Zigzag instability of vortex pairs in stratified and rotating fluids. Part 2. Analytical and numerical analyses

International audienceThe three-dimensional stability of vertical vortex pairs in stratified and rotating fluids is investigated using the analytical approach established in Part 1 and the predictions are compared to the results of previous direct numerical stability analyses for pairs of co-rotating equal-strength Lamb-Oseen vortices and to new numerical analyses for equal-strength counter-rotating vortex pairs. A very good agreement between theoretical and numerical results is generally found, thereby providing a comprehensive description of the zigzag instability. Co-rotating and counter-rotating vortex pairs are most unstable to the zigzag instability when the Froude number Fh =/(2pR 2N) (where is the vortex circulation, R the vortex radius and N the Brunt-Väiäsälä frequency) is lower than unity independently of the Rossby number Ro =/(4pR2?b) (?b is the planetary rotation rate). In this range, the maximum growth rate is proportional to the strain/(2pb2) (b is the separation distance between the vortices) and is almost independent of F h and Ro. The most amplified wavelength scales like Fhb when the Rossby number is large and like Fhb/|Ro| when |Ro| 1, in agreement with previous results. While the zigzag instability always bends equal-strength co-rotating vortex pairs in a symmetric way, the instability is only quasi-antisymmetric for finite Ro for equal-strength counter-rotating vortex pairs because the cyclonic vortex is less bent than the anticyclonic vortex. The theory is less accurate for co-rotating vortex pairs around Ro 2 because the bending waves rotate very slowly for long wavelength. The discrepancy can be fully resolved by taking into account higher-order three-dimensional effects. When Fh is increased above unity, the growth rate of the zigzag instability is strongly reduced because the bending waves of each vortex are damped by a critical layer at the radius where the angular velocity of the vortex is equal to the Brunt-Visl frequency. The zigzag instability, however, continues to exist and is dominant up to a critical Froude number, which mostly depends on the Rossby number. Above this threshold, equal-strength co-rotating vortex pairs are stable with respect to long-wavelength bending disturbances whereas equal-strength counter-rotating vortex pairs become unstable to a quasi-symmetric instability resembling the Crow instability in homogeneous fluids. However, its growth rate is lower than in homogeneous fluids because of the damping by the critical layer. The structure of the critical layer obtained in the computations is in excellent agreement with the theoretical solution. Physically, the different stability properties of vortex pairs in stratified and rotating fluids compared to homogeneous fluids are shown to come from the reversal of the direction of the self-induced motion of bent vortices. © 2010 Cambridge University Press

State recognition for ‘contested languages’: a comparative study of Sardinian and Asturian, 1992–2010

While the idea of a named language as a separate and discrete identity is a political and social construct, in the cases of Sardinian and Asturian doubts over their respective ‘languageness’ have real material consequences, particularly in relation to language policy decisions at the state level. The Asturian example highlights how its lack of official status means that it is either ignored or subjected to repeated challenges to its status as a language variety deserving of recognition and support, reflecting how ‘official language’ in the Spanish context is often understood in practice as synonymous with the theoretically broader category of ‘language’. In contrast, the recent state recognition of Sardinian speakers as a linguistic minority in Italy (Law 482/1999) illustrates how legal recognition served to overcome existing obstacles to the implementation of regional language policy measures. At the same time, the limited subsequent effects of this Law, particularly in the sphere of education, are a reminder of the shortcomings of top-down policies which fail to engage with the local language practices and attitudes of the communities of speakers recognized. The contrastive focus of this article thus acknowledges the continued material consequences of top-down language classification, while highlighting its inadequacies as a language policy mechanism which reinforces artificial distinctions between speech varieties and speakers deserving of recognition

Making it your own by adapting it to what’s important to you”: Plurilingual Critical Literacies to promote L2 Japanese users’ sense of ownership of Japanese

The dichotomy between native speaker (NS) and non-native speaker (NNS) remains ubiquitous across different language-learning contexts despite increasing mobility and multilingualism of society. L2 Japanese learners in particular may find themselves positioned as subordinate to NSs because of the myth of Japan being a homogeneous nation of one race and one language. To help L2 Japanese students counter such positioning and gain a sense of ownership, we implemented “plurilingual critical literacies” in a Japanese language course in the U.S. Critical literacy aims to cultivate students’ awareness that power relationships are at play in language use, and plurilingual pedagogy valorizes students’ multilingual resources. Eleven high-intermediate-level Japanese students mobilized their linguistic and cultural resources to read and discuss authentic texts by transcultural or “culturally mobile” writers (Dagnino 2015). These writers expressed resistance to the status quo and made meaning creatively, as mediators between two languages and cultures. Reading, analyzing, and discussing texts by transcultural writers motivated students to counter ideologies of NS superiority, and to own Japanese in the ways that best suited their transcultural identities

Effects of waves on coastal water dispersion in a small estuarine bay

A three-dimensional wave-current model is used to investigate wave-induced circulations in a small estuarine bay and its impact on freshwater exchanges with the inner shelf, related to stratified river plume dispersion. Modeled salinity fields exhibit a lower salinity surface layer due to river outflows, with typical depth of 1 m inside the bay. The asymmetric wave forcing on the bay circulation, related to the local bathymetry, significantly impacts the river plumes. It is found that the transport initiated in the surf zone by the longshore current can oppose and thus reduce the primary outflow of freshwater through the bay inlets. Using the model to examine a high river runoff event occurring during a high-energy wave episode, waves are found to induce a 24 h delay in freshwater evacuation. At the end of the runoff event, waves have reduced the freshwater flux to the ocean by a factor 5, and the total freshwater volume inside the bay is increased by 40%. According to the model, and for this event, the effect of the surf zone current on the bay flushing is larger than that of the wind. The freshwater balance is sensitive to incident wave conditions. Maximum freshwater retention is found for intermediate offshore wave heights 1 m < Hs < 2 m. For higher-energy waves, the increase in the longshore current reduces the retention, which is two times lower for Hs = 4 m than for Hs = 2 m

Face Work in Spanish Language Service Encounters between Native and Nonnative Speakers in the United States

Linguistic politeness plays an important role in the opinions people form of one another, especially when it offers one of the few clues an individual may have of his or her interlocutor’s disposition, as is the case in anonymous, first-time service encounters. Face is a fundamental con- cept in politeness theory. To borrow a summary from Callahan (2011): Positive face refers to the desire to be liked and appreciated, while negative face refers to the desire to be unimpeded (Brown and Levinson 1987). Positive and negative face are often characterized as corresponding to the dichotomies of involvement vs. independ- ence, intimacy vs. distance, and solidarity vs. deference (Scollon and Scollon 2001). An action or utterance that goes against one’s need for appreciation, in the case of positive face, or autonomy, in the case of negative face, is said to constitute a face-threatening act, or FTA (Brown and Levinson 1987). [...] Speech acts as well as non-linguistic actions that attend to the addressee’s as well as the speaker’s own face, both positive and negative, are described as face work.1 Face work attenuates, or mitigates, the force of face-threatening acts. (28