786 research outputs found
Slower Speed and Stronger Coupling: Adaptive Mechanisms of Self-Organized Chaos Synchronization
We show that two initially weakly coupled chaotic systems can achieve
self-organized synchronization by adaptively reducing their speed and/or
enhancing the coupling strength. Explicit adaptive algorithms for
speed-reduction and coupling-enhancement are provided. We apply these
algorithms to the self-organized synchronization of two coupled Lorenz systems.
It is found that after a long-time self-organized process, the two coupled
chaotic systems can achieve synchronization with almost minimum required
coupling-speed ratio.Comment: 4 pages, 5 figure
Momentum and Heat Transfer in a Laminar Boundary Layer with Slip Flow
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77059/1/AIAA-22968-756.pd
The friction factor of two-dimensional rough-boundary turbulent soap film flows
We use momentum transfer arguments to predict the friction factor in
two-dimensional turbulent soap-film flows with rough boundaries (an analogue of
three-dimensional pipe flow) as a function of Reynolds number Re and roughness
, considering separately the inverse energy cascade and the forward
enstrophy cascade. At intermediate Re, we predict a Blasius-like friction
factor scaling of in flows dominated by the
enstrophy cascade, distinct from the energy cascade scaling of
. For large Re, in the enstrophy-dominated case.
We use conformal map techniques to perform direct numerical simulations that
are in satisfactory agreement with theory, and exhibit data collapse scaling of
roughness-induced criticality, previously shown to arise in the 3D pipe data of
Nikuradse.Comment: 4 pages, 3 figure
Laminar Craya-Curtet jets
This Brief Communication investigates laminar Craya-Curtet flows, formed when a jet with moderately large Reynolds number discharges into a coaxial ducted flow of much larger radius. It is seen that the Craya-Curtet number, C=(J/sub c//J/sub j/)/sup 1/2/, defined as the square root of the ratio of the momentum flux of the coflowing stream to that of the central jet, arises as the single governing parameter when the boundary-layer approximation is used to describe the resulting steady slender jet. The numerical integrations show that for C above a critical value C/sub c/ the resulting streamlines remain aligned with the axis, while for C<C/sub c/ the entrainment demands of the jet cannot be satisfied by the coflow, and a toroidal recirculation region forms. The critical Craya-Curtet number is determined for both uniform and parabolic coflow, yielding C/sub c/=0.65 and C/sub c/=0.77, respectively. The streamlines determined numerically are compared with those obtained experimentally by flow visualizations, yielding good agreement in the resulting flow structure and also in the value of C/sub c
On similarity and pseudo-similarity solutions of Falkner-Skan boundary layers
The present work deals with the two-dimensional incompressible,laminar,
steady-state boundary layer equations. First, we determinea family of velocity
distributions outside the boundary layer suchthat these problems may have
similarity solutions. Then, we examenin detail new exact solutions, called
Pseudo--similarity, where the external velocity varies inversely-linear with
the distance along the surface $ (U_e(x) = U_\infty x^{-1}). The present work
deals with the two-dimensional incompressible, laminar, steady-state boundary
layer equations. First, we determine a family of velocity distributions outside
the boundary layer such that these problems may have similarity solutions.
Then, we examenin detail new exact solutions. The analysis shows that solutions
exist only for a lateral suction. For specified conditions, we establish the
existence of an infinite number of solutions, including monotonic solutions and
solutions which oscillate an infinite number of times and tend to a certain
limit. The properties of solutions depend onthe suction parameter. Furthermore,
making use of the fourth--order Runge--Kutta scheme together with the shooting
method, numerical solutions are obtained.Comment: 15 page
Boundary layer structure in turbulent thermal convection and its consequences for the required numerical resolution
Results on the Prandtl-Blasius type kinetic and thermal boundary layer
thicknesses in turbulent Rayleigh-B\'enard convection in a broad range of
Prandtl numbers are presented. By solving the laminar Prandtl-Blasius boundary
layer equations, we calculate the ratio of the thermal and kinetic boundary
layer thicknesses, which depends on the Prandtl number Pr only. It is
approximated as for and as for
, with . Comparison of the Prandtl--Blasius velocity
boundary layer thickness with that evaluated in the direct numerical
simulations by Stevens, Verzicco, and Lohse (J. Fluid Mech. 643, 495 (2010))
gives very good agreement. Based on the Prandtl--Blasius type considerations,
we derive a lower-bound estimate for the minimum number of the computational
mesh nodes, required to conduct accurate numerical simulations of moderately
high (boundary layer dominated) turbulent Rayleigh-B\'enard convection, in the
thermal and kinetic boundary layers close to bottom and top plates. It is shown
that the number of required nodes within each boundary layer depends on Nu and
Pr and grows with the Rayleigh number Ra not slower than \sim\Ra^{0.15}. This
estimate agrees excellently with empirical results, which were based on the
convergence of the Nusselt number in numerical simulations
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