866 research outputs found
Effect of blade geometry on the aerodynamic loads produced by vertical-axis wind turbines
Accurate aerodynamic modelling of vertical-axis wind turbines poses a significant challenge. The rotation of the turbine induces large variations in the angle of attack of its blades that can manifest as dynamic stall. In addition, interactions between the blades of the turbine and the wake that they produce can result in impulsive changes to the aerodynamic loading. The Vorticity Transport Model has been used to simulate the aerodynamic performance and wake dynamics of three different vertical-axis wind turbine configurations. It is known that vertical-axis turbines with either straight or curved blades deliver torque to their shaft that fluctuates at the blade passage frequency of the rotor. In contrast, a turbine with helically twisted blades delivers a relatively steady torque to the shaft. In this article, the interactions between helically twisted blades and the vortices within their wake are shown to result in localized perturbations to the aerodynamic loading on the rotor that can disrupt the otherwise relatively smooth power output that is predicted by simplistic aerodynamic tools that do not model the wake to sufficient fidelity. Furthermore, vertical-axis wind turbines with curved blades are shown to be somewhat more susceptible to local dynamic stall than turbines with straight blades
Integral points on varieties with infinite \'etale fundamental group
We study integral points on varieties with infinite \'etale fundamental
groups. More precisely, for a number field and a smooth projective
variety, we prove that for any geometrically Galois cover of degree at least , there exists an ample line bundle
on such that for a general member of the complete linear
system , is geometrically irreducible and any set of
-integral points on is finite. We apply this result to
varieties with infinite \'etale fundamental group to give new examples of
irreducible divisors on varieties for which finiteness of integral points is
provable.Comment: 9 pages; comments welcome
On Maltsev digraphs
The original publication is available at www.springerlink.com Copyright SpringerWe study digraphs preserved by a Maltsev operation, Maltsev digraphs. We show that these digraphs retract either onto a directed path or to the disjoint union of directed cycles, showing that the constraint satisfaction problem for Maltsev digraphs is in logspace, L. (This was observed in [19] using an indirect argument.) We then generalize results in [19] to show that a Maltsev digraph is preserved not only by a majority operation, but by a class of other operations (e.g., minority, Pixley) and obtain a O(V G4)-time algorithm to recognize Maltsev digraphs. We also prove analogous results for digraphs preserved by conservative Maltsev operations which we use to establish that the list homomorphism problem for Maltsev digraphs is in L. We then give a polynomial time characterisation of Maltsev digraphs admitting a conservative 2-semilattice operation. Finally, we give a simple inductive construction of directed acyclic digraphs preserved by a Maltsev operation.Peer reviewe
Strength of forelimb lateralization predicts motor errors in an insect
Lateralized behaviours are widespread in both vertebrates and invertebrates, suggesting that lateralization is advantageous. Yet evidence demonstrating proximate or ultimate advantages remains scarce, particularly in invertebrates or in species with individual-level lateralization. Desert locusts (Schistocerca gregaria) are biased in the forelimb they use to perform targeted reaching across a gap. The forelimb and strength of this bias differed among individuals, indicative of individual-level lateralization. Here we show that strongly biased locusts perform better during gap-crossing, making fewer errors with their preferred forelimb. The number of targeting errors locusts make negatively correlates with the strength of forelimb lateralization. This provides evidence that stronger lateralization confers an advantage in terms of improved motor control in an invertebrate with individual-level lateralization
Congruence modularity implies cyclic terms for finite algebras
An n-ary operation f : A(n) -> A is called cyclic if it is idempotent and f(a(1), a(2), a(3), ... , a(n)) = f(a(2), a(3), ... , a(n), a(1)) for every a(1), ... , a(n) is an element of A. We prove that every finite algebra A in a congruence modular variety has a p-ary cyclic term operation for any prime p greater than vertical bar A vertical bar
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The evolution of handedness: why are ant colonies left- and right-handed?
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Differential scaling within an insect compound eye
Environmental and genetic influences cause individuals of a species to differ in size. As they do so, organ size and shape are scaled to available resources whilst maintaining function. The scaling of entire organs has been investigated extensively but scaling within organs remains poorly understood. By making use of the structure of the insect compound eye, we show that different regions of an organ can respond differentially to changes in body size. Wood ant (Formica rufa) compound eyes contain facets of different diameters in different regions. When the animal body size changes, lens diameters from different regions can increase or decrease in size either at the same rate (a ‘grade’ shift) or at different rates (a ‘slope’ shift). These options are not mutually exclusive, and we demonstrate that both types of scaling apply to different regions of the same eye. This demonstrates that different regions within a single organ can use different rules to govern their scaling, responding differently to their developmental environment. Thus, the control of scaling is more nuanced than previously appreciated, diverse responses occurring even among homologous cells within a single organ. Such fine control provides a rich substrate for the diversification of organ morphology
Ground states for a class of deterministic spin models with glassy behaviour
We consider the deterministic model with glassy behaviour, recently
introduced by Marinari, Parisi and Ritort, with \ha\ , where is the discrete sine Fourier transform. The
ground state found by these authors for odd and prime is shown to
become asymptotically dege\-ne\-ra\-te when is a product of odd primes,
and to disappear for even. This last result is based on the explicit
construction of a set of eigenvectors for , obtained through its formal
identity with the imaginary part of the propagator of the quantized unit
symplectic matrix over the -torus.Comment: 15 pages, plain LaTe
Quenching across quantum critical points in periodic systems: dependence of scaling laws on periodicity
We study the quenching dynamics of a many-body system in one dimension
described by a Hamiltonian that has spatial periodicity. Specifically, we
consider a spin-1/2 chain with equal xx and yy couplings and subject to a
periodically varying magnetic field in the z direction or, equivalently, a
tight-binding model of spinless fermions with a periodic local chemical
potential, having period 2q, where q is a natural number. For a linear quench
of the magnetic field strength (or potential strength) at rate 1/\tau across a
quantum critical point, we find that the density of defects thereby produced
scales as 1/\tau^{q/(q+1)}, deviating from the 1/\sqrt{\tau} scaling that is
ubiquitous to a range of systems. We analyze this behavior by mapping the
low-energy physics of the system to a set of fermionic two-level systems
labeled by the lattice momentum k undergoing a non-linear quench as well as by
performing numerical simulations. We also find that if the magnetic field is a
superposition of different periods, the power law depends only on the smallest
period for very large values of \tau although it may exhibit a cross-over at
intermediate values of \tau. Finally, for the case where a zz coupling is also
present in the spin chain, or equivalently, where interactions are present in
the fermionic system, we argue that the power associated with the scaling law
depends on a combination of q and interaction strength.Comment: 13 pages including 11 figure
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