107,556 research outputs found
Forcing and Velocity Correlations in a Vibrated Granular Monolayer
The role of forcing on the dynamics of a vertically shaken granular monolayer
is investigated. Using a flat plate, surprising negative velocity correlations
are measured. A mechanism for this anti-correlation is proposed with support
from both experimental results and molecular dynamics simulations. Using a
rough plate, velocity correlations are positive, and the velocity distribution
evolves from a gaussian at very low densities to a broader distribution at high
densities. These results are interpreted as a balance between stochastic
forcing, interparticle collisions, and friction with the plate.Comment: 4 pages, 5 figure
Isotropisation at small scales of rotating helically-driven turbulence
We present numerical evidence of how three-dimensionalization occurs at small
scale in rotating turbulence with Beltrami (ABC) forcing, creating helical
flow. The Zeman scale at which the inertial and eddy turn-over
times are equal is more than one order of magnitude larger than the dissipation
scale, with the relevant domains (large-scale inverse cascade of energy, dual
regime in the direct cascade of energy and helicity , and dissipation)
each moderately resolved. These results stem from the analysis of a large
direct numerical simulation on a grid of points, with Rossby and
Reynolds numbers respectively equal to 0.07 and . At scales
smaller than the forcing, a helical wave-modulated inertial law for the energy
and helicity spectra is followed beyond by Kolmogorov spectra
for and . Looking at the two-dimensional slow manifold, we also show
that the helicity spectrum breaks down at , a clear sign of
recovery of three-dimensionality in the small scales.Comment: 13 pages, 6 figure
Large amplitude forcing of a high speed 2-dimensional jet
The effect of large amplitude forcing on the growth of a high speed two dimensional jet was investigated experimentally. Two forcing techniques were utilized: mass flow oscillations and a mechanical system. The mass flow oscillation tests were conducted at Strouhal numbers from 0.00052 to 0.045, and peak to peak amplitudes up to 50 percent of the mean exit velocity. The exit Mach number was varied in the range 0.15 to 0.8. The corresponding Reynolds numbers were 8,400 and 45,000. The results indicate no significant change of the jet growth rate or centerline velocity decay compared to the undisturbed free jet. The mechanical forcing system consists of two counter rotating hexagonal cylinders located parallel to the span of the nozzle. Forcing frequencies up to 1,500 Hz were tested. Both symmetric and antisymmetric forcing can be implemented. The results for antisymmetric forcing showed a significant (75 percent) increase of the jet growth rate at an exit Mach number of 0.25 and a Strouhal number of 0.019. At higher rotational speeds, the jet deflected laterally. A deflection angle of 39 deg with respect to the centerline was measured at the maximum rotational speed
Who witnesses The Witness? Finding witnesses in The Witness is hard and sometimes impossible
We analyze the computational complexity of the many types of
pencil-and-paper-style puzzles featured in the 2016 puzzle video game The
Witness. In all puzzles, the goal is to draw a simple path in a rectangular
grid graph from a start vertex to a destination vertex. The different puzzle
types place different constraints on the path: preventing some edges from being
visited (broken edges); forcing some edges or vertices to be visited
(hexagons); forcing some cells to have certain numbers of incident path edges
(triangles); or forcing the regions formed by the path to be partially
monochromatic (squares), have exactly two special cells (stars), or be singly
covered by given shapes (polyominoes) and/or negatively counting shapes
(antipolyominoes). We show that any one of these clue types (except the first)
is enough to make path finding NP-complete ("witnesses exist but are hard to
find"), even for rectangular boards. Furthermore, we show that a final clue
type (antibody), which necessarily "cancels" the effect of another clue in the
same region, makes path finding -complete ("witnesses do not exist"),
even with a single antibody (combined with many anti/polyominoes), and the
problem gets no harder with many antibodies. On the positive side, we give a
polynomial-time algorithm for monomino clues, by reducing to hexagon clues on
the boundary of the puzzle, even in the presence of broken edges, and solving
"subset Hamiltonian path" for terminals on the boundary of an embedded planar
graph in polynomial time.Comment: 72 pages, 59 figures. Revised proof of Lemma 3.5. A short version of
this paper appeared at the 9th International Conference on Fun with
Algorithms (FUN 2018
Analyzing the Welfare Impacts of Full-line Forcing Contracts
Theoretical investigations have examined both anti-competitive and efficiency-inducing rationales for vertical bundling, making empirical evidence important to understanding its welfare implications. We use an extensive dataset on full-line forcing contracts between movie distributors and video retailers to empirically measure the impact of vertical bundling on welfare. We identify and measure three primary effects of fullline forcing contracts: market coverage, leverage, and efficiency. We find that bundling increases market coverage and efficiency, but has little impact on one distributor gaining leverage over another. As a result, we estimate that full-line forcing contracts increased consumer and producer surplus in this application.
The effect of modulated driving on non-rotating and rotating turbulent plane Couette flow
Direct numerical simulations of turbulent non-rotating and rotating plane Couette flow with a periodically modulated plate velocity are conducted to study the effect of modulated forcing on turbulent shear flows. The time-averaged shear Reynolds number is fixed at Re-s = 3 x 10(4), which results in a frictional Reynolds number of approximately Re-tau approximate to 400. The modulating frequency is varied in the range Wo is an element of (20, 200), while the modulating amplitude is kept fixed at 10 % of the shear velocity except to demonstrate that varying this parameter has minimal effect. The resulting shear at the plates is found to be independent of the forcing frequency, and equal to the non-modulated baseline. For the non-rotating simulations, two clear flow regions can be seen: a near-wall region that follows Stokes' theoretical solution, and a bulk region that behaves similar to Stokes' solutions but with an increased effective viscosity. For high driving frequencies, the amplitude response follows the scaling laws for modulated turbulence of von der Heydt et al. (Phys. Rev. E, vol. 67, 2003, 046308). Cyclonic rotation is not found to modify the system's behaviour in a substantial way, but anti-cyclonic rotation changes significantly the system's response to periodic forcing. We find that the persistent axial inhomogeneities introduced by mild anti-cyclonic rotation make it impossible to measure the propagation of the modulation adequately, while stronger anti-cyclonic rotation creates regions where the modulation travels instantaneously
Deterministic SR in a Piecewise Linear Chaotic Map
The phenomenon of Stochastic Resonance (SR) is observed in a completely
deterministic setting - with thermal noise being replaced by one-dimensional
chaos. The piecewise linear map investigated in the paper shows a transition
from symmetry-broken to symmetric chaos on increasing a system parameter. In
the latter state, the chaotic trajectory switches between the two formerly
disjoint attractors, driven by the map's inherent dynamics. This chaotic
switching rate is found to `resonate' with the frequency of an externally
applied periodic perturbation (multiplicative or additive). By periodically
modulating the parameter at a specific frequency , we observe the
existence of resonance where the response of the system (in terms of the
residence-time distribution) is maximum. This is a clear indication of SR-like
behavior in a chaotic system.Comment: 6 pages LaTex, 4 figure
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