Presence Of Large-scale Structures Research Articles

Experimental Study of Energy Contribution by Coherent Structures in a Turbulent Boundary Layer

Kang, Y.-D. and Koo, B., 2021. Experimental study of energy contribution by coherent structures in a turbulent boundary layer. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 579–583. Coconut Creek (Florida), ISSN 0749-0208. An experimental study was carried out to investigate a contribution of large coherent structures in turbulent flow. Both dynamometer and Clauser fitting method were adopted to calculate the frictional velocity near the wall having uncertainty. In order to quantify coherent structures, Particle Image Velocimetry (PIV) system was used to reveal their characteristics connecting through boundary layer. Variable Interval Time Averaging (VITA) technique scanned through flow fluctuations at the edge of boundary layer, whether there was a large deviation from the background flow. According to the presence of large scale structures, the flow field was divided into weak and strong cases to validate their existence. Changing a free-stream velocity from 0.5 m/s to 1.0 m/s increased the Reynolds stress more than 3 times. Proper Orthogonal Decomposition (POD) analysis was also applied to compare the energy budgets between them.

Coherent large-scale structures from the linearized Navier–Stokes equations

The wall-normal extent of the large-scale structures modelled by the linearized Navier–Stokes equations subject to stochastic forcing is directly compared to direct numerical simulation (DNS) data. A turbulent channel flow at a friction Reynolds number of $Re_{\unicode[STIX]{x1D70F}}=2000$ is considered. We use the two-dimensional (2-D) linear coherence spectrum (LCS) to perform the comparison over a wide range of energy-carrying streamwise and spanwise length scales. The study of the 2-D LCS from DNS indicates the presence of large-scale structures that are coherent over large wall-normal distances and that are self-similar. We find that, with the addition of an eddy viscosity profile, these features of the large-scale structures are captured by the linearized equations, except in the region close to the wall. To further study this coherence, a coherence-based estimation technique, spectral linear stochastic estimation, is used to build linear estimators from the linearized Navier–Stokes equations. The estimator uses the instantaneous streamwise velocity field or the 2-D streamwise energy spectrum at one wall-normal location (obtained from DNS) to predict the same quantity at a different wall-normal location. We find that the addition of an eddy viscosity profile significantly improves the estimation.

Study of the effect of external periodic flow perturbations on heat transfer intensity in the impact jet

The aim of this work is to experimentally study unsteady heat transfer in an impact jet flowing onto a heated surface with the presence of large-scale structures and separated flows, as well as to search for possible mechanisms for controlling heat transfer by introducing external disturbances into the flow. In the work, a series of experiments was carried out to study an axisymmetric jet flowing normally onto a heated surface. The jet was both under natural conditions with different flow regimes and under external excitation at various frequencies and amplitudes. The instantaneous and average velocity and temperature fields in the near-wall part of the jet are measured using the PIV and PLIF methods. The correlations of velocity fluctuations and temperature fluctuations, which characterize the intensity of turbulent heat transfer, are calculated. It is shown that at different frequencies of external excitation of the jet, the intensity of turbulent transfer in the region of jet impingement on the wall changes. In the region of jet impingement on the wall, flow separation was recorded. It is shown that the interaction of the coherent structure with the wall leads to a local increase in the outflow of heat from the heated surface. The distributions of the heat transfer coefficient along the heated surface were also measured using a thermal imaging camera for various flow regimes, distances from the nozzle exit to the barrier, frequencies and amplitudes of the external flow disturbance. It is shown that when the jet is excited at different frequencies of the jet excitation, the heat transfer increases or decreases.

Experimental investigation of the velocity time-traces of the turbulent flow in a rectangular channel with a lateral slot

In the present work it is performed an experimental evaluation of turbulent, incompressible air flow at the steady state in compound channel. The purpose is to evaluate the influence of the geometrical parameters over the fluid dynamic behavior of the turbulent flow such as the generation of coherent structures and Strouhal numbers. In order to perform this investigation it is employed the classic approach Power Spectral Density Function (PSD) and Time–Frequency tools for spectral analysis of velocity fluctuations. Hot wire probe was employed to measure time-series velocities. Test sections were formed by two parallel plates attached on a side wall of an aerodynamic channel, forming a slot with width D and depth P. In this paper twelve test sections were investigated covering the P/D- ratio ranging from 3.33 up to 11.00. For every test sections the Reynolds number is considered constant, ReDh=7.98×104. The main results showed that the dynamic characteristics of the turbulent flow in these structures are not dependent only on the P/D-ratio, but also the test section’s length along with the gap width. Periodic patterns in the time-series of velocity fluctuations, that suggests the presence of large scale structures, could be seen as the channel becomes longer and the Strouhal number tends to those previously noticed in the literature. The spectrogram of Continuous Wavelet Transform (CWT) revealed that different frequencies are excited as the time passes causing the broadness of the main frequency peak in PSD. Furthermore, the scattering of the higher energetic coefficients from CWT seems to increase as the width of the slot increases, showing that the large vortices formation might present some degree of unsteadiness

Studying structure formation and evolution with strong-lensing galaxy groups

AbstractWe present the analysis of a sample of strong-lensing galaxy group candidates. Our main findings are: confirmation of group-scale systems, complex light distributions, presence of large-scale structures in their surroundings, and evidence of a strong-lensing bias in the mass-concentration relation. We also report the detection of the first 'Bullet group'.

Ultra Deep Catalogue of Galaxy Structures in the Cosmic Evolution Survey field

This paper presents a large sample of intermediate- to high-redshift galaxy groups and clusters detected using a fully automated search in the Cosmic Evolution Survey field. The detection algorithm is based on density peak extraction from a density distribution sampled using Voronoi tessellation within overlapping slices in the photometric redshift space. The cluster catalogue contains 1780 structures covering the redshift range 0.2 0.4 and many even below this threshold show very prominent substructure indicating that z~ 0.4 marks the slow emergence of virialized clusters in this field in agreement with published findings for other regions of the sky. The redshift distribution of detected structures shows strong variations with prominent peaks suggesting the presence of large-scale structures across the whole range covered by this catalogue. Supercluster candidates have been identified at redshifts z= 0.35, 0.72, 0.94, 1.12, 1.27, 1.45, 2.0 and 2.52. At z= 2.9 we identified a compact agglomeration of galaxy groups and clusters suggesting the presence of another supercluster-like structure which has been the highest redshift candidate so far. Out of the nine supercluster candidates found in this study, six are new detections. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

The development of large-scale instability in Keplerian stellar accretion disks

Our previous studies of large-scale vortical flows arising in shear flows of stellar accretion disks with Keplerian azimuthal velocity distributions as a result of the development of small perturbations are continued. The development of large-scale instability in an accretion disk is investigated via mathematical modeling. One result obtained is the change of the disk flow structure due to the formation of large vortices. In the limiting case, sufficiently long evolution leads to the formation of several asymmetric spiral structures of the flow of disk matter. The presence of large-scale structures leads to angular-momentum redistribution in the disk.

Three-dimensional conditional structure of a high-Reynolds-number turbulent boundary layer

An array of surface hot-film shear-stress sensors together with a traversing hot-wire probe is used to identify the conditional structure associated with a large-scale skin-friction event in a high-Reynolds-number turbulent boundary layer. It is found that the large-scale skin-friction events convect at a velocity that is much faster than the local mean in the near-wall region (the convection velocity for large-scale skin-friction fluctuations is found to be close to the local mean at the midpoint of the logarithmic region). Instantaneous shear-stress data indicate the presence of large-scale structures at the wall that are comparable in scale and arrangement to the superstructure events that have been previously observed to populate the logarithmic regions of turbulent boundary layers. Conditional averages of streamwise velocity computed based on a low skin-friction footprint at the wall offer a wider three-dimensional view of the average superstructure event. These events consist of highly elongated forward-leaning low-speed structures, flanked on either side by high-speed events of similar general form. An analysis of small-scale energy associated with these large-scale events reveals that the small-scale velocity fluctuations are attenuated near the wall and upstream of a low skin-friction event, while downstream and above the low skin-friction event, the fluctuations are significantly amplified. In general, it is observed that the attenuation and amplification of the small-scale energy seems to approximately align with large-scale regions of streamwise acceleration and deceleration, respectively. Further conditional averaging based on streamwise skin-friction gradients confirms this observation. A conditioning scheme to detect the presence of meandering large-scale structures is also proposed. The large-scale meandering events are shown to be a possible source of the strong streamwise velocity gradients, and as such play a significant role in modulating the small-scale motions.

Power law correlations in galaxy distribution and finite volume effects from the Sloan Digital Sky Survey Data Release Four

We discuss the estimation of galaxy correlation properties in several volume limited samples, in different sky regions, obtained from the Fourth Data Release of the Sloan Digital Sky Survey. The small scale properties are characterized through the determination of the nearest neighbor probability distribution. By using a very conservative statistical analysis, in the range of scales [ 0.5, ~30] Mpc/h we detect power-law correlations in the conditional density in redshift space, with an exponent . This behavior is stable in all the different samples we considered; thus it does not depend on galaxy luminosity. In the range of scales [ ~30, ~100] Mpc/h we find evidence for systematic unaveraged fluctuations and we discuss in detail the problems induced by finite volume effects on the determination of the conditional density. We conclude that in such a range of scales there is evidence for a smaller power-law index of the conditional density. However we cannot distinguish between two possibilities: (i) that a crossover to homogeneity (corresponding to in the conditional density) occurs before 100 Mpc/h; (ii) that correlations extend to scales of order 100 Mpc/h (with a smaller exponent ). We emphasize that galaxy distributions in these samples present large fluctuations at the largest scales probed, corresponding to the presence of large scale structures extending up to the boundaries of the present survey. We discuss several differences between the behavior of the conditional density in mock galaxy catalogs built from cosmological N-body simulations and real data. We discuss some theoretical implications of such differences considering also the super-homogeneous features of primordial density fields.

Spectroscopically confirmed large-scale structures associated to a z= 0.83 cluster

We present a discovery of definitive large-scale structures around RXJ0152.7-1352 at z = 0.83 based on spectroscopic redshifts. In our previous papers, we reported a photometric identification of the large-scale structures at z ∼ 0.8. A spectroscopic follow-up observation was carried out on eight selected regions covering the most prominent structures to confirm their association to the main cluster. In six out of the eight fields, a well-isolated peak is identified in the distribution of spectroscopic redshifts at or near the cluster redshift. This is strong evidence for the presence of large-scale structures associated to the main cluster at z = 0.83. It seems that there are two large filaments of galaxies at z ∼ 0.837 and ∼0.844 crossing in this field. We then investigate stellar populations of galaxies in the structures. The composite spectra are constructed from a number of red member galaxies on the colour-magnitude sequence. We consider three representative environments - cluster, group and field - to investigate the environmental dependence of their star formation histories. We quantify the strengths of the 4000-A break (D 4000 ) and the Hδ absorption features and compare them with model predictions. The 'cluster' red galaxies do not show any sign of on-going or recent star formation activities and the passive evolution can naturally link them to the present-day red sequence galaxies in the Sloan Digital Sky Survey. In contrast, the red galaxies in 'groups' and in the 'field' tend to show signs of remaining and/or recent star formation activities characterized by weak [OII] emissions and/or strong Hδ absorptions. Our current data seem to favour a scenario that star formation is truncated in a short time-scale (<1 Gyr). This would imply that galaxy-galaxy interactions are responsible for the truncation of star formation.

Rotationally modulated X-ray emission from the single O starζOphiuchi

Archived measurements by the X-ray Telescope on board the ASCA satellite of the single runaway O9.5V star, ζOphiuchi, are analysed. The data set is unique as it covers just more than one full rotational period of the star. We report a clearly detected periodic X-ray flux variability with amplitude ~20% in the ASCA passband (0.5-10 keV). The detected period possibly indicates a connection with the recurrence time () of the discrete absorption components (DACs) in UV spectra of the star, thought to be due to the presence of large scale structures in the stellar wind modulated by rotation. We attribute the X-ray fluctuation with an uneven distribution of X-ray absorbing material. We also report that an analysis of similar ASCA observations of ζPuppis failed to confirm earlier reported variability of X-rays from this star based on Rosat observations.

A PIV study of a supersonic impinging jet

The characteristics of supersonic impinging jets are investigated using Particle Image Velocimetry (PIV). The purpose of the experiments is to understand the jet induced forces on STOVL aircraft while hovering close to the ground. For this purpose, a large diameter circular plate was attached at the nozzle exit. The oscillations of the impinging jet generated due to a feedback loop are captured in the PIV images. The instantaneous velocity field measurements are used to describe flow characteristics of the impinging jet. The important flow features such as oscillating shock waves, slipstream shear layers and large scale structures are captured clearly by the PIV. The presence of large scale structures in the impinging jet induced high entrainment velocity in the near hydrodynamic field, which resulted in lift plate suction pressures. A passive control device is used to interfere with the acoustic waves travelling in the ambient medium to suppress the feedback loop. As a consequence, the large scale vortical structures disappeared completely leading to a corresponding reduction in the entrainment.

A STUDY OF MAGNETIC COMPLEXITY USING HURST'S RESCALED RANGE ANALYSIS

A fractal analysis using the classical Hurst method has been applied to artificial data, simulated sunspot magnetic field data, and to data acquired with NASA/Marshall Space Flight Center's vector magnetograph. The main goals of this study are to quantify the complexity of an active region and to determine if significant changes in complexity are associated with flare activity. We tested the analysis using three basic types of two-dimensional synthetic data: (1) data composed of gaussians with various types of superimposed features, (2) random data, and (3) synthetic sunspots created from a basic, simple configuration on which are placed increasingly smaller structures. Our results confirm that the Hurst method of analysis is sensitive to the presence of large-scale structures within a two-dimensional image. When the large-scale structure has been removed, the value of the Hurst exponent is inversely proportional to increasing complexity in the image. The Hurst exponent of magnetograph data with the large-scale structure of the sunspot removed, shows a tantalizing variation in the shear parameter five minutes prior to a flare.

High-speed cinematography of compressible mixing layers

Experiments are performed using high-speed film cinematography to temporally resolve compressible planar mixing layer structures using shadowgraphs and planar light sheet visualization. The technique is relatively inexpensive and allows multiple images. The time-dependent shadowgraph and Mie scattering images are documented with a rotating mirror camera operating at approximately 350 kHz. The results show the presence of large scale structures in the mixing layer which flatten as they convect downstream. Both spatial and temporal covariances have been obtained through digital image processing which yield, on average, elliptical structures with convective speeds above the isentropic prediction, and non-isotropic streamwise and transverse scalar transport fluctuations.

Observations of large-scale structures in wakes behind axisymmetric bodies

Wakes behind disk-shaped axisymmetric bodies of varying solidity are studied using flow visualization and two-dimensional Fourier decomposition of velocity measurements. Evidence of a reverse flow region behind some of the bodies is observed to coincide with the presence of large-scale structures in the near and far wake. Fourier analysis shows that these large-scale structures are predominately helical (m= ±1) and occur at a characteristic frequency which corresponds to their wavelength as observed from flow visualization. Our measured value for this characteristic frequency agrees with vortex shedding frequencies observed for these types of wakes.

Reduced NO x emissions and enhanced large scale turbulence from a precessing jet burner

The performance characteristics of a Precessing Jet (PJ) burner, developed at the University of Adelaide, have been investigated in Furnace No. 1 of the International Flame Research Foundation firing natural gas with air pre-heated to 320°C. Results showed that the PJ burner, operating without air swirl, fuel staging or FGR, produces NOx emissions of about 57 ppm (dry, 3% O2) which is comparable to the performance of a swirled flame operated at optimised conditions with either flue gas recirculation, FGR, or fuel staging. The emissions are significantly lower than the 134 ppm measured for the base-line swirl burner, but greater than the 37 ppm measured for the swirl burner operated with both FGR and fuel staging. The flame was investigated by means of in-flame measurement of temperature and species concentration using suction probes and by laser sheet visualisation and subsequently derived probability density functions of jet fluid concentration. These techniques show that the mixing and combustion processes are dominated by the presence of large scale structures. It is anticipated that a more complete understanding of the interaction between the large scale turbulence and the combustion process will lead to further improvements in the performance of the burner.

The role of large scale structures in turbulent jets

The presence of large scale structures in circular jets is well documented in the literature over a wide range of Reynolds numbers. This presentation explores in more detail the generation and interaction and the role they play in the development of the flow field. It is based on experiments carried out, both in water and air jets, at Reynolds numbers up to 100 000.1,2 It is found, as reported by others, that the frequency of generation, as well as the various modes of the large scale vortices, can be predicted by linear stability theory. However, their further development as they are convected downstream is most difficult to study in detail. Conventional correlation measurements or sampling techniques have proven to be inadequate. In fact, the purpose for doing parallel water and jet experiments is to enable one to develop an adequate experimental method that could provide better information on the interaction process between adjacent structures. In a qualitative way, the measurements so far suggest that the induced flow field of the structures must play an important role in the interaction process and in the flow development. In the region of the potential core there is apparently an upstream influence due to the induced pressure field. This affects and, perhaps, controls such flow characteristics as the length of the potential core and the passage frequency of the structures at the end of the core. By introducing artificial disturbances into the flow further work is being carried out to clarify the above conjecture. This research was supported by the National Aeronautics and Space Administration under Grant NASA-NSG-1221.