15,536 research outputs found
Predictive validity of the Hand Assessment for Infants in infants at risk of unilateral cerebral palsy
No full textAim: To evaluate the sensitivity, specificity, and predictive value of the Hand Assessment for Infants (HAI) in identifying infants at risk of being diagnosed with unilateral cerebral palsy (CP), and to determine cut-off values for this purpose. Method: A convenience sample of 203 infants (106 females, 97 males) was assessed by the HAI at 3, 6, 9, and 12 months. Sensitivity, specificity, predictive values, and likelihood ratios were calculated using receiver operating characteristic curve analysis. Cut-off values were derived for different ages. The clinical outcome (unilateral CP yes/no) at 24 months or more served as an external criterion to investigate the predictive validity of HAI. Results: Half of the infants developed unilateral CP. The area under the curve ranged from 0.77 (95% CI [confidence interval] 0.63–0.91) to 0.95 (95% CI 0.90–1.00) across HAI scales and age intervals. Likewise, sensitivity ranged from 63% to 93%, specificity from 62% to 91%, and accuracy from 73% to 94%. Interpretation: HAI scores demonstrated overall accuracy that ranged from very good to excellent in predicting unilateral CP in infants at risk aged between 3.5 and 12 months. This accuracy increased with age at assessment and the earliest possible prediction was at 3.5 months of age, when appropriate HAI cut-off values for different ages were applied
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
Taylor-Couette flow with asymmetric end-walls boundary conditions
No full textIn the paper the authors present the results obtained during a numerical (Direct Numerical Simulation/Spectral Vanishing Viscosity method - DNS/SVV) and experimental investigations (Kalliroscope, PIV) of the Taylor-Couette flow with asymmetric boundary conditions. In the paper attention is focused on the laminar-turbulent transition process. The main purpose of the research is to investigate the influence of different parameters (aspect ratio, curvature parameter, end-walls boundary conditions) on the flow structure and on the flow characteristics. The transverse current Jω is computed from the velocity field obtained numerically. The λ2 criterion has been used for numerical visualization
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-
Measurement of the ratio of prompt χ c to J / ψ production in pp collisions at √s = 7 TeV
No full textThe prompt production of charmonium χ c and J / ψ states is studied in proton-proton collisions at a centre-of-mass energy of √s = 7 TeV at the Large Hadron Collider. The χ c and J / ψ mesons are identified through their decays χ c → J / ψ γ and J / ψ → μ + μ - using 36 pb - 1 of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for χ c and J / ψ, σ (χ c → J / ψ γ) / σ (J / ψ), is determined as a function of the J / ψ transverse momentum in the range 2 < p T J / ψ < 15 GeV / c. The results are in excellent agreement with next-to-leading order non-relativistic expectations and show a significant discrepancy compared with the colour singlet model prediction at leading order, especially in the low p T J / ψ region
Is isotropy restored at small scales in freely decaying strongly stratified turbulence?
No full textWe analyse the scale-dependent anisotropy of homogeneous stratified turbulence. The Ozmidov scale l_N (Ozmidov 1965) helps to compare the relative effects of inertia and of the buoyancy force, and thus to quantify the rise of anisotropy in different scale ranges: at large scales l >> l_N the anisotropy due to strong stratification is dominant, whereas at small scales l << l_N, universal 3D isotropic characteristic of turbulence appear to be restored. We investigate the corresponding dynamics using Direct Numerical Simulations (DNS) in freely decaying turbulence at different stratification rates. We confirm the return to isotropy of the small scales by analyzing the orientation-dependent power spectrum and poloidal/toroidal/density energy modes. To some extent, many characteristics of isotropic universality are restored at small scales but, surprisingly, the density spectrum (also potential energy spectrum) plays a particular role
Wavenumber-frequency spectra in the logarithmic layer of wall turbulence
No full textWe study space-time correlations of wall-bounded turbulence in terms of wavenumber-frequency spectra of the streamwise velocity component. The spectra are obtained from Large Eddy Simulations (LES), which provide a full space-time record of the flow. We find that the frequency distributions exhibit a Doppler shift, which is a consequence of mean flow advection, as well as a considerable Doppler broadening, consistent with the Kraichnan-Tennekes random sweeping hypothesis. For wall-bounded turbulence, both of these effects vary with the wall distance and are closely related to the logarithmic behavior of the mean velocity profile and the velocity fluctuation profiles. We incorporate these observations into a simple analytical model for the wavenumber-frequency spectrum based on an advection equation featuring advection of the small-scale velocity fluctuations with a mean and a large-scale random-sweeping velocity. The model is found to be in very good agreement with the LES data. Potential applications of the model spectrum, e.g., to quantify the spatio-temporal structure of fluctuations in wind energy conversion, will be discussed
A new spectral model for shear-driven homogeneous anisotropic turbulent flows
No full textA new system of governing equations for spherically-averaged descriptors, which allows to calculate incompressible homogeneous turbulent flows, is derived in the present study. Remarkable features of this model are that it makes a distinction between directional and polarization anisotropies, which are treated separately, and that no heuristic tuning of arbitrary constants is required. Spherical averaging allows to obtain a model for anisotropic turbulence which is as versatile as the classical Eddy-Damped Quasi-Normal Markovian (EDQNM) model for isotropic turbulence, i.e. this model can calculate anisotropic turbulent flows at both very high and low Reynolds numbers, with good resolution of both large and small scales and over very long evolution times. The present model is particulary suited for the study of shear-driven turbulent flows and their return to isotropy
Effect of linear feedback control on the optimal transient growth in particle-laden channel flow
No full textThe optimal transient growth process in a particle-laden channel flow is studied under the influence of the linear feedback control. The equilibrium Eulerian approach with the assumption that the particles are small and spherical is adopted. The effect of initial distribution of particles on the optimal transient growth of perturbations is discussed. The LQG control of the particle-laden flow system is considered and compared with the no control cases
Contact velocities of small ellipsoids settling in turbulence
No full textCollisions of small and heavy non-spherical particles settling in turbulence are very important for systems such as ice clouds and proto-planetary disks where the particle spectra evolution is strongly dependent on the collision induced growth rate. Still, the influence of the particle shape on the collision probability is virtually unknown. Building on our recent investigation on the collision rate of monodisperse suspensions of ellipsoidal particles (Siewert et al., J. Fluid Mech. 758, 686-701, 2014), we show theoretically and by direct numerical simulations that the behavior of ellipsoids subject to turbulence and gravity is different from the behavior of spheres. Due to the dependence of the particle settling velocity on the particle orientation, the relative velocity at contact is influenced by turbulence. When ellipsoids differ either by mass or shape, their contact velocity is randomized by the randomized particle orientation. For particles much heavier than the fluid these orientation dependent settling velocity differences are larger than the relative velocities directly induced by the turbulent fluctuations
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