Numerical Predictions of Cavitating Flow around Model Scale Propellers by CFD and Advanced Model Calibration

The numerical predictions of the cavitating flow around two model scale propellers in uniform inflow are presented and discussed. The simulations are carried out using a commercial CFD solver. The homogeneous model is used and the influence of three widespread mass transfer models, on the accuracy of the numerical predictions, is evaluated. The mass transfer models in question share the common feature of employing empirical coefficients to adjust mass transfer rate from water to vapour and back, which can affect the stability and accuracy of the predictions. Thus, for a fair and congruent comparison, the empirical coefficients of the different mass transfer models are first properly calibrated using an optimization strategy. The numerical results obtained, with the three different calibrated mass transfer models, are very similar to each other for two selected model scale propellers. Nevertheless, a tendency to overestimate the cavity extension is observed, and consequently the thrust, in the most severe operational conditions, is not properly predicted

Two algorithms for fast 2D node generation: application to RBF meshless discretization of diffusion problems and image halftoning

Mesh generation techniques for traditional mesh based numerical approaches such as FEM and FVM have now reached a good degree of maturity. There is no such an acknowledged background when dealing with node generation techniques for meshless numerical approaches, despite their theoretical simplicity and efficiency; furthermore node generation can be put in connection with some well-known image approximation techniques. Two node generation algorithms are here proposed and employed in the numerical solution of 2D steady state diffusion problems by means of a local Radial Basis Function (RBF) meshless method. Finally, such algorithms are also tested for greyscale image approximation through stippling

Accurate Stabilization Techniques for RBF-FD Meshless Discretizations with Neumann Boundary Conditions

A major obstacle to the application of the standard Radial Basis Function-generated Finite Difference (RBF-FD) meshless method is constituted by its inability to accurately and consistently solve boundary value problems involving Neumann boundary conditions (BCs). This is also due to ill-conditioning issues affecting the interpolation matrix when boundary derivatives are imposed in strong form. In this paper these ill-conditioning issues and subsequent instabilities affecting the application of the RBF-FD method in presence of Neumann BCs are analyzed both theoretically and numerically. The theoretical motivations for the onset of such issues are derived by highlighting the dependence of the determinant of the local interpolation matrix upon the boundary normals. Qualitative investigations are also carried out numerically by studying a reference stencil and looking for correlations between its geometry and the properties of the associated interpolation matrix. Based on the previous analyses, two approaches are derived to overcome the initial problem. The corresponding stabilization properties are finally assessed by succesfully applying such approaches to the stabilization of the Helmholtz-Hodge decomposition

Decoupled CFD-based optimization of efficiency and cavitation performance of a double-suction pump

In this study the impeller geometry of a double-suction pump ensuring the best performances in terms of hydraulic efficiency and reluctance of cavitation is determined using an optimization strategy , which was driven by means of the modeFRONTIER optimization platform. The different impeller shapes (designs) are modified according to the optimization parameters and tested with a computational fluid dynamics (CFD) software, namely ANSYS CFX. The simulations are performed using a decoupled approach, where only the impeller domain region is numerically investigated for computational convenience. The flow losses in the volute are estimated on the base of the velocity distribution at the impeller outlet. The best designs are then validated considering the computationally more expensive full geometry CFD model. The overall results show that the proposed approach is suitable for quick impeller shape optimization

Influence of the Mass Transfer Model on the Numerical Prediction of the Cavitating Flow Around a Marine Propeller

ABSTRACT Cavitating flows, which can occur in a variety of practical applications, can be modelled using a wide range of methods. One strategy consists of using the RANS (Reynolds Averaged Navier Stokes) approach along with an additional transport equation for the liquid volume fraction, where mass transfer rate due to cavitation is modelled by a mass transfer model. In this study, we verify the influence of three widespread mass transfer models, mainly on the numerical predictions of the propeller performances. The models in question share the common feature of employing some empirical coefficients to tune the models of condensation and evaporation processes, which can influence the accuracy and stability of the numerical predictions. For this reason, and for a fair and congruent comparison, the empirical coefficients of the different mass transfer models are first equally well tuned using an optimization strategy. The numerical predictions of the propeller performances based on the three different well-tuned mass transfer models are very close to each other. Unfortunately, the numerical cavitation patterns are slightly overestimated compared to the experimental ones, and the thrust breakdown is not properly predicted either. Finally, we roughly verify that for the prediction of the model scale propulsive performances in the presence of the partial and tip vortex cavitation, the turbulence model, among those considered in this study, plays a minor role

Numerical investigation of the flow in axial water turbines and marine propellers with scale-resolving simulations

The accurate prediction of the performances of axial water turbines and naval propellers is a challenging task, of great practical relevance. In this paper a numerical prediction strategy, based on the combination of a trusted CFD solver and a calibrated mass transfer model, is applied to the turbulent flow in axial turbines and around a model scale naval propeller, under non-cavitating and cavitating conditions. Some selected results for axial water turbines and a marine propeller, and in particular the advantages, in terms of accuracy and fidelity, of ScaleResolving Simulations (SRS), like SAS (Scale Adaptive Simulation) and Zonal-LES (ZLES) compared to standard RANS approaches, are presented. Efficiency prediction for a Kaplan and a bulb turbine was significantly improved by use of the SAS SST model in combination with the ZLES in the draft tube. Size of cavitation cavity and sigma break curve for Kaplan turbine were successfully predicted with SAS model in combination with robust high resolution scheme, while for mass transfer the Zwart model with calibrated constants were used. The results obtained for a marine propeller in non-uniform inflow, under cavitating conditions, compare well with available experimental measurements, and proved that a mass transfer model, previously calibrated for RANS (Reynolds Averaged Navier Stokes), can be successfully applied also within the SRS approaches

Activity and analysis of costs in a dedicated weaning centre.

AIM: To analyse the diagnosis-related characteristics and the costs of treating patients with difficult/prolonged weaning from mechanical ventilation we have undertaken a retrospective observational study. METHODS: The study has considered all the patients admitted to our weaning unit of a regional Rehabilitation department during 3 consecutive periods since the opening date. Characteristics of the admitted patients and the DRG-related cares delivered have been recorded. A cost analysis has been obtained over time. RESULTS: The number of beds allocated to this unit (from 4 in the 1st period to 6 in the 2nd and 3rd periods) and the number of patients cared for (from 32 to 43 and to 65, respectively) increased over time. In particular, the COPD to non-COPD patient ratio (from 2.2 to 1.3 and to 1.0) and the DRG/patient weight (from 3.0 +/- 0.3 to 3.1 +/- 0.2 and to 3.3 +/- 0.2 point) changed significantly (p < 0.05). The daily reimbursement per patient from the public health care system only slightly increased, whereas the operating margin (reimbursement less costs) per patient significantly improved (from -304, to +17 and +55 Euro/pt/day, respectively, p < 0.05) due to a gradual restriction in the variable costs. Length of stay, mortality rate and weaning rate did not change over time. CONCLUSION: The weaning centre is a hospital area where economic burdens should be carefully evaluated. Given the actual reimbursement received on a national level for these patients, variable costs might be better spread, thus optimising the burdens without losing out on clinical outcomes

Interdisciplinary rehabilitation in morbidly obese subjects: an observational pilot study.

BACKGROUND AND AIM: To assess the clinical effectiveness of a interdisciplinary rehabilitation programme (CR), in a population of morbidly obese subjects we have undertaken a observational study. METHODS: The study included fifty-nine adult subjects (18 M, 60+/-10 years, BMI 47+/-8) with sleep-disturbance related symptoms and disabilities. Assessment and correction of sleep disordered breathing (SDB) abnormalities, improvement of exercise tolerance, body weight and associated psychological features were the aims of this CR, which has been carried out over a 1 month period.Lung functions, apnea/hypopnea index (AHI), 6-minute walking distance (6MWD), body weight (BW), quality of life by means of Sat-P questionnaire and serum metabolic data has been recorded at baseline (TO), at the end (Ti) and 6 months after (T2) the CR. RESULTS: The percentage of patients with AHI > 10 declined from 65% (at TO) to 20% (at both T1 and T2). 6MWD and BW significantly improved (p < 0.005) at T1 and still maintained at T2; a significant relationship (r = 0.379, p < 0.01) has been found between changes of BW and 6MWD recorded in between TO and T2. Sat-P item scores dealing with sleep efficiency, problem solving, and social interactions improved (p < 0.01) at T1 and still maintained at T2. CONCLUSIONS: This hospital-based CR provides indication for effectiveness in advanced morbidly obese subjects and warrants further controlled trials to confirm the results

ECMO for COVID-19 patients in Europe and Israel

Since March 15th, 2020, 177 centres from Europe and Israel have joined the study, routinely reporting on the ECMO support they provide to COVID-19 patients. The mean annual number of cases treated with ECMO in the participating centres before the pandemic (2019) was 55. The number of COVID-19 patients has increased rapidly each week reaching 1531 treated patients as of September 14th. The greatest number of cases has been reported from France (n = 385), UK (n = 193), Germany (n = 176), Spain (n = 166), and Italy (n = 136) .The mean age of treated patients was 52.6 years (range 16–80), 79% were male. The ECMO configuration used was VV in 91% of cases, VA in 5% and other in 4%. The mean PaO2 before ECMO implantation was 65 mmHg. The mean duration of ECMO support thus far has been 18 days and the mean ICU length of stay of these patients was 33 days. As of the 14th September, overall 841 patients have been weaned from ECMO support, 601 died during ECMO support, 71 died after withdrawal of ECMO, 79 are still receiving ECMO support and for 10 patients status n.a. . Our preliminary data suggest that patients placed on ECMO with severe refractory respiratory or cardiac failure secondary to COVID-19 have a reasonable (55%) chance of survival. Further extensive data analysis is expected to provide invaluable information on the demographics, severity of illness, indications and different ECMO management strategies in these patients