1,720,974 research outputs found
Feather roughness reduces flow separation during low Reynolds number glides of swifts
No full textSwifts are aerodynamically sophisticated birds with a small arm and large hand wing that provides them with exquisite control over their glide performance. However, their hand wings have a seemingly unsophisticated surface roughness that is poised to disturb flow. This roughness of about 2% chord length is formed by the valleys and ridges of overlapping primary feathers with thick protruding rachides, which make the wing stiffer. An earlier flow study of laminar–turbulent boundary layer transition over prepared swift wings suggests that swifts can attain laminar flow at low angle-of-attack. In contrast, aerodynamic design theory suggests that airfoils must be extremely smooth to attain such laminar flow. In hummingbirds, which have similarly rough wings, flow measurements on a 3D printed model suggests that the flow separates at the leading edge and becomes turbulent well above the rachis bumps in a detached shear layer. The aerodynamic function of wing roughness in small birds is, therefore, not fully understood. Here we perform particle image velocimetry and force measurements to compare smooth versus rough 3D-printed models of the swift hand wing. The high-resolution boundary layer measurements show that the flow over rough wings is indeed laminar at low angle-of-attack and Reynolds number, but becomes turbulent at higher values. In contrast, the boundary layer over the smooth wing forms open laminar separation bubbles that extend beyond the trailing edge. The boundary layer dynamics of the smooth surface varies nonlinear as a function of angle-of-attack and Reynolds number, whereas the rough surface boasts more consistent turbulent boundary layer dynamics. Comparison of the corresponding drag values, lift values, and glide ratios suggests, however, that glide performance is equivalent. The increased structural performance, boundary layer robustness, and equivalent aerodynamic performance of rough wings might have provided small (proto) birds with an evolutionary window to high glide performance
Pollution dispersion from highways: Enhancing particulate matter capture using aerodynamics
No full textPeople living or working close to the highways are at risk to higher exposure levels of different air pollutants, one of which is particulate matter. The Antea group has recently proposed the addition of Electrostatic Precipitators (or ESPs) on the top of sound barriers adjacent to highways, in order to capture the particulate matter and safely discard them. This is an attractive, but expensive, supplement. While estimates of the collection efficiency (i.e. what percentage of particulate matter mass entering the ESP is collected by the device) are known, the aerodynamic efficiency of the ESP (i.e. what percentage of pollutants reach the ESP entrance as compared to the mass emitted) is yet to be determined. The net efficiency of the ESP will be the product of the above two efficiencies, making it necessary to study the latter, prior to large-scale installation.Scaled down experiments were performed in a water channel, housed in the Laboratory for Aero and Hydrodynamics at the Delft University of technology. Simultaneous measurements of the flow velocities and dye intensities in the symmetry-plane of the channel were made through planar Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). A few flow configurations were studied by varying certain parameters. It was found that: (a) ESP installation is more beneficial on top of shorter sound barriers. (b) Slightly raising the ESP from the top of the sound barrier is advantageous. (c) Installation of ESPs in isolated highway canyons should be done with care, as several flow regimes are observed.First order estimates on the aerodynamic efficiencies of the ESP were determined. This was accompanied by the identification of ESP performance trends that should serve as a guideline for the initial testing of the ESPs at full scale. The guidelines include the physical location and entrance orientation of the ESP. It is known that the collection efficiency of the ESP is a function of the incoming flow velocity. It is recommended that the entrance flow velocities reported here be used as an input for calculating the associated particle collection efficiency. Together, the collection and aerodynamic efficiencies can be used to estimate the overall efficiency of the ESP. If the overall efficiency satisfies a minimum desirable threshold, the field experiments yield promising results, the device clears safety requirements, and the benefits outweigh the costs, the ‘Open Air Line ESP’ can be installed along the highways en masse in the future
The effect of mildly rapid strain on turbulent pipe flow
No full textThe way in which mean strain affects the turbulent structures is imperative to understand various natural flows such as flow over a hill, the flow of a river in the delta, jet streams in the upper atmosphere etc. Further, it also has industrial implications viz; flow over bodies such as airfoil, turbomachinery, gas pipelines. The strained pipe flows, in particular, have huge engineering interest due to its prevalence in industrial fittings wherein a larger pipe diameter is connected to a smaller one and vice versa. This subject also has a fundamental interest as strain highlights the interaction of various scales of turbulence. This scale interactionessentially dictates transfer of energy in turbulence and hence is fundamental in understanding turbulence dynamics itself. The present work deals with the experimental study of the response of pipe turbulence to axisymmetric, irrotational strain using high-resolution planar Particle Image Velocimetry (PIV). A mildly rapid strain (s*s ~ 3.2) is imposed on turbulence via a spatial contraction. It is seen that turbulence is suppressed upon straining. As a response to mean strain, transverse Reynold stress increases at the expense of streamwise Reynolds stress and anisotropy is induced in the turbulence. Despite strain being only mildly rapid, Rapid Distortion Theory (RDT) is found to predict the correct trend of normal Reynolds stress although traverse Reynolds stress is over-predicted. The effect of strain on different scales of turbulence is discerned. The large scales of turbulence are seen to get compressed in the radial direction although they do not get affected significantly in the streamwise direction. Near-wall coherent structures which were initially inclined w.r.t. the wall are seen to get aligned with the flow as they also get severely compressed in the radial direction. On the other hand, the small scales of turbulence are found to be spatially organised in the form of sheets or layers. Upon straining, these sheets are found to get aligned with the mean flow. Further, they get elongated in streamwise and compressed in the radial direction. It is observed that the small scales are more severely distorted than the large scales upon staining inside the contraction. At the Reynolds number (Re) range employed in this thesis, there is no substantial difference in which turbulence is strained inside the contraction at disparate Re. Downstream of the contraction, at the axis of the pipe, the anisotropy of Reynolds stress is found to recover slowly. Further, this relaxation is seen to be Re dependent with higher Re turbulence relaxing slightly faster.Mechanical Engineering | Process and Energy Technolog
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Experimental Characterization of a Micro-ramp Wake and its Flow Mixing Properties in Supersonic Flow
No full textMicro-ramps are currently being investigated as a means of reducing negative effects of shock wave-turbulent boundary layer interactions. A micro-ramp enhances the mixing of the ow by introducing vortices in the ow, thus energizing the boundary layer to overcome the adverse pressure gradient due to an impinging shock. The effect of changing micro-ramp height, Mach number and Reynolds number on the mean ow downstream of a micro-ramp's center-line has been studied extensively.Aerospace Engineering | Aerodynamics and Wind Energ
The effect of tripping devices on a shock wave - boundary layer interaction
No full textShock Wave Boundary Layer Interactions (SWBLIs) occur frequently in supersonic and hypersonic flows and, depending on the strength of the shock wave and the state of the boundary layer, may lead to boundary layer separation. This can result in a multitude of problems, such as an increase in drag, a drop in air intake efficiency and high fatigue loads due to the inherent unsteadiness of the ow phenomenon. Since a turbulent boundary layer has a better capability to withstand the adverse pressure gradient that is inherent to the incident shock wave when compared to a laminar boundary layer, it is beneficial to have a turbulent boundary layer at the interaction. By forcing boundary layer transition only a short distance upstream of the interaction, the capability of the boundary layer to remain attached is increased while retaining the low skin friction coefficient of a laminar boundary layer for as long as possible.Aerospace Engineerin
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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