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Small Disturbances (small + disturbance)
Selected AbstractsCounter-current gas-liquid wavy film flow between the vertical plates analyzed using the Navier-Stokes equationsAICHE JOURNAL, Issue 8 2010Yu. Ya. Abstract The article is devoted to a theoretical analysis of counter-current gas-liquid wavy film flow between vertical plates. We consider two-dimensional nonlinear waves on the interface over a wide variation of parameters. The main interest is to analyse the wave structure at the parameter values corresponding to the onset of flooding observed in experiments. We use the Navier-Stokes equations in their full statement to describe the liquid phase hydrodynamics. For the gas phase equations, we use two models: (1) the Navier-Stokes system and (2) the simplified Benjamin-Miles approach where the liquid phase is a small disturbance for the laminar or turbulent gas flow. With the superficial gas velocity increasing and starting from some value of the velocity, the waves demonstrate a rapid decreasing of both the minimal film thickness and the phase wave velocity. We obtain a region of the gas velocity where we have two solutions at one set of the problem parameters and where the flooding takes place. Both the phase wave velocity and the minimal film thickness are positive numbers at such values of the velocity. We calculate the flooding point dependences on the liquid Reynolds number for two different liquids. The wave regime corresponding to the flooding point demonstrates negative u- velocities in the neighbourhood of the interface near the film thickness maximum. At smaller values of the superficial gas velocity, the negative u- velocities take place in the neighbourhood of the film thickness minimum. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Feedbacks between phosphorus deposition and canopy cover: The emergence of multiple stable states in tropical dry forestsGLOBAL CHANGE BIOLOGY, Issue 1 2008MARCIA DeLONGE Abstract Dry forests represent a large percentage of tropical forests and are vulnerable to both anthropogenic and natural disturbances, yet important aspects of their sensitivity to disruption remain poorly understood. It is particularly unclear how changes in land-use or tropical storm patterns may affect the resiliency of phosphorus (P)-limited neotropical forests. In these systems, vegetation is sustained in the long-term by atmospheric P-inputs through rainfall, dust, or fog. Past research supports the idea that dust and fog deposition are dependent on canopy density (e.g. leaf area index). Thus, the canopy may function as a ,trap' for P, enabling a positive feedback between vegetation and P-deposition. We developed a conceptual model to investigate how Neotropical vegetation may respond to reduced P-deposition due to canopy losses. The model suggests that a canopy-deposition feedback may induce bistable vegetation dynamics; under some conditions, forests may be unable to naturally recover from relatively small disturbances. [source] Maintaining diversity through intermediate disturbances: evidence from rodents colonizing rehabilitating coastal dunesAFRICAN JOURNAL OF ECOLOGY, Issue 4 2000S. M. Ferreira Abstract Rodents inhabit the coastal dune forests of KwaZulu-Natal, South Africa. Here habitat rehabilitation following mining of dunes has resulted in coastal dune forest succession similar to that recorded in nonmined forests. We investigated the colonization of rehabilitating stands and evaluate the role of disturbance in maintaining rodent diversity. A trapping programme was established between July 1993 and February 1995 during which rodent colonization, local extinction and species richness were recorded for rehabilitating stands of different ages. Trends in these variables were closely associated with one of three possible outcomes for a disturbed patch over time, with no intervening disturbances following the initial disturbance. Colonization was initially high which led to an increase in species richness. Extinction was lower than colonization, but became higher when the habitat was 3 years old, which led to a decline in richness. We extrapolate this result assuming negligibly small disturbances after the initiation of rehabilitation and suggest that intermediate levels of disturbance maintain rodent species richness in coastal dune forests. Furthermore, our results illustrated species turnover, a prediction of the recorded outcome, with young stands dominated by Mastomys natalensis and older stands by Saccostomus campestris or Aethomys chrysophilus. Résumé Il y a des rongeurs dans les forêts des dunes côtières du KwaZulu-Natal, en Afrique du Sud. Là, la réhabilitation de l'habitat après l'exploitation minière des dunes a abouti à une succession de forêts côtières des dunes semblable à celle qui est observée dans les forêts non exploitées. Nous avons étudié la colonisation des endroits en voie de réhabilitation et évalué le rôle des perturbations dans le maintien de la diversité des rongeurs. On a mis au point un programme de piégeage entre juillet 1993 et février 1995, pendant lequel on a noté la colonisation par les rongeurs, l'extinction locale et la richesse en espèces pour les endroits à différents stades de réhabilitation. Les tendances pour ces variables étaient étroitement associées à l'un des trois résultats possibles que peut conna,^tre avec le temps un endroit qui a été perturbé, lorsque aucune autre perturbation ne suit la première. La colonisation a d'abord été forte, ce qui a causé un enrichissement des espèces. Les extinctions étaient moins fréquentes que les colonisations, mais elles ont augmenté lorsque le nouvel habitat a atteint l'âge de trois ans, ce qui a entra,^né une perte de richesse en espèces. Nous extrapolons ce résultant en supposant que ce sont de petites perturbations négligeables après le démarrage de la réhabilitation et nous suggérons que des taux moyens de perturbation maintiennent la richesse spécifique des rongeurs dans les forêts des dunes côtières. De plus, nos résultats illustrent une rotation des espèces, une prédiction des résultats rapportés, avec les endroits les plus jeunes dominés par Mastomys natalensis et les plus anciens par Saccostomus campestris et Aethomys chrysophilus. [source] Responses of Aquatic Macrophytes to Disturbance by Flash Floods in a Brazilian Semiarid Intermittent Stream,BIOTROPICA, Issue 4 2001Leonardo Maltchik ABSTRACT Resistance and resilience of Najas marina to disturbance by flash floods were studied in a permanent fluvial pool of a Brazilian semiarid intermittent stream. A total of 21 macrophyte samples was collected in the high-rainfall season during two annual cycles (1996,1997). Decreases in macrophyte biomass were positively correlated with flood magnitude (Pearson, P = 0.047), varying from 25 to 53 percent when discharges were lower than or equal to 0.5 m3/ sec and between 70 and 100 percent when discharges were higher than 1.0 m- 3/sec. Macrophyte resilience was greater after floods of low magnitude. After floods of 0.5 m- 3/sec, three weeks were necessary to re-establish 88 percent of biomass lost, and after a flood of 1.4 m-Vsec, six months were needed to initiate A, marina regrowth. This population of N. marina in Avelós stream has higher stability in response to small disturbances, but as expected, its resistance and resilience decreases with larger disturbances. In general, the high resistance and resilience of N. marina m response to small disturbances have been observed in intermittent tropical streams. The absence of large floods during the study period and the low variability of water temperature in this tropical region may have influenced these results. RESUMES A influencia de cheias rápidas na resistência e na resistência de Najas marina foram estudadas em uma poça fluvial permanente de um riacho efêmero do Semi-árido Brasileiro. Vinte e uma coletas de macrófitas aquáticas foram realizadas durante o período de chuvas de dois ciclos anuais (1996 e 1997). A variaçäo da biomassa de macrófitas aquáticas estava diretamente correlacionada com a magnitude da cheia (Pearson, P=0.047), variando entre 25 e 53 por cento quando as vazöes eram inferiores ou iguais à 0.5 m3 -Vsec e entre 70 e 100 por cento quando as vazöes eram superiores à 1.0 m3/sec. A resiliência de macrófitas era maior após às cheias de baixa magnitude. Após a cheia de 0.5 m3 -Vsec, foram necessários seis meses para restabelecer 88 por cento da biomassa perdida, e após a cheia de 1.4 m3/ sec, foram necessáries seis meses para iniciar a resiliência de Najas marina. Esta populaçäo de N. marina do riacho Avelós apresentou maior estabilidade frente ás perturbaçöes hidrológicas de baixa magnitude, mas a resistêncía e a resiliéncia diminuíam à medida que a magnitude da perturbaçao aumentava. A alta resístência e resiliência de N marina frente às perturbaçöes hidrológicas de baixa magnitude, geralmente tem sido observadas em riachos intermi-tentes tropicais. A ausência de grandes cheias durante o periodo estudado e a pequena amplitude térmica da água superficial desta regiäo tropical podem ter favorecido estes resultados. [source] Complex dynamic behavior during transition in a solid combustion model,COMPLEXITY, Issue 6 2009Jun Yu Through examples in a free-boundary model of solid combustion, this study concerns nonlinear transition behavior of small disturbances of front propagation and temperature as they evolve in time. This includes complex dynamics of period doubling, and quadrupling, and it eventually leads to chaotic oscillations. Within this complex dynamic domain we also observe a period six-folding. Both asymptotic and numerical solutions are studied. We show that for special parameters our asymptotic method with some dominant modes captures the formation of coherent structures. Finally,we discuss possible methods to improve our prediction of the solutions in the chaotic case. © 2009 Wiley Periodicals, Inc. Complexity, 2009 [source] | ||||||||||