120 research outputs found
Koopman analysis of the long-term evolution in a turbulent convection cell
We analyse the long-time evolution of the three-dimensional flow in a closed
cubic turbulent Rayleigh-B\'{e}nard convection cell via a Koopman eigenfunction
analysis. A data-driven basis derived from diffusion kernels known in machine
learning is employed here to represent a regularized generator of the unitary
Koopman group in the sense of a Galerkin approximation. The resulting Koopman
eigenfunctions can be grouped into subsets in accordance with the discrete
symmetries in a cubic box. In particular, a projection of the velocity field
onto the first group of eigenfunctions reveals the four stable large-scale
circulation (LSC) states in the convection cell. We recapture the preferential
circulation rolls in diagonal corners and the short-term switching through roll
states parallel to the side faces which have also been seen in other
simulations and experiments. The diagonal macroscopic flow states can last as
long as a thousand convective free-fall time units. In addition, we find that
specific pairs of Koopman eigenfunctions in the secondary subset obey enhanced
oscillatory fluctuations for particular stable diagonal states of the LSC. The
corresponding velocity field structures, such as corner vortices and swirls in
the midplane, are also discussed via spatiotemporal reconstructions.Comment: 32 pages, 9 figures, article in press at Journal of Fluid Mechanic
Statistics of the Energy Dissipation Rate and Local Enstrophy in Turbulent Channel Flow
Using high-resolution direct numerical simulations, the height and Reynolds
number dependence of higher-order statistics of the energy dissipation rate and
local enstrophy are examined in incompressible, fully-developed turbulent
channel flow. The statistics are studied over a range of wall distances,
spanning the viscous sublayer to the channel flow centerline, for friction
Reynolds numbers and . The high resolution of
the simulations allows dissipation and enstrophy moments up to fourth order to
be calculated. These moments show a dependence on wall distance, and Reynolds
number effects are observed at the edge of the logarithmic layer. Conditional
analyses based on locations of intense rotation are also carried out in order
to determine the contribution of vortical structures to the dissipation and
enstrophy moments. Our analysis shows that, for the simulation at the larger
Reynolds number, small-scale fluctuations of both dissipation and enstrophy
become relatively constant for .Comment: Accepted by Physica
Patterned turbulence and relaminarization in MHD pipe and duct flows
We present results of a numerical analysis of relaminarization processes in MHD duct and pipe flows. It is motivated by Julius Hartmann's classical experiments on flows of mercury in pipes and ducts under the influence of magnetic fields. The simulations, conducted both in periodic and non‐periodic settings, provide a first detailed view of flow structures that have not been experimentally accessible. The main novelty of the analysis is very long (tens to hundreds of hydraulic diameters) computational domains that allows to discover new flow regimes with localized turbulent spots near the side walls parallel to the magnetic field. The computed critical parameters for transition as well as the friction coefficients are in good agreement with Hartmann's data. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109939/1/603_ftp.pd
Subcritical instability of liquid metal channel flow in the presence of a spanwise magnetic field
The linear and nonlinear evolution of perturbations is investigated in a magnetohydrodynamic channel flow with electrically insulating walls. The applied magnetic field is parallel to the walls and orthogonal to the stream. Linear optimal perturbations and their maximum amplifications over finite time intervals are computed using a scheme based on the direct and adjoint governing equations. It is shown that dominant optimal perturbations are no more the classical streamwise modes and how the flow is two-dimenzionalized for high enough Hartmann numbers. For fixed Reynolds and Hartmann numbers, direct numerical simulations are applied to investigate how the transition to turbulence is affected by the magnetic field. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/60899/1/4140005_ftp.pd
Direct numerical simulation of transition in MHD duct flow
Transition in the flow of electrically conducting fluid in a square duct with insulating walls is studied by direct numerical simulations. A uniform magnetic field is applied in the transverse direction. Moderate values of the Reynolds ( Re = 5000 ) and Hartmann ( Ha = 0 … 30 ) numbers are considered that correspond to the classical Hartmann & Lazarus [1] experiments. It is shown that the laminarization begins in the Hartmann layers, whereas the sidewall layers remain turbulent. Complete re‐laminarization occurs in the range of R = Re / Ha ≈︁ 220 , which is in agreement with the H. & L. experiments. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/89488/1/659_ftp.pd
Effect of wall conductivity on turbulent channel flow under spanwise magnetic field
The effect of wall conductivity on turbulence in electrically conducting fluid in the presence of a constant magnetic field is considered. A channel flow with a spanwise magnetic field is analyzed using high-resolution direct numerical simulations performed for the case of low magnetic Reynolds number. It is found that the effect of suppression of wall-normal momentum transfer and reduction of wall friction identified earlier for the flow with perfectly insulating walls is enhanced if the walls are electrically conducting. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78302/1/515_ftp.pd
Wall-attached convection under strong inclined magnetic fields
We employ a linear stability analysis and direct numerical simulations to
study the characteristics of wall-modes in thermal convection in a rectangular
box under strong and inclined magnetic fields. The walls of the convection cell
are electrically insulated. The stability analysis assumes periodicity in the
spanwise direction perpendicular to the plane of the homogeneous magnetic
field. Our study shows that for a fixed vertical magnetic field, the imposition
of horizontal magnetic fields results in an increase of the critical Rayleigh
number along with a decrease in the wavelength of the wall modes. The wall
modes become tilted along the direction of the resulting magnetic fields and
therefore extend further into the bulk as the horizontal magnetic field is
increased. Once the modes localized on the opposite walls interact, the
critical Rayleigh number decreases again and eventually drops below the value
for onset with a purely vertical field. We find that for sufficiently strong
horizontal magnetic fields, the steady wall modes occupy the entire bulk and
therefore convection is no longer restricted to the sidewalls. The above
results are confirmed by direct numerical simulations of the nonlinear
evolution of magnetoconvection.Comment: 25 pages, 18 figure
Effects of strong fringing magnetic fields on turbulent thermal convection
We study the influence of fringing magnetic fields on turbulent thermal
convection in a horizontally extended rectangular domain. The magnetic field is
created in the gap between two semi-infinite planar magnetic poles, with the
convection layer located near the edge of the gap. We employ direct numerical
simulations in this setup for fixed Rayleigh and small Prandtl numbers, but
vary the fringe-width by controlling the gap between the magnetic poles and the
convection cell. The magnetic field generated by the magnets is strong enough
to cease the flow in high magnetic flux region of the convection cell. We
observe that as the local vertical magnetic field strength increases, the large
scale structures become thinner and align themselves perpendicular to the
longitudinal sidewalls. We determine the local Nusselt and Reynolds numbers as
functions of the local Hartmann number (based on the vertical component of the
magnetic field) and estimate the global heat and momentum transport. We show
that the global heat transport decreases with increasing fringe-width for
strong magnetic fields but decreases with increasing fringe-width for weak
magnetic fields. In the regions of large vertical magnetic fields, the
convective motion becomes confined to the vicinity of the sidewalls. The
amplitudes of these wall modes show a non-monotonic dependence on the
fringe-width.Comment: 19 pages, 11 figure
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