Visualization of turbulent flow around a sphere at subcritical reynolds numbers

The shear layer evolution and turbulent structure of near-wake behind a sphere atRe= 11,000 and 5,300 were investigated using a smoke-wire visualization method. A laminar flow separation was found to occur near the equator. The smooth laminar shear layers appeared to be axisymmetrically stable to the downstream location of aboutx/d=1.0 atRe=11,000 andx/d= 1.7∼1.8 atRe=5,300, respectively. At Re=11,000, the vortex ring-shaped protrusions were observed with the onset of shear layer instability. Moreover, the transition from laminar to turbulence in the separated flow region occurred earlier at the hiher Reynolds number ofRe=11,000 than atRe=5,300. The PIV measurements in the streamwise and cross-sectional planes atRe=11,000 clearly revealed the turbulent structures of the sphere wake such as recirculating flow, shear layer instability, vortex roll-up, and small-scale turbulent eddies.     

Deposition of inertial particles from turbulent flow in channels at high Reynolds numbers

On the basis of the previously developed asymptotic theory of turbulent particle-laden flow with particle deposition in channels coupled with the transport model for the particle Reynolds stress, an asymptotic solution to the problem on the deposition of particles in the limit of high Reynolds numbers was obtained. The numerical calculations confirmed the presence, in the region of the transition from the diffusion-impaction regime of particle sedimentation to the inertia-moderated regime, bifurcation phenomenon of a solution found previously in earlier studies. Features of particle accumulation in the viscous sublayer are analyzed. On the basis of the numerical solution, correlations for particle deposition velocity were obtained. Boundary conditions of the wall-function type for particle concentration whose use allows widening the applicability limits of the equilibrium Eulerian models in terms of particle inertia are proposed.     

Deviations from the law of velocity defect observed for small reynolds numbers

Deviations from the velocity defect law for a turbulent flow through plane channels and tubes with a circular cross sections are studied by direct numerical simulation based on the results obtained by different authors.     

高瑞利数下水平自然热对流的幂律关系

采用数值模拟方法，研究了高度和宽度比为1∶10的狭长矩形腔内的水平自然热对流. 根据对瑞利数(Rayleigh数)Ra在10⁴ <Ra<10¹¹内情形的计算结果，将流动分为三个不同的区间：线性区、连续过渡区、1/5次幂律区. 虽然流量和努塞尔数(Nusselt数)Nu随瑞利数的变化都包括了三个参数演化区间，但从一个区间到另外一个参数区间的转变时并不是同步的，其中努塞尔数的转变总是超前流量的转变. 对比前人的研究发现，流量1/3次幂律的结果是由于瑞利数不够高所致. 此外，模拟结果也表明Siggers等的理论分析过高估计了热通量强度，实际的温度边界层内努塞尔数和瑞利数为1/5次幂律关系.     

Chaos from turbulence: stochastic-chaotic equilibrium in turbulent convection at high Rayleigh numbers

It is shown that the correlation function of the mean wind velocity generated by a turbulent thermal convection (Rayleigh number Ra ～ 10(11)) exhibits exponential decay with a very long correlation time, while the corresponding largest Lyapunov exponent is certainly positive. These results together with the reconstructed phase portrait indicate the possible presence of chaotic component in the examined mean wind. Telegraph approximation is also used to study the relative contribution of the chaotic and stochastic components to the mean wind fluctuations and an equilibrium between these components has been studied in detail.     

Influence of the reynolds number on the plane-channel turbulent flow of a fluid

Using the data on direct numerical simulation of the plane-channel fluid flow obtained by several authors, the influence of the Reynolds number on the location and maximal values of the tangential and normal Reynolds stresses is investigated. The flow in a viscous sublayer is studied. Deviations from the law of the wall are noted.     

The two-level element free Galerkin method for MHD flow at high Hartmann numbers

In this Letter, a new element free Galerkin method, namely the two-level element free Galerkin method, is presented for solving the governing equations of steady magnetohydrodynamic duct flow. Because this element free Galerkin method makes use of the nodal point configurations which do not require a mesh, therefore it differs from FEM-like approaches by avoiding the need of meshing, a very demanding task for complicated geometry problems. Another distinguished feature of the proposed method is the resolving capability of high gradients near the layer regions without local or adaptive refinements. Numerical results indicate that no matter how large the Hartmann number is, this method has the ability to produce the satisfactory results for the velocity and the magnetic field simultaneously. That is to say, the presented method has some excellent properties, such as better stability and accuracy.     

Stability of a jet in confined pressure-driven biphasic flows at low reynolds numbers

Motivated by its importance for microfluidic applications, we study the stability of jets formed by pressure-driven concentric biphasic flows in cylindrical capillaries. The specificity of this variant of the classical Rayleigh-Plateau instability is the role of the geometry which imposes confinement and Poiseuille flow profiles. We experimentally evidence a transition between situations where the flow takes the form of a jet and regimes where drops are produced. We describe this as the transition from convective to absolute instability, within a simple linear analysis using lubrication theory for flows at low Reynolds number, and reach remarkable agreement with the data.     

Noise of a turbulent boundary layer flow over smooth and rough plates at low mach numbers

The spectral levels of the quadrupole noise generated by a boundary layer flow over a smooth surface are calculated. Explicit dependences of the noise levels on the Reynolds number are obtained for the low-frequency and high-frequency ranges. It is shown that the logarithmic zone of the velocity profile is responsible for the region of the quadrupole noise spectrum with a hyperbolic dependence on frequency. A method of calculating the dipole noise of a boundary layer flow over a rough surface is developed. The method is based on the use of the combined probability density for the turbulent velocity fluctuations and the random dimensions of protuberances of the rough surface. The two constants involved in this theory are determined from a special experiment. It is shown that the surface roughness noticeably increases the radiation levels of a boundary layer flow in a certain frequency range.     

MAS-NMR at very high temperatures

We report MAS-NMR experiments at temperatures of approx. 1200 K using a CO(2) laser as the heating device. An internal NMR thermometer based on the (7)Li T1 data of Li(0.24)La(0.54)TiO(3) is used for temperature calibration. Using this setup, temperatures as high as 1191 K could be reached under MAS conditions as confirmed by the melting of Li(2)B(4)O(7) at 1191 K which could be followed by (7)Li-MAS-NMR.     

A bubble-stabilized least-squares finite element method for steady MHD duct flow problems at high Hartmann numbers

In this paper we devise a stabilized least-squares finite element method using the residual-free bubbles for solving the governing equations of steady magnetohydrodynamic duct flow. We convert the original system of second-order partial differential equations into a first-order system formulation by introducing two additional variables. Then the least-squares finite element method using C⁰$C^0$ linear elements enriched with the residual-free bubble functions for all unknowns is applied to obtain approximations to the first-order system. The most advantageous features of this approach are that the resulting linear system is symmetric and positive definite, and it is capable of resolving high gradients near the layer regions without refining the mesh. Thus, this approach is possible to obtain approximations consistent with the physical configuration of the problem even for high values of the Hartmann number. Before incoorperating the bubble functions into the global problem, we apply the Galerkin least-squares method to approximate the bubble functions that are exact solutions of the corresponding local problems on elements. Therefore, we indeed introduce a two-level finite element method consisting of a mesh for discretization and a submesh for approximating the computations of the residual-free bubble functions. Numerical results confirming theoretical findings are presented for several examples including the Shercliff problem.     

Crisis of hydrodynamic drag of drops in the two-phase turbulent flow of a spray produced by a mechanical nozzle at transition reynolds Numbers

When processing experimental data for the hydrodynamics of a two-phase flow in a spray produced by a mechanical nozzle, we revealed an anomaly in the behavior of the hydrodynamic drag of drops: the drag coefficient turns out to be four to seven times lower than the previously known values. Several hypotheses are put forward to explain the anomaly. It is found that, when the gas flows around drops under highly turbulent conditions, an “early” (i.e., observed even at transition Reynolds numbers, Re>50) crisis of drag resistance of drops takes place. This new physical phenomenon allows us to account for a number of features of the two-phase flow that are observed in the experiment. Among these features is, in particular, the fact that the momentum transferred to the gas is roughly half the initial momentum of the liquid jet.     

Microfluidic valves for flow control at low Reynolds numbers

Fluidics is a technology of generating and controlling fluid flows &#x2014; preferably without the action of mechanical moving components. Microfluidics perform these tasks in small, typically micronsized structures. Essential part of almost all microfluidic systems are flow control valves. The basic problem is the low Reynolds numberRe: inertial effects used in large-scale fluidics are too small relative to viscous dissipation. New approaches, such as pressure or electrokinetic driving are required. In the subdynamic, viscosity dominated flow regime, Re ceases to be of importance and for pressure-driven valves a new characterisation number was to be introduced. An example of a diverter valve, developed by the author, is described and the meaning of the new dimensionless parameter is demonstrated.     

Gamma-ray bursts at high and very high energies

Gamma-Ray Bursts (GRBs) are extra-galactic and extremely energetic transient emissions of gamma rays, which are thought to be associated with the death of massive stars or the merger of compact objects in binary systems. Their huge luminosities involve the presence of a newborn stellar-mass black hole emitting a relativistic collimated outflow, which accelerates particles and produces non-thermal emissions from the radio domain to the highest energies. In this article, I review recent progresses in the understanding of GRB jet physics above 100 MeV, based on Fermi observations of bright GRBs. I discuss the physical implications of these observations and their impact on GRB modeling, and I present some prospects for GRB observation at very high energies in the near future.     

Quantum gravity at very high energies

The problem of time and the quantization of three-dimensional gravity in the strong coupling regime is studied following path integral methods. The time is identified with the volume of spacetime. We show that the effective action describes an infinite set of massless relativistic particles moving in a curved three-dimensional target space, i.e., a tensionless 3-brane on a curved background. If the cosmological constant is zero the target space is flat and there is no ‘‘ graviton" propagation (i.e., G[g_ij(2),g_ij(1)]=0). If the cosmological constant is different from zero, 3D gravity is both classical and quantum mechanically soluble. Indeed, we find the following results: (i) the general exact solutions of the Einstein equations are singular at t=0 showing the existence of a big-bang in this regime and (ii) the propagation amplitude between two geometries 〈g_ij(2),t₂|g_ij(1),t₁〉 vanishes as t→0, suggesting that big-bang is suppressed quantum mechanically. This result is also valid for D>3.     

Two-photon processes at very high energies

Formulae are presented by which a numerically stable computer program for two-photon processes above $\text{s}\text{= 100}\text{GeV}$ can be constructed. For this purpose a new method for evaluating matrix elements is introduced. It is then shown that two-photon processes should be no background to Drell-Yan signals in pp collisions. A comparison is made between two-photon physics at e⁺e^?, ep and pp collisions.