Probability density function of turbulent velocity fluctuations in rough-wall boundary layer

The probability density function of single-point velocity fluctuations in turbulence is studied systematically using Fourier coefficients in the energy-containing range. In ideal turbulence where energy-containing motions are random and independent, the Fourier coefficients tend to Gaussian and independent of each other. Velocity fluctuations accordingly tend to Gaussian. However, if energy-containing motions are intermittent or contaminated with bounded-amplitude motions such as wavy wakes, the Fourier coefficients tend to non-Gaussian and dependent of each other. Velocity fluctuations accordingly tend to non-Gaussian. These situations are found in our experiment of a rough-wall boundary layer.Comment: 6 pages, to appear in Physical Review

Nonlinear description of transversal motion in a laminar boundary layer with streaks

The nonlinear streamwise growth of a spanwise periodic array of steady streaks in a flat plate boundary layer is numerically computed using the well known Reduced Navier-Stokes formulation. It is found that the flow configuration changes substantially when the amplitude of the streaks grows and the nonlinear effects come into play. The transversal motion (in the wall normal-spanwise plane), which is normally not considered, becomes non-negligible in the nonlinear regime, and it strongly distorts the streamwise velocity profiles, which end up being quite different from those predicted by the linear theory. We analyze in detail the resulting flow patterns for the nonlinearly saturated streaks, and compare them with available experimental results

Analysis of shared heritability in common disorders of the brain

Paroxysmal Cerebral Disorder

Scaling of mixed structure functions in turbulent boundary layers

The scaling of the anisotropic components of the hierarchy of correlation tensors in the logarithmic region of a flat plate turbulent boundary layer is addressed. We isolate the anisotropic observables by means of a recent theory based on the SO(3) symmetry group of rotations. Employing a dataset made of velocity signals obtained by a multi-probe setup, we demonstrate that the behavior of the anisotropic fluctuations throughout the boundary layer may be understood in terms of the superposition of two distinct regimes, with the transition being controlled by the magnitude of the mean shear and identified with the shear scale. Below the shear scale an isotropy-recovering behavior occurs, characterized by a set of universal exponents which roughly match dimensional predictions based on a first-order expansion in terms of the shear magnitude. Above the shear scale, the competition between energy production and dissipation mechanisms gives rise to a completely different scenario with significant differences in the observed scaling laws. This aspect has profound implications for the correct parameterization of anisotropic behavior in the near-wall region, since approaching the wall, an increasingly larger fraction of the scaling interval tends to conform to the shear-dominated power-law

On the passive control of the near-field of coaxial jets by means of vortex shedding

The present paper confirms experimentally the theoretical result by Talamelli and Gavarini (Flow, Turbul. & Combust., 2006), who proposed that the wake behind a separation wall between two streams of a coaxial jet creates the condition for an absolute instability. This instability, by means of the induced vortex shedding, provides a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers has been demonstrated and its effect towards increased turbulence activity has been shown

Passive control of mixing in a coaxial jet

An experimental investigation regarding interacting shear layers in a coaxial jet geometry has been performed. The present paper confirms experimentally the theoretical result by Talamelli and Gavarini (2006), who proposed that the wake behind the separation wall between the two stream of a coaxial jet creates the condition for an absolute instability. This instability, by means of the induced vortex shedding, may provide a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers has been demonstrated

Scaling of mixed structure functions in turbulent boundary layers

We address the issue of the scaling of the anisotropic components of the hierarchy of correlation tensors in the logarithmic region of a turbulent boundary layer over a flat plate, at Re15 000. We isolate the anisotropic observables by means of decomposition tools based on the SO3 symmetry group of rotations. By employing a dataset made of velocity signals detected by two X probes, we demonstrate that the behavior of the anisotropic fluctuations throughout the boundary layer may be understood in terms of the superposition of two distinct regimes. The transition is controlled by the magnitude of the mean shear and occurs in correspondence with the shear scale. Below the shear scale, an isotropy-recovering behavior occurs, which is characterized by a set of universal exponents which roughly match dimensional predictions based on Lumley\u2019s argument J. L. Lumley, Phys.Fluids 8, 1056 1965. Above the shear scale, the competition between energy production and transfer mechanisms gives rise to a completely different scenario with strong alterations of the observed scaling laws. This aspect has significant implications for the correct parametrization of the anisotropy behavior in the near wall region since, approaching the wall, an increasingly larger fraction of the scaling interval tends to conform to the shear-dominated power laws. \ua9 2008 American Institute of Physics