10,276 research outputs found
Computing Groebner Fans
This paper presents algorithms for computing the Groebner fan of an arbitrary
polynomial ideal. The computation involves enumeration of all reduced Groebner
bases of the ideal. Our algorithms are based on a uniform definition of the
Groebner fan that applies to both homogeneous and non-homogeneous ideals and a
proof that this object is a polyhedral complex. We show that the cells of a
Groebner fan can easily be oriented acyclically and with a unique sink,
allowing their enumeration by the memory-less reverse search procedure. The
significance of this follows from the fact that Groebner fans are not always
normal fans of polyhedra in which case reverse search applies automatically.
Computational results using our implementation of these algorithms in the
software package Gfan are included.Comment: 26 page
Polygons on a Rotating Fluid Surface
We report a novel and spectacular instability of a fluid surface in a
rotating system. In a flow driven by rotating the bottom plate of a partially
filled, stationary cylindrical container, the shape of the free surface can
spontaneously break the axial symmetry and assume the form of a polygon
rotating rigidly with a speed different from that of the plate. With water we
have observed polygons with up to 6 corners. It has been known for many years
that such flows are prone to symmetry breaking, but apparently the polygonal
surface shapes have never been observed. The creation of rotating internal
waves in a similar setup was observed for much lower rotation rates, where the
free surface remains essentially flat. We speculate that the instability is
caused by the strong azimuthal shear due to the stationary walls and that it is
triggered by minute wobbling of the rotating plate. The slight asymmetry
induces a tendency for mode-locking between the plate and the polygon, where
the polygon rotates by one corner for each complete rotation of the plate
The Circuit Ideal of a Vector Configuration
The circuit ideal, \ica, of a configuration \A = \{\a_1, ..., \a_n\}
\subset \Z^d is the ideal generated by the binomials {\x}^{\cc^+} -
{\x}^{\cc^-} \in \k[x_1, ..., x_n] as \cc = \cc^+ - \cc^- \in \Z^n varies
over the circuits of \A. This ideal is contained in the toric ideal, \ia,
of \A which has numerous applications and is nontrivial to compute. Since
circuits can be computed using linear algebra and the two ideals often
coincide, it is worthwhile to understand when equality occurs. In this paper we
study \ica in relation to \ia from various algebraic and combinatorial
perspectives. We prove that the obstruction to equality of the ideals is the
existence of certain polytopes. This result is based on a complete
characterization of the standard pairs/associated primes of a monomial initial
ideal of \ica and their differences from those for the corresponding toric
initial ideal. Eisenbud and Sturmfels proved that \ia is the unique minimal
prime of \ica and that the embedded primes of \ica are indexed by certain
faces of the cone spanned by \A. We provide a necessary condition for a
particular face to index an embedded prime and a partial converse. Finally, we
compare various polyhedral fans associated to \ia and \ica. The Gr\"obner
fan of \ica is shown to refine that of \ia when the codimension of the
ideals is at most two.Comment: 25 page
Fish schooling as a basis for vertical axis wind turbine farm design
Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the
high power coefficient (mechanical power output divided by the power of the
free-stream air through the turbine cross-sectional area) of an isolated
turbine. However when in close proximity to neighbouring turbines, HAWTs suffer
from a reduced power coefficient. In contrast, previous research on vertical
axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience
only small decreases (or even increases) in an individual turbine's power
coefficient when placed in close proximity to neighbours, thus yielding much
higher power outputs for a given area of land. A potential flow model of
inter-VAWT interactions is developed to investigate the effect of changes in
VAWT spatial arrangement on the array performance coefficient, which compares
the expected average power coefficient of turbines in an array to a
spatially-isolated turbine. A geometric arrangement based on the configuration
of shed vortices in the wake of schooling fish is shown to significantly
increase the array performance coefficient based upon an array of 16x16 wind
turbines. Results suggest increases in power output of over one order of
magnitude for a given area of land as compared to HAWTs.Comment: Submitted for publication in BioInspiration and Biomimetics. Note:
The technology described in this paper is protected under both US and
international pending patents filed by the California Institute of Technolog
Age-related differences in adaptation during childhood: The influences of muscular power production and segmental energy flow caused by muscles
Acquisition of skillfulness is not only characterized by a task-appropriate application of muscular forces but also by the ability to adapt performance to changing task demands. Previous research suggests that there is a different developmental schedule for adaptation at the kinematic compared to the neuro-muscular level. The purpose of this study was to determine how age-related differences in neuro-muscular organization affect the mechanical construction of pedaling at different levels of the task. By quantifying the flow of segmental energy caused by muscles, we determined the muscular synergies that construct the movement outcome across movement speeds. Younger children (5-7 years; n = 11), older children (8-10 years; n = 8), and adults (22-31 years; n = 8) rode a stationary ergometer at five discrete cadences (60, 75, 90, 105, and 120 rpm) at 10% of their individually predicted peak power output. Using a forward dynamics simulation, we determined the muscular contributions to crank power, as well as muscular power delivered to the crank directly and indirectly (through energy absorption and transfer) during the downstroke and the upstroke of the crank cycle. We found significant age Ă— cadence interactions for (1) peak muscular power at the hip joint [Wilks' Lambda = 0.441, F(8,42) = 2.65, p = 0.019] indicating that at high movement speeds children produced less peak power at the hip than adults, (2) muscular power delivered to the crank during the downstroke and the upstroke of the crank cycle [Wilks' Lambda = 0.399, F(8,42) = 3.07, p = 0.009] indicating that children delivered a greater proportion of the power to the crank during the upstroke when compared to adults, (3) hip power contribution to limb power [Wilks' Lambda = 0.454, F(8,42) = 2.54, p = 0.023] indicating a cadence-dependence of age-related differences in the muscular synergy between hip extensors and plantarflexors. The results demonstrate that in spite of a successful performance, children construct the task of pedaling differently when compared to adults, especially when they are pushed to their performance limits. The weaker synergy between hip extensors and plantarflexors suggests that a lack of inter-muscular coordination, rather than muscular power production per se, is a factor that limits children's performance ranges
How to Improve University Orientation: Seven Good Practice Strategies for South Africa
There is a great deal of variability in the practice of orientation across the country at South Africa’s universities, and there is limited knowledge of what exactly constitutes good practice in orientation. Many areas of enquiry remain unexplored, and remain blind spots for South Africa’s higher education sector. The article addresses this central question: What constitutes good practice for orientation programmes in South Africa? The article argues that a structured and informed orientation strategy is critical in terms of matters of student retention and, in fact, may serve as the key linchpin of students’ decision to stay or exit the higher education system. Accordingly, seven strategies to improve nationalorientation practice are proposed in this article
Grey-box Modeling of Reversible Solid Oxide Cell Stack’s Electrical Dynamics Based on Electrochemical Impedance Spectroscopy
This paper aims to design a lumped-capacity modelof a reversible solid oxide cell stack for hydrogen electrolysis.The lumped-capacity model needs to have an adequate representationof the electrical dynamics over a wide operatingrange and a model structure suitable for the design of a physicalemulator. The grey-box model is based on data obtained by electrochemicalimpedance spectroscopy conducted on a commercialsolid oxide cell stack for four different gas compositions at sixaging stages. In addition, a comparison of the experimental andsimulated voltage response of the reversible solid oxide cell stackin cyclic reversible operation mode was conducted at differentaging levels of the stack
Radiomics for Everyone: A New Tool Simplifies Creating Parametric Maps for the Visualization and Quantification of Radiomics Features
Aim was to develop a user-friendly method for creating parametric maps that would provide a comprehensible visualization and allow immediate quantification of radiomics features. For this, a self-explanatory graphical user interface was designed, and for the proof of concept, maps were created for CT and MR images and features were compared to those from conventional extractions. Especially first-order features were concordant between maps and conventional extractions, some even across all examples. Potential clinical applications were tested on CT and MR images for the differentiation of pulmonary lesions. In these sample applications, maps of Skewness enhanced the differentiation of non-malignant lesions and non-small lung carcinoma manifestations on CT images and maps of Variance enhanced the differentiation of pulmonary lymphoma manifestations and fungal infiltrates on MR images. This new and simple method for creating parametric maps makes radiomics features visually perceivable, allows direct feature quantification by placing a region of interest, can improve the assessment of radiological images and, furthermore, can increase the use of radiomics in clinical routine
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