Flow over a system of several ribs under conditions of high free-stream turbulence

Results of an experimental hydrodynamic and heat-transfer study of the turbulent separated flow developing over a system of several cross-flow ribs are reported. Conditions with low and high free-stream turbulence are considered. Visualization and heat-transfer data are presented, and a comparison for two turbulence levels is given. In the system of three or more ribs, a very unstable flow in the second inter-rib cell was observed under low-turbulence conditions. Under a high level of free-stream turbulence, the flow in the first inter-rib cell is unstable; this observation is supported by measured pressure distributions. Addition of each next rib makes the separation flow region behind the last rib and the pressure recovery region less extended, and decreases the coordinate at which the rate of heat transfer attains its maximum. In the high-turbulent flow, the heat-transfer intensification in the second inter-rib cell amounts to 30 %. This work was financially supported by the Russian Foundation for Basic Research (Grants Nos. 04-02-16070 and 06-08-00300).     

Free stream turbulence effect on the flow structure over the finite span straight wing

Flow visualization tests have been performed to examine the structure of the near-wall flow over a low-aspect ratio straight wing installed at various angles of attack a and chord Reynolds numberRe _c=U_∞c/ν=1.76×10⁵. The experiments were carried out at two free-stream turbulence levels, ε=0.1% and ε=1%, the latter one having been achieved using a baffling grid. To visualize the flow, termochromic cholesteric liquid crystals and digital processing of video images were used. At the low turbulence level and α=27°, a flow stall on the lee side of the wing was observed, with a pair of largescale vortices rotating in the wing plane. Simultaneously, no vortex structures were observed on the windward wing surface. It was found the flow patterns on either side of the wing significantly changed with increasing free-stream turbulence level. A separation bubble appeared near the leading edge on the lee side of the airfoil at ε=1%, and large-scale stationary longitudinal vortices originated over the wing windward surface. The number and sizes of the longitudinal structures were found to be dependent on the angle of attack.     

Flow visualization and local mass transfer studies for turbulent flow in a wind tunnel with chamfered ribs

The paper presents results of flow visualization and mass transfer studies for fully developed turbulent flow of air in a square section wind tunnel with repeated chamfered rib roughness on the bottom of the tunnel (rib head chamfer angles ϕ of -15°, 0°, and 15°; relative roughness pitchp/e = 3, 5, 7.5, and 10). Direct video recordings of flow patterns were made using a simple technique of particles visualization. For the positively chamfered closely spaced ribs (p/e <- 5) vigorous vortex shedding has been seen compared to the square or negatively chamfered ribs, which is found to be a function of the Reynolds number. For the widely spaced ribs, the study shows flow separation at the ribs and reattachment in the inter-rib region. Local mass transfer studies, based on the variation in colour of cobaltous chloride solution impregnated paper due to evaporation of water, showed a significant improvement in mass transfer rate in the recirculating region in the wake of ribs with the change in the chamfer angle from -15° to 15°. The positively chamfered 15° ribs are found to be better than square section ribs atp/e <- 7.5. The performance of negatively chamfered ribs is found to be poor compared to other ribs irrespective of the relative roughness pitch.     

Experimental investigation of the effect of small-obstacle-induced vortex sheet on the separated flow in cavity

In the present paper, we report results of an experimental study of the influence which a vortex-generating element installed upstream of the main obstacle has on the separated flow and heat transfer in a cross-flow cavitytrench. The element was a small cross-flow rib whose height was an order of magnitude smaller than the depth of the cavity. In the experiments, the variable parameters were the angle of inclination of the frontal and rear walls of the cavity, the rib height, and the rib-to-cavity distance. It is shown that the introduction of additional vortical perturbations into the recirculation zone leads to a substantial modification of both the vortex production process and the distributions of pressure and heat-transfer coefficients. Optimal height of the mini-turbulizer and its optimal location are defined by the fall of the re-attachment point of mini-rib-generated flow onto the rear wall of cavity. In the latter situation, the maximal value of the heat-transfer coefficient increases as compared to the case with no vortex generator used, the increase amounting to 30 %.     

Turbulent mixing of small-obstacle-induced perturbations with the separated shear layer behind a backward-facing step

In the present paper, we consider one of the most efficient and simple methods to additionally intensify the exchange processes and heat transfer in the separated flow behind a backward-facing step. The method uses small obstacles installed upstream the step; such obstacle act as turbulators smaller in size than the main obstacle. As the turbulators, solid mini ribs, comb ribbings, and wall-detached mini ribs were used. Intensification of the turbulent mixing process behind the main obstacle occurs due to the introduction of small-obstacle-induced 2D and 3D perturbations into the separated shear layer behind the step. Results of a detailed experimental study of the distributions of pressure and heat transfer for different heights of the small intensifier and its positions with respect to the step are reported. The influence of intensifier shape on the thermal and dynamic characteristics of the flow has been analyzed. The distributions of pressure and heat-transfer coefficients were used to evaluate the effectiveness of the various mini obstacles and the limits of their action on the drag and heat transfer.     

Structure of the channel flow behind a surface-mounted rib under conditions of laminar-turbulent transition

The PIV technique was used to measure the instantaneous vector fields of flow velocity and vorticity behind a thin cross-flow rib installed in a channel with laminar, transient, or turbulent flow. The data were treated statistically to determine the fields of mean longitudinal flow velocity and the correlations of pulsating velocity components 〈u′ν′〉 and 〈u′u′〉. Some features of the flow structure developing under conditions of laminar-turbulent transition behind the rib have been revealed.     

An experimental study of the forerunners of boundary-layer localized disturbances at an enhanced turbulence level

Turbulence production processes in boundary layer at a high level of free-stream turbulence have been studied. The tests were carried out in the MT-324 subsonic wind tunnel of ITAM, SB RAS, on models of straight and 45° swept wings at Reynolds numbers Re_c1 = 97000 and Re_c2 = 137000, and also at low (Tu = 0.18 % U _∞) and high (Tu = 0.79 and 2.31 % U _∞) levels of free-stream turbulence. The longitudinal localized disturbances developing in the boundary layer under the action of free-stream turbulence were artificially modeled using local air suction through a slot on the model surface. Wave packets, or forerunners, produced in the boundary layer, in the region preceding the abrupt local change of flow velocity near the localized-disturbance fronts, were examined. The high level of free-stream turbulence was found to accelerate the downstream evolution of the wave packets and their transformation into turbulent spots.     

Non-collinear vortex states in twinned YBa2Cu3O7?δ single crystals

The magnetic moments parallel and perpendicular to thec-axis (measured simultaneously) have been studied as a function of direction of applied magnetic fieldH in twinned and de-twinned YBa₂Cu₃O_7−δ single crystals. They show pronounced anomalies when the field direction approaches thec-axis. These allow clear identification of the angle ϕ_L at which vortices are locked into twin planes. Complete shielding of theH _ab field component (transverse Meissner effect) was observed in the locked state. For larger angles, up to ϕ_T, the vortices continue to be non-collinear with the applied field, but their direction deviates from the trapping plane. ϕ_L exhibits a 1/H field-dependence, whereas ϕ_T shows only weak logarithmic variations withH.     

Duality and the cosmological constant

A dynamical theory is studied in which a scalar field ϕ in Einstein-Minkowski space is coupled to the four-velocityN _μ of a preferred inertial observer in that space. As a consistent requirement on this coupling we study a principle of duality invariance of the dynamical mass term of ϕ at some universal length in the small-distance regime. In the large-distance regime duality breaking can be introduced by giving a background value to ϕ and a background direction toN _μ. It is shown that, in an appropriate approximation, duality breaking can be related to the emergence of a characteristic phase in which the condensation of the ground state allows massive excitations with a characteristic scale of squared mass which agrees with the present observational bound for the cosmological constant.     

Numerical and experimental investigations on the mach 2 pseudo-shock wave in a square duct

This paper describes numerical and experimental investigations for the multiple shock wave/turbulent boundary layer interaction in a Mach 2 supersonic square duct. The numerical simulation is carried out with the Harten-Yee second-order accuracy TVD scheme and the Baldwin-Lomax turbulence model. The flow conditions are a free-stream Mach number ofM _∞≈=2.0 and a Reynolds number ofRe _∞;=2.5×10⁷ and the flow confinements are δ_∞/h=0.15 (case A) and δ_∞/h=0.25 (case B), respectively. The computational results for both cases show good agreement with the experimental results. Based on these agreements, the flow quantities, which are very difficult to obtain experimentally, are analyzed by numerical simulation. Moreover, the effect of flow confinement on the pseudo-shock wave characteristics is also presented.     

Thermoelectric effects in a double-dot Aharonov-Bohm interferometer with Rashba spin-orbit interaction

We investigate the thermoelectric effects in a double-dot Aharonov-Bohm interferometer coupled to ferromagnetic leads held at different temperatures. The interplay of Rashba spin-orbit interaction (RSOI) and magnetic flux ϕ induces various interesting spin-dependent interference phenomena. The thermoelectric transport oscillates with ϕ. The peak of the thermopower S and figure of merit ZT splits into two new peaks and its splitting increases with the Rashba induced phase factor φ _R . With increasing φ _R S and ZT at ϕ = ± 2nπ (n = 0,1,2,...) exhibit a conversion from a peak to a valley. In the presence of the interplay of RSOI and ϕ by increasing spin polarization the splitting peaks of S (ZT) become asymmetric and ZT is greatly enhanced. The influence of the quantum dot levels on thermoelectric effects is also analyzed.     

前缘曲率对三维边界层内被激发出非定常横流模态的影响研究

三维边界层感受性问题是三维边界层层流向湍流转捩的初始阶段,是实现三维边界层转捩预测与控制的关键环节.在高湍流度的环境下,非定常横流模态的失稳是导致三维边界层流动转捩的主要原因;但是,前缘曲率对三维边界层感受性机制作用的研究也是十分重要的课题之一.因此,本文采用直接数值模拟方法研究在自由来流湍流作用下具有不同椭圆形前缘三维（后掠翼平板）边界层内被激发出非定常横流模态的感受性机制;揭示不同椭圆形前缘曲率对三维边界层内被激发出非定常横流模态的扰动波波包传播速度、传播方向、分布规律、感受性系数以及分别提取获得一组扰动波的幅值、色散关系和增长率等关键因素的影响;建立在不同椭圆形前缘曲率情况下,三维边界层内被激发出非定常横流模态的感受性问题与自由来流湍流的强度和运动方向变化之间的内在联系;详细分析了不同强度各向异性的自由来流湍流在激发三维边界层感受性机制的物理过程中起着何种作用等.通过上述研究将有益于拓展和完善流动稳定性理论,为三维边界层内层流向湍流转捩的预测与控制提供依据.     

Kroll-Watson-type theorem for potential scattering in a bichromatic laser field

Summary We show that for potential scattering of electrons in a bichromatic laser field a Kroll-Watson type of theorem can be derived in which the scatteringT-matrix element is composed of a renormalized on-shell matrix element for scattering without a field times a generalized Bessel function factor. The radiation field has two components of frequency ω₁ and sω₁ (s=2,3,…) and both components are out of phase by an angle ϕ. Our paper is a generalization of earlier investigations which were performed in the first-order Born approximation.     

The dynamical mixing of light and pseudoscalar fields

We solve the general problem of mixing of electromagnetic and scalar or pseudoscalar fields coupled by axion-type interactions L _int = g _ϕ ϕε _μναβ F ^μν F ^αβ . The problem depends on several dimensionful scales, including the magnitude and direction of background magnetic field, the pseudoscalar mass, plasma frequency, propagation frequency, wave number, and finally the pseudoscalar coupling. We apply the results to the first consistent calculations of the mixing of light propagating in a background magnetic field of varying directions, which show a great variety of fascinating resonant and polarization effects.      

The sign of interband interaction and stability of the superconducting ground state in the multiband model

Summary It is stressed that the stability of the superconducting ground state in the two-band model is guaranteed for both signs of the leading interband interactionW. Thereby the requirement for the energy minimum fixed the phase differences of two order parameters as |ϕ₁-ϕ₂|=0,2π, … ifW<0 and |ϕ₁-ϕ₂|=π 3π, … ifW>0, and this difference is reflected in the ground-state wave function.     

Power law distributions and dynamic behaviour of stock markets

A simple agent model is introduced by analogy with the mean field approach to the Ising model for a magnetic system. Our model is characterised by a generalised Langevin equation = F ϕ + G ϕ t where t is the usual Gaussian white noise, i.e.: t t ^′ = 2Dδ t-t ^′ and t = 0. Both the associated Fokker Planck equation and the long time probability distribution function can be obtained analytically. A steady state solution may be expressed as P ϕ = exp{ - Ψϕ - ln G(ϕ)} where Ψϕ = - F/ G dϕ and Z is a normalization factor. This is explored for the simple case where F ϕ = Jϕ + bϕ² - cϕ³ and fluctuations characterised by the amplitude G ϕ = ϕ + ɛ when it readily yields for ϕ≫ɛ, a distribution function with power law tails, viz: P ϕ = exp{2bϕ-cϕ² /D}. The parameter c ensures convergence of the distribution function for large values of ϕ. It might be loosely associated with the activity of so-called value traders. The parameter J may be associated with the activity of noise traders. Output for the associated time series show all the characteristics of familiar financial time series providing J < 0 and D≈ | J|. Received 25 July 2000     

Potentials of pointlike particles in gauge theory with a dilaton

I examine the potential of a pointlike particle carrying SU (N _c) charge in a gauge theory with a dilaton. The potential depends on boundary conditions imposed on the dilaton: For a dilaton that vanishes at infinity the resulting potential is a regulatized Coulomb potential of the form (r+r _ϕ)⁻¹, withr _ϕ, inversely proportional to the decay constant of the dilaton. Another natural constraint on the dialaton ϕ is independence of (1/g ²) exp(ϕ/f_ϕ) from the gauge couplingg. This requirement yields a confining potential proportional tor.     

Absorption of <Emphasis Type="Italic">ϕ</Emphasis> mesons in near-threshold proton-nucleus reactions

In the framework of the nuclear-spectral-function approach for incoherent primary proton-nucleon and secondary pion-nucleon production processes, we study the inclusive ϕ-meson production in the interaction of 2.83-GeV protons with nuclei. In particular, the A dependences of the absolute and relative ϕ-meson yields are investigated within the different scenarios for its in-medium width as well as for the cross-section ratio σ _pn→pnϕ /σ _pp→ppϕ . Our model calculations take into account the acceptance window of the ANKE facility used in a recent experiment performed at COSY. They show that the pion-nucleon production channel contributes distinctly to the ϕ creation in heavy nuclei in the chosen kinematics and, hence, has to be taken into consideration on close examination of the dependences of the ϕ-meson yields on the target mass number with the aim to get information on its width in the medium. They also demonstrate that the experimentally unknown ratio ϕ _pn→pnϕ /σ _pp→ppϕ has a weak effect on the A dependence of the relative ϕ-meson-production cross section at incident energy of present interest, whereas it is found to be appreciably sensitive to the ϕ in-medium width, which means that this relative observable can indeed be useful to help determine the above width from the direct comparison of the results of our calculations with the future data from the respective ANKE-at-COSY experiment. The text was submitted by the authors in English.     

Liquid crystal heat transfer measurements in a rectangular channel with solid and slit rib

An experimental investigation on the heat transfer effectiveness of solid and slit ribs mounted on the bottom surface of a rectangular channel has been carried out at Reynolds numbers of 13400, 22600, 32100 and 40800. The rib height to hydraulic diameter ratio (e/D _h)set during experiment is equal to 0.0624. The surface Nusselt number results from transient liquid crystal thermography are presented. The heat transfer enhancement performance analysis has been carried out using entropy generation principle. The slit rib is superior to solid rib from both heat transfer augmentation and pressure penalty point of view. The performance of the slit rib is a function of the open area ratio (β) and the location of the slit (b) from the bottom test surface. The optimum open area ratio is 20% and the slit located symmetrically from the top and bottom surface of the rib is the optimum location of the slit. The heat transfer augmentation of the slit rib (β=20%) is 61% in comparison to 40% for the solid rib at Re=32100 and the pressure penalty for the slit rib is 7% lower than the solid rib. The entropy generation for the slit rib is 33% less than that of the solid rib.     

Slow Motion of Charges¶Interacting Through the Maxwell Field

We study the Abraham model for N charges interacting with the Maxwell field. On the scale of the charge diameter, R _ϕ, the charges are a distance ɛ^-1 R _ϕ apart and have a velocity with ɛ a small dimensionless parameter. We follow the motion of the charges over times of the order ɛ^-3/2 R _ϕ/c and prove that on this time scale their motion is well approximated by the Darwin Lagrangian. The mass is renormalized. The interaction is dominated by the instantaneous Coulomb forces, which are of the order ɛ². The magnetic fields and first order retardation generate the Darwin correction of the order ɛ³. Radiation damping would be of the order ɛ^7/2. Received: 13 January 2000 / Accepted: 4 February 2000