Luoyi Tao

In this work we establish a link between a Reynolds averaged turbulence modeling methodology, containing interactions up to the second order correlations between the velocity fluctuations at various scales, and a multi-objective optimization problem with the constraints expressed in terms of equality and inequality, imposed by the given boundary conditions and the positive semi-definiteness of the Reynolds stress tensor, etc. The information unavailability and uncertainty associated with the boundary conditions for the fluctuation correlations of various orders is delineated, and the information from the Navier-Stokes equations is utilized to the extent allowed by the available input data necessary for simulations; turbulence from the perspective of systems simulation is explored and some objective functions are proposed. Finally, the challenges faced by the formulation and the issues yet to be resolved are discussed. © 2009.

Dispersion of active Brownian particles is a fundamental issue in biological, environmental, and related applications. However, due to the restriction in former models, a detailed analysis of Taylor dispersion of gyrotactic microorganisms in a horizontal plane Poiseuille flow is still lacking. In the present paper, with a recently proposed method [Jiang and Chen, J. Fluid Mech. 877, 1 (2019)JFLSA70022-112010.1017/jfm.2019.562], we illustrate the influences of the swimming ability, gyrotaxis intensity, shape anisotropy of microorganisms, and velocity of the ambient fluid on the dispersion process. Compared with nongyrotactic ones, there is a double accumulation mechanism for gyrotactic microorganisms: gravitactic focusing and wall accumulation. By using different boundary conditions, we show the effects of gravitactic focusing alone and double accumulation together. The variations of vertical distribution, overall drift, and effective dispersivity are characterized by changing the characteristic parameters of the microorganisms and the flow. Consisting of a swimming-induced part and an advection-induced part, the overall drift and effective dispersivity are coupled with the shape factor, flow Péclet number, and swimming Péclet number, which leads to nonmonotonic variations as functions of these parameters.

As the largest CH4 emitter, China produces CH4 at an increasing rate, especially from its energy activities. Presented in this paper is a detailed inventory and analysis of CH4 emissions from energy activities in China from 1980 to 2007 covering all the significant sources. The total energy-related CH4 emissions in China tripled during the period with an average annual increase rate of 4.7% and reached 21,943.1 Gg in 2007, 2.4 times of that in USA. As the largest emission source, coal mining increased its share from 69.2% (4559.5 Gg) in 1980 to 85.8% (18,825.5 Gg) in 2007; The second biggest source was fuel combustion, mainly bio-fuel combustion (2370.3 Gg in 2007); Oil and natural gas system leakage was a minor source but at a rapidly increasing rate. This transient emission structure is quite different from the steady structure of USA, which is dominated by the fugitive emissions from natural gas and oil systems. According to the lower IPCC Global Warming Potential, the annual energy-related CH4 emissions were equivalent to 9.1%-11.7% of China's energy-related CO2 emissions, amounting to 548.6 Mt CO2-eq in 2007 which is greater than the nationwide gross CO2 emissions in many developed countries. © 2013 Elsevier Ltd. All rights reserved.

We investigate the applicability of Poincaré's and the quantum recurrence theorems to the physical systems such as a container filled with a gas from a physical perspective. We discuss certain difficulties with regard to the definition of the Hamiltonian for such systems and the existence of one-to-one mappings as solutions to the equations of motion. We examine a typical proof of Poincaré's recurrence theorem and notice that a key part of the proof appeals to the motion of a neighborhood of finite volume in a phase space and this neighborhood possesses infinite kinetic energy, raising another question concerning the applicability of the theorem to physical systems. The same situations also present themselves in the application of Liouville's theorem. © 2008 Elsevier B.V. All rights reserved.

Dispersion of substances with reactive boundaries is relevant to a wide range of chemical, biological, and geophysical processes. A supplied crossflow, or equivalently sedimentation of the substance, is also expected to affect the dispersion process. We consider a setting with two infinite parallel plates, where the diffusive substance is adsorbed at the lower plate, simultaneously advected longitudinally by a main flow and vertically by a crossflow. Although the same configuration has been studied previously with the generalized Taylor dispersion (GTD) theory [M. Shapiro and H. Brenner, AIChE J. 33, 1155 (1987)10.1002/aic.690330710] and a dual-perturbation method [T. Y. Lin and E. S. G. Shaqfeh, Phys. Rev. Fluids 4, 034501 (2019)10.1103/PhysRevFluids.4.034501], both of them focused on the long-time asymptotic dispersion regime, exclusive of the important transient dispersion process. As an extension of these works to the transient dispersion process, we utilize the classical method of moments along with the eigenfunction expansion to calculate the moments up to fourth order, and thus the effects of non-Gaussian properties can be reflected. Compared with the result of Brownian dynamics simulations, the present work is shown sufficient to cover the preasymptotic dispersion regime out of reach of the GTD and dual-perturbation method. Strong non-Gaussian properties are found in the preasymptotic regime, as reflected by the nonzero skewness and kurtosis as well as asymmetric longitudinal concentration distribution. Additionally, it is found that the duration of the preasymptotic regime is extended in the presence of both the crossflow and wall adsorption. Considering that most of the substance may have been adsorbed during the preasymptotic regime, it is necessary to use higher-order dispersion models such as the one presented herein.

Shear-induced migration of elongated micro-swimmers exhibiting anisotropic Brownian diffusion at a population scale is investigated analytically in this work. We analyse the steady motion of confined ellipsoidal micro-swimmers subject to coupled diffusion in a general setting within a continuum homogenisation framework, as an extension of existing studies on macro-transport processes, by allowing for the direct coupling of convection and diffusion in local and global spaces. The analytical solutions are validated successfully by comparison with numerical results from Monte Carlo simulations. Subsequently, we demonstrate from the probability perspective that symmetric actuation does not yield net vertical polarisation in a horizontal flow, unless non-spherical shapes, external fields or direct coupling effects are harnessed to generate steady locomotion. Coupled diffusivities modify remarkably the drift velocity and vertical migration of motile micro-swimmers exposed to fluid shear. The interplay between stochastic swimming and preferential alignment could explain the diverse concentration and orientation distributions, including rheological formations of depletion layers, centreline focusing and surface accumulation. Results of the analytical study shed light on unravelling peculiar self-propulsion strategies and dispersion dynamics in active-matter systems, with implications for various transport problems arising from the fluctuating shape, size and other external or inter-particle interactions of swimmers in confined environments.