191 research outputs found
Destabilization of free convection by weak rotation
This study offers an explanation of a recently observed effect of
destabilization of free convective flows by weak rotation. After studying
several models where flows are driven by a simultaneous action of convection
and rotation, it is concluded that the destabilization is observed in the cases
where centrifugal force acts against main convective circulation. At relatively
low Prandtl numbers this counter action can split the main vortex into two
counter rotating vortices, where the interaction leads to instability. At
larger Prandtl numbers, the counter action of the centrifugal force steepens an
unstable thermal stratification, which triggers Rayleigh-B\'enard instability
mechanism. Both cases can be enhanced by advection of azimuthal velocity
disturbances towards the axis, where they grow and excite perturbations of the
radial velocity. The effect was studied considering a combined
convective/rotating flow in a cylinder with a rotating lid and a parabolic
temperature profile at the sidewall. Next, explanations of the destabilization
effect for rotating magnetic field driven flow and melt flow in a Czochralski
crystal growth model were derived
Constrained flow around a magnetic obstacle
Many practical applications exploit an external local magnetic field --
magnetic obstacle -- as an essential part of their constructions. Recently, it
has been demonstrated that the flow of an electrically conducting fluid
influenced by an external field can show several kinds of recirculation. The
present paper reports a 3D numerical study whose some results are compared with
an experiment about such a flow in a rectangular duct.Comment: accepted to JFM, 26 pages, 14 figure
On the role of vortex stretching in energy optimal growth of three dimensional perturbations on plane parallel shear flows
The three dimensional optimal energy growth mechanism, in plane parallel
shear flows, is reexamined in terms of the role of vortex stretching and the
interplay between the span-wise vorticity and the planar divergent components.
For high Reynolds numbers the structure of the optimal perturbations in
Couette, Poiseuille, and mixing layer shear profiles is robust and resembles
localized plane-waves in regions where the background shear is large. The waves
are tilted with the shear when the span-wise vorticity and the planar
divergence fields are in (out of) phase when the background shear is positive
(negative). A minimal model is derived to explain how this configuration
enables simultaneous growth of the two fields, and how this mutual
amplification reflects on the optimal energy growth. This perspective provides
an understanding of the three dimensional growth solely from the two
dimensional dynamics on the shear plane
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