914 research outputs found

    Dynamics of automatic stations' descent in planetary atmospheres as means of measurement data control

    Get PDF
    Automatic stations descent in planetary atmospheres as means of measurement data contro

    Constraints on scalar diffusion anomaly in three-dimensional flows having bounded velocity gradients

    Full text link
    This study is concerned with the decay behaviour of a passive scalar θ\theta in three-dimensional flows having bounded velocity gradients. Given an initially smooth scalar distribution, the decay rate d/dtd/dt of the scalar variance is found to be bounded in terms of controlled physical parameters. Furthermore, in the zero diffusivity limit, κ0\kappa\to0, this rate vanishes as κα0\kappa^{\alpha_0} if there exists an α0(0,1]\alpha_0\in(0,1] independent of κ\kappa such that <<\infty for αα0\alpha\le\alpha_0. This condition is satisfied if in the limit κ0\kappa\to0, the variance spectrum Θ(k)\Theta(k) remains steeper than k1k^{-1} for large wave numbers kk. When no such positive α0\alpha_0 exists, the scalar field may be said to become virtually singular. A plausible scenario consistent with Batchelor's theory is that Θ(k)\Theta(k) becomes increasingly shallower for smaller κ\kappa, approaching the Batchelor scaling k1k^{-1} in the limit κ0\kappa\to0. For this classical case, the decay rate also vanishes, albeit more slowly -- like (lnPr)1(\ln P_r)^{-1}, where PrP_r is the Prandtl or Schmidt number. Hence, diffusion anomaly is ruled out for a broad range of scalar distribution, including power-law spectra no shallower than k1k^{-1}. The implication is that in order to have a κ\kappa-independent and non-vanishing decay rate, the variance at small scales must necessarily be greater than that allowed by the Batchelor spectrum. These results are discussed in the light of existing literature on the asymptotic exponential decay eγt\sim e^{-\gamma t}, where γ>0\gamma>0 is independent of κ\kappa.Comment: 6-7 journal pages, no figures. accepted for publication by Phys. Fluid

    Comparative experimental study of local mixing of active and passive scalars in turbulent thermal convection

    Full text link
    We investigate experimentally the statistical properties of active and passive scalar fields in turbulent Rayleigh-B\'{e}nard convection in water, at Ra1010Ra\sim10^{10}. Both the local concentration of fluorescence dye and the local temperature are measured near the sidewall of a rectangular cell. It is found that, although they are advected by the same turbulent flow, the two scalars distribute differently. This difference is twofold, i.e. both the quantities themselves and their small-scale increments have different distributions. Our results show that there is a certain buoyant scale based on time domain, i.e. the Bolgiano time scale tBt_B, above which buoyancy effects are significant. Above tBt_B, temperature is active and is found to be more intermittent than concentration, which is passive. This suggests that the active scalar possesses a higher level of intermittency in turbulent thermal convection. It is further found that the mixing of both scalar fields are isotropic for scales larger than tBt_B even though buoyancy acts on the fluid in the vertical direction. Below tBt_B, temperature is passive and is found to be more anisotropic than concentration. But this higher degree of anisotropy is attributed to the higher diffusivity of temperature over that of concentration. From the simultaneous measurements of temperature and concentration, it is shown that two scalars have similar autocorrelation functions and there is a strong and positive correlation between them.Comment: 13 pages and 12 figure

    Passive Scalar Structures in Supersonic Turbulence

    Full text link
    We conduct a systematic numerical study of passive scalar structures in supersonic turbulent flows. We find that the degree of intermittency in the scalar structures increases only slightly as the flow changes from transonic to highly supersonic, while the velocity structures become significantly more intermittent. This difference is due to the absence of shock-like discontinuities in the scalar field. The structure functions of the scalar field are well described by the intermittency model of She and L\'{e}v\^{e}que [Phys. Rev. Lett. 72, 336 (1994)], and the most intense scalar structures are found to be sheet-like at all Mach numbers.Comment: 4 pages, 3 figures, to appear in PR

    Evidence for Bolgiano-Obukhov scaling in rotating stratified turbulence using high-resolution direct numerical simulations

    Get PDF
    We report results on rotating stratified turbulence in the absence of forcing, with large-scale isotropic initial conditions, using direct numerical simulations computed on grids of up to 4096^3 points. The Reynolds and Froude numbers are respectively equal to Re=5.4 x 10^4 and Fr=0.0242. The ratio of the Brunt-V\"ais\"al\"a to the inertial wave frequency, N/f, is taken to be equal to 4.95, a choice appropriate to model the dynamics of the southern abyssal ocean at mid latitudes. This gives a global buoyancy Reynolds number R_B=ReFr^2=32, a value sufficient for some isotropy to be recovered in the small scales beyond the Ozmidov scale, but still moderate enough that the intermediate scales where waves are prevalent are well resolved. We concentrate on the large-scale dynamics, for which we find a spectrum compatible with the Bolgiano-Obukhov scaling, and confirm that the Froude number based on a typical vertical length scale is of order unity, with strong gradients in the vertical. Two characteristic scales emerge from this computation, and are identified from sharp variations in the spectral distribution of either total energy or helicity. A spectral break is also observed at a scale at which the partition of energy between the kinetic and potential modes changes abruptly, and beyond which a Kolmogorov-like spectrum recovers. Large slanted layers are ubiquitous in the flow in the velocity and temperature fields, with local overturning events indicated by small Richardson numbers, and a small large-scale enhancement of energy directly attributable to the effect of rotation is also observed.Comment: 19 pages, 9 figures (including compound figures

    Diffusion of passive scalar in a finite-scale random flow

    Full text link
    We consider a solvable model of the decay of scalar variance in a single-scale random velocity field. We show that if there is a separation between the flow scale k_flow^{-1} and the box size k_box^{-1}, the decay rate lambda ~ (k_box/k_flow)^2 is determined by the turbulent diffusion of the box-scale mode. Exponential decay at the rate lambda is preceded by a transient powerlike decay (the total scalar variance ~ t^{-5/2} if the Corrsin invariant is zero, t^{-3/2} otherwise) that lasts a time t~1/\lambda. Spectra are sharply peaked at k=k_box. The box-scale peak acts as a slowly decaying source to a secondary peak at the flow scale. The variance spectrum at scales intermediate between the two peaks (k_box0). The mixing of the flow-scale modes by the random flow produces, for the case of large Peclet number, a k^{-1+delta} spectrum at k>>k_flow, where delta ~ lambda is a small correction. Our solution thus elucidates the spectral make up of the ``strange mode,'' combining small-scale structure and a decay law set by the largest scales.Comment: revtex4, 8 pages, 4 figures; final published versio

    Kraichnan model of passive scalar advection

    Full text link
    A simple model of a passive scalar quantity advected by a Gaussian non-solenoidal ("compressible") velocity field is considered. Large order asymptotes of quantum-field expansions are investigated by instanton approach. The existence of finite convergence radius of the series is proved, a position and a type of the corresponding singularity of the series in the regularization parameter are determined. Anomalous exponents of the main contributions to the structural functions are resummed using new information about the series convergence and two known orders of the expansion.Comment: 21 page

    Lagrangian statistics in forced two-dimensional turbulence

    Full text link
    We report on simulations of two-dimensional turbulence in the inverse energy cascade regime. Focusing on the statistics of Lagrangian tracer particles, scaling behavior of the probability density functions of velocity fluctuations is investigated. The results are compared to the three-dimensional case. In particular an analysis in terms of compensated cumulants reveals the transition from a strong non-Gaussian behavior with large tails to Gaussianity. The reported computation of correlation functions for the acceleration components sheds light on the underlying dynamics of the tracer particles.Comment: 8 figures, 1 tabl

    From non-Brownian Functionals to a Fractional Schr\"odinger Equation

    Full text link
    We derive backward and forward fractional Schr\"odinger type of equations for the distribution of functionals of the path of a particle undergoing anomalous diffusion. Fractional substantial derivatives introduced by Friedrich and co-workers [PRL {\bf 96}, 230601 (2006)] provide the correct fractional framework for the problem at hand. In the limit of normal diffusion we recover the Feynman-Kac treatment of Brownian functionals. For applications, we calculate the distribution of occupation times in half space and show how statistics of anomalous functionals is related to weak ergodicity breaking.Comment: 5 page

    Fractal dimension crossovers in turbulent passive scalar signals

    Get PDF
    The fractal dimension δg(1)\delta_g^{(1)} of turbulent passive scalar signals is calculated from the fluid dynamical equation. δg(1)\delta_g^{(1)} depends on the scale. For small Prandtl (or Schmidt) number Pr<102Pr<10^{-2} one gets two ranges, δg(1)=1\delta_g^{(1)}=1 for small scale r and δg(1)\delta_g^{(1)}=5/3 for large r, both as expected. But for large Pr>1Pr> 1 one gets a third, intermediate range in which the signal is extremely wrinkled and has δg(1)=2\delta_g^{(1)}=2. In that range the passive scalar structure function Dθ(r)D_\theta(r) has a plateau. We calculate the PrPr-dependence of the crossovers. Comparison with a numerical reduced wave vector set calculation gives good agreement with our predictions.Comment: 7 pages, Revtex, 3 figures (postscript file on request
    corecore