1,720,965 research outputs found
Inertial particles in homogeneous shear turbulence: Experiments and direct numerical simulation
Full text linkThe properties of the transport of heavy inertial particles in a uniformly sheared turbulent flow have been investigated by combining experimental and numerical data at particle Stokes number St ≈ 0.3 ÷ 0.5 respectively. As in isotropic turbulence, particles are observed to avoid zones of intense enstrophy and to cluster in strain-dominated regions, resulting in highly intermittent spatial distributions. Moreover, the anisotropy of the mean flow is found to imprint a clear preferential orientation of the particle clusters in the direction of the maximum mean strain. These features are observed both in the numerics and in the experiments, and have been consistently quantified by a number of complementary statistical tools, such as the Voronoï tessellations and the pair correlation function. The latter quantity has been generalized in the form of the Angular Distribution Function and has allowed to evaluate the anisotropy content of the particle field at each scale. The behavior of this observable exhibits the same trend in the two datasets and suggests that, owing to increased inertia, the particle distribution starts to recover isotropy at scales smaller than the carrier velocity field. A proper rescaling of the two datasets in terms of their respective values of the shear scale allows to account for differences in the Reynolds number of experiments and numerics in the range of scales dominated by the mean shear. © 2013 Springer Science+Business Media Dordrecht
Scaling properties in the production range of shear dominated flows
No full textIn large Reynolds number turbulence, isotropy is recovered as the scale is reduced and homogeneous-isotropic scalings are eventually observed. This picture is violated in many cases, e.g., wall bounded flows, where, due to the shear, different scaling laws emerge. This effect has been ascribed to the contamination of the inertial range by the larger anisotropic scales. The issue is addressed here by analyzing both numerical and experimental data for a homogeneous shear flow. In fact, under strong shear, the alteration of the scaling exponents is not induced by the contamination from the anisotropic sectors. Actually, the exponents are universal properties of the isotropic component of the structure functions of shear dominated flows. The implications are discussed in the context of turbulence near solid walls, where improved closure models would be advisable
Experimental assessment of a new form of scaling law for new-wall turbulence
Full text linkScaling laws and intermittency in the wall region of a turbulent flow are addressed by analyzing data obtained by single component hot wire anemometry in the boundary layer of a flat plate at Reθ = 2200. The article aims in particular at the experimental validation of a new form of refined similarity recently proposed for the shear dominated range of turbulence, where the classical Kolmogorov-Oboukhov inertial range theory is inappropriate. An approach inspired by the extended self-similarity allows for the extraction of the different power laws for the longitudinal structure functions at several wall normal distances. A double scaling regime is found in the logarithmic region, confirming previous experimental results. Approaching the wall, the scaling range corresponding to the classical cascade-dominated range tends to disappear and, in the buffer layer, a single power law is found to describe the available range of scales. The double scaling is shown to be associated with two different forms of refined similarity. The classical form holds below the shear scale Ls. The other, originally introduced on the basis of direct numerical simulation data for a turbulent channel, is experimentally confirmed to set up above Ls. Given the experimental difficulties in the evaluation of the instantaneous dissopation rate, some care is devoted to check that Taylor hypothesis and the one-dimensional surrogate do not bias the results. The increased intermittency as the wall is approached is experimentally found entirely consistent with the failure of the refined Kolmogorov-Oboukhov similarity and the establishment of its new form near the wall. © 2002 American Institute of Physics
The residual of anisotropy at small scales in high shear turbulence
No full textIt has always been believed that turbulence in fluids can achieve a universal state at small scales with fluctuations that, becoming statistically isotropic, are characterized by universal scaling laws. In fact, in different branches of physics it is common to find conditions such that statistical isotropy is never recovered and the anisotropy induced by large scale shear contaminates the entire range of scales up to velocity gradients. We address this issue here, of particular significance, for wall bounded flows. The systematic decomposition in spherical harmonics of the correlation functions of velocity fluctuations enables us to extract the different anisotropic contributions. They vanish at small scale at a relatively fast rate under weak shear. Under strong shear instead they keep a significant amplitude up to viscous scales, thus leaving a persistent signature on the gradients which can be detected even in the statistics of low order, e.g., in the energy dissipation tensor. (C) 2007 American Institute of Physics
Double scaling in shear dominated flows
No full textThe nature of intermittency in shear dominated flows changes with respect to homogeneous and Isotropic conditions since the process of energy transfer is affected by the turbulent kinetic energy production associated with the Reynolds stresses. For these flows, a new form of refined similarity law is able to describe the increased level of intermittency. Ideally a length scale associated with the mean shear separates the two ranges, i.e., the classical Kolmogorov-like inertial range, below, and the shear dominated range, above. In the present paper we give evidence of the coexistence of the two regimes and we support the conjecture that the statistical properties of the dissipation field are practically insensible to the mean shear. This allows for a theoretical prediction of the scaling exponents of structure functions in the shear dominated range based on the known intermittency corrections for isotropic flows. The prediction is found to closely match the available numerical and experimental data. The analysis shows that the larger anisotropic scales of shear turbulence display universality, and determines the modality by which the dissipation field fixes the properties of turbulent fluctuations in the shear dominated range
Scaling of mixed structure functions in turbulent boundary layers
Full text linkWe 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 Re?15000. We isolate the anisotropic observables by means of decomposition tools based on the SO(3) 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's 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
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