1,330 research outputs found
Taylor's (1935) dissipation surrogate reinterpreted
New results from direct numerical simulation of decaying isotropic turbulence show that Taylor’s expression for the viscous dissipation rate ε = CεU3/L is more appropriately interpreted as a surrogate for the inertial energy flux. As a consequence, the well known dependence of the Taylor prefactor Cε on Reynolds number Cε(RL)→Cε,∞ can be understood as corresponding to the onset of an inertial range
Eulerian spectral closures for isotropic turbulence using a time-ordered fluctuation-dissipation relation
Procedures for time-ordering the covariance function, as given in a previous
paper (K. Kiyani and W.D. McComb Phys. Rev. E 70, 066303 (2004)), are extended
and used to show that the response function associated at second order with the
Kraichnan-Wyld perturbation series can be determined by a local (in wavenumber)
energy balance. These time-ordering procedures also allow the two-time
formulation to be reduced to time-independent form by means of exponential
approximations and it is verified that the response equation does not have an
infra-red divergence at infinite Reynolds number. Lastly, single-time
Markovianised closure equations (stated in the previous paper above) are
derived and shown to be compatible with the Kolmogorov distribution without the
need to introduce an ad hoc constant.Comment: 12 page
Non-local modulation of the energy cascade in broad-band forced turbulence
Classically, large-scale forced turbulence is characterized by a transfer of
energy from large to small scales via nonlinear interactions. We have
investigated the changes in this energy transfer process in broad-band forced
turbulence where an additional perturbation of flow at smaller scales is
introduced. The modulation of the energy dynamics via the introduction of
forcing at smaller scales occurs not only in the forced region but also in a
broad range of length-scales outside the forced bands due to non-local triad
interactions. Broad-band forcing changes the energy distribution and energy
transfer function in a characteristic manner leading to a significant
modulation of the turbulence. We studied the changes in this transfer of energy
when changing the strength and location of the small-scale forcing support. The
energy content in the larger scales was observed to decrease, while the energy
transport power for scales in between the large and small scale forcing regions
was enhanced. This was investigated further in terms of the detailed transfer
function between the triad contributions and observing the long-time statistics
of the flow. The energy is transferred toward smaller scales not only by
wavenumbers of similar size as in the case of large-scale forced turbulence,
but by a much wider extent of scales that can be externally controlled.Comment: submitted to Phys. Rev. E, 15 pages, 18 figures, uses revtex4.cl
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