Bifurcation of a Newtonian-fluid flow in a planar channel with sudden contraction and expansion

The features of a Newtonian-fluid flow in a two-dimensional channel with sudden contraction and expansion are investigated by numerical modeling. The kinetics of the bifurcation transition from the symmetric mode to steady-state asymmetric flow on the outlet from the zone of contraction of the channel is analyzed. The linear dependence of the degree of asymmetry of flow on the Reynolds number is established.     

Turbulent gas-dispersed flow in a pipe with sudden expansion: numerical simulation

A mathematical model was developed to simulate two-phase gas-dispersed flow moving through a pipe with axisymmetric sudden expansion. In the model, the two-fluid Euler approach was used. The model is based on solving Reynolds-averaged Navier — Stokes equations for a two-phase stream. In calculating the fluctuating characteristics of the dispersed phase, equations borrowed from the models by Simonin (1991), Zaichik et al. (1994), and Derevich (2002) were used. Results of a comparative analysis with previously reported experimental and numerical data on two-phase flows with separation past sudden expansion in a plane channel and in a pipe are given. This work was supported by the President of the Russian Federation through the Foundation for Young Candidates of Sciences under Grant MK-186.2007.8 and by the Russian Foundation for Basic Research (Grants Nos. 05-08-33586 and 06-08-00967).     

突扩燃烧室湍流预混反应流的数值模拟

对突扩燃烧室这一典型工程燃烧装置内的湍流预混反应流进行了数值模拟。时平均控制方程组的封闭采用k-ε湍流输运模型和EBU-Arhenius湍流反应模型。模拟结果给出了突扩燃烧室内湍流预混反应流的气体时均流场、组分浓度场与温度场的分布。通过数值模拟结果与实验的比较对EBU-Arhenius模型进行了讨论与评价。     

Simulation of turbulent non-isothermal polydisperse bubbly flow behind a sudden tube expansion

The results of numerical simulation of the structure of non-isothermal polydisperse bubbly turbulent flow and heat transfer behind a sudden tube expansion are presented. The study was carried out at a change in the initial diameter of the air bubbles within d _m1 = 1–5 mm and their volumetric void fraction β = 0–10 %. Small bubbles are available in almost the entire cross section of the tube, while the large bubbles pass mainly through the flow core. An increase in the size of dispersed phase causes the growth of turbulence in the liquid phase due to flow turbulization, when there is a separated flow of liquid past the large bubbles. Adding the air bubbles causes a significant reduction in the length of the separation zone and heat transfer enhancement, and these effects increase with increasing bubble size and their gas volumetric flow rate ratio.     

Peculiarities of propagation of the dispersed phase in a gas-droplet flow downstream of a pipe sudden expansion

The results of numerical simulation of the propagation of the dispersed phase in a gas-droplet flow downstream of a pipe sudden expansion for small initial mass concentrations of particles (M _L1 = 0–0.1) are presented. Fine-dispersed droplets with the Stokes numbers Stk < 1 are entrained by a separated flow and are present in the whole cross section of the pipe. The near-wall region of the pipe is free of fine particles due to intense evaporation. Heavy particles (Stk > 1) do not get in the recirculation flow region and are present only in the mixing layer and in the flow core. It is shown that the addition of fine-dispersed droplets suppresses the energy of the gaseous phase turbulence in the separated flow. The results are compared with the experimental data for two-phase separated flows and are found to be in the conformity with these data.     

Experimental and numerical study of heat transfer enhancement in a turbulent bubbly flow in a sudden pipe expansion

Results of an experimental and numerical simulation of heat transfer in an upward bubbly flowin a sudden pipe expansion are presented. The experimental study of the heat transfer has been performed using infrared thermography. The measurements of the bubble size before the pipe expansion area were carried out by the shadow photographymethod. The numerical simulation of the bubbly flow structure in the sudden pipe expansion has been performed using the Eulerian approach in the presence of heat transfer between the two-phase flow and the wall surface. The model uses the system of Reynolds-averaged Navier–Stokes equations in an axisymmetric approximation, written with consideration of the back effect of bubbles on the averaged and pulsation characteristics of the flow. It has been experimentally and numerically shown that addition of air bubbles causes a significant (up to 3-fold) increase in the heat transfer intensity, these effects growing with bubble concentration. The largest rise in the heat transfer has been revealed in the region of flow relaxation downstream of the flow attachment point.     

Stable regimes for hard disks in a channel with twisting walls

We study a gas of N hard disks in a box with semi-periodic boundary conditions. The unperturbed gas is hyperbolic and ergodic (these facts are proved for N=2 and expected to be true for all N≥2). We study various perturbations by twisting the outgoing velocity at collisions with the walls. We show that the dynamics tends to collapse to various stable regimes, however we define the perturbations, and however small they are.     

The effect of thermal prehistory on turbulent separated flow at sudden tube expansion

Results of numerical investigation of the effect of heat boundary layer thickness in front of a sudden expansion of a round tube on turbulent transfer in the zone of flow separation, attachment, and relaxation are presented. Before separation the flow was hydrodynamically stable, and the heat layer in front of expansion could change its thickness in maximally possible limits: from zero to a half of tube diameter. The Reynolds number varied from 6.7·10³ to 1.33·10⁵. It was found that the growth of heat layer thickness leads to reduction of heat transfer intensity in the separation area and moving away of the coordinate of maximal heat transfer from the place of tube expansion. Generalizing dependence for the maximum Nusselt number is given for variation of the heat layer thickness. Comparison with experimental data of [1] proved the main behavior tendencies of heat and mass transfer processes in separation flows behind a backward-facing step with different thermal prehistory.     

Influence of mean flow profile and geometrical ratios on scattering of sound at a sudden area expansion in a duct

The scattering of sound at a sudden area expansion in a duct with subsonic mean flow has been modelled with a multimodal method. Technological applications are for instance internal combustion engine exhaust silencers and silencers in industrial duct systems. Both 2D rectangular and 2D cylindrical geometries are considered.The influence of the mean flow profile, and the—in this method—associated application of an acoustic Kutta condition at the edge of the area discontinuity, is investigated. The scattering coefficients for the plane waves are found to change smoothly as the flow profile is changed gradually from one, where the acoustic Kutta condition is applied to one where it is not applied. Furthermore, for high Strouhal numbers no difference is observed in the results for the scattering coefficients obtained for different flow profiles. Also, at low Strouhal numbers the magnitudes of the scattering coefficients are the same for different profiles.The influence of the ratio of the heights (in 2D rectangular geometry), respectively, radii (in 2D cylindrical geometry), of the ducts upstream and downstream of the area expansion on the scattering coefficients is examined. Around a certain Strouhal number, a specific feature in the scattering coefficients is observed when the ratio of the duct heights or radii is less than 0.5. This is found to be connected to a strong interaction between the first evanescent acoustic mode and the hydrodynamic instability mode. For non-uniform flow even an apparent jump between the first evanescent acoustic mode and the hydrodynamic unstable mode and a corresponding jump in scattering coefficients is observed, when employing causality analysis according to the Briggs-Bers or Crighton-Leppington procedure. This implies that in fact an absolute instability occurs.     

Four regimes of decaying grid turbulence in a finite channel

Attenuation of second sound in helium II has been used to observe up to 6 orders of magnitude of decaying vorticity displaying four distinctly different regimes of decaying grid turbulence in a finite channel. A purely classical spectral model for homogeneous and isotropic turbulence describes most of the decay of helium II vorticity in the temperature range 1.2相似文献   

Biomagnetic fluid flow in a channel with stenosis

In this study, the fundamental problem of the biomagnetic (blood) fluid flow in a channel with stenosis under the influence of a steady localized magnetic field is studied. The mathematical model used for the formulation of the problem is consistent with the principles of ferrohydrodynamics (FHD) and magnetohydrodynamics (MHD). Blood is considered as a homogeneous Newtonian fluid and is treated as an electrically conducting magnetic fluid which also exhibits magnetization. For the numerical solution of the problem, which is described by a coupled, non-linear system of PDEs, with appropriate boundary conditions, the stream function-vorticity formulation is adopted. The solution is obtained by the development of an efficient pseudotransient numerical methodology using finite differences. This methodology is based on the development of a semi-implicit numerical technique, transformations and stretching of the grid and proper construction of the boundary conditions for the vorticity. Results concerning the velocity and temperature field, skin friction and rate of heat transfer indicate that the presence of the magnetic field influences the flow field considerably.     

Distribution regimes of intense laser beam in a self-consistent plasma channel

In the present work, we investigate the distributed regimes of an intense laser beam in a self-consistent plasma channel. As the intensity of the laser beam increases, the relativistic mass effect as well as the ponderomotive expulsion of electrons modifies the dielectric function of the medium due to which the medium exhibits nonlinearity. Based on Wentzel–Kramers–Brillouin and paraxial ray theory, the steady-state solution of an intense, Gaussian electromagnetic beam is studied. A differential equation of the beamwidth parameter with the distance of propagation is derived, including the effects of relativistic self-focusing (SF) and ponderomotive self-channeling. The nature of propagation and radial dynamics of the beam in plasma depend on the power, width of the beam, and Ω _p, the ratio of plasma to wave frequency. For a given value of Ω _p (<1), the distribution regimes have been obtained in beampower–beamwidth plane, characterizing the regimes of propagation as steady divergence, oscillatory divergence, and SF. The related focusing parameters are optimized introducing plasma density ramp function, and spot size of the laser beam is analyzed for inhomogeneous plasma. This results in overcoming the diffraction and guiding the laser beam over long distance. Numerical computations are performed for typical parameters of relativistic laser–plasma interaction studies.     

Preturbulent regimes in graphene flow

We provide numerical evidence that electronic preturbulent phenomena in graphene could be observed, under current experimental conditions, through current fluctuations, echoing the detachment of vortices past localized micron-sized impurities. Vortex generation, due to micron-sized constriction, is also explored with special focus on the effects of relativistic corrections to the normal Navier-Stokes equations. These corrections are found to cause a delay in the stability breakout of the fluid as well as a small shift in the vortex shedding frequency.     

Transitions between flow regimes of baroclinic flow in a rotating annulus of fluid

Baroclinic flow in a rotating annulus of fluid shows remarkable transitions of flow patterns as do Rayleigh–Benard convection and Taylor vortices. There are four flow regimes in two nondimensional parameter space, called a symmetric regime (Hadley regime), a steady wave regime (Rossby regime), a vacillating wave regime and a geostrophic turbulence regime. Laminar flow in a symmetric regime is formed between the balance of a horizontal pressure gradient force and a Coriolis torque (geostrophic balance), and this flow becomes unstable when one of the nondimensional parameters, the thermal Rossby number, becomes less than the critical value. In this paper, the characteristic features of the four flow regimes are reviewed including recent findings about the behavior of geostrophic turbulence.     

Couette flow regimes with heat transfer in rarefied gas

Based on numerical solution of the Boltzmann equation by direct statistic simulation, the Couette flow with heat transfer is studied in a broad range of ratios of plate temperatures and Mach numbers of a moving plate. Flow regime classification by the form of the dependences of the energy flux and friction stress on the Knudsen number Kn is proposed. These dependences can be simultaneously monotonic and nonmonotonic and have maxima. Situations are possible in which the dependence of the energy flux transferred to a plate on Kn has a minimum, while the dependence of the friction stress is monotonic or even has a maximum. Also, regimes exist in which the dependence of the energy flux on Kn has a maximum, while the dependence of the friction stress is monotonic, and vice versa.     

Plasma expansion and current flow in a vacuum arc with a smallanode

A low-density plasma flow in a vacuum arc with a small anode, which intercepts only part of the cathodic plasma jet, was studied theoretically using a two-dimensional approximation. The plasma expansion was modeled using the sourceless steady-state hydrodynamic equations, where the free boundary of the plasma was determined by a self-consistent solution of the gasdynamic and electrical current equations. Magnetic forces from the azimuthal self-magnetic field were taken into account. The influence of the ratio of the anode radius to initial plasma jet radius on the plasma density, velocity, current distribution, and anode sheath potential drop is analyzed. It is shown that the mass and current flow in a 500 A arc are compressed near the axis. This leads to an increase in the plasma density by a factor of two and in the axial current density by a factor of 1.5     

Flow separation behind a rib in a channel with laminar flow

Visualization data and results of combined measurements of flow quantities in flow with separation past a rib at nominally laminar regime of channel flow are reported. In the separation region, the flow is found to be essentially three-dimensional and unsteady, exhibiting a distinct cellular structure and flow zones with transverse motion. It is shown that the rib-induced flow separation gives rise to low-frequency fluctuations of flow velocity and initiates the turbulence transition in the channel flow. The critical Reynolds number at which flow instability starts developing in the channel is estimated. It is shown that at Reynolds numbers higher than the critical Reynolds number the linear integral scale of flow velocity fluctuations in the channel is defined by the duct size.