Experimental study of the fine-scale structure of conserved scalar mixing in turbulent shear flows. Part 1. <i>Sc</i> [Gt      ] 1

Buch, Kenneth A.; Dahm, Werner J. A.

doi:10.1017/s0022112096000651

Cited by 161 publications

(79 citation statements)

References 60 publications

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“…Following [9], λ ν is taken as the full width of the vortical structure at which ω z has decreased to one-fifth of its peak value, for which α = 4 ln 5. When diffusion of the vorticity is in equilibrium [9] with the background strain, then…”

Section: A Strain Rate Tensormentioning

confidence: 99%

“…An understanding of how vortical structures are stretched by the strain rate field S ij (x) is thus essential to any description of the energetics of such flows. Over the last two decades, direct numerical simulations (DNS) [5,6,7] and experimental studies [8,9,10,11,12] of the fine-scale structure of turbulence have revealed a preferred alignment of the vorticity with the intermediate eigenvector of the strain rate tensor. This result has been widely regarded as surprising.…”

Section: Introductionmentioning

confidence: 99%

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Local and nonlocal strain rate fields and vorticity alignment in turbulent flows

2008

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Local and nonlocal contributions to the total strain rate tensor Sij at any point x in a flow are formulated from an expansion of the vorticity field in a local spherical neighborhood of radius R centered on x. The resulting exact expression allows the nonlocal (background) strain rate tensor S B ij (x) to be obtained from Sij (x). In turbulent flows, where the vorticity naturally concentrates into relatively compact structures, this allows the local alignment of vorticity with the most extensional principal axis of the background strain rate tensor to be evaluated. In the vicinity of any vortical structure, the required radius R and corresponding order n to which the expansion must be carried are determined by the viscous lengthscale λν. We demonstrate the convergence to the background strain rate field with increasing R and n for an equilibrium Burgers vortex, and show that this resolves the anomalous alignment of vorticity with the intermediate eigenvector of the total strain rate tensor. We then evaluate the background strain field S B ij (x) in DNS of homogeneous isotropic turbulence where, even for the limited R and n corresponding to the truncated series expansion, the results show an increase in the expected equilibrium alignment of vorticity with the most extensional principal axis of the background strain rate tensor.

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Section: A Strain Rate Tensormentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Local and nonlocal strain rate fields and vorticity alignment in turbulent flows

2008

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“…For combustion, mixing of fuel/oxidizer is critical. Scalar mixing is strongly affected by small-scale turbulence structures [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Therefore, when the droplet/ligament size is comparable to the turbulence scale, it is expected that droplets/ligaments would change the small-scale structures, hence mixing.…”

Section: Introductionmentioning

confidence: 99%

Droplet/ligament modulation of local small-scale turbulence and scalar mixing in a dense fuel spray

Shinjo

Xia

Umemura

2015

Proceedings of the Combustion Institute

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In this study, the modulation of turbulence and scalar mixing by finite-size droplets/ligaments in a dense fuel spray is investigated using a DNS (Direct Numerical Simulation) dataset. Ejected from a spray nozzle with a high speed, a liquid-fuel jet deforms and the fuel spray is atomized into many ligaments and droplets. During these processes, the gas flow becomes turbulent due to droplet/ligament dynamics. At the same time, droplet evaporation and mixing with ambient air are affected by the small-scale gas turbulence. An understanding of the mixing characteristics in the dense spray zone is important for modeling spray combustion. In a region where the droplet number density is relatively low, a universal feature of isotropic turbulence was found, although the alignments of strain eigenvectors with vorticity and the mixture fraction gradient are slightly modulated by the presence of droplets, which is a characteristic of particle-laden flows. In gas-phase regions close to droplet surfaces, where the dissipation rate of turbulent kinetic energy is strongly increased, the alignments are more modulated, especially those of the scalar gradient with strain eigenvectors. This can also be seen in the topology similarity among energy dissipation, enstrophy and scalar dissipation in the near field of droplet/ligament surfaces. For the first time, it is found that droplets whose size is comparable to turbulence scales do affect the mixing characteristics in a realistic turbulent spray. This finding has shed new light upon the modeling of flow turbulence and scalar mixing in an evaporating and atomizing fuel spray.

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“…Several previous researches reported these spikes and their characteristics. The turbulent structure relating to the occurrence of the spikes was investigated in a grid turbulence and turbulent shear flow with a spatially uniform mean concentration gradient [10] [11] [12] [13]. The high concentration spikes occurred at a saddle point of the velocity field based on the large-scale structure of turbulent eddies.…”

Section: Introductionmentioning

confidence: 99%

Diffusion Process of High Concentration Spikes in a Quasi-Homogeneous Turbulent Flow

Endo¹,

Shao²,

Tsukahara³

et al. 2016

OJFD

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When a mass spreads in a turbulent flow, areas with obviously high concentration of the mass compared with surrounding areas are formed by organized structures of turbulence. In this study, we extract the high concentration areas and investigate their diffusion process. For this purpose, a combination of Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) techniques was employed to obtain simultaneously the two fields of the concentration of injected dye and of the velocity in a water turbulent channel flow. With focusing on a quasi-homogeneous turbulence in the channel central region, a series of PLIF and PIV images were acquired at several different downstream positions. We applied a conditional sampling technique to the PLIF images to extract the high concentration areas, or spikes, and calculated the conditional-averaged statistics of the extracted areas such as length scale, mean concentration, and turbulent diffusion coefficient. We found that the averaged length scale was constant with downstream distance from the diffusion source and was smaller than integral scale of the turbulent eddies. The spanwise distribution of the mean concentration was basically Gaussian, and the spanwise width of the spikes increased linearly with downstream distance from the diffusion source. Moreover, the turbulent diffusion coefficient was found to increase in proportion to the spanwise distance from the source. These results reveal aspects different from those of regular mass diffusion and let us conclude that the diffusion process of the spikes differs from that of regular mass diffusion.

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Experimental study of the fine-scale structure of conserved scalar mixing in turbulent shear flows. Part 1. Sc [Gt ] 1

Cited by 161 publications

References 60 publications

Local and nonlocal strain rate fields and vorticity alignment in turbulent flows

Local and nonlocal strain rate fields and vorticity alignment in turbulent flows

Droplet/ligament modulation of local small-scale turbulence and scalar mixing in a dense fuel spray

Diffusion Process of High Concentration Spikes in a Quasi-Homogeneous Turbulent Flow

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