1,720,960 research outputs found
Comparing Compressibility Effects in Turbulence at Various Mach Numbers
Get PDFCompressibility effects in a high speed turbulent flow were examined experimentally within a novel pressure vessel equipment setup. The equipment was assembled, validated and utilized in a series of air and SF6 test cases. Using these two gases allowed Reynolds Number to be adjusted separately from Mach Number. In this way, the boundary conditions and nature of the turbulence determined by Reynolds Number were maintained, however the compressibility effects determined by Mach Number were changed. The turbulent flows consisted of a solenoidal component generated by a high speed fan and a dilatational component by high sound pressure level (SPL) speakers. Large scale turbulence dynamics, including axial, radial and power profiles, remained similar for both gas cases. However, small scale turbulence dynamics, including velocity energy spectra and velocity derivative skewness, revealed compressibility effects at work for the high speed and high SPL cases in SF6
The Stability of the Bottom Boundary Layer Under a Model Mode-1 Internal Tide
Get PDF The instability properties of the bottom boundary layer (BBL) under a model mode-1 internal tide in linearly stratified finite-depth water are studied, using 2-D fully nonlinear and non-hydrostatic direct numerical simulations (DNS) based on a spectral multidomain penalty method model. Low-mode internal tides are known to transport large amounts of energy throughout the oceans. One possible mechanism, among others, through which the energy of the particular tidal waves can be directly dissipated, without transfer to higher modes, is through wave-BBL interactions, where strong near-bottom shear layers develop, leading to localized instabilities and ultimately mixing. In the model problem, the stability response of the time-dependent wave-induced BBL is examined by introducing low-amplitude perturbations near the bed. For the linear stage of instability evolution, the time-dependent perturbation energy growth rates are computed by tracking the largest perturbation energy density in the domain through the wave-modulated shear and stratification, ultimately the formation of distinct localized near-bed Kelvin Helmholtz billows are observed. The average growth rate, σ, is then compared to the time, Tw , that a parcel of fluid is subject to a local Richardson number less than 1/4, resulting in a nondimensional criterion for instability, σ Tw. A stability boundary is then constructed as a function of the three non-dimensional parameters that characterize the flow, the wave steepness, aspect ratio and Reynolds number. It is shown that the nondimensional growth rate can be written as a function of these parameter, σ Tw = F( Re, st, AR). Additionally the minimum initial perturbation amplitude that is shown to cause overturning of isodensity surfaces for each parameter set is also shown to be a function of the wave parameters, Ac = F( Re, st, AR). </p
Unsteady Aerodynamics of Sailing Maneuvers and Kinetic Techniques
No full textSmall sailboat kinetics are significantly affected by bodyweight motions of the sailor. Highly dynamic motions of the boat lead to unsteady aerodynamics around the sail. These unsteady flows are studied using a two-part approach. First, on-the-water tests in a modified Laser sailboat capture the motion of the boat and sail. Subsequently, these characteristic motions are the basis for towing tank experiments exploring fundamental 2D flows around representative sail sections.
The “Sail-Flicking”, “S-Turn”, and “Roll-Tacking” kinetic techniques are explored in depth. Each technique is used by racing sailors to improve the performance during specific portions of a race and are most effective in select wind conditions.
Sail Flicking consists of repeated small amplitude sail pulses which increase straight-line speed in upwind and cross-wind sailing. In the 2D case, a vortex pair is shed during each individual “flick”. The benefit in driving force is maximized at apparent wind angles near 45°. Sail Flicking effectiveness scales with the non-dimensional parameters reduced frequency and heave-to-chord ratio, making the technique most effective in light wind.
The S-Turning technique uses coordinated turns, rolls, and sail adjustments while sailing downwind. It is named after the boat's “S” shaped path through the water. On-the-water tests show a measurable increase in downwind Velocity Made Good, VMG, while laboratory experiments indicate constructive interaction between the sail and vortices shed during each motion period.
Sailboats tack back and forth to make progress into the wind. During a Roll-Tack, sailors allow the boat to reach a high heel angle during the turn. An aggressive bodyweight shift then rolls the boat upright, sweeping the sail through the air and increasing driving force. On-the-water tests show that Roll-Tacking in light wind increases windward VMG relative to straight-line sailing and relative to tacks which do not roll the boat. 2D laboratory experiments representing a slice of the sail show the formation of a strong vortex pair associated with a six fold increase in driving force
Vortex Dynamics of a Vortex Pair in Wall Effect and Flapping Airfoil Propulsion
No full textVorticity provides the means to exchange momentum between objects and their surrounding fluid. As such, vorticity can be potentially harmful, as seen in the hazard posed by strong counter-rotating vortex pairs generated behind aircraft in flight. Conversely, the production of strong vortical wakes is beneficial to birds and fish, which exploit their generation to produce thrust. In this work, we will examine two classes of flows in which vorticity plays a key role. In the first flow, we simulate the trailing vortex pair of an aircraft in a water tank. Such counter-rotating vortex pairs are commonly observed to undergo an instability involving a sinusoidal displacement, which ultimately leads to the generation of a series of large vortex rings. As these trailing vortices are most dangerous when aircraft are in close proximity, such as near the ground, we study the behavior of this instability when it is interrupted by a solid wall. In the presence of the wall, the interaction between the primary vortex pair and secondary vorticity generated in the boundary layer leads to significant topological changes and the production of arrays of smaller-scale vortex rings, the precise configuration of which depends on the extent of development of the instability before wall interaction. In the second flow, we examine the behavior of an airfoil oscillating with pitching and heaving motions. Many animal studies have shown that such flapping airfoils enable high agility and are also quite effective in producing thrust. Similar vehicles are under development for applications such as search and rescue, environmental monitoring, and reconnaissance. In order to reduce weight and complexity of the propulsive mechanism, however, we equip the airfoil with an actuator in the heave direction but allow it to pitch passively under the control of a torsion spring. In this system, variation of the spring stiffness and the chordwise pivot location allows control of the vortex wake produced by the airfoil, enabling it to produce either thrust or drag as desired. If these parameters are well selected, performance can even be comparable to that of a system with two actively controlled degrees of freedom. Finally, the majority of existing studies of flapping airfoils use a fixed incoming flow velocity, achieved either by towing the vehicle at a constant speed or by placing it in a water channel. This imposed velocity does not necessarily correspond to that at which the vehicle would naturally travel, determined by a balance of the vehicle's drag and the thrust produced by the airfoil. Using a cyber-physical fluid dynamics technique, in which force-feedback determines the acceleration of the airfoil in real time, we can simulate self-propulsion for a wide range of heaving and pitching amplitudes. For a given cruising velocity, we determine the combination of pitch and heave that enables the most efficient propulsion and examine the characteristics of the forces and vorticity produced that correspond to this condition
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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