Anisotropy Of Turbulent Flow Research Articles

Quantifying small-scale anisotropy in turbulent flows

Quantifying small-scale anisotropy in turbulent flows

On the scaling law of ramp structures in scalar turbulence

Ramp structures widely exist in scalar turbulence, such as temperature, water vapor, and carbon dioxide (CO2), which refer to the phenomenon that the physical quantity increases slowly with time and then suddenly drops. ramp structures lead to large gradients on a small scale and result in intermittency and anisotropy of turbulent flows. in this paper, wavelet analysis is used to analyze observed data from the beijing 325-m meteorological tower to extract ramp structures in temperature, water vapor, and CO2 signals. ramp structures in CO2 signals are different from those in temperature and water vapor in terms of the averaged temporal scale and normalized amplitude, and the ramp duration almost equals the cliff duration, which means ramp structures in CO2 signals are not easy to generate and different physical mechanisms may exist. in addition, both the ascending and descending part of ramp structures are linearly fitted. it is found that a scaling law exists between the slope and duration in the ascending part in the three scalar signals. the corresponding power exponents are slightly different. furthermore, the same rule exists in the descending part of ramp structures, which indicates that self-similarity may be a universal law in scalar turbulence. moreover, the maxima of selected ramp structures show the same pattern, i.e. there are ramp structures in the maximum sequence, which proves that small-scale ramp structures are superimposed on large-scale ramp structures.

Generation of anisotropy in turbulent flows subjected to rapid distortion.

A computational tool for the anisotropic time-evolution of the spectral velocity correlation tensor is presented. We operate in the linear, rapid distortion limit of the mean-field-coupled equations. Each term of the equations is written in the form of an expansion to arbitrary order in the basis of irreducible representations of the SO(3) symmetry group. The computational algorithm for this calculation solves a system of coupled equations for the scalar weights of each generated anisotropic mode. The analysis demonstrates that rapid distortion rapidly but systematically generates higher-order anisotropic modes. To maintain a tractable computation, the maximum number of rotational modes to be used in a given calculation is specified a priori. The computed Reynolds stress converges to the theoretical result derived by Batchelor and Proudman [Quart. J. Mech. Appl. Math. 7, 83 (1954)QJMMAV0033-561410.1093/qjmam/7.1.83] if a sufficiently large maximum number of rotational modes is utilized; more modes are required to recover the solution at later times. The emergence and evolution of the underlying multidimensional space of functions is presented here using a 64-mode calculation. Alternative implications for modeling strategies are discussed.

The role of directionality on the structure and dynamics of strongly anisotropic turbulent flows

Strong anisotropy in turbulent flows may be induced by body forces, Coriolis, buoyancy, Lorentz, and/or by large-scale gradients. These effects combined to the redistribution pressure terms are first identified by an angle dependence of the wave vector in Fourier space, the directionality. The resulting anisotropic structure is not taken into account in classical phenomenological theory, using essentially ‘isotropised’ dimensional analysis. Besides, it is generally hidden in practical engineering models by means of tuned constants, which may vary if the flow changes of nature. In this paper, different examples of anisotropic turbulence are revisited and compared to each other in order to shed light on fundamental aspects of this specific turbulence. To begin with, flows without energy production like rotating turbulence are considered. In this case, isotropy is broken by mean of third-order correlations in the equations. These correlations quantify the interscale energy transfer, and must be investigated at three-point, or triad by triad in Fourier space. This allows to account for the role of typical anisotropic frequency 2Ωcos θ k , with θ k the angle of the wave vector to the axis of rotation, and to simultaneously restore the role of phase coherence. We pursue the discussion with a second flow case, with production, quasi-static magnetohydrodynamics. This illustrates turbulence forced towards two-dimensional structure by an explicit Ohmic dissipation term. Linear dynamics displays an angle (called Moreau, or Shebalin) capable of reflecting the basic anisotropy in models as simple as . In the final phase of transition towards 2D structure, however, dynamics are essentially driven by third-order velocity correlations, and both successive linear and nonlinear phases yield counter-intuitive anisotropic results. The last case considered here is the turbulent mixing induced by a Rayleigh-Taylor instability. It is shown that anisotropy plays a central role in the dynamics of the mixing zone by means of an angular dimensionality parameter similar to the Moreau angle but for the density field, and appearing in a global model of buoyancy-drag equation.

Invariant Analysis of Turbulent Pipe Flow

The anisotropy analysis of Lumley provides a useful tool to quantify the degree of anisotropy in turbulent flows. Also included in the analysis are relations which may be used to check if the flow is axisymmetric or two-dimensional. However, the method does not provide any scale information about the structures. The analysis has therefore been extended here to Fourier space, which allows scale information to be derived. The method was applied to fully developed pipe flow and it was shown that the large-scale motion is everywhere close to axisymmetric. The intermediate scales are strongly influenced by the restrictions posed by the pipe walls. At the centre line, the flow structure appears axisymmetric at all scales, but the measurement sindicate that true axisymmetry is lost very quickly away from the centre line. The structure of the smallest scales could not be determined reliably due to a singularity in the analysis which develops as the scales go to zero.