications, further support is deduced that pressure measurements can help to extend our understanding of the turbulence structure in a number of flows where hitherto mainly hot-wire techniques were applied.

Acoustic emission from subsonic turbulent jets

A method is proposed for determining the noise intensity of subsonic jets. The results are given from a calculation of the far-field intensity level of a jet for various Mach numbers at the nozzle orifice. The results of the calculation are compared with the experimental data.

Fluctuation Intensity of Passive Species in a Turbulent Subsonic Jet

The equation for the turbulent fluctuation intensity of a passive scalar for incompressible shear flow was applied to a subsonic circular jet, using phenomenological diffusivities, for both the diffusion of the scalar mean and mean square fluctuation of the scalar, to determine the values of these diffusivities which give the best agreement between the calculated self-similar radial profile of the fluctuation intensity and experimental measurements. From experimental values in smoke jets and heated jets of the ratio of the fluctuation intensity to mean on the centerline and their radial profiles, it appears that the turbulent diffusivity for the mean square fluctuations is approximately equal to the turbulent diffusivity for the mean values of the scalar. The coefficient for turbulent dissipation which was treated as an eigenvalue in the calculation is in good agreement with the expected value.

Conversion of Lighthill's Equation on Jet-Noise Generation to an Impedance Concept

M. J. Lighthill derived an equation of proportionality for the generation of sound by subsonic turbulent jets. This question is modified to an impedance form by extracting thrust, impedance, and nondimensional parameters. The equation is dependent upon information that is generally available to engineers for any particular jet engine. The equation also is modified empirically for use with supersonic jets. Some dimensionless jet-noise spectra are extrapolated into a frequency-dependent equation that is substituted into the modified Lighthill's equation. The result is a prediction method for the acoustic power spectrum at its source.

The Generation of Sound by Aerodynamic Means

SummaryA summary is given of some of the more important experimental results relating to the noise radiated from a cold subsonic turbulent jet. These are then related to the predictions of Lighthill's general theory of aerodynamic noise.