Journal of Fluid Mechanics
SCOPUS (1956-2023)SCIE-ISI
1469-7645
0022-1120
Anh Quốc
Cơ quản chủ quản: Cambridge University Press , CAMBRIDGE UNIV PRESS
Lĩnh vực:
Mechanics of MaterialsCondensed Matter PhysicsMechanical EngineeringApplied Mathematics
Các bài báo tiêu biểu
On the identification of a vortexConsiderable confusion surrounds the longstanding question of what
constitutes a vortex, especially in a turbulent flow. This question,
frequently misunderstood as academic, has recently acquired particular
significance since coherent structures (CS) in turbulent flows are now
commonly regarded as vortices. An objective definition of a vortex should
permit the use of vortex dynamics concepts to educe CS, to explain formation
and evolutionary dynamics of CS, to explore the role of CS in turbulence
phenomena, and to develop viable turbulence models and control strategies
for turbulence phenomena. We propose a definition of a vortex in an
incompressible flow in terms of the eigenvalues of the symmetric tensor
${\bm {\cal S}}^2 + {\bm
\Omega}^2$ ; here ${\bm {\cal S}}$ and ${\bm \Omega}$ are respectively the
symmetric and antisymmetric parts of the velocity gradient tensor
${\bm \Delta}{\bm
u}$ . This definition captures the pressure minimum
in a plane perpendicular to the vortex axis at high Reynolds numbers, and
also accurately defines vortex cores at low Reynolds numbers, unlike a
pressure-minimum criterion. We compare our definition with prior
schemes/definitions using exact and numerical solutions of the Euler and
Navier–Stokes equations for a variety of laminar and turbulent flows. In
contrast to definitions based on the positive second invariant of
${\bm \Delta}{\bm
u}$ or the complex eigenvalues of
${\bm \Delta}{\bm
u}$ , our definition accurately identifies the
vortex core in flows where the vortex geometry is intuitively
clear.
Tập 285 - Trang 69-94 - 1995
Turbulence statistics in fully developed channel flow at low Reynolds numberA direct numerical simulation of a turbulent channel flow is performed. The unsteady Navier-Stokes equations are solved numerically at a Reynolds number of 3300, based on the mean centreline velocity and channel half-width, with about 4 × 106 grid points (192 × 129 × 160 inx, y, z ). All essential turbulence scales are resolved on the computational grid and no subgrid model is used. A large number of turbulence statistics are computed and compared with the existing experimental data at comparable Reynolds numbers. Agreements as well as discrepancies are discussed in detail. Particular attention is given to the behaviour of turbulence correlations near the wall. In addition, a number of statistical correlations which are complementary to the existing experimental data are reported for the first time.
Tập 177 - Trang 133-166 - 1987
Phân Tích Chế Độ Động Của Dữ Liệu Số Học và Thực Nghiệm Dịch bởi AI Việc mô tả các đặc điểm nhất quán của dòng chảy là cần thiết để hiểu các quá trình động học và vận chuyển chất lỏng. Một phương pháp được giới thiệu có khả năng trích xuất thông tin động lực học từ các trường dòng chảy được tạo ra bởi mô phỏng số trực tiếp (DNS) hoặc được hình ảnh hóa/đo lường trong một thí nghiệm vật lý. Các chế độ động được trích xuất, có thể được hiểu như sự tổng quát hóa của các chế độ ổn định toàn cục, có thể được sử dụng để miêu tả các cơ chế vật lý cơ bản được thể hiện trong chuỗi dữ liệu hoặc để chuyển đổi các vấn đề quy mô lớn sang một hệ thống động lực học với ít bậc tự do hơn đáng kể. Việc tập trung vào các tiểu vùng của trường dòng chảy, nơi động học có liên quan được kỳ vọng, cho phép phân chia dòng chảy phức tạp thành các vùng có hiện tượng bất ổn cục bộ và thêm một minh họa về sự linh hoạt của phương pháp, như việc mô tả động lực học trong một khung không gian. Các minh họa của phương pháp này được trình bày bao gồm dòng chảy trong kênh phẳng, dòng chảy qua một khoang hai chiều, dòng chảy sau màng linh hoạt và luồng khí đi qua giữa hai xi lanh.
Tập 656 - Trang 5-28 - 2010
#chế độ động #dòng chảy số #mô phỏng #bất ổn cục bộ #cơ chế vật lý #phương pháp phân tích động #miền phụ
On density effects and large structure in turbulent mixing layersPlane turbulent mixing between two streams of different gases (especially nitrogen and helium) was studied in a novel apparatus. Spark shadow pictures showed that, for all ratios of densities in the two streams, the mixing layer is dominated by large coherent structures. High-speed movies showed that these convect at nearly constant speed, and increase their size and spacing discontinuously by amalgamation with neighbouring ones. The pictures and measurements of density fluctuations suggest that turbulent mixing and entrainment is a process of entanglement on the scale of the large structures; some statistical properties of the latter are used to obtain an estimate of entrainment rates. Large changes of the density ratio across the mixing layer were found to have a relatively small effect on the spreading angle; it is concluded that the strong effects, which are observed when one stream is supersonic, are due to compressibility effects, not density effects, as has been generally supposed.
Tập 64 Số 4 - Trang 775-816 - 1974
The lift on a small sphere in a slow shear flowIt is shown that a sphere moving through a very viscous liquid with velocity V relative to a uniform simple shear, the translation velocity being parallel to the streamlines and measured relative to the streamline through the centre, experiences a lift force 81·2μVa 2 k ½ /v ½ + smaller terms perpendicular to the flow direction, which acts to deflect the particle towards the streamlines moving in the direction opposite to V . Here, a denotes the radius of the sphere, κ the magnitude of the velocity gradient, and μ and v the viscosity and kinematic viscosity, respectively. The relevance of the result to the observations by Segrée & Silberberg (1962) of small spheres in Poiseuille flow is discussed briefly. Comments are also made about the problem of a sphere in a parabolic velocity profile and the functional dependence of the lift upon the parameters is obtained.
Tập 22 Số 2 - Trang 385-400 - 1965
A refinement of previous hypotheses concerning the local structure of turbulence in a viscous incompressible fluid at high Reynolds numberThe hypotheses concerning the local structure of turbulence at high Reynolds number, developed in the years 1939-41 by myself and Oboukhov (Kolmogorov 1941 a,b,c ; Oboukhov 1941 a,b ) were based physically on Richardson's idea of the existence in the turbulent flow of vortices on all possible scales l < r < L between the ‘external scales’ L and the ‘internal scale’ l and of a certain uniform mechanism of energy transfer from the coarser-scaled vortices to the finer.
Tập 13 Số 1 - Trang 82-85 - 1962
The disintegration of wave trains on deep water Part 1. TheoryThe phenomenon in question arises when a periodic progressive wave train with fundamental frequency ω is formed on deep water—say by radiation from an oscillating paddle—and there are also present residual wave motions at adjacent side-band frequencies ω(1 ± δ), such as would be generated if the movement of the paddle suffered a slight modulation at low frequency. In consequence of coupling through the non-linear boundary conditions at the free surface, energy is then transferred from the primary motion to the side bands at a rate that, as will be shown herein, can increase exponentially as the interaction proceeds. The result is that the wave train becomes highly irregular far from its origin, even when the departures from periodicity are scarcely detectable at the start. In this paper a theoretical investigation is made into the stability of periodic wave trains to small disturbances in the form of a pair of side-band modes, and Part 2 which will follow is an account of some experimental observations in accord with the present predictions. The main conclusion of the theory is that infinitesimal disturbances of the type considered will undergo unbounded magnification if
\[
0 < \delta \leqslant (\sqrt{2})ka,
\]
where k and a are the fundamental wave-number and amplitude of the perturbed wave train. The asymptotic rate of growth is a maximum for δ = ka .
Tập 27 Số 3 - Trang 417-430 - 1967
Numerical simulations of particulate suspensions via a discretized Boltzmann equation. Part 1. Theoretical foundationA new and very general technique for simulating solid–fluid suspensions is described; its most important feature is that the computational cost scales linearly with the number of particles. The method combines Newtonian dynamics of the solid particles with a discretized Boltzmann equation for the fluid phase; the many-body hydrodynamic interactions are fully accounted for, both in the creeping-flow regime and at higher Reynolds numbers. Brownian motion of the solid particles arises spontaneously from stochastic fluctuations in the fluid stress tensor, rather than from random forces or displacements applied directly to the particles. In this paper, the theoretical foundations of the technique are laid out, illustrated by simple analytical and numerical examples; in a companion paper (Part 2), extensive numerical tests of the method, for stationary flows, time-dependent flows, and finite-Reynolds-number flows, are reported.
Tập 271 - Trang 285-309 - 1994