43,991 research outputs found
On the relevance of Reynolds stresses in resolvent analyses of turbulent wall-bounded flows
The ability of linear stochastic response analysis to estimate coherent
motions is investigated in turbulent channel flow at friction Reynolds number
Re = 1007. The analysis is performed for spatial scales characteristic
of buffer-layer and large-scale motions by separating the contributions of
different temporal frequencies. Good agreement between the measured
spatio-temporal power spectral densities and those estimated by means of the
resolvent is found when the effect of turbulent Reynolds stresses, modelled
with an eddy-viscosity associated to the turbulent mean flow, is included in
the resolvent operator. The agreement is further improved when the flat forcing
power spectrum (white noise) is replaced with a power spectrum matching the
measures. Such a good agreement is not observed when the eddy-viscosity terms
are not included in the resolvent operator. In this case, the estimation based
on the resolvent is unable to select the right peak frequency and wall-normal
location of buffer-layer motions. Similar results are found when comparing
truncated expansions of measured streamwise velocity power spectral densities
based on a spectral proper orthogonal decomposition to those obtained with
optimal resolvent modes
Langevin PDF simulation of particle deposition in a turbulent pipe flow
The paper deals with the description of particle deposition on walls from a
turbulent flow over a large range of particle diameter, using a Langevin PDF
model. The first aim of the work is to test how the present Langevin model is
able to describe this phenomenon and to outline the physical as- pects which
play a major role in particle deposition. The general features and
characteristics of the present stochastic model are first recalled. Then,
results obtained with the standard form of the model are presented along with
an analysis which has been carried out to check the sensitivity of the
predictions on different mean fluid quantities. These results show that the
physical repre- sentation of the near-wall physics has to be improved and that,
in particular, one possible route is to introduce specific features related to
the near-wall coherent structures. In the following, we propose a simple
phenomenological model that introduces some of the effects due to the presence
of turbulent coherent structures on particles in a thin layer close to the
wall. The results obtained with this phenomenological model are in good
agreement with experimental evidence and this suggests to pursue in that
direction, towards the development of more general and rigorous stochastic
models that provide a link between a geometrical description of turbulent flow
and a statistical one.Comment: 40 pages, 8 figure
On the transition to turbulence of wall-bounded flows in general, and plane Couette flow in particular
The main part of this contribution to the special issue of EJM-B/Fluids
dedicated to Patrick Huerre outlines the problem of the subcritical transition
to turbulence in wall-bounded flows in its historical perspective with emphasis
on plane Couette flow, the flow generated between counter-translating parallel
planes. Subcritical here means discontinuous and direct, with strong
hysteresis. This is due to the existence of nontrivial flow regimes between the
global stability threshold Re_g, the upper bound for unconditional return to
the base flow, and the linear instability threshold Re_c characterized by
unconditional departure from the base flow. The transitional range around Re_g
is first discussed from an empirical viewpoint ({\S}1). The recent
determination of Re_g for pipe flow by Avila et al. (2011) is recalled. Plane
Couette flow is next examined. In laboratory conditions, its transitional range
displays an oblique pattern made of alternately laminar and turbulent bands, up
to a third threshold Re_t beyond which turbulence is uniform. Our current
theoretical understanding of the problem is next reviewed ({\S}2): linear
theory and non-normal amplification of perturbations; nonlinear approaches and
dynamical systems, basin boundaries and chaotic transients in minimal flow
units; spatiotemporal chaos in extended systems and the use of concepts from
statistical physics, spatiotemporal intermittency and directed percolation,
large deviations and extreme values. Two appendices present some recent
personal results obtained in plane Couette flow about patterning from numerical
simulations and modeling attempts.Comment: 35 pages, 7 figures, to appear in Eur. J. Mech B/Fluid
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