10,702 research outputs found

    A Generalized Compressible Cavitation Model

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    A new multi-phase model for low speed gas/liquid mixtures is presented; it does not require ad-hoc closure models for the variation of mixture density with pressure and yields thermodynamically correct acoustic propagation for multi-phase mixtures. The solution procedure has an interface-capturing scheme that incorporates an additional scalar transport equation for the gas void fraction. Cavitation is modeled via a finite rate source term that initiates phase change when liquid pressure drops below its saturation value. The numerical procedure has been implemented within a multi-element unstructured framework CRUNCH that permits the grid to be locally refined in the interface region. The solution technique incorporates a parallel, domain decomposition strategy for efficient 3D computations. Detailed results are presented for sheet cavitation over a cylindrical headform and a NACA 66 hydrofoil

    Feedback control of unstable steady states of flow past a flat plate using reduced-order estimators

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    We present an estimator-based control design procedure for flow control, using reduced-order models of the governing equations, linearized about a possibly unstable steady state. The reduced models are obtained using an approximate balanced truncation method that retains the most controllable and observable modes of the system. The original method is valid only for stable linear systems, and we present an extension to unstable linear systems. The dynamics on the unstable subspace are represented by projecting the original equations onto the global unstable eigenmodes, assumed to be small in number. A snapshot-based algorithm is developed, using approximate balanced truncation, for obtaining a reduced-order model of the dynamics on the stable subspace. The proposed algorithm is used to study feedback control of 2-D flow over a flat plate at a low Reynolds number and at large angles of attack, where the natural flow is vortex shedding, though there also exists an unstable steady state. For control design, we derive reduced-order models valid in the neighborhood of this unstable steady state. The actuation is modeled as a localized body force near the leading edge of the flat plate, and the sensors are two velocity measurements in the near-wake of the plate. A reduced-order Kalman filter is developed based on these models and is shown to accurately reconstruct the flow field from the sensor measurements, and the resulting estimator-based control is shown to stabilize the unstable steady state. For small perturbations of the steady state, the model accurately predicts the response of the full simulation. Furthermore, the resulting controller is even able to suppress the stable periodic vortex shedding, where the nonlinear effects are strong, thus implying a large domain of attraction of the stabilized steady state.Comment: 36 pages, 17 figure

    Cumulene Molecular Wire Conductance from First Principles

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    We present first principles calculations of current-voltage characteristics (IVC) and conductance of Au(111):S2-cumulene-S2:Au(111) molecular wire junctions with realistic contacts. The transport properties are calculated using full self-consistent ab initio NEGF-DFT methods under external bias. The conductance of the cumulene wires shows oscillatory behavior depending on the number of carbon atoms (double bonds). Among all conjugated oligomers, we find that cumulene wires with odd number of carbon atoms yield the highest conductance with metallic-like ballistic transport behavior. The reason is the high density of states in broad LUMO levels spanning the Fermi level of the electrodes. The transmission spectrum and the conductance depend only weakly on applied bias, and the IVC is nearly linear over a bias region from +1 to -1 V. Cumulene wires are therefore potential candidates for metallic connections in nanoelectronic applications.Comment: Accepted in Phys. Rev. B; 5 pages and 6 figure

    Investigation of hypersonic shock-induced combustion in a hydrogen-air system

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    A numerical study is conducted to simulate the ballistic range experiments at Mach 5.11 and 6.46. The flow field is found to be unsteady with periodic instabilities originating in the stagnation zone. The unsteadiness of the flow field decreased with increase in the Mach number, thus indicating that it is possible to stabilize such flow fields with a high degree of overdrive. The frequency of periodic instability is determined using Fourier power spectrum and is found to be in good agreement with the experimental data. The physics of the instability is explained by the wave interaction models available in the literature

    Modeling and optimization of production and distribution of drinking water at VMW

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    We develop and discuss an operational planning model aiming at minimizing production and distribution costs in large drinking water networks containing buffers with free inflow. Modeling drinking water networks is very challenging due of the presence of complex hydraulic constraints, such as friction losses and pump curves. Non-linear, non-convex constraints result from the relationships between pressure and flow in power terms. Also, binary variables are needed to model the possibility of free inflow or re-injection of water at reservoirs. The resulting model is thus a non-convex Mixed-Integer Non-Linear Program (MINLP). A discrete-time setting is proposed to solve the problem over a finite horizon made of several intervals. A commercial solver, BONMIN, suited for convex MINLP models is used to heuristically solve the problem. We are able to find a good solution for a small part of an existing network operated by the Vlaamse Maatschappij voor Watervoorziening (VMW), a major drinking water company in Flanders

    Robust Video Watermarking Scheme Based on Intra-Coding Process in MPEG-2 Style

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    The proposed scheme implemented a semi blind digital watermarking method for video exploiting MPEG-2 standard. The watermark is inserted into selected high frequency coefficients of plain types of discrete cosine transform blocks instead of edge and texture blocks during intra coding process. The selection is essential because the error in such type of blocks is less sensitive to human eyes as compared to other categories of blocks. Therefore, the perceptibility of watermarked video does not degraded sharply. Visual quality is also maintained as motion vectors used for generating the motion compensated images are untouched during the entire watermarking process. Experimental results revealed that the scheme is not only robust to re-compression attack, spatial synchronization attacks like cropping, rotation but also strong to temporal synchronization attacks like frame inserting, deleting, swapping and averaging. The superiority of the anticipated method is obtaining the best sturdiness results contrast to the recently delivered schemes

    Professional identity and anxiety in architect-client interactions

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    © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Large-scale construction projects increasingly have powerful and knowledgeable clients as project owners with whom professionals, such as architects, must interact. In such contexts, clients may have a significant impact on the constitution of a coherent and stable professional identity. Based on qualitative interviews with 50 architects across four large multidisciplinary professional service firms (PSFs) located in Sydney, Australia, supplemented by ethnographic observations, this article explores how architects constitute their identity in interactions with clients. The findings led us to conceptualise professional–client interactions in terms of two overarching discursive strategies deployed by architects in attempts to manage clients that are powerful and knowledgeable: best for client and best for project. We illustrate the anxieties that architects experience and suggest that attempts to secure professional identity may result in (re)producing an enduring sense of anxiety with unintended consequences for project outcomes and organisational performance

    Emotions and identity work: Emotions as discursive resources in the constitution of junior professionals’ identities

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    © The Author(s) 2018. For junior professionals, notions of professional identity established during their education are often called into question in the early stages of their professional careers. The workplace gives rise to identity challenges that manifest in significant emotional struggles. However, although extant literature highlights how emotions trigger and accompany identity work, the constitutive role of emotions in identity work is under-researched. In this article, we analyse how junior professionals mobilize emotions as discursive resources for identity work. Drawing on an empirical study of junior architects employed in professional service firms, we examine how professional identities, imbued with varying forms of discipline and agency, are discursively represented. The study makes two contributions to the literature on emotions and identity work. First, we identify three key identity work strategies (idealizing, reframing and distancing) that are bound up in junior architects’ emotion talk. We suggest that these strategies act simultaneously as a coping mechanism and as a disciplinary force in junior architects’ efforts to constitute themselves as professionals. Second, we argue that identity work may not always lead to the accomplishment of a positive sense of self but can express a sense of disillusionment that leads to the constitution of dejected professional identities
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