Short Chord Research Articles

Acoustic modes of self-excited oscillations in struts

The acoustic modes of self-excited oscillations are investigated numerically for two-dimensional generic struts. In profiles with short chord lengths, hydrodynamic instability is responsible for wake bifurcation and for generation of narrow band tones. However, it is found, another mechanism is the major factor for periodic oscillations in long profiles. Two points of wall contour develops an acoustic feedback between each other. These points are the inflection point and the trailing edge point. Consequently, limits of two different modes of periodic oscillations are outlined. Moreover, it is determined that acoustic radiation has dipole character which agrees well with the theory of sound radiation for acoustically compact surfaces.

A note on Duchet's conjecture

A kernel in a digraph is a set of vertices that is both an absorbing and independent set. In [4] P. Duchet conjectured that a digraph such that every odd circuit has two short chords is kernel perfect digraph (i.e. every induced subdigraph has a kernel) and he proved the conjecture for digraphs where every 3 -circuit is reversal. We give another simple proof of this result by using reorientation and constructive methods. We also show that the conjecture is valid for digraphs D where the underlying undirected spanning subgraph deduced from the spanning subdigraph of D with only pairs of symmetric arcs is a comparability graph.

On Clique-Transversals and Clique-Independent Sets

A clique-transversal of a graph G is a subset of vertices intersecting all the cliques of G. A clique-independent set is a subset of pairwise disjoint cliques of G. Denote by τ C (G) and α C (G) the cardinalities of the minimum clique-transversal and maximum clique-independent set of G, respectively. Say that G is clique-perfect when τ C (H)=α C (H), for every induced subgraph H of G. In this paper, we prove that every graph not containing a 4-wheel nor a 3-fan as induced subgraphs and such that every odd cycle of length greater than 3 has a short chord is clique-perfect. The proof leads to polynomial time algorithms for finding the parameters τ C (G) and α C (G), for graphs belonging to this class. In addition, we prove that to decide whether or not a given subset of vertices of a graph is a clique-transversal is Co-NP-Complete. The complexity of this problem has been mentioned as unknown in the literature. Finally, we describe a family of highly clique-imperfect graphs, that is, a family of graphs G whose difference τ C (G)−α C (G) is arbitrarily large.

Tone Profiles following Short Chord Progressions: Top-down or Bottom-up?

Three experiments explored the relationship between chroma-salience profiles of individual chords and tone profiles obtained after short chord progressions. Musicians' tone profiles for diatonic progressions of one, two, and three chords were compared with predictions of three models: a bottom- up stimulus model (number of times each chroma occurs in the progression), a top-down or schema-driven key model (best-fitting key profile of C. L. Krumhansl & E. J. Kessler, 1982), and an intermediate pitch model that includes both top-down and bottom-up components (cumulative pitch salience; R. Parncutt, 1989, 1993). For single chords, all predictors significantly matched tone profiles, except the key model applied to the diminished triad. For pairs of chords, the pitch and key models consistently outperformed the stimulus model, consistent with the assumption that a (top- down) key had been established; in the pitch model, the second chord influenced the tone profile more than the first (recency effect). Progressions of three chords comprised forward (e. g., FG-C) and backward (C-G-F) cadences in major and minor keys. The pitch and key models were successful for all progressions, but the key model predicted the tonic of backward cadences in C major and minor to be F. Predictions of the stimulus model were clearly worse than those of the other models, especially for backward cadences. Both primacy and recency effects were observed. In summary, the pitch model was the most consistently successful model over all experiments. To successfully predict tone profiles following chord progressions, it was necessary to account not only for recency (and primacy) but also for variations in pitch salience within chords. Results are consistent with a model of tonality induction in which bottom-up processes interact in real time with top-down processes of two kinds: recognition of harmonic pitch patterns and recognition of key profiles.

Posttensioned and Shaped Hypar Space Trusses

This paper describes a kind of innovative posttensioned and shaped hypar space truss characterized by an economical fabrication and construction process. The hypar truss, assembled initially in a planar square layout on the ground, is posttensioned at the short bottom chords at one diagonal. As a result, the top chords become curved and thereby a nonzero Gaussian curvature surface of a hypar space truss is shaped. A distinguishing feature of posttensioned and shaped hypar trusses, compared with conventional metal trusses, is that the shape formation and erection is completed by a simple posttensioning process, rather than by the traditional techniques involving the use of cranes and scaffolding. The essential aspects of shape formation are discussed and a small-scale posttensioned and shaped hypar is formed in this paper. Results of experiment and nonlinear finite-element analysis indicate that, even within the yield limit of the members, a planar square layout can be deformed to a significant hypar shape...

Wing Shape in Insectivorous Passerines Inhabiting New Guinea and Australian Rain Forests and Eucalypt Forest/Eucalypt Woodlands

AssTAcr.-Based on museum skins of 47 bird species in 17 genera, I evaluated the suppositions for members of various taxonomic groups in New Guinea and Australian rain forest compared to open eucalypt forest/eucalypt woodland that in rain forest birds compared to eucalypt forest/eucalypt woodland counterparts in each group: (1) outer primary 9 tends to be proportionately shorter; (2) secondary 1 is proportionally longer; and (3) the longest primary is more proximally placed relative to the body in its series. The postulations are largely confirmed. The results are presaged by the findings of earlier writers that species operating in confined spaces should have broad wings with rounded tips, with low wing loading conferring high manoeuvrability, and that distance flyers should have wings with long wingspans and pointed tips for economy of effort. Received 1 April 1993, accepted 4 September 1994. WHEN MEASURING SKINS of New Guinea and Australian tropical rain-forest insectivorous passerines, and eucalypt forest/woodland counterparts, I found regularly occurring differences in wing form. The former tended to have more rounded wings, with the outer primary 9 relatively short, while the longest primary in the series was placed more proximally than the others in its series relative to the body in the rain-forest species. Examples were found in foliage-gleaning warblers (Gerygone), whistlers (Pachycephala), ground-feeding robins, and several other groups, suggesting that a general habitat adaptation was operating, as has been shown for several body structures by Winkler and Leisler (1992). In this paper, I quantitatively explored this observation and postulated connection in a diverse set of ecomorphological types of birds. An early paper by Savile (1957) documented the occurrence of two alternative wing types in small birds-elliptical wings found in birds that lived in confined spaces, and wings characteristic of species that remain in the air for long periods. In the former group the wing is curved to increase negative pressure and, hence, lift above, with slotting of the tips serving to minimize turbulence and ensure uniform pressure over the entire surface. The result is high lift, as well as good control and manoeuvrability. By contrast, the high-speed wing has a lower camber, a slender tapering tip, a sweepback of the leading edge, and a short chord. These features reduce drag and conserve power.

Perception of musical tension in short chord sequences: The influence of harmonic function, sensory dissonance, horizontal motion, and musical training

This study investigates the effect of four variables (tonal hierarchies, sensory chordal consonance, horizontal motion, and musical training) on perceived musical tension. Participants were asked to evaluate the tension created by a chord X in sequences of three chords [C major-->X-->C major] in a C major context key. The X chords could be major or minor triads major-minor seventh, or minor seventh chords built on the 12 notes of the chromatic scale. The data were compared with Krumhansl's (1990) harmonic hierarchy and with predictions of Lerdahl's (1988) cognitive theory, Hutchinson and Knopoff's (1978) and Parncutt's (1989) sensory-psychoacoustical theories, and the model of horizontal motion defined in the paper. As a main outcome, it appears that judgments of tension arose from a convergence of several cognitive and psychoacoustics influences, whose relative importance varies, depending on musical training.

Three-dimensional Ising system with long-range interactions: A computer model of Vycor glass.

A computer model for Vycor glass using the three-dimensional Ising model with long range interactions is presented. The effect of the interaction range on the resulting Vycor microstructure is also investigated by measuring the pore size distribution for the q=6 (nearest neighbor) as well as longer-range interaction, i.e., q=26 and 124 Ising models. The discrepancy between the nearest-neighbor Ising model and the experimental results is interpreted in terms of a lattice-induced interfacial energy anisotropy. The influence of the range of interaction on the degree of interfacial energy anisotropy is illustrated by means of two- and three-dimensional Wulff plots (interfacial energy versus interface orientation) for each of the ranges of interaction considered. The shape of the pore size distributions obtained from the longer range Ising models is in agreement with the experimental results. However, the simulation distributions display a shift in the peak location observed as compared to the experimental distribution. The shift in peak location present in the simulation results is explained by a relative abundance of short chords combined with a lack of the large chords, with respect to the experimental results. This abundance of short chords is attributed to the interface roughness introduced by the longer range interaction, while the lack of large chords is attributed to the finite lattice sizes used in this study. A minimum system size and maximum interaction range are suggested, which should lead to a pore size distribution in close agreement with experimental results.

Disease of the left ventricle in pulmonary atresia with intact ventricular septum: The limiting factor for long-lasting successful surgical intervention?

Eight hearts with pulmonary atresia with intact ventricular septum were studied grossly and microscopically. The right ventricle was composed of inlet, infundibular, and trabecular portions in two hearts, without trabecular portions in three hearts, and consisted of an inlet part only in another three hearts. Pulmonary atresia was classified as membranous in two hearts; the other six hearts had muscular atresia. The left ventricle showed a range of abnormalities, albeit mostly discrete. A mitral valve with short tendinous chords was present in four hearts; one of these had a cleft mitral valve. All hearts (except the heart of a 1-day-old neonate) had a prominent subaortic septal bulge. All hearts had left ventricular hypertrophy, which exceeded twice the standard deviation of normal in two patients. None of the hearts had histologic features of hypertrophic cardiomyopathy. The density of intramyocardial coronary arteries was normal in each patient, but medial thickening (20 to 100 μm diameter arteries) had occurred in four patients. Signs of acute myocardial ischemia (hydropic cell swelling) were present in all; one heart (the oldest case of 6 months) contained areas of replacement fibrosis and calcifications indicative of previous infarction. Quantification of the volume density of endomysial (interfiber) collagen (Picrosirius red microspectrophotometry) showed high levels in all, within the range of normal in five patients, but exceeding twice the standard deviation of normal in three patients. In all cases the values obtained in the subendocardial layers were higher than those obtained subepicardially. The observations show that the left ventricle in hearts with pulmonary atresia with intact ventricular septum contains abnormalities that may render the left ventricle less capable to cope with a volume load. The high values of endomysial (interfiber) collagen suggest chronic ischemia in relation to left ventricular hypertrophy as the main mechanism involved. Therefore, as a late consequence, the left ventricle could be the limiting factor for long-lasting successful surgical intervention. (J THORACCARDIOVASCSURG1994;108:1-8)J Thoracic Cardiovasc Surg 1994;108:1-8

Short-chorded and perfect graphs

A graph is short-chroded (a.k.a. Raspail) if every odd cycle of length at least 5 has a short chord, which is a chord joining vertices distance 2 apart in the cycle. A subclass of short-chorded graphs, not contained in any of the known classes of perfect graphs, will be proved perfect.

Rediscovery of the Sooty Rail (Porzana tabuensis) in American Samoa

The Sooty Rail (Porzana tabuensis) is widely distributed in the Pacific, but in American Samoa has been found only on the island of Ta'i. This population was discovered in 1923 when 10 specimens were taken (Murphy 1924), but it has not been recorded since (Banks 1984). No Sooty Rails were found on Ta'ui during a survey conducted in 1975-1976 (Amerson et al. 1982: 64), and, as a result, it was believed to be extirpated (Muse and Muse 1982: 83). United States Fish and Wildlife Service biologists rediscovered this bird in 1985-1986, when they spent 8 days surveying birds on Ta'fu. On 27 June 1985 Engbring found a fresh, roadkilled Sooty Rail on a dirt side road about 1 km east of Ta'ii Village on the northwest side of the island. We heard or saw at least three more individuals in July 1986, all near the main road within 2 km of where the specimen was found. The birds were located by their soft but rather distinctive chattering calls. One was viewed briefly as it scooted across a small path, but only after we made considerable effort to flush it. The 1985 specimen was deposited at the Bernice P. Bishop Museum (BPBM 161855). Its external measurements (mm) were: tail, 39; wing chord, 74; exposed culmen, 16.9; and tarsus, 24.6. After comparing its plumage with other specimens in the collection, we determined the bird was an adult. The skull was fully ossified. The bird was crushed when killed, and it was impossible to determine the sex internally. Culmen and tarsus measurements, when compared with those of other P. tabuensis collected in Samoa (Amadon 1942), suggest it is a female. The short chord measurement indicates that the specimen belongs to the short-winged population, P. t. tabuensis, found in Tonga, Fiji, Niue, and Samoa (Banks 1984). Biologists with. the Whitney South Sea Expedition found the Sooty Rail on Ta'ui in marshy habitat on the northwest side of the island (Banks 1984). The rails that we found in 1985-1986 were also in the northwest portion of the island; they occupied rank grass and weed patches in semi-open agricultural forest. We did not find any marsh or reeds here, though there were densely vegetated swales where puddles formed after rains. The population of Sooty Rails on Ta'u has evidently never been large and may be confined to the northwest portion of the island. Residents of Ta'u did not recognize the bird we salvaged and thought it was a young Purple Swamphen (Porphyrio porphyrio), a relatively common and well-known bird on Ta'ui. The rails that we found were associated with habitat created by subsistence agriculture, an activity that has diminished over the last several decades on Ta'u. The decrease in subsistence agriculture, along with a possible loss of wetland habitat, probably has decreased Sooty Rail numbers on Ta'u. We thank staff from the Office of Marine and Wildlife Resources, American Samoa, especially R. Tulafono, B. Emsley, K. So'oto, and F. Niumata. U.S. Fish and Wildlife Service biologists who collected field data were C. Aguon, P. Ashman, and P. Pyle. M. LeCroy looked up museum numbers and remeasured specimens at the American Museum of Natural History. R. Banks and R. Clapp reviewed the manuscript.

A theorem about a conjecture of H. Meyniel on kernel-perfect graphs

A diagraph D is said to be an R-digraph (kernel-perfect graph) if all of its induced subdigraphs possesses a kernel (independent dominating subset). I show in this work that a digraph D, without directed triangles all of whose odd directed cycles C = (1,2,…,2 n + 1, 1), possesses two short chords (that means there exist two arcs of D of the form: ( q, q +2) and ( q′, q′ + 2)) is an R-digraph.

Two-dimensional echo Doppler study of congenital disorders of the mitral valve

To define the use of two-dimensional echocardiography (2DE) and Doppler methods for diagnosis of forms of congenital mitral stenosis, we studied 16 children, age range 2 months to 13 years, with congenital deformities of the mitral valve documented at cardiac catheterization. Thirteen had additional congenital heart defects, most commonly aortic stenosis or aortic coarctation. In eight patients features of mitral valve anatomy were observed and described during cardiac surgery and in one child the anatomy was verified by postmortem examination. 2DE studies allowed anatomic subclassification of 10 valves which had asymmetric or single dominant papillary muscles (parachute) and six which had arcade mitral valve attached by short chords to multiple diminutive papillary muscle heads. All patients' echoes exhibited shortened and thickened mitral chordae and doming of the mitral leaflets during diastole, and seven children had restricted mitral orifices imaged on the short-axis imaging plane. All seven of the patients studied by Doppler echocardiography had increased maximal transmitral inflow velocity (range 111 to 260 cm/sec) greater than the 95% confidence limits for mitral flow velocities in 34 normal children who served as the control group. Our study suggests that 2DE studies, especially when combined with Doppler interrogation, are sensitive for defining forms of congenital mitral stenosis.

The interaction of entropy fluctuations with turbine blade rows; a mechanism of turbojet engine noise

The theory relating to the interaction of entropy fluctuations (‘hot spots’), as well as vorticity and pressure, with blade rows is described. A basic feature of the model is that the blade rows have blades of sufficiently short chord that this is negligible in comparison with the wavelength of the disturbances. For the interaction of entropy with a blade row to be important, it is essential that the steady pressure change across the blade row should be large, although all unsteady perturbations are assumed small. A number of idealized examples have been calculated, beginning with isolated blade rows, progressing to single and then to several turbine stages. Finally, the model has been used to predict the low-frequency rearward-radiated acoustic power from a commercial turbojet engine. Following several assumptions, together with considerable empirical data, the correct trend and level are predicted, suggesting the mechanism to be important at low jet velocities.

The Development of the Standard Design for Self-Supporting Radio Towers for the United Fruit and Tropical Radio Telegraph Companies

This paper is an historical review of the United Fruit Company's Radio Towers, and describes the origin of a 1906 design for 200 ft. (61 m.) towers; of 50 ft. (15 m.) extension added in 1910; of the hurricane wrecks of 1914, 1915, and 1916, of data deduced from them, and of the reconstruction of the towers. It discusses the effect of a new policy that "radio communication should be made so reliable that, when all other means fail, the public can confidently look to the radio stations to maintain service, which can be done if only the towers remain standing." The preliminary designs for three-legged towers made in 1915 are described, and their economy and advantages as compared with four-legged towers, the saving due to adjustable rod bracing and the low cost of shop work effected by making all leg sections equal short chords of a circular arc, are pointed out. The design of the Tegucigalpa (Honduras) towers, with antenna bridges and the latest revised, "Standard Plan of 400 ft (122 m.) self-supporting towers, with 150 ft. (46 m.) antenna bridges," are shown with the specified loads, unit stresses, and notes on the most favorable direction of wire pull in relation to cross-section of tower. The design of sections and details are briefly referred to, with the limiting heights of towers to which similar sections are applicable. Two pairs of 100 ft. (30.5 m.