589 research outputs found
Electrically driven convection in a thin annular film undergoing circular Couette flow
We investigate the linear stability of a thin, suspended, annular film of
conducting fluid with a voltage difference applied between its inner and outer
edges. For a sufficiently large voltage, such a film is unstable to
radially-driven electroconvection due to charges which develop on its free
surfaces. The film can also be subjected to a Couette shear by rotating its
inner edge. This combination is experimentally realized using films of smectic
A liquid crystals. In the absence of shear, the convective flow consists of a
stationary, azimuthally one-dimensional pattern of symmetric, counter-rotating
vortex pairs. When Couette flow is applied, an azimuthally traveling pattern
results. When viewed in a co-rotating frame, the traveling pattern consists of
pairs of asymmetric vortices. We calculate the neutral stability boundary for
arbitrary radius ratio and Reynolds number of the shear
flow, and obtain the critical control parameter and the critical azimuthal mode number . The
Couette flow suppresses the onset of electroconvection, so that . The calculated suppression is
compared with experiments performed at and .Comment: 17 pages, 2 column with 9 included eps figures. See also
http://mobydick.physics.utoronto.c
Experimental investigation of laminar turbulent intermittency in pipe flow
In shear flows turbulence first occurs in the form of localized structures
(puffs/spots) surrounded by laminar fluid. We here investigate such spatially
intermittent flows in a pipe experiment showing that turbulent puffs have a
well defined interaction distance, which sets the minimum spacing of puffs as
well as the maximum observable turbulent fraction. Two methodologies are
employed here. Starting from a laminar flow puffs can be created by locally
injecting a jet of fluid through the pipe wall. When the perturbation is
applied periodically at low frequencies, as expected, a regular sequence of
puffs is observed where the puff spacing is given by the ratio of the mean flow
speed to the perturbation frequency. On the other hand, at large frequencies
puffs are found to interact and annihilate each other. Varying the perturbation
frequency an interaction distance can be determined. In the second set of
experiments, the Reynolds number is reduced suddenly from fully developed
turbulence to the intermittent regime.The resulting flow reorganizes itself to
a sequence of constant size puffs which, unlike in Couette and Taylor Couette
flow are randomly spaced. The minimum distance between the turbulent patches is
identical to the puff interaction length. The puff interaction length is found
to be in excellent agreement with the wavelength of regular stripe and spiral
patterns in plane Couette and Taylor-Couette flow. We propose that the same
interaction mechanism is present in these flows
On the self-sustained nature of large-scale motions in turbulent Couette flow
Large-scale motions in wall-bounded turbulent flows are frequently
interpreted as resulting from an aggregation process of smaller-scale
structures. Here, we explore the alternative possibility that such large-scale
motions are themselves self-sustained and do not draw their energy from
smaller-scale turbulent motions activated in buffer layers. To this end, it is
first shown that large-scale motions in turbulent Couette flow at Re=2150
self-sustain even when active processes at smaller scales are artificially
quenched by increasing the Smagorinsky constant Cs in large eddy simulations.
These results are in agreement with earlier results on pressure driven
turbulent channels. We further investigate the nature of the large-scale
coherent motions by computing upper and lower-branch nonlinear steady solutions
of the filtered (LES) equations with a Newton-Krylov solver,and find that they
are connected by a saddle-node bifurcation at large values of Cs. Upper branch
solutions for the filtered large scale motions are computed for Reynolds
numbers up to Re=2187 using specific paths in the Re-Cs parameter plane and
compared to large-scale coherent motions. Continuation to Cs = 0 reveals that
these large-scale steady solutions of the filtered equations are connected to
the Nagata-Clever-Busse-Waleffe branch of steady solutions of the Navier-Stokes
equations. In contrast, we find it impossible to connect the latter to buffer
layer motions through a continuation to higher Reynolds numbers in minimal flow
units
Institute for Computational Mechanics in Propulsion (ICOMP)
The Institute for Computational Mechanics in Propulsion (ICOMP) is a combined activity of Case Western Reserve University, Ohio Aerospace Institute (OAI) and NASA Lewis. The purpose of ICOMP is to develop techniques to improve problem solving capabilities in all aspects of computational mechanics related to propulsion. The activities at ICOMP during 1991 are described
Linear non-normal energy amplification of harmonic and stochastic forcing in turbulent channel flow
The linear response to stochastic and optimal harmonic forcing of small coherent perturbations to the turbulent channel mean flow is computed for Reynolds numbers ranging from Re_tau=500 to Re_tau=20000. Even though the turbulent mean flow is linearly stable, it is nevertheless able to sustain large amplifications by the forcing. The most amplified structures consist of streamwise elongated streaks that are optimally forced by streamwise elongated vortices. For streamwise elongated structures, the mean energy amplification of the stochastic forcing is found to be, to a first approximation, inversely proportional to the forced spanwise wavenumber while it is inversely proportional to its square for optimal harmonic forcing in an intermediate spanwise wavenumber range. This scaling can be explicitly derived from the linearised equations under the assumptions of geometric similarity of the coherent perturbations and of logarithmic base flow. Deviations from this approximate power-law regime are apparent in the premultiplied energy amplification curves that reveal a strong influence of two different peaks. The dominant peak scales in outer units with the most amplified spanwise wavelength of while the secondary peak scales in wall units with the most amplified . The associated optimal perturbations are almost independent of the Reynolds number when respectively scaled in outer and inner units. In the intermediate wavenumber range the optimal perturbations are approximatively geometrically similar. Furthermore, the shape of the optimal perturbations issued from the initial value, the harmonic forcing and the stochastic forcing analyses are almost indistinguishable. The optimal streaks corresponding to the large-scale peak strongly penetrate into the inner layer, where their amplitude is proportional to the mean-flow profile. At the wavenumbers corresponding to the large-scale peak, the optimal amplifications of harmonic forcing are at least two orders of magnitude larger than the amplifications of the variance of stochastic forcing and both increase with the Reynolds number. This confirms the potential of the artificial forcing of optimal large-scale streaks for the flow control of wall-bounded turbulent flows
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Rossby waves and two-dimensional turbulence in a large-scale zonal jet
The theory of homogeneous barotropic beta-plane turbulence is here extended to include effects arising from spatial inhomogeneity in the form of a zonal shear flow. Attention is restricted to the geophysically important case of zonal flows that are barotropically stable and are of larger scale than the resulting transient eddy field.
Because of the presumed scale separation, the disturbance enstrophy is approximately conserved in a fully nonlinear sense, and the (nonlinear) wave-mean-flow interaction may be characterized as a shear-induced spectral transfer of disturbance enstrophy along lines of constant zonal wavenumber k. In this transfer the disturbance energy is generally not conserved. The nonlinear interactions between different disturbance components are turbulent for scales smaller than the inverse of Rhines's cascade-arrest scale κβ[identical with] (β0/2urms)½ and in this regime their leading-order effect may be characterized as a tendency to spread the enstrophy (and energy) along contours of constant total wavenumber κ [identical with] (k2 + l2)½. Insofar as this process of turbulent isotropization involves spectral transfer of disturbance enstrophy across lines of constant zonal wavenumber k, it can be readily distinguished from the shear-induced transfer which proceeds along them. However, an analysis in terms of total wavenumber K alone, which would be justified if the flow were homogeneous, would tend to mask the differences.
The foregoing theoretical ideas are tested by performing direct numerical simulation experiments. It is found that the picture of classical beta-plane turbulence is altered, through the effect of the large-scale zonal flow, in the following ways: (i) while the turbulence is still confined to K Kβ, the disturbance field penetrates to the largest scales of motion; (ii) the larger disturbance scales K u2 rather than vice versa; (iii) the initial spectral transfer rate away from an isotropic intermediate-scale source is significantly enhanced by the shear-induced transfer associated with straining by the zonal flow. This last effect occurs even when the large-scale shear appears weak to the energy-containing eddies, in the sense that dU/dy [double less-than sign] κ for typical eddy length and velocity scales
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