1,721,016 research outputs found
Long-term analysis of stochastic θ-methods for damped stochastic oscillators
No full textWe analyze long-term properties of stochastic θ-methods for damped linear stochastic oscillators. The presented a-priori analysis of the error in the correlation matrix allows to infer the long-time behaviour of stochastic θ-methods and their capability to reproduce the same long-term features of the continuous dynamics. The theoretical analysis is also supported by a selection of numerical experiments
Nearly conservative multivalue methods with extended bounded parasitism
No full textThe paper is focused on the analysis of parasitism for multivalue numerical methods intended as geometric numerical integrators for Hamiltonian problems. In particular, the main topic is the design of multivalue numerical methods whose parasitic components remain bounded over certain time intervals, opening the path to the development of nearly conservative multivalue methods able to guarantee a control of parasitism in the long time. The analysis of parasitism as well as the development of the corresponding methods is the core of the treatise. The effectiveness of the approach is also confirmed on selected Hamiltonian problems
Sensitivity analysis and passive control of the secondary instability in the wake of a cylinder
No full textThe stability properties of selected flow configurations, usually denoted as base flows, can be significantly altered by small modifications of the flow, which can be caused, for instance, by a non-intrusive passive control. This aspect is amply demonstrated in the literature by ad hoc sensitivity studies which, however, focus on configurations characterised by a steady base flow. Nevertheless, several flow configurations of interest are characterised by a time-periodic base flow. To this purpose, we propose here an original theoretical framework suitable to quantify the effects of base-flow variations in the stability properties of saturated time-periodic limit cycles. In particular, starting from a Floquet analysis of the linearised Navier–Stokes equations and using adjoint methods, it is possible to estimate the variation of a selected Floquet exponent caused by a generic structural perturbation of the base-flow equations. This link is expressed concisely using the adjoint operators coming from the analysis, and the final result, when applied to spatially localised disturbances, is used to build spatial sensitivity and control maps. These maps identify the regions of the flow where the placement of a infinitesimal small object produces the largest effect on the Floquet exponent and may also provide a quantification of this effect. Such analysis brings useful insights both for passive control strategies and for further characterising the investigated instability. As an example of application, the proposed analysis is applied here to the three-dimensional flow instabilities in the wake past a circular cylinder. This is a classical problem which has been widely studied in the literature. Nevertheless, by applying the proposed analysis we derive original results comprising a further characterisation of the instability and related control maps. We finally show that the control maps obtained here are in very good agreement with control experiments documented in the literature
External acoustic control of the laminar vortex shedding past a bluff body
No full textThis paper deals with the active control of the compressible flow past a bluff-body by means of external acoustic sources. We successfully suppress the transition leading to the von-Karman vortex street. The derived adjoint-based framework allows the determination of the corresponding acoustic sources for generic bluff-body shapes. In particular, we determine the optimal spatial position for the control, i.e. where the acoustic source is most efficient, and the associated temporal signal. For the proposed example, a two-dimensional cylindrical body, the resulting acoustic actuation is anti-symmetric and harmonic. We found that the limits of our control approach are related to the limits of linear acoustics. The methodology and the results proposed in this paper can be used to design a control strategy coupled with a continuation procedure able to obtain the unstable solutions beyond the critical threshold
Recreational drugs: a new health hazard for patients with concomitant chronic liver diseases.
No full textOptimal explicit Runge-Kutta methods for compressible Navier-Stokes equations
No full textWe focus our attention on the numerical simulations of compressible flows obtained by using Finite Difference in time /Finite Element in space approximation. In particular, we determine optimal explicit Runge-Kutta methods capable to maximize the stability features of the resulting numerical scheme. Two different regimes characterized by low and moderate Mach numbers have been taken into account. In the former regime, we have determined an explicit Runge-Kutta method of fourth order that is approximately 15% more efficient than classical ERK(4,4) schemes. For moderate Mach numbers, Ma=0.4, and transitional Reynolds numbers we have determined ERK schemes that outperform classic
ERK(3,3) or ERK(4,4). Optimal ERK have a reduced CFL approximatively four or five times larger than classical ones. These optimized ERK schemes are then promising for the study of transitional flows for global stability or transient growth analyses
Erratum to: Error sensitivity to refinement: a criterion for optimal grid adaptation (Theoretical and Computational Fluid Dynamics, (2017), 31, 5-6, (595-605), 10.1007/s00162-016-0413-x)
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Findings from studies are congruent with obesity having a viral origin, but what about obesity-related nafld?
Full text linkInfection has recently started receiving greater attention as an unusual causative/inducing factor of obesity. Indeed, the biological plausibility of infectobesity includes direct roles of some viruses to reprogram host metabolism toward a more lipogenic and adipogenic status. Furthermore, the probability that humans may exchange microbiota components (virome/virobiota) points out that the altered response of IFN and other cytokines, which surfaces as a central mechanism for adipogenesis and obesity-associated immune suppression, is due to the fact that gut microbiota uphold intrinsic IFN signaling. Last but not least, the adaptation of both host immune and metabolic system under persistent viral infections play a central role in these phenomena. We hereby discuss the possible link between adenovirus and obesity-related nonalcoholic fatty liver disease (NAFLD). The mechanisms of adenovirus-36 (Ad-36) involvement in hepatic steatosis/NAFLD consist in reducing leptin gene expression and insulin sensitivity, augmenting glucose uptake, activating the lipogenic and pro-inflammatory pathways in adipose tissue, and increasing the level of macrophage chemoattractant protein-1, all of these ultimately leading to chronic inflammation and altered lipid metabolism. Moreover, by reducing leptin expression and secretion Ad-36 may have in turn an obesogenic effect through increased food intake or decreased energy expenditure via altered fat metabolism. Finally, Ad-36 is involved in upregulation of cAMP, phosphatidylinositol 3-kinase, and p38 signaling pathways, downregulation of Wnt10b expression, increased expression of CCAAT/enhancer binding protein-beta, and peroxisome proliferator-activated receptor gamma 2 with consequential lipid accumulation
A-stability preserving perturbation of Runge–Kutta methods for stochastic differential equations
No full textThe paper is focused on analyzing the linear stability properties of stochastic Runge–Kutta (SRK) methods interpreted as a stochastic perturbation of the corresponding deterministic Runge–Kutta methods. In particular, we give a condition such that deterministic A-stability is automatically inherited by stochastic Runge–Kutta methods as mean-square A-stability. This issue provides classes of mean-square A-stable SRK methods straightforwardly
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