scholarly journals Acoustic, hydrodynamic and thermal modes in a supersonic cold jet

2016 ◽  
Vol 800 ◽  
pp. 387-432 ◽  
Author(s):  
S. Unnikrishnan ◽  
Datta V. Gaitonde
Keyword(s):  
Shear Layer ◽  
Acoustic Field ◽  
Large Scale ◽  
Acoustic Mode ◽  
Layer Growth ◽  
Decay Rates ◽  
Time Resolved ◽  
Acoustic Frequency ◽  
Eddy Simulation ◽  
Radiation Spectra

Large-eddy simulation data for a Mach 1.3 round jet are decomposed into acoustic, hydrodynamic and thermal components using Doak’s momentum potential theory. The decomposed fields are then analysed to examine the properties of each mode and their dynamics based on the transport equation for the total fluctuating enthalpy. The solenoidal fluctuations highlight hydrodynamic components of the jet and capture the shear layer growth and breakdown process. The acoustic mode exhibits a jittering coherent wavepacket structure in the turbulent region and consequent highly directional downstream radiation. The expected radial decay rates, $r^{-6}$ for hydrodynamic and $r^{-2}$ for acoustic, are recovered and closely follow the universal radiation spectra in the sideline and downstream directions. The scalogram of the acoustic mode in the near-acoustic-field region is consistent with that of the pressure perturbation signal in the acoustic-frequency range, but effectively removes the hydrodynamic and thermal content. The time-resolved and mean behaviour of terms in the total fluctuating enthalpy equation is analysed in detail. A large-scale intermittent event in the near-acoustic field is shown to be associated with an intrusion of vortices from the shear layer into the core of the jet. Acoustic sources are created when the resulting negative fluctuations in the solenoidal component interact with positive fluctuations in the Coriolis acceleration term. The latter are associated with regions of high vorticity on the inner side of the shear layer. In contrast, sinks result from the interaction of solenoidal momentum fluctuations with positive entropy gradients along entrainment streaks.

scholarly journals Analysis of V-Gutter Reacting Flow Dynamics Using Proper Orthogonal and Dynamic Mode Decompositions

Energies ◽  
2020 ◽  
Vol 13 (18) ◽  
pp. 4886 ◽  
Author(s):  
Yang Yang ◽  
Xiao Liu ◽  
Zhihao Zhang
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Dynamic Mode Decomposition ◽  
Dynamic Mode ◽  
Reacting Flow ◽  
Eddy Simulation ◽  
Mode Decomposition ◽  
Wake Instability ◽  
Shedding Frequency ◽  
Proper Orthogonal

The current work is focused on investigating the potential of data-driven post-processing techniques, including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) for flame dynamics. Large-eddy simulation (LES) of a V-gutter premixed flame was performed with two Reynolds numbers. The flame transfer function (FTF) was calculated. The POD and DMD were used for the analysis of the flame structures, wake shedding frequency, etc. The results acquired by different methods were also compared. The FTF results indicate that the flames have proportional, inertial, and delay components. The POD method could capture the shedding wake motion and shear layer motion. The excited DMD modes corresponded to the shear layer flames’ swing and convect motions in certain directions. Both POD and DMD could help to identify the wake shedding frequency. However, this large-scale flame oscillation is not presented in the FTF results. The negative growth rates of the decomposed mode confirm that the shear layer stabilized flame was more stable than the flame possessing a wake instability. The corresponding combustor design could be guided by the above results.

scholarly journals Scaling of separated shear layers: an investigation of mass entrainment

2017 ◽  
Vol 826 ◽  
pp. 851-887 ◽  
Author(s):  
Francesco Stella ◽  
Nicolas Mazellier ◽  
Azeddine Kourta
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Separated Flow ◽  
Layer Growth ◽  
Upper And Lower Bounds ◽  
Stream Velocity ◽  
Physical Parameters ◽  
Separated Shear Layer ◽  
Mass Entrainment ◽  
Recirculation Length

We report an experimental investigation of the separating/reattaching flow over a descending ramp with a $25^{\circ }$ expansion angle. Emphasis is given to mass entrainment through the boundaries of the separated shear layer emanating from the upper edge of the ramp. For this purpose, the turbulent/non-turbulent interface and the separation line inferred from image-based analysis are used respectively to mark the upper and lower bounds of the separated shear layer. The main objective of this study is to identify the physical parameters that scale the development of the separated shear layer, by giving a specific emphasis to the investigation of mass entrainment. Our results emphasise the multiscale nature of mass entrainment through the separated shear layer. The recirculation length $L_{R}$, step height $h$ and free-stream velocity $U_{\infty }$ are the dominant scales that organise the separated flow (and related large-scale quantities as pressure distribution or shear layer growth rate) and set mean mass fluxes. However, local viscous mechanisms seem to be responsible for most of local mass entrainment. Furthermore, it is shown that large-scale mass entrainment is driven by incoming boundary layer properties, since $L_{R}$ scales with $Re_{\unicode[STIX]{x1D703}}$, and in particular by its turbulent state. Surprisingly, the relationships evidenced in this study suggest that these dependencies are established over a large distance upstream of separation and that they might also extend to small scales, at which viscous entrainment is dominant. If confirmed by additional studies, our findings would open new perspectives for designing effective separation control systems.

Large Eddy Simulation of Self-Sustained Cavity Oscillation for Subsonic and Supersonic Flows

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
K. M. Nair ◽  
S. Sarkar
Keyword(s):  
Large Eddy Simulation ◽  
Shear Layer ◽  
Acoustic Waves ◽  
Large Scale ◽  
Hydrodynamic Instability ◽  
Supersonic Flows ◽  
Large Scale Structures ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Scale Structures

The primary objective is to perform a large eddy simulation (LES) using shear improved Smagorinsky model (SISM) to resolve the large-scale structures, which are primarily responsible for shear layer oscillations and acoustic loads in a cavity. The unsteady, three-dimensional (3D), compressible Navier–Stokes (N–S) equations have been solved following AUSM+-up algorithm in the finite-volume formulation for subsonic and supersonic flows, where the cavity length-to-depth ratio was 3.5 and the Reynolds number based on cavity depth was 42,000. The present LES resolves the formation of shear layer, its rollup resulting in large-scale structures apart from shock–shear layer interactions, and evolution of acoustic waves. It further indicates that hydrodynamic instability, rather than the acoustic waves, is the cause of self-sustained oscillation for subsonic flow, whereas the compressive and acoustic waves dictate the cavity oscillation, and thus the sound pressure level for supersonic flow. The present LES agrees well with the experimental data and is found to be accurate enough in resolving the shear layer growth, compressive wave structures, and radiated acoustic field.

Identification and Analysis of Vortical Structures in a Ribbed Channel

2008 ◽  
Author(s):  
Lei Wang ◽  
Mirko Salewski ◽  
Bengt Sunde´n
Keyword(s):  
Shear Layer ◽  
Turbulent Flows ◽  
Large Scale ◽  
Transport Processes ◽  
Vortical Structures ◽  
Ribbed Channel ◽  
Eddy Simulation ◽  
Separated Shear Layer ◽  
Image Velocimetry ◽  
Flow Features

Vortical motions, usually called sinews and muscles of fluid motions, constitute important features of turbulent flows and form the base for large-scale transport processes. In this study, we present a variety of flow decomposition techniques to identify and analyze the vortical structures in a ribbed channel. To this end, the instantaneous velocity fields are measured by means of a two-dimensional particle image velocimetry (PIV). Firstly, the implementation of Galilean-, Reynolds- and large-eddy simulation (LES) decompositions on the instantaneous flow fields allows one to perceive the coherent vortices embedded in the separated shear layer. In addition, the proper orthogonal decomposition (POD) is employed to extract the underlying flow features out of the fluctuating velocity and vorticity fields, respectively. For velocity-based decomposition, the first two POD modes show that the shear layer is highly unstable and associated with the ‘flapping’ motion. For vorticity-based decomposition, the first two POD modes are characterized by the distinct horizontal bands which manifest the coherent structures in the shear layer. In order to interpret the flow structures in a convenient way, a linear combination of POD modes (reconstruction) is also carried out in the present study. The result shows that a large-scale, pronounced vortex is recognizable in the region downstream of rib.

Vortex dynamics and sound emission in excited high-speed jets

2018 ◽  
Vol 839 ◽  
pp. 313-347 ◽  
Author(s):  
Michael Crawley ◽  
Lior Gefen ◽  
Ching-Wen Kuo ◽  
Mo Samimy ◽  
Roberto Camussi
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Noise Source ◽  
Near Field ◽  
Velocity Fields ◽  
Source Mechanism ◽  
Time Resolved ◽  
Large Scale Structures ◽  
Coherent Ring ◽  
Scale Structures

This work aims to study the dynamics of and noise generated by large-scale structures in a Mach 0.9 turbulent jet of Reynolds number $6.2\times 10^{5}$ using plasma-based excitation of shear layer instabilities. The excitation frequency is varied to produce individual or periodic coherent ring vortices in the shear layer. First, two-point cross-correlations are used between the acoustic near field and far field in order to identify the dominant noise source region. The large-scale structure interactions are then investigated by stochastically estimating time-resolved velocity fields using time-resolved near-field pressure traces and non-time-resolved planar velocity snapshots (obtained by particle image velocimetry) by means of an artificial neural network. The estimated time-resolved velocity fields show multiple mergings of large-scale structures in the shear layer, and indicate that disintegration of coherent ring vortices is the dominant aeroacoustic source mechanism for the jet studied here. However, the merging of vortices in the initial shear layer is also identified as a non-trivial noise source mechanism.

Numerical simulation of the compressible mixing layer past an axisymmetric trailing edge

2007 ◽  
Vol 591 ◽  
pp. 215-253 ◽  
Author(s):  
FRANCK SIMON ◽  
SEBASTIEN DECK ◽  
PHILIPPE GUILLEN ◽  
PIERRE SAGAUT ◽  
ALAIN MERLEN
Keyword(s):  
Numerical Simulation ◽  
Shear Layer ◽  
Large Scale ◽  
Free Stream ◽  
Mixing Layer ◽  
Adverse Pressure Gradient ◽  
Layer Growth ◽  
Trailing Edge ◽  
Azimuthal Mode ◽  
Compressible Mixing Layer

Numerical simulation of a compressible mixing layer past an axisymmetric trailing edge is carried out for a Reynolds number based on the diameter of the trailing edge approximately equal to 2.9 × 106. The free-stream Mach number at separation is equal to 2.46, which corresponds to experiments and leads to high levels of compressibility. The present work focuses on the evolution of the turbulence field through extra strain rates and on the unsteady features of the annular shear layer. Both time-averaged and instantaneous data are used to obtain further insight into the dynamics of the flow. An investigation of the time-averaged flow field reveals an important shear-layer growth rate in its initial stage and a strong anisotropy of the turbulent field. The convection velocity of the vortices is found to be somewhat higher than the estimated isentropic value. This corroborates findings on the domination of the supersonic mode in planar supersonic/subsonic mixing layers. The development of the shear layer leads to a rapid decrease of the anisotropy until the onset of streamline realignment with the axis. Due to the increase of the axisymmetric constraints, an adverse pressure gradient originates from the change in streamline curvature. This recompression is found to slow down the eddy convection. The foot shock pattern features several convected shocks emanating from the upper side of the vortices, which merge into a recompression shock in the free stream. Then, the flow accelerates and the compressibility levels quickly drop in the turbulent developing wake. Some evidence of the existence of large-scale structures in the near wake is found through the domination of the azimuthal mode m = 1 for a Strouhal number based on trailing-edge diameter equal to 0.26.

Investigation of Combustion Oscillations of Premixed Dump Combustor Using Time-Resolved Particle Image Velocimetry

2014 ◽  
Author(s):  
Ramgopal Sampath ◽  
Vikram Ramanan ◽  
S. R. Chakravarthy
Keyword(s):  
Heat Release ◽  
Shear Layer ◽  
Large Scale ◽  
Velocity Fields ◽  
Small Scale ◽  
Pressure Amplitude ◽  
Time Resolved ◽  
Dump Combustor ◽  
Large Scale Vortex ◽  
Chemiluminescent Intensity

The present work deals with time-resolved investigation of the flow field during acoustic self-excitation by a lean premixed flame in a dump combustor with varying equivalence ratio at a constant air flow rate. Simultaneous measurements of pressure fluctuations, velocity fields using Time resolved Particle imaging velocimetry (TR-PIV) and CH* chemiluminescence were performed. The pressure, velocity and chemiluminescent intensity time traces were Fourier transformed to estimate the frequency and amplitudes. Conditions of maximum pressure amplitude correspond to the prevalence of intermittent bursts in pressure, velocity, and chemiluminescent intensity. Further, Proper orthogonal decomposition (POD) is applied to the chemiluminescent intensity and velocity fields. The POD mode shapes are able to capture the modes pertaining to both the acoustic and vortex mode of flame/flow oscillations. The burst oscillations are understood by examining the sequence of time-resolved velocity and chemiluminescent intensity during their growth and decay regimes. The growth of oscillations is promoted by the flame heat release fluctuations following the pattern of the large-scale vortex roll-up in the recirculation zone downstream of the dump plane, causing a tendency of acoustic excitation at the vortex mode. As the amplitude rises, the natural acoustic mode of the duct is simultaneously amplified, leading to small-scale vortices shed from the step corner at the acoustic time scale. These small-scale vortices adversely interact with the large-scale vortex controlling the heat release, resulting in its weakening and hence the decay of oscillations. This behavior was further observed in the spatially averaged vorticity along the shear layer. In addition to this, the time traces of the pressure and the velocity fluctuations at the shear layer and located half step height from the separation point were overlapped. The overlapped time traces showed a drift in the instantaneous phase during which the growth and decay of the oscillations were observed.

scholarly journals Turbulent shear-layer mixing at high Reynolds numbers: effects of inflow conditions

1998 ◽  
Vol 376 ◽  
pp. 115-138 ◽  
Author(s):  
M. D. SLESSOR ◽  
C. L. BOND ◽  
P. E. DIMOTAKIS
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Layer Growth ◽  
Chemical System ◽  
Turbulent Shear ◽  
Layer Formation ◽  
Molecular Mixing ◽  
Chemically Reacting ◽  
High Reynolds ◽  
Inflow Conditions

We report on the results from a set of incompressible, shear-layer flow experiments, at high Reynolds number (Reδ≡ρΔUδT(x)/ μ≃2×105), in which the inflow conditions of shear-layer formation were varied (δT is the temperature-rise thickness for chemically-reacting shear layers). Both inert and chemically-reacting flows were investigated, the latter employing the (H2+NO)/F2 chemical system in the kinetically-fast regime to measure molecular mixing. Inflow conditions were varied by perturbing each, or both, boundary layers on the splitter plate separating the two freestream flows, upstream of shear-layer formation. The results of the chemically-reacting ‘flip experiments’ reveal that seemingly small changes in inflow conditions can have a significant influence not only on the large-scale structure and shear-layer growth rate, as had been documented previously, but also on molecular mixing and chemical-product formation, far downstream of the inflow region.

scholarly journals The influence of initial conditions on turbulent mixing due to Richtmyer–Meshkov instability

2010 ◽  
Vol 654 ◽  
pp. 99-139 ◽  
Author(s):  
B. THORNBER ◽  
D. DRIKAKIS ◽  
D. L. YOUNGS ◽  
R. J. R. WILLIAMS
Keyword(s):  
Kinetic Energy ◽  
Volume Fraction ◽  
Initial Conditions ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Layer Growth ◽  
Decay Rates ◽  
Fine Grid ◽  
Eddy Simulation ◽  
Energy Decay Rates

This paper investigates the influence of different three-dimensional multi-mode initial conditions on the rate of growth of a mixing layer initiated via a Richtmyer–Meshkov instability through a series of well-controlled numerical experiments. Results are presented for large-eddy simulation of narrowband and broadband perturbations at grid resolutions up to 3 × 109 points using two completely different numerical methods, and comparisons are made with theory and experiment. It is shown that the mixing-layer growth is strongly dependent on initial conditions, the narrowband case giving a power-law exponent θ ≈ 0.26 at low Atwood and θ ≈ 0.3 at high Atwood numbers. The broadband case uses a perturbation power spectrum of the form P(k) ∝ k−2 with a proposed theoretical growth rate of θ = 2/3. The numerical results confirm this; however, they highlight the necessity of a very fine grid to capture an appropriately broad range of initial scales. In addition, an analysis of the kinetic energy decay rates, fluctuating kinetic energy spectra, plane-averaged volume fraction profiles and mixing parameters is presented for each case.

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