Unsteady wake modelling for tidal current turbines

The authors present a numerical model for three-dimensional unsteady wake calculations for tidal turbines. Since wakes are characterised by the shedding of a vortex sheet from the rotor blades, the model is based on the vorticity transport equations. A vortex sheet may be considered a jump contact discontinuity in tangential velocity with, in inviscid hydrodynamic terms, certain kinematic and dynamic conditions across the sheet. The kinematic condition is that the sheet is a stream surface with zero normal fluid velocity; the dynamic condition is that the pressure is equal on either side of the sheet. The dynamic condition is explicitly satisfied at the trailing edge only, via an approximation of the Kutta condition. The shed vorticity is the span-wise derivative of bound circulation, and the trailed vorticity is the time derivative of bound circulation, and is convected downstream from the rotors using a finite-volume solution of vorticity transport equations thus satisfying the kinematic conditions. Owing to an absence in the literature of pressure data for marine turbines, results from the code are presented for the NREL-UAE Phase IV turbine. Axial flow cases show a close match in pressure coefficients at various spanwise stations; however, yawed flow cases demonstrate the shortcomings of a modelling strategy lacking viscosity

Obtaining Self-similar Scalings in Focusing Flows

The surface structure of converging thin fluid films displays self-similar behavior, as was shown in the work by Diez et al [Q. Appl. Math 210, 155, 1990]. Extracting the related similarity scaling exponents from either numerical or experimental data is non-trivial. Here we provide two such methods. We apply them to experimental and numerical data on converging fluid films driven by both surface tension and gravitational forcing. In the limit of pure gravitational driving, we recover Diez' semi-analytic result, but our methods also allow us to explore the entire regime of mixed capillary and gravitational driving, up to entirely surface tension driven flows. We find scaling forms of smoothly varying exponents up to surprisingly small Bond numbers. Our experimental results are in reasonable agreement with our numerical simulations, which confirm theoretically obtained relations between the scaling exponents.Comment: 11 pages, 11 figures, accepted for Phys Rev

Falling : Song

https://digitalcommons.library.umaine.edu/mmb-vp/1420/thumbnail.jp

Polyetyleeniä ja n-heksaania sisältävien systeemien faasitasapaino eri olosuhteissa

Phase equilibria knowledge of polymer – solvent systems is vital for the development of solution polymerization processes. When producing the polymer, the phase state of the reaction mixture can impact reaction rates and product properties. Also, when separating the polymer from the solvent, phase separation can be used. The goal of this work is to investigate liquid-liquid phase boundaries in systems containing polyethylene and n-hexane by measuring points of liquid-liquid separation, also known as cloud points. Measurements were conducted for temperatures from 150 to 250 degrees Celsius, for pressures up to 100 bar and for systems with 10-36 mass percentage (m-%) polymer concentration. Six different polymer types were used in the experiments. Cloud point measurements were conducted for binary systems containing polyethylene and n-hexane, as well as for multicomponent systems containing polyethylene, ethylene monomer, 1-butene or 1-octene comonomer, butane or iso-octane, and a hexane solvent. A pressure and temperature controlled variable volume cell was used for the measurements. Lower critical solution temperature (LCST) behavior was observed in the investigated temperature range. Increasing polymer concentration was found to increase the solubility of the polymer, meaning the polymer concentrations investigated were larger than the critical polymer concentrations. Increasing differences between the properties of the polymer and solvent, such as molar mass and density, was found to decrease polymer solubility, meaning the system showed more tendency to separate into two liquid phases.Polymeeri – liuotinsysteemien faasitasapainon tunteminen on välttämätöntä kehitettäessä liuospolymerointiprosesseja. Polymeerejä tuotettaessa reaktioliuoksen faasitila voi vaikuttaa reaktionopeuksiin ja tuotteiden ominaisuuksiin. Myös, kun polymeeriä erotetaan liuottimesta, faasien erottuvuutta voidaan hyödyntää. Tämän työn tavoite on tutkia polyetyleeniä ja n-heksaania sisältävien systeemien neste-neste faasitasapainoa mittaamalla kyseisten systeemien samepisteitä. Mittaukset suoritettiin lämpötilavälillä 150 – 200 Celsiusastetta paineen ollessa alle 100 bar, ja polymeerikonsentraation vaihdellessa välillä 10-36 massaprosenttia (m-%). Mittauksia tehtiin kuudella eri polymeerityypillä. Samepisteitä mitattiin vain polyetyleeniä ja n-heksaania sisältävistä binääriseoksista, sekä monikomponenttiseoksista, jotka sisälsivät polyetyleeniä, etyleenimonomeeriä, 1-buteeni- tai 1-okteenikomonomeeriä, butaania tai iso-oktaania, ja heksaaniliuotinta. Mittaukset suoritettiin tilavuussäädettävällä kennolla, jonka lämpötilaa ja painetta voitiin hallita. Mitatulla lämpötilavälillä systeemeissä havaittiin alemman lämpötilarajan (LCST – lower critical solution temperature) tyyppistä neste-neste faasikäyttäytymistä. Polymeerikonsentraation noustessa polymeerin liukoisuus parani, tarkoittaen sitä, että käytetyt polymeerikonsentraatiot olivat suurempia kuin systeemien kriittiset polymeerikonsentraatiot. Polymeerin ja liuotinseoksen välisten ominaisuuksien, kuten moolimassojen ja tiheyksien erojen kasvaessa polymeerin liukoisuus huononi, ja systeemi erottui helpommin kahdeksi nestefaasiksi

Inter-patch movement in an experimental system: the effects of life history and the environment

An important process for the persistence of populations subjected to habitat loss and fragmentation is the dispersal of individuals between habitat patches. Dispersal involves emigration from a habitat patch, movement between patches through the surrounding landscape, and immigration into a new suitable habitat patch. Both landscape and physical condition of the disperser are known to influence dispersal ability, although disentangling these effects can often be difficult in the wild. In one of the first studies of its kind, we used an invertebrate model system to investigate how dispersal success is affected by the interaction between the habitat condition, as determined by food availability, and life history characteristics (which are also influenced by food availability). Dispersal of juvenile and adult mites (male and female) from either high food or low food natal patches were tested separately in connected three patch systems where the intervening habitat patches were suitable (food supplied) or unsuitable (no food supplied). We found that dispersal success was reduced when low food habitat patches were coupled to colonising patches via unsuitable intervening patches. Larger body size was shown to be a good predictor of dispersal success, particularly when the intervening landscape is unsuitable. Our results suggest that there is an interaction between habitat fragmentation and habitat suitability in determining dispersal success: if patches degrade in suitability and this affects the ability to disperse successfully then the effective connectance across landscapes may be lowered. Understanding these consequences will be important in informing our understanding of how species, and the communities in which they are embedded, may potentially respond to habitat fragmentation