Experiments in a rotating Hele-Shaw cell

We introduce a modification to Hele-Shaw flows consisting of a rotating cell. A viscous fluid (oil) is injected at the rotation axis of the cell, which is open to air. The morphological instability of the oil-air interface is thus driven by centrifugal force and is controlled by the density (not viscosity) difference. We derive the linear dispersion relation and verify the maximum growth rate selection of initial patterns within experimental uncertainty. The nonlinear growth regime is studied in the case of vanishing injection rate. Several characteristic lengths are studied to quantify the patterns obtained. Experimental data exhibit good collapse for two characteristic lengths, namely, the radius of gyration and the radial finger length, which in the nonlinear regime appear to grow linearly in time.

Intrinsic noise-induced phase transitions: Beyond the noise interpretation

We discuss intrinsic noise effects in stochastic multiplicative-noise partial differential equations, which are qualitatively independent of the noise interpretation (Itô vs Stratonovich), in particular in the context of noise-induced ordering phase transitions. We study a model which, contrary to all cases known so far, exhibits such ordering transitions when the noise is interpreted not only according to Stratonovich, but also to Itô. The main feature of this model is the absence of a linear instability at the transition point. The dynamical properties of the resulting noise-induced growth processes are studied and compared in the two interpretations and with a reference Ginzburg-Landau-type model. A detailed discussion of a different numerical algorithm valid for both interpretations is also presented.

Radial displacement of a fluid annulus in a rotating Hele¿Shaw cell

The radial displacement of a fluid annulus in a rotating circular Hele-Shaw cell has been investigated experimentally. It has been found that the flow depends sensitively on the wetting conditions at the outer interface. Displacements in a prewet cell are well described by Darcy's law in a wide range of experimental parameters, with little influence of capillary effects. In a dry cell, however, a more careful analysis of the interface motion is required; the interplay between a gradual loss of fluid at the inner interface, and the dependence of capillary forces at the outer interface on interfacial velocity and dynamic contact angle, result in a constant velocity for the interfaces. The experimental results in this case correlate in the form of an empirical scaling relation between the capillary number Ca and a dimensionless group, related to the ratio of centrifugal to capillary forces, which spans about three orders of magnitude in both quantities. Finally, the relative thickness of the coating film left by the inner interface, alpha i, is obtained as a function of Ca.

Interfacial instabilities of a fluid annulus in a rotating Hele-Shaw cell

We have studied the interfacial instabilities experienced by a liquid annulus as it moves radially in a circular Hele-Shaw cell rotating with angular velocity Omega. The instability of the leading interface (oil displacing air) is driven by the density difference in the presence of centrifugal forcing, while the instability of the trailing interface (air displacing oil) is driven by the large viscosity contrast. A linear stability analysis shows that the stability of the two interfaces is coupled through the pressure field already at a linear level. We have performed experiments in a dry cell and in a cell coated with a thin fluid layer on each plate, and found that the stability depends substantially on the wetting conditions at the leading interface. Our experimental results of the number of fingers resulting from the instability compare well with the predictions obtained through a numerical integration of the coupled equations derived from a linear stability analysis. Deep in the nonlinear regime we observe the emission of liquid droplets through the formation of thin filaments at the tip of outgrowing fingers.

Noves aportacions a la flora de les comarques meridionals de Catalunya.

Data about 89 taxa are presented. Arabis nova Vill. subsp. iberica Rivas Mart. ex Talavera, Cistus × nigricans Pourr. and Phacelia tanacetifolia Benth. are new records for the Catalan flora, and 15 taxa (Allium oporinanthum Brullo et al., Apera interrupta (L.) Beauv., Asplenium trichomanes L. subsp. trichomanes, Campanula affinis Schult., Carex sylvatica Huds. subsp. paui (Sennen) A. and O. Bolòs, Epilobium angustifolium L., Limonium hibericum Erben, Melilotus messanensis (L.) All., Minuartia funkii (Jord.) Graebner, Onosma triceros-perma subsp. catalaunica (Sennen) O. Bolòs and Vigo, Orchis palustris Jacq., Orchis simia Lam., Selaginella denticulata (L.) Spring, Stellaria neglecta Weihle in Bluff and Fingerh. and Vaccinium myrtillus L. are new records for Southern Catalonia.

L'aliança Polygonion avicularis Br.-Bl. ex Dich. 1933 als Pirineus Catalans

L'anàlisi de nombrosos inventaris procedents dels Pirineus centrals i orientals ens ha dut a realitzar una revisió de I'aliança Polygonion avicularis Br.-Bl. ex Dich 1933 en aquesta àrea. Reconeixem fins al moment sis associacions per a les quals donem la seva caracterització florística i ecologica, així com la seva distribució als Pirineus catalans. Descrivim, a més, una subass. scleranthetosum uncinatae del Rumici-Spergularietum rubrae, comunitat altimontana i subalpina pròpia dels sòls calcigats.

Glyceria declinata Breb.

Alt Empordà: Les Salines, Agullana, sobre Sta. Eugènia, 400 m 31T DG 89, al Glycerio-Sparganion,). Bou i R. M. Manobens, 23-VI-1984, BC 649.115.

Noves aportacions a la flora de les comarques meridionals de Catalunya.

Data about 89 taxa are presented. Arabis nova Vill. subsp. iberica Rivas Mart. ex Talavera, Cistus × nigricans Pourr. and Phacelia tanacetifolia Benth. are new records for the Catalan flora, and 15 taxa (Allium oporinanthum Brullo et al., Apera interrupta (L.) Beauv., Asplenium trichomanes L. subsp. trichomanes, Campanula affinis Schult., Carex sylvatica Huds. subsp. paui (Sennen) A. and O. Bolòs, Epilobium angustifolium L., Limonium hibericum Erben, Melilotus messanensis (L.) All., Minuartia funkii (Jord.) Graebner, Onosma triceros-perma subsp. catalaunica (Sennen) O. Bolòs and Vigo, Orchis palustris Jacq., Orchis simia Lam., Selaginella denticulata (L.) Spring, Stellaria neglecta Weihle in Bluff and Fingerh. and Vaccinium myrtillus L. are new records for Southern Catalonia.

Syntaxonomic conspectus of the vegetation of Catalonia and Andorra. I: Hygrophilous herbaceous communities.

The first part of a general survey of the vegetation of Catalonia andAndorra, this paper reports all the phytocoenological associations and subassociations recorded in this area. For each community, we provide the correct name and usual synonyms, its typification (where appropriate), all the references including relevés, and the most outstanding features of its structure, species composition, ecology, distribution and diversity. Moreover, associations and subassociations are ordered appropriately in a syntaxonomic scheme. Syntaxonomic ranks are considered in a fairly broad, conservative sense. This classification established 101 associations, which correspond to the classes Lemnetea, Zosteretea, Potametea, Littorelletea, Montio-Cardaminetea, Phragmiti-Magnocaricetea, Scheuchzerio-Caricetea, Isoeto-Nanojuncetea and Molinio-Arrhenatheretea.

Evaluación del interés natural del Parque Nacional de Aigüestortes i estany de Sant Maurici a partir de la cartografía de hábitats CORINE

diverse and in most cases including lakes, snow beds and fens.

Las comunidades de Salix pyrenaica en el valle de Aran.

Se aportan datos referentes a dos comunidades de la alianza Primulion intricatae en que participa el subarbusto Salix pyrenaica: Dryado-Salicetum pyrenaicae Chouard ex Vanden Berghen 1970 y Carici-Geetum pyrenaici Chouard ex Vanden Berghen 1970. Ambas se conocían de áreas más atlánticas de la cordillera pirenaica, y principalmente dentro de territorio francés, si bien se disponía de muy pocos inventarios fitosociológicos concretos. En el valle de Aran, aparecen localmente en los afloramientos calizos, y contienen diversos táxones poco comunes o raros en los Pirineos, como por ejemplo Arctostaphylos alpina. Además, ambas comunidades muestran un carácter relativamente particular en comparación con las afines descritas previamente. Por ello, se propone considerarlas como sendas nuevas subasociaciones: Dryado- Salicetum pyrenaicae salicetosum reticulatae, subass. nov., y Carici-Geetum pyrenaici anthyllidetosum boscii, subass. nov.

Plantas vasculares del piso alpino de los Pirineos Catalanes raras o amenazadas

Como contribución al conocimiento de la flora supraforestal pirenaica y a la política de protección medioambiental, presentamos una relación de los táxones cormofíticos raros o amenazados, existentes en el piso alpino de los Pirineos dentro de los territorios que dependen administrativamente de la Generalitat de Cataluña, Andorra y el departamento de los Pyrénées- Orientales. Dichos táxones se agrupan en seis subapartados, según su área de distribución y su grado de rareza en el territorio considerado. Las plantas especialmente raras o vulnerables se señalan mediante un asterisco situado a continuación del nombre científico. Para la mayoría de los táxones aportamos datos más concretos referentes a distribución y frecuencia.