TY - JOUR

T1 - Characteristics of backward and forward two-particle relative dispersion in turbulence at different Reynolds numbers

AU - Buaria, Dhawal

AU - Sawford, Brian Lewis

AU - Yeung, Pui-Kuen

PY - 2015

Y1 - 2015

N2 - A new algorithm based on postprocessing of saved trajectories has been developed and applied to obtain wellsampled backward and forward relative dispersion statistics in stationary isotropic turbulence, over a range of initial separations ranging from Kolmogorov to energy-containing scales. Detailed results are obtained over a range of Taylorscale Reynolds numbers, up to 1000, which is higher than in recent work in the literature. Backward dispersion is faster, especially at intermediate times after the ballistic range and before longtime diffusive behavior is reached. Richardson scaling has been demonstrated for the meansquared separation, and forward and backward Richardson constants estimated to be gf = 0.55 and gb = 1.5, which are close to or comparable to other estimates. However, because of persistent dissipation infrange effects no corresponding scaling was observed for higher order moments of the separation. Analysis of the separation probability density function showed only transitory agreement with the wellknown Richardson prediction. The strong exponential growth of the separation on dissipation infrange scales was analyzed in terms of a central limit theory approximation. The resulting predictions for the ratio of the growth rates of the third- and fourth- order moments are reasonably consistent with the theory. The backward growth rates, corresponding to the ratio of the magnitude of the smallest to largest Lyapunov exponents, are about 50 greater than the forward growth rates, somewhat higher than other estimates. The predicted asymmetry between backward and forward relative displacements at early times, manifested in a t3 variation of the difference in the backward and forward meansquare relative displacement, was confirmed numerically and explicitly traced to Eulerian properties at the small scales. However, this t3 growth is not simply connected to the t3 growth in the Richardson regime and the asymmetry manifested there by the difference in the backward and forward Richardson constants. Asymmetry in time for higher order moments was also explained using a Taylor-series analysis at early times

AB - A new algorithm based on postprocessing of saved trajectories has been developed and applied to obtain wellsampled backward and forward relative dispersion statistics in stationary isotropic turbulence, over a range of initial separations ranging from Kolmogorov to energy-containing scales. Detailed results are obtained over a range of Taylorscale Reynolds numbers, up to 1000, which is higher than in recent work in the literature. Backward dispersion is faster, especially at intermediate times after the ballistic range and before longtime diffusive behavior is reached. Richardson scaling has been demonstrated for the meansquared separation, and forward and backward Richardson constants estimated to be gf = 0.55 and gb = 1.5, which are close to or comparable to other estimates. However, because of persistent dissipation infrange effects no corresponding scaling was observed for higher order moments of the separation. Analysis of the separation probability density function showed only transitory agreement with the wellknown Richardson prediction. The strong exponential growth of the separation on dissipation infrange scales was analyzed in terms of a central limit theory approximation. The resulting predictions for the ratio of the growth rates of the third- and fourth- order moments are reasonably consistent with the theory. The backward growth rates, corresponding to the ratio of the magnitude of the smallest to largest Lyapunov exponents, are about 50 greater than the forward growth rates, somewhat higher than other estimates. The predicted asymmetry between backward and forward relative displacements at early times, manifested in a t3 variation of the difference in the backward and forward meansquare relative displacement, was confirmed numerically and explicitly traced to Eulerian properties at the small scales. However, this t3 growth is not simply connected to the t3 growth in the Richardson regime and the asymmetry manifested there by the difference in the backward and forward Richardson constants. Asymmetry in time for higher order moments was also explained using a Taylor-series analysis at early times

UR - http://goo.gl/rfKEID

U2 - 10.1063/1.4931602

DO - 10.1063/1.4931602

M3 - Article

VL - 27

SP - 1

EP - 24

JO - Physics of Fluids

JF - Physics of Fluids

SN - 1089-7666

IS - 10

ER -