The macroscopic behavior of dense suspensions of neutrally-buoyant spheres in turbulent plane channel flow is examined. We show that particles larger than the smallest turbulence scales cause the suspension to deviate from the continuum limit in which its dynamics is well described by an effective suspension viscosity. This deviation is caused by the formation of a particle layer close to the wall with significant slip velocity. By assuming two distinct transport mechanisms in the near-wall layer and the turbulence in the bulk, we define an effective wall location such that the flow in the bulk can still be accurately described by an effective suspension viscosity. We thus propose scaling laws for the mean velocity profile of the suspension flow, together with a master equation able to predict the increase in drag as function of the particle size and volume fraction.
We present experimental observations of the spatial distribution of large inertial particles suspended in a turbulent swirling flow at high Reynolds number. The plastic particles, which are tracked using several high speed cameras, are heavier than the working fluid so that their dynamics results from a competition between gravitational effects and turbulent agitation. We observe two different regimes of suspension. At low rotation rate, particles are strongly confined close to the bottom and are not able to reach the upper region of the tank whatever their size or density. At high rotation ra…
The power laws in the overlap region of two layer theory of turbulent boundary layer have been obtained by Izakson-Millikan-Kolmogorov hypothesis. The solution of the open functional equation is not unique. The power laws for velocity \& skin friction have equivalence with log laws for velocity \& skin friction, for large Reynolds numbers.
Microswimmer suspensions self-organize into complex spatio-temporal flow patterns, including vortex lattices and mesoscale turbulence. Here we explore the consequences for the motion of passive tracers, based on a continuum model for the microswimmer velocity field. We observe two qualitatively different regimes distinguished via the dimensionless Kubo number $K$. At advection strengths right above the transition to turbulence, the flow field evolves very slowly ($K \gg 1$) and the spatial vortex structures lead to dominant trapping effects. In contrast, deep in the turbulent state, much faste…
The diffusive transport in two-dimensional incompressible turbulent fields is investigated with the aid of high-quality direct numerical simulations. Three classes of turbulence spectra that are able to capture both short and long-range time-space correlations and oscillating features are employed. We report novel scaling laws that depart from the $γ=7/10$ paradigm of percolative exponents and are dependent on the features of turbulence. A simple relation between diffusion in the percolative and frozen regimes is found. The importance of discerning between differential and integral characteris…
Relative velocities in bi-disperse turbulent aerosols: simulations and theory
我们进行直接的数值模拟的双分散悬浮的重球形颗粒在强制，均匀，各向同性的三维湍流。我们计算了不同尺寸颗粒之间的相对颗粒距离和纵向相对速度的联合分布，并将结果与最近的理论预测[Meibohm et al. Phys. Rev. E 96（2017）061102]进行了比较。我们还计算了相对速度的时刻作为粒子分离的函数，并与理论预测进行比较。我们遵守良好的协议。
Homogeneity and isotropy in a laboratory turbulent flow
我们提出了一种新的设计，搅拌槽，迫使由两个平行的平面阵列的随机致动的合成射流。这种布置在高雷诺数和低平均流量下产生湍流。最重要的是，它表现出一个区域的3D均匀各向同性湍流，是显着大于积分的长度尺度。这些功能是必要的，使实验室测量湍流悬浮液。我们使用定量成像，以确认各向同性在大，小，和中等规模的检查一-和两-点统计在坦克中心。然后，我们重复这些相同的测量，以确认在罐中心测量的值在大的均匀区域内是恒定的。在垂直于对称平面的方向上，我们的测量表明，均匀区域延伸至少两倍的积分长度尺度$L=9.5$ cm。在平行于对称平面的方向上，该区域至少是整体长度尺度的四倍，并且在该方向上的范围仅受罐的尺寸限制。在均匀的各向同性区域内，我们测量的湍流动能为$6.07 \times 10^{-4} $ m $^2$ s $^{-2}$，耗散率为$4.65 \times 10^{-5} $ m $^2$ s $^{-3}$，泰勒尺度雷诺数为$R_λ=334$。该水箱的大均匀区域，结合其高雷诺数和非常低的平均流量，提供了最好的近似均匀各向同性湍流实现在实验室流动的日期。这些特性使搅拌槽成为研究湍流和湍流悬浮液基本动力学的最佳设备。
We study the fully-coupled dynamics between a fully-developed turbulent flow and an ensemble of immersed flexible fibers. We vary the concentration of the suspension, the mechanical properties and the length of the fibers in a vast parametric range. For all configurations, the fiber dynamics falls in only two possible dynamical states: (i) the fiber manifests its natural response to the flow forcing or (ii) its motion fully synchronizes to the hydrodynamic timescales of the turbulent flow. This scenario holds for both a dilute condition, where the carrier flow is not affected by the fluid-stru…
We present results of interface-resolved simulations of heat transfer in suspensions of finite-size neutrally-buoyant spherical particles for solid volume fractions up to 35% and bulk Reynolds numbers from 500 to 5600. An Immersed Boundary-Volume of Fluid method is used to solve the energy equation in the fluid and solid phase. We relate the heat transfer to the regimes of particle motion previously identified, i.e. a viscous regime at low volume fractions and low Reynolds number, particle-laden turbulence at high Reynolds and moderate volume fraction and particulate regime at high volume frac…
We describe an immersed boundary method in which the fluid-structure coupling is achieved in an Eulerian framework. The method is an improved extension of the immersed boundary method originally developed by Kajishima et al. [1], which accounts for the inertia of the fictitious fluid inside the particle volume and is thus able to reproduce the behaviour of particles both in the case of neutrally-buoyant objects and in the presence of density difference between the particles and the fluid. The method is capable to handle the presence of multiple suspended objects, i.e., a suspension, by includi…
Scaling Laws for Thermodiffusively Unstable Lean Premixed Turbulent Hydrogen-Air Flames
RWTH Aachen University（亚琛工业大学）；Loughborough University（拉夫堡大学）；Vrije Universiteit Brussel（布鲁塞尔自由大学）；Sandia National Laboratories（桑迪亚国家实验室）；Norwegian University of Science and Technology（挪威科技大学）；SINTEF Energy Research（辛特夫能源研究）；University of Edinburgh（爱丁堡大学）；Newcastle University（纽卡斯尔大学）
In most results concerning bounds on the heat transport in the Rayleigh-Bénard convection problem no-slip boundary conditions for the velocity field are assumed. Nevertheless it is debatable, whether these boundary conditions reflect the behavior of the fluid at the boundary. This problem is important in theoretical fluid mechanics as well as in industrial applications, as the choice of boundary conditions has effects in the description of the boundary layers and its properties. In fact, different boundary conditions may inhibit or enhance heat transport. This review presents a selection of co…
Similarity scaling, when it can be justified, is a powerful tool for predicting properties of fluid flows and reducing the computational load when using mathematical models. Numerous publications describe different applications of this method, using often different scaling laws with one or more scaling parameters. The justification for these laws is often based on some assumptions or references to experimental results. In this paper, we base the scaling law on basic physical principles of classical Newtonian physics (Galilei group) and derive some predictions that we apply to a simple model fo…
