396 research outputs found
Three-dimensional instabilities in a discretely heated annular flow: onset of spatio-temporal complexity via defect dynamics
The transition to three-dimensional and unsteady flow in an annulus with a discrete heat source on the inner cylinder is studied numerically. For large applied heat flux through the heater (large Grashof number Gr), there is a strong wall plume originating at the heater that reaches the top and forms a large scale axisymmetric wavy structure along the top. For Gr approximate to 6 x 109, this wavy structure becomes unstable to three-dimensional instabilities with high azimuthal wavenumbers m similar to 30, influenced by mode competition within an Eckhaus band of wavenumbers. Coexisting with some of these steady three-dimensional states, solution branches with localized defects break parity and result in spatio-temporal dynamics. We have identified two such time dependent states. One is a limit cycle that while breaking spatial parity, retains spatio-temporal parity. The other branch corresponds to quasi-periodic states that have globally broken parity. (C) 2014 AIP Publishing LLC.Postprint (published version
Numerical simulation of double‐diffusive Marangoni convection
This is the published version. Copyright © 1986 American Institute of PhysicsMarangoni convection is important in a variety of physical systems and occurs as a result of surface tension gradients at a liquid free surface. In general, liquid surface tension varies with temperature and species concentration in a binary fluid. If the temperature and concentration distributions make opposing contributions to the overall surface tension gradient at a free surface, convective motion, as well as heat and mass transfer within the system, is shown to depend on double‐diffusive effects. This situation is analogous to double‐diffusive natural convection, in that convection may occur, even though the overall surface tension difference along the free surface suggests stagnant fluid conditions
Conductive and convective heat transfer in fluid flows between differentially heated and rotating cylinders
The flow of fluid confined between a heated rotating cylinder and a cooled
stationary cylinder is a canonical experiment for the study of heat transfer in
engineering. The theoretical treatment of this system is greatly simplified if
the cylinders are assumed to be of infinite length or periodic in the axial
direction, in which cases heat transfer occurs only through conduction as in a
solid. We here investigate numerically heat transfer and the onset of
turbulence in such flows by using both periodic and no-slip boundary conditions
in the axial direction. We obtain a simple linear criterion that determines
whether the infinite-cylinder assumption can be employed. The curvature of the
cylinders enters this linear relationship through the slope and additive
constant. For a given length-to-gap aspect ratio there is a critical Rayleigh
number beyond which the laminar flow in the finite system is convective and so
the behaviour is entirely different from the periodic case. The criterion does
not depend on the Prandtl number and appears quite robust with respect to the
Reynolds number. In particular, it continues to work reasonably in the
turbulent regime.Comment: 25 pages, 9 figure
Biharmonic pattern selection
A new model to describe fractal growth is discussed which includes effects
due to long-range coupling between displacements . The model is based on the
biharmonic equation in two-dimensional isotropic defect-free
media as follows from the Kuramoto-Sivashinsky equation for pattern formation
-or, alternatively, from the theory of elasticity. As a difference with
Laplacian and Poisson growth models, in the new model the Laplacian of is
neither zero nor proportional to . Its discretization allows to reproduce a
transition from dense to multibranched growth at a point in which the growth
velocity exhibits a minimum similarly to what occurs within Poisson growth in
planar geometry. Furthermore, in circular geometry the transition point is
estimated for the simplest case from the relation
such that the trajectories become stable at the growing surfaces in a
continuous limit. Hence, within the biharmonic growth model, this transition
depends only on the system size and occurs approximately at a distance far from a central seed particle. The influence of biharmonic patterns on
the growth probability for each lattice site is also analysed.Comment: To appear in Phys. Rev. E. Copies upon request to
[email protected]
An Asymptotic-Preserving all-speed scheme for the Euler and Navier-Stokes equations
We present an Asymptotic-Preserving 'all-speed' scheme for the simulation of
compressible flows valid at all Mach-numbers ranging from very small to order
unity. The scheme is based on a semi-implicit discretization which treats the
acoustic part implicitly and the convective and diffusive parts explicitly.
This discretization, which is the key to the Asymptotic-Preserving property,
provides a consistent approximation of both the hyperbolic compressible regime
and the elliptic incompressible regime. The divergence-free condition on the
velocity in the incompressible regime is respected, and an the pressure is
computed via an elliptic equation resulting from a suitable combination of the
momentum and energy equations. The implicit treatment of the acoustic part
allows the time-step to be independent of the Mach number. The scheme is
conservative and applies to steady or unsteady flows and to general equations
of state. One and Two-dimensional numerical results provide a validation of the
Asymptotic-Preserving 'all-speed' properties
Heterogeneous nanofluids: natural convection heat transfer enhancement
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case
GPU Accelerated Multiple-Relaxation-Time Lattice Boltzmann Simulation of Convective Flows in a Porous Media
A two-dimensional (2D) multiple-relaxation-time (MRT)-lattice Boltzmann method (LBM) is used for porous media with the Brinkman–Forchheimer extended Darcy model to investigate the natural and mixed convection flows in a square cavity. This Brinkman–Forchheimer model is directly applied by using the forcing moments as a source term. A Tesla K40 NVIDIA graphics card has been used for the present graphics processing unit (GPU) parallel computing via compute unified device architecture (CUDA) C platform. The numerical results are presented in terms of velocity, temperature, streamlines, isotherms, and local and average Nusselt numbers. For the wide range of Rayleigh numbers, (Ra = 103 to 1010), Reynolds numbers, Darcy numbers, and porosities, the average Nusselt number is compared with the available results computed by finite element method (FEM) and single-relaxation-time (SRT) lattice Boltzmann method-LBM and, showing great compliance. The results are also validated with the previous experimental results. The simulations speed up to a maximum of 144x using CUDA C in GPU compared with the time of FORTRAN 90 code using a single core CPU simulation
Parameter-free symmetry-preserving regularization modeling of a turbulent differentially heated cavity
Impact of different internal convection control strategies in a non-evacuated CPC collector performance
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