Dynamics of m = 0 and m = 1 modes and of streamwise vortices in a turbulent axisymmetric mixing layer

2012 ◽  
Vol 709 ◽  
pp. 408-444 ◽  
Author(s):  
S. Davoust ◽  
L. Jacquin ◽  
B. Leclaire
Keyword(s):  
High Speed ◽  
Spatial Organization ◽  
Near Field ◽  
Mixing Layer ◽  
Streamwise Vortices ◽  
Temporal Correlations ◽  
Dynamical Features ◽  
The Mean ◽  
Spatio Temporal ◽  
Proper Orthogonal

AbstractThe near field of a Reynolds number $R{e}_{0} = 2. 14\ensuremath{\times} 1{0}^{5} $ and low-Mach-number cylindrical jet has been investigated by means of a high-speed stereo PIV setup that provides the spatio-temporal velocity field in a transverse plane, two diameters downstream of the jet exit. Proper orthogonal decomposition (POD) and spatio-temporal correlations are used to identify some of the main dynamical features of this flow. We show that the flow is dominated by streamwise vortices whose production and spatial organization can be related to $m= 0$ and $m= 1$ perturbations, and to the mean shear of the mixing layer. A dynamical scenario is proposed which describes this interaction, in accordance with our observations.

Studies on high speed jets from nozzles with internal grooves

2004 ◽  
Vol 108 (1079) ◽  
pp. 43-50 ◽  
Author(s):  
S. Elangovan ◽  
E. Rathakrishnan
Keyword(s):  
High Speed ◽  
Cell Structure ◽  
Pressure Profile ◽  
Equivalent Diameter ◽  
Streamwise Vortices ◽  
Shock Cell ◽  
Underexpanded Jets ◽  
Pitot Pressure ◽  
Mean Pressure ◽  
The Mean

Experiments were carried out on jets issuing from circular nozzles with grooved exits and the results compared with those of the plain nozzle. The plain nozzle had an exit diameter of 10mm. Because of the introduction of semi-circular grooves at the exit, the effective or equivalent diameter of the grooved nozzles was 10·44mm. The groove lengths were varied as 3, 5 and 8mm. The nozzles were operated at fully expanded sonic and underexpanded exit conditions. The corresponding fully expanded Mach numbers were 1·0 and 1·41. The shock cell structure of the underexpanded jets from grooved nozzles appeared to be weaker than that of the plain nozzle, as indicated by lesser amplitudes of the cyclic variation of the Pitot pressure. The iso-Mach contours indicate that the jet spread along the grooved plane is significantly higher than that along the ungrooved plane. Off-centre peaks were observed in the mean pressure profile of underexpanded jets from grooved nozzles. They were probably due to the streamwise vortices shed from the grooves.

ESTIMATION OF CONVECTION SPEED THROUGH PIV MEASUREMENTS IN TURBULENT PLANAR MIXING LAYER

2009 ◽  
Vol 23 (03) ◽  
pp. 353-356
Author(s):  
CHIUAN-TING LI ◽  
KEH-CHIN CHANG ◽  
MUH-RONG WANG
Keyword(s):  
Particle Image ◽  
Mixing Layer ◽  
Temporal Correlation ◽  
Dominant Frequency ◽  
Piv Measurements ◽  
Temporal Correlations ◽  
Taylor’S Hypothesis ◽  
Image Velocimetry ◽  
Spatio Temporal ◽  
Convection Speed

The spatio-temporal correlations in a turbulent planar mixing layer are acquired using the particle image velocimetry. Estimation of convection speed is recommended to be made with the spatio-temporal correlations of fluctuating vorticity. The spatial correlation can be deduced from the temporal correlation through the use of the Taylor's hypothesis when applied to the region without apparent dominant frequency.

On the periodically excited plane turbulent mixing layer, emanating from a jagged partition

2007 ◽  
Vol 589 ◽  
pp. 479-507 ◽  
Author(s):  
E. KIT ◽  
I. WYGNANSKI ◽  
D. FRIEDMAN ◽  
O. KRIVONOSOVA ◽  
D. ZHILENKO
Keyword(s):  
Turbulent Mixing ◽  
Mixing Layer ◽  
Two Dimensional ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Trailing Edge ◽  
Mean Velocity ◽  
Turbulent Mixing Layer ◽  
Arithmetic Average ◽  
The Mean

The flow in a turbulent mixing layer resulting from two parallel different velocity streams, that were brought together downstream of a jagged partition was investigated experimentally. The trailing edge of the partition had a short triangular ‘chevron’ shape that could also oscillate up and down at a prescribed frequency, because it was hinged to the stationary part of the partition to form a flap (fliperon). The results obtained from this excitation were compared to the traditional results obtained by oscillating a two-dimensional fliperon. Detailed measurements of the mean flow and the coherent structures, in the periodically excited and spatially developing mixing layer, and its random constituents were carried out using hot-wire anemometry and stereo particle image velocimetry.The prescribed spanwise wavelength of the chevron trailing edge generated coherent streamwise vortices while the periodic oscillation of this fliperon locked in-phase the large spanwise Kelvin–Helmholtz (K-H) rolls, therefore enabling the study of the inter- action between the two. The two-dimensional periodic excitation increases the strength of the spanwise rolls by increasing their size and their circulation, which depends on the input amplitude and frequency. The streamwise vortices generated by the jagged trailing edge distort and bend the primary K-H rolls. The present investigation endeavours to study the distortions of each mode as a consequence of their mutual interaction. Even the mean flow provides evidence for the local bulging of the large spanwise rolls because the integral width (the momentum thickness, θ), undulates along the span. The lateral location of the centre of the ensuing mixing layer (the location where the mean velocity is the arithmetic average of the two streams,y0), also suggests that these vortices are bent. Phase-locked and ensemble-averaged measurements provide more detailed information about the bending and bulging of the large eddies that ensue downstream of the oscillating chevron fliperon. The experiments were carried out at low speeds, but at sufficiently high Reynolds number to ensure naturally turbulent flow.

The response of a mixing layer formed between parallel streams to a concomitant excitation at two frequencies

2001 ◽  
Vol 441 ◽  
pp. 139-168 ◽  
Author(s):  
MING DE ZHOU ◽  
I. WYGNANSKI
Keyword(s):  
High Speed ◽  
Mixing Layer ◽  
Major Axis ◽  
Velocity Profiles ◽  
Single Frequency ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Coherent Vortices ◽  
The Mean ◽  
Change In Mean

Simultaneous excitation of a turbulent mixing layer by two frequencies, a fundamental and a subharmonic, was investigated experimentally. Plane perturbations were introduced to the flow at its origin by a small oscillating flap. The results describe two experiments that differ mainly in the amplitudes of the imposed perturbations and both are compared to the data acquired while the mixing layer was forced at a single frequency.Conventional statistical quantities such as: mean velocity profiles, widths of the flow, turbulent intensities, spectra, phase-locked velocity and vorticity fields, as well as streaklines were computed. The rate of spread of the flow under concomitant excitation at the two frequencies was much greater than under a single frequency, although it remained dominated by two-dimensional eddies. The Reynolds stresses and turbulence production are associated with the deformation and orientation of the large coherent vortices. When the major axis of the coherent vortices starts leaning forward on the high-speed side of the flow, the production of turbulent energy changes sign (i.e. becomes negative) and this results in the flow thinning in the direction of streaming. It also indicates that energy is extracted from the turbulence to the mean motion. Resonance phenomena play an important role in the evolution of the flow. A vorticity budget showed that the change in mean vorticity was mainly caused by the nonlinear interaction between coherent vorticities. Nevertheless, the locally dominant frequency scales the mean growth rate, the inclination and distortion of the mean velocity profiles as well as the phase-locked vorticity contours.

Added Sound Sources in Jets; Theory and Simulation

2009 ◽  
Vol 8 (6) ◽  
pp. 511-534 ◽  
Author(s):  
Philippe R. Spalart ◽  
Michael L. Shur ◽  
Michael Kh. Strelets
Keyword(s):  
Near Field ◽  
Mixing Layer ◽  
Sound Level ◽  
Local Source ◽  
Mean Flow ◽  
Sound Sources ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Noise Measurements ◽  
The Mean

The Large-Eddy Simulation (LES) system established over the last six years is reviewed in terms of its progress in accuracy and transition physics, and is then used to explore the transmission of noise from a known local source through a turbulent region, here the shear layer of a jet. For this, weak artificial monopole sources are added to an LES, and their sound tracked in detail both in the near-field and far-field. Sources are placed in the potential cores of the primary and secondary streams, as well as in the mixing layer and outside the jet with various locations relative to the observer, and different frequencies. Simple Ray Acoustics theory based on the mean flow field and assuming full conservation of wave action via the Blokhintsev equation is quite successful, both in terms of wave-fronts and sound level, even at a diameter Strouhal number St of only 0.5. The principal difference is that LES predicts a gradual cone of silence upstream, which theory does not. The abrupt downstream cones of silence agree. Thus, even crossing a mixing layer with a Mach 0.9 difference does not appear to alter the sound much. Cases with a dual nozzle and hot core stream return similar findings. This will be helpful when creating lower-order prediction tools, and correcting noise measurements made outside a co-flow.

Dynamics of the wing-tip vortex in the near field of a NACA 0012 aerofoil

2011 ◽  
Vol 115 (1166) ◽  
pp. 229-239 ◽  
Author(s):  
C. del Pino ◽  
J.M. López-Alonso ◽  
L. Parras ◽  
R. Fernandez-Feria
Keyword(s):  
Unstable Mode ◽  
Near Field ◽  
Reynolds Numbers ◽  
Tip Vortex ◽  
Vortex Centre ◽  
Wing Tip Vortex ◽  
Spatio Temporal ◽  
Naca 0012 ◽  
Proper Orthogonal ◽  
Wing Tip

AbstractThe dynamics of the wing tip vortex in the near-field of a NACA 0012 aerofoil has been analysed by means of flow visualisations in a water tunnel. Different axial distances near the wing up to four chords, Reynolds numbers up to 42,000 and three angles-of-attack are studied to characterise the behaviour of the vortex meandering. The spatio-temporal vortex centre positions show distorted elliptical shapes in a (x,y)-plane. The Reynolds number has no significant influence on the axial evolution of the meandering amplitude. In addition, the flow visualisations obtained with a low speed camera are analysed by the singular value or proper orthogonal decomposition. Thus, the most energetic displacement modes are obtained. The frequency associated to these modes is computed byFFT. In all the cases studied, our results show that the most unstable mode corresponds to the azimuthal wavenumber |n| = 1 in the so-called Kelvin helical modes and the frequency is lower or close to 1Hz.

The effect of streamwise vortices on the aeroacoustics of a Mach 0.9 jet

2007 ◽  
Vol 578 ◽  
pp. 139-169 ◽  
Author(s):  
MEHMET B. ALKISLAR ◽  
A. KROTHAPALLI ◽  
G. W. BUTLER
Keyword(s):  
Flow Field ◽  
Shear Layer ◽  
High Frequency ◽  
High Speed ◽  
Sound Pressure ◽  
Near Field ◽  
Stereoscopic Particle Image Velocimetry ◽  
Far Field ◽  
Streamwise Vortices ◽  
Uniform Noise

The role of the streamwise vortices on the aeroacoustics of a Mach 0.9 axisymmetric jet is investigated using two different devices to generate streamwise vortices: microjets and chevrons. The resultant acoustic field is mapped by sideline microphones and a microphone phased array. The flow-field characteristics within the first few diameters of the nozzle exit are obtained using stereoscopic particle image velocimetry (PIV). The flow-field measurements reveal that the counter-rotating streamwise vortex pairs generated by microjets are located primarily at the high-speed side of the initial shear layer. In contrast, the chevrons generate vortices of greater strength that reside mostly on the low-speed side. Although the magnitude of the chevron's axial vorticity is initially higher, it decays more rapidly with downstream distance. As a result, their influence is confined to a smaller region of the jet. The axial vorticity generated by both devices produces an increase in local entrainment and mixing, increasing the near-field turbulence levels. It is argued that the increase in high-frequency sound pressure levels (SPL) commonly observed in the far-field noise spectrum is due to the increase in the turbulence levels close to the jet exit on the high-speed side of the shear layer. The greater persistence and lower strength of the streamwise vortices generated by microjets appear to shift the cross-over frequencies to higher values and minimize the high-frequency lift in the far-field spectrum. The measured overall sound pressure level (OASPL) shows that microjet injection provides relatively uniform noise suppression for a wider range of sound radiation angles when compared to that of a chevron nozzle.

Examination of large-scale structures in a turbulent plane mixing layer. Part 1. Proper orthogonal decomposition

1999 ◽  
Vol 391 ◽  
pp. 91-122 ◽  
Author(s):  
J. DELVILLE ◽  
L. UKEILEY ◽  
L. CORDIER ◽  
J. P. BONNET ◽  
M. GLAUSER
Keyword(s):  
Proper Orthogonal Decomposition ◽  
Large Scale ◽  
Mixing Layer ◽  
Orthogonal Decomposition ◽  
Plane Mixing Layer ◽  
Large Scale Structures ◽  
The Mean ◽  
Scale Structures ◽  
Turbulent Plane ◽  
Proper Orthogonal

Large-scale structures in a plane turbulent mixing layer are studied through the use of the proper orthogonal decomposition (POD). Extensive experimental measurements are obtained in a turbulent plane mixing layer by means of two cross-wire rakes aligned normal to the direction of the mean shear and perpendicular to the mean flow direction. The measurements are acquired well into the asymptotic region. From the measured velocities the two-point spectral tensor is calculated as a function of separation in the cross-stream direction and spanwise and streamwise wavenumbers. The continuity equation is then used for the calculation of the non-measured components of the tensor. The POD is applied using the cross-spectral tensor as its kernel. This decomposition yields an optimal basis set in the mean square sense. The energy contained in the POD modes converges rapidly with the first mode being dominant (49% of the turbulent kinetic energy). Examination of these modes shows that the first mode contains evidence of both known flow organizations in the mixing layer, i.e. quasi-two-dimensional spanwise structures and streamwise aligned vortices. Using the shot-noise theory the dominant mode of the POD is transformed back into physical space. This structure is also indicative of the known flow organizations.

Articulatory Behavior Pre and Post Full-Mouth Tooth Extraction and Alveoloplasty

1980 ◽  
Vol 23 (3) ◽  
pp. 630-645 ◽  
Author(s):  
Gerald Zimmermann ◽  
J.A. Scott Kelso ◽  
Larry Lander
Keyword(s):  
Control Subject ◽  
High Speed ◽  
Tooth Extraction ◽  
Kinematic Parameters ◽  
Experimental Conditions ◽  
Mean Values ◽  
Articulatory Movements ◽  
The Mean ◽  
Experimental Subjects ◽  
Normal Speech

High speed cinefluorography was used to track articulatory movements preceding and following full-mouth tooth extraction and alveoloplasty in two subjects. Films also were made of a control subject on two separate days. The purpose of the study was to determine the effects of dramatically altering the structural dimensions of the oral cavity on the kinematic parameters of speech. The results showed that the experimental subjects performed differently pre and postoperatively though the changes were in different directions for the two subjects. Differences in both means and variabilities of kinematic parameters were larger between days for the experimental (operated) subjects than for the control subject. The results for the Control subject also showed significant differences in the mean values of kinematic variables between days though these day-to-day differences could not account for the effects found pre- and postoperatively. The results of the kinematic analysis, particularly the finding that transition time was most stable over the experimental conditions for the operated subjects, are used to speculate about the coordination of normal speech.

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