Large-scale Organized Motions Research Articles

Development and Application of Bivariate 2D-EMD for the Analysis of Instantaneous Flow Structures and Cycle-to-Cycle Variations of In-cylinder Flow

The bivariate two dimensional empirical mode decomposition (Bivariate 2D-EMD) is extended to estimate the turbulent fluctuations and to identify cycle-to-cycle variations (CCV) of in-cylinder flow. The Bivariate 2D-EMD is an adaptive approach that is not restricted by statistical convergence criterion, hence it can be used for analyzing the nonlinear and non-stationary phenomena. The methodology is applied to a high-speed PIV dataset that measures the velocity field within the tumble symmetry plane of an optically accessible engine. The instantaneous velocity field is decomposed into a finite number of 2D spatial modes. Based on energy considerations, the in-cylinder flow large-scale organized motion is separated from turbulent fluctuations. This study is focused on the second half of the compression stroke. For most of the cycles, the maximum of turbulent fluctuations is located between 50 and 30 crank angle degrees before top dead center (TDC). In regards to the phase-averaged velocity field, the contribution of CCV to the fluctuating kinetic energy is approximately 55% near TDC.

Bivariate 2D empirical mode decomposition for analyzing instantaneous turbulent velocity field in unsteady flows

We introduce and demonstrate the bivariate two-dimensional empirical mode decomposition (bivariate 2D-EMD) for the decomposition of a turbulent instantaneous velocity field to separate spatial large-scale organized motion from random turbulent fluctuations. To validate this approach, it was applied to an experimental homogeneous and isotropic turbulent flow (HIT), perturbed by a synthetic Lamb–Oseen vortex that mimics the feature of organized motion. Through different test cases, the scale, the amplitude and the position of the synthetic vortex with respect to the turbulent velocity field were changed. By applying an energy criterion on the modes which resulted from the decomposition process, the initial HIT flow was separated from synthetic perturbation. It is important to point out that in this approach the decomposition as well as the distinction of different parts of the flow are free from any prior and objective assumptions and it requires just one instantaneous velocity field of the flow under analysis. The proposed methodology could be used for analyzing 2D velocity fields obtained from experimental measurement or CFD in different configurations (in-cylinder flow, channel flows, etc.).

Vortices and Large-Scale Structures in a Rough Open-Channel Flow Subjected to Bed Suction and Injection

The effect of seepage (injection and suction) on the dynamics of vortices and large-scale structures in a rough open channel flow was examined experimentally. The velocity measurements in the open channel flow were carried out using particle image velocimetry. A swirling strength-based vortex identification algorithm was used to extract the location, size, and strength of vortices in the flow. Proper orthogonal decomposition (POD) was used to analyze the dynamics of large-scale structures in the flow. The results indicate that suction induces a greater change compared with an equivalent amount of injection on the open channel flow. Suction dampens the size and strength of the vortices compared to the no-seepage case. POD analysis indicates that seepage tends to introduce more small-scale structures in the flow and destroys the large-scale organized motion present otherwise in the flow. DOI: 10.1061/(ASCE)EM.1943-7889.0000354. © 2012 American Society of Civil Engineers. CE Database subject headings: Seepage; Open channel flow; Vortices; Decomposition. Author keywords: Seepage; Open channel flow; Vortices; Large-scale-structures; Swirling strength; Proper orthogonal decomposition.

Size and effect on the mean flow of large-scale horizontal coherent structures in open-channel flows: an experimental studyThis paper is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

The size of the largest horizontal coherent structures (HCSs) of turbulence in open-channel flows is investigated experimentally on the basis of three series of flow velocity measurements. These are further used to explore the dynamics and morphological consequences of HCSs. The flow velocity measurements were carried out in a 21 m long and 1 m wide channel, with a bed formed by sand with average grain size of 2 mm. The bed surface was flat. The turbulent and subcritical flow under investigation was uniform, with a flow depth of 4 cm. The bed slope of 0.0015 was such that, for the present flow depth, the bed shear stress acting on the bed was substantially below the threshold for initiation of motion, thus ensuring that the bed remained flat throughout the measurements. To the knowledge of the writers, this work is a first attempt to systematically investigate HCSs in open-channel flows. It should be viewed as an extension to the case of horizontal structures of work previously carried out by a number of authors on large-scale organized turbulence motion in open-channel flows, so far focusing exclusively on vertical coherent structures (VCSs). The horizontal burst length was found to be between five and seven times the flow width. A slight internal meandering of the flow caused by the superimposition of burst sequences on the mean flow was detectable. Both of these findings lend support to the longstanding belief expressed by many prominent researchers that the formation of large-scale river forms is directly related to the large-scale turbulence. In particular, the present measurements for the first time provide some direct evidence in support of hypotheses previously raised by Yalin and da Silva regarding the formation of alternate bars and meanders through the action of HCSs on the mean flow and the mobile bed and banks.

A method for the transverse modulation of reactive flows with application to combustion instability

A method allowing the transverse excitation of non-reactive or reactive flows is described. The method relies on a characteristic wave modulation applied on the lateral sides of the computational domain. It is shown that this procedure can be used to induce a transverse sloshing motion in the region of interest. Two two-dimensional geometries are studied: in the first, the flow features one or two wakes embedded in a high-speed stream; the second configuration involves a premixed reactive jet flame. The excited flow structure calculated in this last case is found to be similar to that observed in an experiment carried out previously. As the simulations are performed in two dimensions, they cannot describe many of the processes taking place in a turbulent flow. They are, however, valuable when the flow is dominated by a large-scale organized motion induced by a transverse acoustic modulation. The present calculations indicate that the method in combination with a large eddy simulation flow solver could be used to study combustion response to transverse acoustic perturbations. With additional developments this might be used to study liquid propellant rocket motor instabilities coupled by transverse acoustic modes in the high-frequency range.

COMPUTATIONAL VISUALIZATION OF SEPARATED/REATTACHED TRANSITIONAL FLOWS ON A BLUNT PLATE

It is well known that large-scale organized motions, usually called coherent structures, exist in many transitional and turbulent flows (if not all). The topology and range of scales of those large-scale structures change widely from flow to flow such as counter-rotating vortices in wake flows, streaks and hairpin vortices in turbulent boundary layers. However, it is not well established what kind of large-scale structures exists in separated/reattached transitional flows. Large-eddy simulation with a dynamic subgrid-scale model is employed in the current study to investigate the physics of transitional separating/reattaching flows over a blunt plate held normal to a uniform stream. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6500. Statistics of the LES are found to be in acceptable agreement with the available experimental data in the laminar, transitional, and turbulent flow regions. The entire transition process leading to breakdown to turbulence has been shown by flow visualization and large-scale structures have been identified at different stages of the transition process of the separating/reattaching flow. The well-known hairpin vortices commonly associated with boundary-layer transition on a flat plate have been clearly shown at about the mean reattachment point and more large-scale structures are presented as well. The large-scale structures are noticed to persist a considerable distance downstream of reattachment before they eventually break into smaller-scale turbulent structures. In addition, it may shed light on the transition mechanisms and the nature of instabilities involved to understand the formation of these large-scale structures, their spatial and temporal evolution and eventual break-up into smaller structures

Large-Scale Structures and Growth of a Flat Plate Compressible Wake

Experiments conducted on a flat plate wake in a compressible flow at Mach 2 are described. Large-scale organized motions are revealed both in the separating boundary layer upstream and in the wake downstream. The organized motions in the wake contained within them embeded vortex structures, whose periodicity was found to be weak. The broadband spectra contained a distinct frequency with a Strouhal number of 0.3. The study of mean flow characteristics showed that some qualitative similarity exists in the near- and intermediate-wake regions. The relative Mach number distribution showed the compressibility effects to be small

Laminar and turbulent Rayleigh–Bénard convection in a perfectly conducting cubical cavity

This paper discusses flow structures and heat transfer rates generated by Rayleigh–Bénard convective motions of a Boussinesq fluid with a Prandtl number of 0.7 in a perfectly conducting cubical cavity. Complete numerical simulations of laminar flows were conducted in the range of Rayleigh numbers 7×10 3⩽ Ra⩽10 5. The large-eddy simulation (LES) technique was used for the simulations at two high Rayleigh numbers ( Ra=10 6 and 10 8). LES were carried out using a second-order accurate finite volume code with a dynamic localized one-equation subgrid-scale (SGS) model with constant SGS Prandtl number. In the laminar regime, two single roll structures and a four-roll structure in which the axis of each roll is perpendicular to one sidewall were found to be stable. LES of Rayleigh–Bénard convection in an infinite fluid layer were initially carried out and results were seen to be in agreement with direct numerical simulations (DNS) reported in the literature. At Ra=10 6 and 10 8, the instantaneous velocity and temperature fields present strong fluctuations with respect to the time-averaged flow field. The confining effect of the conductive lateral walls of the cavity generates, in the unsteady flows at Ra=10 6 and 10 8, persistent vertical currents near these walls. The recirculation of these ascending and descending flows towards the central region of the cavity produces large-scale organized rolling motions, which imprint the topology of the time-averaged flow field in form of two vortex ring structures located near the horizontal walls.

Simulations of local extinction phenomena in bluff-body stabilized diffusion flames with a Lagrangian reactedness model

Two-dimensional large-eddy simulations of bluff-body stabilized flames of methane and propane, exhibiting significant finite-rate chemistry effects, are presented. A partial equilibrium/two-scalar exponential probability density function (PDF) combustion submodel is applied at the subgrid level. Subgrid scale motions are modelled with a first-order closure employing an anisotropic subgrid eddy-viscosity and two equations for the subgrid turbulent kinetic and scalar energies. Statistical independence of the joint PDF scalars is avoided and the necessary moments are obtained from an extended scale-similarity assumption. Extinction is accounted for by comparing the local turbulent Damköhler number against a ‘critical’ local limit related to the Gibson scalar scale and the reaction zone thickness in mixture fraction space. The post-extinction regime is modelled via a Lagrangian transport equation for a reactedness progress variable which follows a linear deterministic relaxation to its mean value (interaction by exchange with the mean model; IEM).Comparisons between simulations and measurements suggested the ability of the adopted methodology to represent the experimental variations in the momentum and scalar fields at conditions close to the lean or the rich blow-out limit. Favourable agreement was achieved in the calculation of the recirculation lengths and the peak temperature and turbulence levels in the near-wake region. Significant experimental trends, such as the suppression of the large-scale organized motions in the developing wake at low and medium fuel injection rates, and the re-emergence of the quasi-periodic shedding activity close to the lean limit, were also reproduced. Quantitative discrepancies increased in the prediction of major species, but the measured trends due to the effects of partial extinction were adequately recovered.

The large-scale turbulent structure of nonpremixed planar jet flames

A comparative study was conducted of the large-scale turbulent structure of nonpremixed planar jet flames and nonreacting planar jets of varying density. The diagnostic techniques used included planar laser-induced fluorescence of OH and planar laser Mie scattering (PLMS) from alumina particles seeded into the jet fluid. The PLMS imaging shows that the nonreacting jets at all density ratios studied exhibit large-scale organized motions as observed in previous studies at a density ratio of unity. Such organized structures are not nearly as dominant in the jet flames at comparable local Reynolds numbers. To quantitatively investigate the large-scale motions, ensembles of PLMS images were used to compute root-mean-square (RMS) intermittency fields. The RMS intermittency profiles (normalized by the local jet width) are very similar for all of the nonreacting cases, while the flame profile was considerably narrower. Furthermore, plan view PLMS images reveal quasi-two-dimensional organization for the nonreacting case, but more random three-dimensional structure for the flame. These differences argue for the substantial modification of the large-scale turbulent structure owing to the presence of the flame, and do not appear to be due to global Reynolds number effects or global density changes resulting from heat release.

Motions in Jovian Hot Spot–Plume Regions Using Voyager Images

We have measured the motions and wind velocity vectors in six jovian hot spot–plume areas (latitudes 0° to +12°) using high resolution Voyager 1 and 2 images separated by one jovian rotation (∼10 hr). Rapid changes in cloud forms take place in a few hours at scales from 100 to 2000 km. No large scale organized motions, e.g., diagonal convergence along the plume tails toward hot spots or circulation patterns, such as those recently reported using Galileo images separated by 1 hr, have been found. We show that particle evaporation and cloud element mixing in the upper ammonia cloud could be fast enough to dissipate the small cloud elements, such as those used as tracers by the Galileo SSI team, in less than 10 hr.

Wave Number Frequency Spectra of a Lifting Wake for Broadband Noise Prediction

Measurements of wave number frequency spectra of velocity fluctuations have been made using two three-component hot-wire probes in the wake of a rectangular NACA 0012 half-wing at a 5-deg angle of attack. Spectra were measured along spanwise lines in the near two-dimensional part of the wake, in the stretched region where the wake begins to wind around the vortex, directly above the vortex core, and radially from the core center. Dominated by the large-scale organized motions present in the wake, the spectra show little similarity with the von Karman isotropic turbulence spectrum. Outside the core, upwash spectra contain a single maximum at the passage frequency of these large structures. Motions associated with this peak are highly anisotropic, both in terms of velocity components and length scales. Similar anisotropy is also seen at higher wave numbers, suggesting either that the smaller scale turbulent motions are organized by the large eddies or that the nonsinusoidal components of those eddies contribute significantly to the spectrum at higher wave numbers. The implications of these results for broadband noise prediction have been assessed by considering the sound radiation from this turbulent flow over a semi-infinite flat plate. The major conclusion is that the correlation length scales of the flow are small compared to the acoustic wavelength and so are well represented by their values at zero spanwise wave number. However, the blade response function has a variation with spanwise wave number that cannot be ignored and significantly influences the spanwise directionality.

Wave number frequency spectra of a lifting wake for broadband noise prediction

Measurements of wave number frequency spectra of velocity fluctuations have been made using two three-component hot-wire probes in the wake of a rectangular NACA 0012 half-wing at a 5-deg angle of attack. Spectra were measured along spanwise lines in the near two-dimensional part of the wake, in the stretched region where the wake begins to wind around the vortex, directly above the vortex core, and radially from the core center. Dominated by the large-scale organized motions present in the wake, the spectra show little similarity with the von Karman isotropic turbulence spectrum. Outside the core, upwash spectra contain a single maximum at the passage frequency of these large structures. Motions associated with this peak are highly anisotropic, both in terms of velocity components and length scales. Similar anisotropy is also seen at higher wave numbers, suggesting either that the smaller scale turbulent motions are organized by the large eddies or that the nonsinusoidal components of those eddies contribute significantly to the spectrum at higher wave numbers. The implications of these results for broadband noise prediction have been assessed by considering the sound radiation from this turbulent flow over a semi-infinite flat plate. The major conclusion is that the correlation length scales of the flow are small compared to the acoustic wavelength and so are well represented by their values at zero spanwise wave number. However, the blade response function has a variation with spanwise wave number that cannot be ignored and significantly influences the spanwise directionality.

95/03938 Numerical modeling of turbulent jet diffusion flames in the atmospheric surface layer

High-speed visual observations of a turbulent, momentum-driven, acetylene diffusion flame are presented. It is found that the flame tip burnout can be interpreted as a consequence of the large-scale organized motion in the flow—at times the tip burnout is axisymmetric in nature while at other times it appears more helical. These observations are similar to and were motivated by, the measurements by Dahm and Dimotakis on liquid “flames.”. The results are consistent with previous soot measurements of Magnussen on similar turbulent flames.

The structure of a supersonic turbulent boundary layer subjected to concave surface curvature

This paper reports an experimental investigation of the instantaneous structure of a supersonic turbulent boundary layer (M= 2.86,Reθ= 82000) over a short region of longitudinal concave surface curvature. The radius of curvature was 12 initial boundary-layer thicknesses and the turning angle was 16°. Severe distortion of the boundary layer occurred, as evidenced by an alteration of the mean velocity profiles and an increase in wall shear stress of 125%. The large-scale organized motions in the boundary layer were significantly altered as illustrated by changes in the character of the mass flux ‘fronts’ (large gradients in the fluctuating streamwise mass flux).

Large-scale dynamics in high Reynolds number jets and jet flames

The far-field large-scale dynamics of a momentum-driven Re = 2 × 108 non-reacting jet and a Re = 3 × 107 jet diffusion flame are presented and compared. The results are derived from computer graphic volume rendering of a set of sequential images of each flow. When compared to conventional display techniques, volume rendering, by allowing many frames of a movie sequence to be presented simultaneously, more clearly shows the detailed flow evolution. For the non-reacting jet we see the passage and growth of large-scale organized structures up through the jet column, the axial velocity decay of the structures, the fluid entrainment patterns, and occasional pairing events. A rendering of a non-sequential set of images shows no discernible organized component. Volume rendering of the reacting jet shows a similar pattern of burning large-scale organized structures which convert over considerable axial distances but without the corresponding velocity decay, similar to observations of laboratory flames. The images presented here are believed to be some of the most direct visual evidence to date for large-scale organized motions in the far-field of high Reynolds number, fully developed jets and jet flames. Since conditional sampling techniques are not used, we believe that the volume renderings seen here are likely to be representative of the natural development of jet flows.

Convection velocity in supersonic turbulent boundary layers

The convection velocity of large-scale organized motions was measured in a Mach 3 boundary layer using streamwise-separated hot wires. The broadband convection velocity, determined from space-time correlations, is 0.9U∞ across the entire outer region of the boundary layer. The convection velocity of individual structures was deduced using a pattern recognition technique, and this revealed that the majority of the large-scale structures convect at nearly the same velocity. A comparison of the present results to previous measurements indicates negligible Reynolds number and Mach number effects on convection velocity.

Topology and transport properties of large-scale organized motion in a slightly heated rough wall boundary layer

Large-scale temperature discontinuities are detected in a high Reynolds number boundary layer over a slightly heated rough wall. In the streamwise and normal plane of the measurements, the discontinuities form (as for smooth wall boundary layers) the boundaries between successive spanwise vortexlike structures, flattened against the wall. The regions immediately downstream from the discontinuities are particularly important for the transport of heated, low-momentum fluid away from the wall. The proportions contributed by the detected organized motion to the velocity and temperature variances are similar to those in smooth wall boundary layers. The proportions contributed to mean products, especially to the Reynolds shear stress, are significantly higher than over smooth walls at similar Reynolds numbers. The detected organized motion in the rough wall boundary layer resembles that in a smooth wall boundary layer of similar skin friction coefficient (and hence much lower Reynolds number).

The Visible Structure of Turbulent Jet Diffusion Flames: Large-Scale Organization and Flame Tip Oscillation

Abstract High speed visual observations of free turbulent jet diffusion flames are presented. Computer graphic volume rendering, whereby many frames from a movie sequence are displayed simultaneously, is used to provide a unique view of the flow evolution. Two fuels were used, acetylene and ethylene, to span Ihe range from the momentum-driven to the buoyancy-driven regime. The data show that, for all flow regimes, the flames exhibit a large-scale organized motion in the far-field of the jet as manifested by a progression of organized structures up through the length of the visible flame. Occasional pairings of structures are also observed. These structures eventually become the flame tip, which burns out in a quasi-periodic manner. Under no conditions was a random, small-scale breakup and disappearance of the flame tip ever observed. In all cases the speed of the organized structure is seen to be constant and equal to 12 ± 2° of the jet exit velocity. The constancy of speed occurs in spite of the axial de...

Visual observations of a turbulent diffusion flame

High-speed visual observations of a turbulent, momentum-driven, acetylene diffusion flame are presented. It is found that the flame tip burnout can be interpreted as a consequence of the large-scale organized motion in the flow—at times the tip burnout is axisymmetric in nature while at other times it appears more helical. These observations are similar to and were motivated by, the measurements by Dahm and Dimotakis on liquid “flames.”. The results are consistent with previous soot measurements of Magnussen on similar turbulent flames.