Bypass transition and spot nucleation in boundary layers

The spatio-temporal aspects of the transition to turbulence are considered in the case of a boundary layer flow developing above a flat plate exposed to free-stream turbulence. Combining results on the receptivity to free-stream turbulence with the nonlinear concept of a transition threshold, a physically motivated model suggests a spatial distribution of spot nucleation events. To describe the evolution of turbulent spots a probabilistic cellular automaton is introduced, with all parameters directly fitted from numerical simulations of the boundary layer. The nucleation rates are then combined with the cellular automaton model, yielding excellent quantitative agreement with the statistical characteristics for different free-stream turbulence levels. We thus show how the recent theoretical progress on transitional wall-bounded flows can be extended to the much wider class of spatially developing boundary-layer flows

Turbulence transition in the asymptotic suction boundary layer

We study the transition to turbulence in the asymptotic suction boundary layer (ASBL) by direct numerical simulation. Tracking the motion of trajectories intermediate between laminar and turbulent states we can identify the invariant object inside the laminar-turbulent boundary, the edge state. In small domains, the flow behaves like a travelling wave over short time intervals. On longer times one notes that the energy shows strong bursts at regular time intervals. During the bursts the streak structure is lost, but it reforms, translated in the spanwise direction by half the domain size. Varying the suction velocity allows to embed the flow into a family of flows that interpolate between plane Couette flow and the ASBL. Near the plane Couette limit, the edge state is a travelling wave. Increasing the suction, the travelling wave and a symmetry-related copy of it undergo a saddle-node infinite-period (SNIPER) bifurcation that leads to bursting and discrete-symmetry shifts. In wider domains, the structures localize in the spanwise direction, and the flow in the active region is similar to the one in small domains. There are still periodic bursts at which the flow structures are shifted, but the shift-distance is no longer connected to a discrete symmetry of the flow geometry. Two different states are found by edge tracking techniques, one where structures are shifted to the same side at every burst and one where they are alternatingly shifted to the left and to the right.Comment: Conference TSFP8, Poitiers 2013. TSFP-8 conference proceedings 2013, http://www.tsfp-conference.org/proceedings

Seismostratigraphy of the Middle St. Lawrence Esturary: A Late Quaternary Glacial Marine to Estuarine Depositional/Erosional Record

A buried bedrock trough 350 m deep extends 100 km above Saguenay Fjord beneath the North Channel of the middle estuary. Four of five regional seismostratigraphic units are recognized in and adjacent to the trough; unit 1 (glacial ice-contact) and older sediments might also be present beneath the largely unpenetrated trough axis. Units 2 and 3 represent thick glacial marine sediments deposited in the >550 m deep waters of the Goldthwait Sea after glacial withdrawal ca. 13 ka BP: lower draped muds 10-20 m thick (unit 2) suggest deposition proximal to a retreating ice margin, while upper onlapping muds > 290 m thick (unit 3) record distal basin-filling; lateral transition to a coarse-grained proximal wedge 5*260 m thick (unit 2) is indicated by unit 3 reflectors rising and strengthening towards the Saguenay entrance, where a stable ice-margin ca. 13-11 ka BP supplied sediment to the lower and middle estuary. Unit 4 corresponds to lobes over 30 m thick on both sides of the upper North Channel, recording marginal input from glacial fluvio-deltaic sources. Unit 5 (estuarine sands, gravels and muds =£30 m thick) unconformably overlies glacial units. A smooth unconformity surface records erosion (at least 15 m, to axial depths >150m) by strong currents; irregular relief above depths of 25-50 m might relate to relative sea levels below present ca. 7-6 ka BP. Sand bedforms (apparently inactive) occur at the estuary floor, and possibly buried beneath estuarine muds; buried bedforms would imply an early Holocene genesis. Greatest thicknesses of estuarine mud coincide with adjacent fluvial discharges. Sandy/gravelly veneers form the estuary floor in most places. Mass displacement has disturbed units 3 and 5 along the northern, and locally southern, walls of the North Channel.Une fosse enfouie à socle rocheux de plus de 350 m de profondeur s'étend sur 100 km en amont du fjord du Saguenay sous le chenal du Nord. Quatre des cinq unités sismostratigraphiques quaternaires ont été identifiées à l'intérieur ou à la périphérie de la fosse. L'unité n° 1 (de contact glaciaire) et des sédiments plus anciens sont peut-être aussi présents sous l'axe de cette fosse en grande partie insondée. Les unités nos 2 et 3 sont constituées d'une grande épaisseur de sédiments glacio-marins mis en place dans les eaux de la Mer de Goldthwait (> 550 m) après le retrait glaciaire, vers 13 ka BP; les boues de recouvrement de la couche inférieure, de 10 à 20 m d'épaisseur (unité n° 2), témoignent d'une mise en place à proximité de la marge en recul, alors que les boues sus-jacentes de 290 m d'épaisseur (unité n° 3) reflètent un remplissage distal du bassin. Le passage latéral à une accumulation proximale de matériel à grains grossiers de 5*260 m d'épaisseur (unité n° 2) est indiqué par l'élévation et le renforcement des réflecteurs vers l'entrée du Saguenay, où une marge glaciaire stable a fourni des sédiments à l'estuaire inférieur et moyen de 13 à 11 ka BP environ. L'unité n° 4 correspond à des lobes de plus de 30 m d'épaisseur de chaque côté du chenal du Nord, témoignant d'une accumulation marginale à partir de sources fluvio-deltaïques glaciaires. L'unité n° 5 (sables, graviers et boues estuariennes de =£30 m d'épaisseur) recouvre en discordance les unités glaciaires. Une surface de discordance unie témoigne d'une érosion (de 15 m, à des profondeurs de >150m dans l'axe) par de forts courants; la topographie irrégulière de 25-50 m de profondeur pourrait être liée à l'abaissement des niveaux marins, vers 7-6 ka BP.Vergrabenes anstehendes Gestein erstreckt sien ùber 100 km in > 350 m Tiefe oberhalb des Saguenay-Fjords unter dem Nord-Kanal des mittleren Mùndungsbeckens. Vier von fùnf regionalen seismostratigraphischen Einheiten sind in und neben dem Trog erkannt worden; Einheit 1 (glazialer Eiskontakt) und altère Sedimente kônnten auch unter der weitgehend nichtergrùndeten Trog-Achse vorhan-den sein. Die Einheiten 2 ind 3 bestehen aus dicken glazialen marinen Sedimenten, welche in dem 290 m Dicke (Einheit 3) distale Becken-Fùllung belegt; eine latérale Ùberleitung zu einem grobkôrnigen proximalen Keil von 3--260 m Dicke (Einheit 2) wird durch die Anhebung und Verstàrkung der Einheit 3 - Reflektoren zum Eingang des Saguenay hin angezeigt, wo ein stabiler Eissaum etwa 13-11 ka v.u.Z. Sediment zum unteren und mittleren Mùndungsbecken brachte. Einheit 4 entspricht ùber 30 m dicken Loben beiderseits des oberen Nord-Kanals, und belegt eine Anlagerung von glazialen FluB-Delta-Quellen am Rand. Einheit 5 (Mùndungsbecken-Sand, Kies und Schlamm =s30 m ûick) ùberlagert in Diskordanz die glazialen Einheiten. Eine glatte Diskoranz-Oberflâche belegt eine Erosion (mindestens 15 m, bis zu Tiefen von > 150 m in der Achse) durch starke Strômungen; das unregelmàBige Relief ùber den Tiefen von 25-50 m kônnte mit relativen Meeresniveaus, die etwa um 7-6 ka v.u.Z. unter den gegenwàrtigen lagen, zusammenhàngen

The MANTA: An RPV design to investigate forces and moments on a lifting surface

The overall goal was to investigate and exploit the advantages of using remotely powered vehicles (RPV's) for in-flight data collection at low Reynold's numbers. The data to be collected is on actual flight loads for any type of rectangular or tapered airfoil section, including vertical and horizontal stabilizers. The data will be on a test specimen using a force-balance system which is located forward of the aircraft to insure an undisturbed air flow over the test section. The collected data of the lift, drag and moment of the test specimen is to be radioed to a grand receiver, thus providing real-time data acquisition. The design of the mission profile and the selection of the instrumentation to satisfy aerodynamic requirements are studied and tested. A half-size demonstrator was constructed and flown to test the flight worthiness of the system

Awareness of energy drink intake guidelines and associated consumption practices: a cross-sectional study

BACKGROUND: Despite concern regarding harms of energy drink (ED) consumption, no research has been conducted to determine awareness and compliance with ED intake guidelines displayed on product packaging in Australia (a novel approach internationally). METHODS: A convenience sample of 1922 people completed an online survey. Participants reported their knowledge of maximum recommended daily ED intake according to Australian guidelines. RESULTS: Guideline awareness was reported by 38, 23 and 19 % of past year consumers, lifetime, and non-consumers, respectively. Amongst past year consumers, \u27accurate estimators\u27 reported greater ED intake and were more likely to exceed intake guidelines and consume alcohol mixed with ED (AmED). After controlling for demographics and frequency of use, guideline awareness predicted increased likelihood of exceeding guidelines in ED sessions, but was not associated with exceeding ED guidelines in AmED sessions. CONCLUSIONS: Australia is considered to have the most stringent regulatory approach to EDs internationally. However, advisory statements are not associated with greater awareness and compliance with intake guidelines. Failure to comply with standards for efficacious product labelling, and absence of broader education regarding guidelines, needs to be addressed