A Strategy for Parallel Simulation of Declarative Object-Oriented Models of Generalized Physical Networks

For several years now, most of the growth of comput- ing power has been made possible by exploiting paral- lel CPUs on the same chip; unfortunately, state-of-the-art software tools for the simulation of declarative, object- oriented models still generate single-threaded simulation code, showing an increasingly disappointing performance. This paper presents a simple strategy for the efficient com- putation of the right-hand-side of the ordinary differen- tial equations resulting from the causalization of object- oriented models, which is often the computational bottle- neck of the executable simulation code. It is shown how this strategy can be particularly effective in the case of gen- eralized physical networks, i.e., system models built by the connection of components storing certain quantities and of components describing the flow of such quantities between them

Il ruolo della simulazione dinamica nell'ingegneria di processo: stato dell'arte e prospettive

La modalità tradizionale di progetto degli impianti a fluido (ad. es. impianti per l'energia, condizionamento e raffrescamento aziendale, chimico e petrolchimico) si pone normalmente il problema del sistema di controllo nelle ultime fasi del progetto; spesso, la verifica delle prestazioni dinamiche avviene direttamente sul campo, durante la messa in servizio. In particolare nel caso di impianti innovativi, questo tipo di procedura non è soddisfacente, perché le possibili criticità sia a livello di processo che di strategie di controllo si evidenziano quando è ormai troppo tardi per attuare interventi correttivi. La simulazione dinamica di sistema può accompagnare l'intero arco della progettazione dell'impianto, dal progetto di massima (in cui possibilmente si valutano alternative progettuali), passando per l'ingegneria di dettaglio, fino alla messa in opera, all'esercizio, e possibilmente al revamping. In questo modo si ha sempre chiaro quali sono le prestazioni attese dell'impianto accoppiato al suo sistema di controllo, ed è possibile ottenere prestazioni ottimali senza sgradite sorprese. A tale scopo risultano particolarmente adatte le moderne metodologie e strumenti di modellazione e simulazione orientata agli oggetti. Nella memoria vengono riassunte le caratteristiche salienti di tali ambienti, con una panoramica delle attività di ricerca passate, presenti e future presso il Dipartimento di Elettronica e Informazione del Politecnico di Milano

On the choice of initial guesses for the Newton-Raphson algorithm

The initialization of equation-based differential-algebraic system models, and more in general the solution of many engineering and scientific problems, require the solution of systems of nonlinear equations. Newton-Raphson's method is widely used for this purpose; it is very efficient in the computation of the solution if the initial guess is close enough to it, but it can fail otherwise. In this paper, several criteria are introduced to analyze the influence of the initial guess on the evolution of Newton-Raphson's algorithm and to identify which initial guesses need to be improved in case of convergence failure. In particular, indicators based on first and second derivatives of the residual function are introduced, whose values allow to assess how much the initial guess of each variable can be responsible for the convergence failure. The use of such criteria, which are based on rigorously proven results, is successfully demonstrated in three exemplary test cases.Comment: Version accepted for publication on Applied Mathematics and Computatio

Modelling and Simulation of Self-regulating Pneumatic Valves

In conventional aircraft energy systems, self-regulating pneumatic valves (SRPVs) are used to control the pressure and mass flow of the bleed air. The dynamic behavior of these valves is complex and dependent on several physical phenomena. In some cases, limit cycles can occur, deteriorating performance. This paper presents a complex multi-physical model of SRPVs implemented in Modelica. First, the working-principle is explained, and common challenges in control-system design-problems related to these valves are illustrated. Then, a Modelica-model is presented in detail, taking into account several physical domains. It is shown, how limit cycle oscillations occurring in aircraft energy systems can be reproduced with this model. The sensitivity of the model regarding both solver options and physical parameters is investigated

Steady-state initialization of object-oriented thermo-fluid models by homotopy methods

The steady-state initialization of large object-oriented thermo-hydraulic networks is a difficult problem, because of the sensitivity of the convergence to the initial guesses of the iteration variables. This paper proposes an approach to this problem based on homotopy transformation, detailing specific criteria for model simplifications in this physical domain. The approach is successfully demonstrated on large power plant test cases, having several hundreds of iteration variables

The effect of ERAS management in gastric cancer: assessment from the IMIGASTRIC study

Background:Establish protocols to enhance the surgical management (ERAS) can improve outcomes, shortening hospital stay and save resources. Several studies have carried out for colorectal surgery, while a lack of evidence for gastrectomy remains.This study aims to evaluate the impact of ERAS strategies in a large series of patients underwent gastric cancer surgery.Methods:This is a propensity score-matched case-control study, comparing an ERAS group with a control group. Data were recorded through a tailored and protected web-based system. Primary outcomes: hospital stay, complications rate. Among the secondary outcomes, there are: POD of mobilization, POD of starting liquid diet and soft solid diet.Results:Patients in the ERAS and control groups were 1:1 matched by the closest propensity score on the logit scale and with a Caliber = 0.2. The successful matching resulted in a total sample of 440 patients. The two groups showed no differences in all baseline patients characteristics, type of surgery (P=0.31) and stage of the disease (P=0.61). A benefit in favor of the ERAS management was found in the length of hospital stay (P=0.0004) and complications rate (P=0.001).Conclusion:An ERAS program can safely be established in referral centers for gastric cancer, enabling to significantly improve the main clinical outcomes