16,198 research outputs found
Real-space Manifestations of Bottlenecks in Turbulence Spectra
No full textAn energy-spectrum bottleneck, a bump in the turbulence spectrum between the inertial and dissipation ranges, is shown to occur in the non-turbulent, one-dimensional, hyperviscous Burgers equation and found to be the Fourier-space signature of oscillations in the real-space velocity, which are explained by boundary-layer-expansion techniques. Pseudospectral simulations are used to show that such oscillations occur in velocity correlation functions in one- and three-dimensional hyperviscous hydrodynamical equations that display genuine turbulence
Reynolds number dependence of the dimensionless dissipation rate in stationary magnetohydrodynamic turbulence
No full textResults on the Reynolds number dependence of the dimensionless total dissipation rate C_ε are presented, obtained from medium to high resolution direct numerical simulations (DNSs) of mechanically forced stationary homogeneous magnetohydrodynamic (MHD) turbulence in the absence of a mean magnetic field, showing that C_ε -> const with increasing Reynolds number. Furthermore, a model equation for the Reynolds number dependence of the dimensionless dissipation rate is derived from the real-space energy balance equation by asymptotic expansion in terms of Reynolds number of the second- and third-order correlation functions of the Elsässer fields z± = u ± b. At large Reynolds numbers we find that a model of the form C_ε = C_ε,∞ + C/R describes the data well, while at lower Reynolds numbers the model needs to be extended to second order in 1/R in order to obtain a good fit to the data, where R is a generalised Reynolds number with respect to the Elsässer field z-
Universal Statistical Properties of Inertial-particle Trajectories in Three-dimensional, Homogeneous, Isotropic, Fluid Turbulence
No full textWe obtain new universal statistical properties of heavy-particle trajectories in three-dimensional, statistically steady, homogeneous, and isotropic turbulent flows by direct numerical simulations. We show that the probability distribution functions (PDFs) P(Φ), of the angle Φ between the Eulerian velocity u and the particle velocity v, at a point and time, scales as P(Φ) ∼Φ−, with a new universal exponent ≃ 4
Turbulent stratified shear flow experiments: Length scale comparison
No full textStratified shear flows are ubiquitous in geophysical systems such as oceanic overflows, wind-driven thermoclines, and atmo- spheric inversion layers. The stability of such flows is governed by the Richardson Number Ri which represents a balance between the stabilizing influence of stratification and the destabilizing influence of shear. For a shear flow with velocity difference U, density difference ∆ρ and characteristic length H, one has Ri = g(∆ρ/ρ)H/U^2 which is often used when detailed information about the flow is not available. A more precise definition is the gradient Richardson Number Rig = N^2/S^2 where the buoyancy frequency N = ((g/ρ)∂ρ/∂z)^{1/2}, the mean strain S = ∂U/∂z in which z is parallel to gravity and suitable ensemble or time averages define the gradients. We explore the stability and mixing properties of a wall-bounded shear flow over a range 0.1< Rig <1 using simultaneous planar measurements of density and velocity fields using Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), respectively. The flow, confined from the top by glass horizontal boundary, is a lighter alcohol-water mixture injected from a nozzle into quiescent heavier salt-water fluid with velocity between 5 and 10 cm/s and with a relative fractional density difference of 0.0026 or 0.0052. The injected flow is turbulent with Taylor Reynolds number between 50 and 100. We compare a set of length scales that characterize the mixing properties of our turbulent stratified shear flow including the Thorpe Length L_T, the Ozmidov Length L_o, the Ellison Length L_E, and turbulent mixing lengths L_m and L_ρ
Measurements of small radius ratio turbulent Taylor-Couette flow
No full textIn Taylor-Couette flow, the radius ratio () is one of the key parameters of the system. For small , the asymmetry of the inner and outer boundary layer becomes more important, affecting the general flow structure and boundary layer characteristics. Using high-resolution particle image velocimetry we measure flow profiles for a radius ratio of 0.5 and Taylor number of up to . By measuring at varying heights, roll structures are characterized for two different rotation ratios of the inner and outer cylinder. In addition, we investigate how the turbulent bursts coming from the inner and outer cylinder affect the flow profiles. These results exemplify how curvature affects flow in strongly turbulent Taylor-Couette Flow
Parts of speech systems as a basic typological parameter.
No full textThis paper argues that the word order possibilities of a language are partly determined by the parts-of-speech system of that language. In languages in which lexical items are specialized for certain functionally defined syntactic slots (e.g. the modifier slot within a noun phrase), the identifiability of these slots is ensured by the nature of the lexical items (e.g. adjectives) themselves. As a result, word order possibilities are relatively unrestricted in these languages. In languages in which lexical items are not specialized for certain syntactic slots, in that these items combine the functions of two or more of the traditional word classes, other strategies have to be invoked to enhance identifiability. In these languages word order constraints are used to make syntactic slots identifiable on the basis of their position within the clause or phrase. Hence the word order possibilities are rather restricted in these languages. Counterexamples to the latter claim all involve cases in which identifiability is ensured by morphological rather than syntactic means. This shows that there is a balanced trade-off between the syntactic, morphological, and lexical structure of a language
Liftings for noncomplete probability spaces
No full textThe current state of knowledge concerning liftings for noncomplete probability spaces is discussed. This is a somewhat expanded version of the author's talk given at the 1991 Summer Conference on General Topology and Applications in Honor of Mary Ellen Rudin and Her Work.PT: S; CR: BURKE MR, IN PRESS P AM MATH S BURKE MR, 1991, ISRAEL J MATH, V73, P33 BURKE MR, 1992, ISRAEL J MATH, V79, P289 CARLSON T, THEOREM LIFTING CHRISTENSEN JPR, 1974, TOPOLOGY BOREL STRUC FREMLIN DH, 1989, HDB BOOLEAN ALGEBRAS, P877 INOESCUTULCEA A, 1966, 5TH P BERK S MATH ST, V2 IONESCUTULCEA A, 1967, CONTRIBUTIONS PROB 1, P63 IONESCUTULCEA A, 1969, TOPICS THEORY LIFTIN JECH TJ, 1978, SET THEORY JOHNSON RA, 1980, P AM MATH SOC, V80, P234 JUST W, IN PRESS T AM MATH S KUPKA J, 1983, INDIANA U MATH J, V32, P717 LOSERT V, 1983, LNM, V1080, P95 MAHARAM D, 1958, P AM MATH SOC, V9, P987 SHELAH S, 1983, ISRAEL J MATH, V45, P90 TALAGRAND M, 1982, P AM MATH SOC, V84, P379 VONNEUMANN J, 1931, CRELLES J MATH, V165, P109; NR: 18; TC: 0; J9: ANN N Y ACAD SCI; PG: 4; GA: BZ86BSource type: Electronic(1
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
Studies on genetic abnormalities in childhood leukaemia
No full textDr U R Kees$AUD 207,525.74NHMRC Project GrantsStandard Project Gran
Energy flux in isotropic turbulence under large variations of external forcing
No full textWe investigate the response of energy flux in isotropic turbulence to step-function like perturbation in external forcing at large length scales. From both physical experiments and direct numerical simulations, we measured the evolution of the Eulerian velocity structure functions, such as , , before and after the perturbation in forcing. In both cases, we observed the cascade of the energy excess at large scales cascade through scales to the dissipative range, which can be used to study the dynamics of the cascade, and in particular, to estimate the relevant time scales
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