This work examines the fluid dynamic structure of underexpanded gas jets by using a highperformance computing (HPC) methodology in order to untangle the question of whether it is necessary to include the real gas assumption dealing with hydrogen jets. The answer to this question is needed in order to guarantee accurate numerical simulations of such jets in practical engineering applications, such as direct-injection hydrogen engines. An axial symmetric turbulent flow model, which solves the Favre-averaged Navier-Stokes equations for a multicomponent gas mixture, has been implemented and validated. The flow model has been assessed by comparing spreading and centreline property decay rates of subsonic jets at different Mach numbers with those obtained by both theoretical considerations and experimental measurements. Besides, the Mach disk structure of underexpanded jets has been recovered, thus confirming the suitability and reliability of the computational model. To take into account the effects of real gases, both van der Waals and Redlich-Kwong equations of state have been implemented. The analysis of a highly underexpanded hydrogen jet with total pressure equal to 75 MPa, issuing into nitrogen at 5 MPa, shows that the use of real gas equations of state affects significantly the jet structure in terms of temperature, pressure, and Mach number profiles along the jet centerline and also in terms of jet exit conditions, with differences up to 38%.
We study numerically the behaviour of a two-dimensional mixture of a passive isotropic fluid and an active polar gel, in the presence of a surfactant favouring emulsification. Focussing on parameters for which the underlying free energy favours the lamellar phase in the passive limit, we show that the interplay between nonequilibrium and thermodynamic forces creates a range of multifarious exotic emulsions. When the active component is contractile (e.g., an actomyosin solution), moderate activity enhances the efficiency of lamellar ordering, whereas strong activity favours the creation of passive droplets within an active matrix. For extensile activity (occurring, e.g., in microtubule-motor suspensions), instead, we observe an emulsion of spontaneously rotating droplets of different size. By tuning the overall composition, we can create high internal phase emulsions, which undergo sudden phase inversion when activity is switched off. Therefore, we find that activity provides a single control parameter to design composite materials with a strikingly rich range of morphologies.
Resumo Neste artigo são analisadas as múltiplas dimensões da atuação dos burocratas de nível de rua no contexto brasileiro da implementação de políticas públicas. O objetivo do ensaio é contribuir para uma maior compreensão do tema com uma abordagem analítica expandida. Para tanto, foi realizada uma pesquisa bibliográfica em bases digitais acadêmicas que permitiu identificar e selecionar trabalhos nacionais e internacionais de acordo com critérios metodologicamente definidos. As publicações obtidas foram analisadas e organizadas em razão das seguintes categorias conceituais: discricionariedade, estruturas, ação individual e relações. Da tensão sobre regras e discricionariedade na administração pública, a análise e a interpretação desses textos evidenciaram: a) a necessidade de cotejar, separadamente e em conjunto, três diferentes perspectivas da atuação dos burocratas de nível de rua (a perspectiva estrutural, a de ação individual e a relacional); b) a necessidade de compreender a articulação dessas perspectivas no próprio processo de implementação. Após este trabalho de revisão bibliográfica foram formuladas hipóteses para uma análise expandida da atuação dos burocratas implementadores, traçando-se linhas para pesquisas futuras sobre o tema. Este ensaio contribui para o debate sobre o tema com um construto teórico inédito, que busca integrar as três diferentes perspectivas analíticas apresentadas numa síntese dialética dos conceitos centrais da literatura sobre a implementação de políticas públicas pelos burocratas de nível de rua.
We present hybrid lattice Boltzmann simulations of extensile and contractile active fluids where we incorporate phenomenologically the tendency of active particles such as cell and bacteria, to move, or swim, along the local orientation. Quite surprisingly, we show that the interplay between alignment and activity can lead to completely different results, according to geometry (periodic boundary conditions or confinement between flat walls) and nature of the activity (extensile or contractile). An interesting generic outcome is that the alignment interaction can transform stationary active patterns into continuously moving ones: the dynamics of these evolving patterns can be oscillatory or chaotic according to the strength of the alignment term. Our results suggest that flow-polarisation alignment can have important consequences on the collective dynamics of active fluids and active gel.
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