Outer Mixing Region Research Articles

AN ANALYSIS ON COAXIAL JET FLOWS USING DIFFERENT DECOMPOSITION TECHNIQUES

In this study, the Fourier transform, wavelet transform and turbulence filter methods have been applied to coaxial jet flows for various downstream positions in the inner and outer mixing regions. The data were obtained from cross-wire measurements, so axial and radial velocity components were acquired. Attention is focused on the characteristics of these signals in the initial region of the jet-flow field. The effects of some basic mechanisms of the vorticity dynamics on the velocity time histories, and on different statistical and spectral quantities, are studied.

Effects of Forced Excitation on Double Coaxial Pipe Jets.

Circular coaxial jets have been experimentally studied, and the outer annular jet is excited by the sound wave from speakers. We have measured velocity, turbulent intensity distributions and the power spectra of the fluctuating velocity of the coaxial jets for different configurations of the outer nozzle with a pipe length of L/D0=0 to 5 and for various values of the velocity ratios of inner jet to outer jet, in order to examine the effects of excitation. For the nozzles with the pipe length of L/D0≥2 and the forcing frequency of f0>200Hz, the recirculating region shrinks and moves upstream. The intensity of fluctuating velocity in the outer mixing region for the jets of L/D0=0 and 0.5 increases at f0=300∼400 Hz. It is confirmed that the increase of turbulence intensity is related to that of the fundamental frequency component. For the jet of L/D0=1, turbulence suppression is observed at the forcing frequency of about 600 Hz. It is found that the suppression effect is a consequence of earlier transition of the shear layer vortices. For the nozzle with L/D0=2, the vortex pairing event has been recorded through phase-locked measurements at the forcing frequencies of about 300∼400 Hz. The velocity fluctuation intensity is confirmed to be enhanced by vortex pairing.

Coherent structure interactions in an unexcited coaxial jet

Based on a double triggering criterion, the axisymmetric and azimuthal structures within the initial region of an unexcited coaxial jet of mean velocity ratio of 0.3 (inner to outer) were recovered. Outer vortices, two trains of inner vortices and their amalgamation are found in the outer and inner mixing regions. Bifurcation of the amalgamated inner vortices occurs within the first two diameters of the jet and is responsible for the occurrence of the azimuthal structures in the inner jet region further downstream. Co-dominance of the symmetrical and azimuthal structures having different convection velocities is observed in the fully merged zone. Spatial separation between vortical structures and their relative vorticities are important parameters in affecting their convection velocities, growth and decay.

Study on Double Coaxial Pipe Jets. 2nd Report. Vortex Formation in Nozzle.

Flow of double coaxial pipe jets which consist of a central round air jet (an inner jet) and an annular air jet (an outer jet) has been experimentally studied. Several nozzles with outer pipe lengths of L/D0=0 to 2 and different widths B of the outer annular jet were employed to examine the effects of self-excitation by the outer pipe. The strong intensity of fluctuating velocity appears in the nozzle with the pipe lengths of L/D0=0. 5 to 1. The characteristics of the jet have great resemblance to those of a self-excited axisymmetric jet driven by a whistler nozzle (i. e., pipe-collar). It is found that the values of the Strouhal number StL based on the pipe length L are constant, and that these values increase in a stepwise manner to the values equal to integral multiples, with further increase in the L/Bratio. The results of the measured distributions of vorticity and the flow visualization show the coherent structures in the inner and outer mixing regions of the jets. It is confirmed that the strong intensity of fluctuating velocity is related to the subharmonic component by vortex pairing.

Flow structures in initial region of two interacting parallel plane jets

The initial region of two interacting parallel plane jets with a truncated bluff body at the plane of symmetry is studied with velocity measurements in the frequency and time domains. The mean flows of the two plane jets are nearly parallel to the plane of symmetry, and their inner and outer mixing regions are found. Within these mixing regions their respective trains of coherent structures, which are of vortical form, are established. The inner vortices are inwardly rotational, while the outer vortices rotate outwardly. The successive initial vortices in the inner mixing region seem to undergo two processes of either pairing or amalgamation. The former results in the formation of a nearly circular coherent structure, while the latter results in the elongated structure. Both the inner and other structures experience fairly rapid decay. The process of decay also seems to be either the usual decay or the division of the coalesced structure back into the individual vortical structures followed by their decay.

Flow structures of coaxial jet of mean velocity ratio 0.5

THE presence of two trains of coherent structures in the inner and outer mixing regions of homogeneous coaxial jet of mean velocity ratio X (outer to inner) greater than unity has been established.-'2 At mean velocity ratio X of 0.5, both the inner and outer structures were observed to undergo substantial development. The present investigation was intended to study the development of the two coherent structures and their possible mutual interaction in a coaxial jet of X = 0.5. The experimental techniques employed included conditional samplings of both structures and an azimuthal mode expansion scheme on the outer structures.

Coaxial jets of different mean velocity ratios, part 2

This second part of the investigation presents the flow characteristics of coaxial jets of mean velocity ratios (inner to outer) 0·15<λ−1<1. An attempt to correlate data between coaxial jets and its limiting cases of a single jet and an annular jet has been made. The flow structures originating from each of the mixing regions are investigated. For the coherent structure in the outer mixing region a comparison with that of the single jet is attempted. For the inner coherent structures (identified as coflowing-wake-vortices), investigations into their dependence on λ−1 is also made. The inner potential core length, and hence the elongation function, is shown to play an important role in correlating data from coaxial jets of different λ−1. At low λ−1, coalescence of the coflowing-wake-vortices and their evolution into another train of vortices that resembles the wake-vortices found in the annular jet emphasizes the role of the coaxial jet as an intermediary between the single jet and the annular jet.

Flow structures of a basic annular jet

This paper describes the modal expansion and conditional sampling results of static pressure fluctuations in the inner and outer mixing regions of a basic annular jet. An understanding of the mechanism of the wake shed- ding process from the internal recirculating region behind the blunt interface is attempted by conditionally sampled pressure signals. The evolution and characteristics of the shedding wake, wake-induced vortices, and jet vortices in both shear layers are measured and presented. The evolution of the wake-induced vortices in the outer shear layer as a result of excitation by the disturbances associated with the shedding wakes is further supported. In the case of the basic annular jet, another type of coherent structure was found in the outer mixing region, in addition to those associated with the outer jet vortices.5 This additional structure, termed the wake-induced vortex, seemed to be the consequence of the presence of the internal recirculating region and the shedding of the standing vortices or the wake vortices downstream of the blunt interface.5-6 Both the outer jet vortices and wake-induced vortices have, besides the axisymmetrical mode constituents, azimuthal constituents. The constituent of the first mode could be as dominant as that of the axisymmetrical.7 The existence and dominance of the wake-induced vortices depended on the diameter ratio D//Z>0. For large diameter ratios, the wake- induced vortices seemed to be more dominant. The investigation of the influence of the shedding wakes and their associated disturbances on the initial outer shear layers and the subsequent evolution of the induced structures may help to further clarify the phenomena of shear layer ex- citation and large-scale structure dynamics which are fun- damental in the study of turbulent free shear flows. Moreover, the inner region resembles that of a near wake behind an axisymmetric blunt body. Similar studies are rare and the ones reported are limited primarily to the case of a disk in cross flow. The present investigation was an attempt to understand the mechanism of the wake shedding process from the inter- nal recirculating region in the near wake of the blunt inter- face and the mode of evolution of the wake-induced vortices in the outer mixing region. This paper documents the results and interpretations based primarily on a mode expansion technique and a time domain analysis on the static pressure fluctuations. Measurements of the velocity fluctuations are being carried out and will be reported separately.

Pressure Measurements of Coaxial Jet of High Mean-Velocity Ratio

This paper describes an experimental investigation of the initial region of a subsonic cold coaxial jet at a mean-velocity ratio λ, outer to inner, of 1.25. Detailed measurements in the initial region have shown that similarity of the pressure intensity profiles exists in the three zones: the initial merging zone, the intermediate zone and the fully merged zone. Spectral measurements of the pressure fluctuations confirm the existence of coherent structures in the outer mixing region. Comparison of the coaxial jet results with those of the single jet has been attempted.

Cross-correlation of velocity and temperature in a premixed turbulent flame

Gradient models of turbulent transport have been widely used without justification in the mathematical modeling of the turbulent flame. The velocity and the temperature of a turbulent premixed flame were measured simultaneously by a laser Doppler anemometer and a compensated fine thermocouple, and their cross-correlation was calculated with a microcomputer. A turbulent heat transport vector was estimated from the measured values in the whole region of the flame. It is directed from the low-temperature side to the high-temperature side in the turbulent reaction zone, which is opposite to the direction expected by the usual gradient transport model and proves the occurrence of the unusual countergradient transport of heat. In the mixing region outside and downstream of the flame it is directed roughly from the high to the low temperature side. However, it was not always transverse to the isotherms but almost parallel in the region near the burner exit, which may suggest that the usual gradient transport assumption is also invalid in the mixing region. Sources of the cross-correlation in both regions were not ascertained from the experimental results, but it was guessed that they may be different between the reaction zone and the outer mixing region.

Experimental investigation of turbulent walljets in the presence of adverse pressure gradients in a rectangular diffuser

An experimental study of wall static pressure distributions and mean velocity profiles along a duct and diffuser downstream of wall-jet injection was conducted over a range of diffuser total angles from 15 to 40° at injection to core flow mass flux ratios from 0 to 6. Pressure recovery in the diffuser increased with injection ratio and decreased with diffuser total angle. Peak velocities in the wall-jet decayed along the flow and the inner shear layer and outer mixing region grew in thickness along the flow. The inner layer was near similarity condition, but non-similar variations were found in the outer layer. Estimated wall shear stresses depended upon injection mass fluxes, downstream distance and diffuser total angle. Greater decay of peak velocity and larger friction coefficients were found in the diffuser than indicated by correlations from data for a wall-jet without a pressure gradient. At the largest diffuser total angle and the highest injection ratio flow reversal occurred in the core region.

The inner regions of annular jets

Further experiments on the detailed study of annular jets are described in which the mean and fluctuating properties in the inner region have been measured. The experiments in the conical jetf have shown, besides the jet† vortices in the outer mixing region, another train of vortices in the inner region. This train of vortices is due to the wake formed by the boundary layer on the surface of the conical bullet.The experiments in the inner region of the basic annular jet have similar mean velocity profiles to those in the internal recirculating region. Good correlation is found for the location of the vortex centre, the location of reattachment, the minimum and maximum mean static pressure and their locations with the non-dimensional available pressure M°/AiPatm for entrainment behind the interface. A train of wake vortices is generated in the internal recirculating region and a train of jet vortices is found in the inner mixing region. Both types of vortices in the inner region seem to decay fairly rapidly within a distance of one outer diameter D° downstream.The disturbances associated with the wake vortices in the inner region seem to excite the outer mixing region. This results in another wave or train of vortices observed in the outer shear layer in addition to the jet vortices which are already in existence.

Coherent structures in the outer mixing region of annular jets

This paper describes in detail the overall pressure and spectral results in the outer mixing region of three annular jets. The outer mixing region can be considered as the result of the shearing of a single jet by the ambient air. The coherent structures found consist of an array of toroidal jet vortices convecting downstream in the outer mixing region. The structures tend to agree with the ones for a single jet.

The initial region of subsonic coaxial jets

This paper describes part of a detailed study of the initial region of coaxial jets of three different mean-velocity ratios. Similarity of the mean-velocity and turbulent-intensity profiles within the two mixing regions inside the initial merging zone, and within the mixing region inside the fully-merged zone, has been observed. Similarity with single-jet results has been obtained. Overall pressure measurements and hot-wire and microphone spectra inside coaxial jets yield two pronounced peaks, suggesting the existence of two types of vortices at different frequencies. The higher-frequency or primary vortices are found to be generated in the primary or inner mixing region, and the lower-frequency or secondary ones in the secondary or outer mixing region. It seems that the former are generated further upstream than the latter. Both types of vortices are found in the initial merging zone. However, their growth or decay and their dominance depend on the mean-velocity ratio. Low mean-velocity ratios indicate the dominance of the high-frequency vortices. At high mean-velocity ratios, the low-frequency vortices are dominant. The Strouhal number of the vortices plays an important part in their growth and decay. The complicated flow structure of coaxial jets can be very simply related to, and described by, the much simpler structure of single jets.

Preliminary Investigation of the Aeroacoustics of Jets Perturbed by Screens

It is observed that the placement of a screen across the exit plane of a jet nozzle results in a substantial reduction in noise intensity over what would be realized with an equivalent unperturbed jet. The general features of this phenomenon are analyzed experimentally utilizing both acoustic and velocity (hot wire) measurements. Acoustic radiation data from a jet operating in the Much number range 0.2–0.6 indicate that the insertion of a screen into the flow can reduce the maximum intensity by as much as 14 dB. Associated with this noise reduction is a loss of directivity and a substantial flattening of the power spectrum. Comparison with the spectrum of unperturbed jets indicates that the noise radiation from both the mixing and adjustment regions of the jet is affected. Hot wire data indicate that the predominant change in the mixing characteristics occurs at the interface between the potential core and the outer mixing region. Turbulence scale and mean shear are reduced which, on theoretical grounds, is consistent with the acoustic observations.

Turbulence measurements in a cylindrical wall jet

A cylindrical wall jet is obtained when the jet emerges from a converging nozzle and flows along a cylinder placed concentric with the nozzle. The results of hot wire measurements in the developed zone of such a cylindrical wall jet are presented. The results obtained are compared with similar measurements in a free circular jet and in a plane wall jet. The comparisons show the effect of the cylinder in the inner region, but do not reveal any effect of the cylinder in the outer mixing region. Further, it is shown that the currently available method of determining eddy diffusivity distribution in jet flows is misleading.