Search

There have been impressive advances in the physical and mathematical modelling of complex physiological systems in the last few decades, with the potential to revolutionise personalised healthcare with patient-specific evidence-based diagnosis, risk assessment and treatment decision support using digital twins. However, practical progress and genuine clinical impact hinge on successful model calibration, parameter estimation and uncertainty quantification, which calls for novel innovative adaptions and methodological extensions of contemporary state-of-the-art inference techniques from Statistics and Machine Learning. In the present study, we focus on two computational fluid-dynamics (CFD) models of the blood systemic and pulmonary circulation. We discuss state-of-the-art emulation techniques based on deep learning and Gaussian processes, which are coupled with established inference techniques based on greedy optimisation, simulated annealing, Markov Chain Monte Carlo, History Matching and rejection sampling for computationally fast inference of unknown parameters of the CFD models from blood flow and pressure data. The inference task was set as a competitive challenge which the participants had to conduct within a limited time frame representative of clinical requirements. The performance of the methods was assessed independently and objectively by the challenge organisers, based on a ground truth that was unknown to the method developers. Our results indicate that for the systemic challenge, in which an idealised case of noise-free data was considered, the relative deviation from the ground-truth in parameter space ranges from 10−5% (highest-performing method) to 3% (lowest-performing method). For the pulmonary challenge, for which noisy data was generated, the performance ranges from 0.9% to 7% deviation for the parameter posterior mean, and from 35% to 570% deviation for the parameter posterior variance.

Inclined submerged diffusers that are used to dilute hypersaline and highly contaminated brine, discharged from desalination plants, in receiving marine waters are commonly modeled via semi-empirical, integral, and 3D Computational Fluid Dynamic (CFD) models. The first two models are computationally simple and efficient, but not enough accurate in many cases, and 3D-CFD models which show good agreement with experimental data are time-consuming. To avoid computational costs of 3D models and to present a more precise model than simple ones, a modified 2D-CFD model for stagnant and dynamic ambient is suggested in this study. The results showed that the proposed model can predict the jet behavior in both ambients more accurately than integral models and in shorter computing time than 3D models. The results of this study can be used in order to design environmentally friendly discharge systems by engineers and practitioners for brine or pollutant dilution in the receiving marine waters. [ABSTRACT FROM AUTHOR]

There have been impressive advances in the physical and mathematical modelling of complex physiological systems in the last few decades, with the potential to revolutionise personalised healthcare with patient-specific evidence-based diagnosis, risk assessment and treatment decision support using digital twins. However, practical progress and genuine clinical impact hinge on successful model calibration, parameter estimation and uncertainty quantification, which calls for novel innovative adaptions and methodological extensions of contemporary state-of-the-art inference techniques from Statistics and Machine Learning. In the present study, we focus on two computational fluid-dynamics (CFD) models of the blood systemic and pulmonary circulation. We discuss state-of-the-art emulation techniques based on deep learning and Gaussian processes, which are coupled with established inference techniques based on greedy optimisation, simulated annealing, Markov Chain Monte Carlo, History Matching and rejection sampling for computationally fast inference of unknown parameters of the CFD models from blood flow and pressure data. The inference task was set as a competitive challenge which the participants had to conduct within a limited time frame representative of clinical requirements. The performance of the methods was assessed independently and objectively by the challenge organisers, based on a ground truth that was unknown to the method developers. Our results indicate that for the systemic challenge, in which an idealised case of noise-free data was considered, the relative deviation from the ground-truth in parameter space ranges from 1 0 − 5 % (highest-performing method) to 3% (lowest-performing method). For the pulmonary challenge, for which noisy data was generated, the performance ranges from 0.9% to 7% deviation for the parameter posterior mean, and from 35% to 570% deviation for the parameter posterior variance. [ABSTRACT FROM AUTHOR]

A theoretical fluid mechanical model is proposed for the investigation of myocardial perfusion in healthy and stenotic conditions. The model hinges on Terzaghi's consolidation theory and reformulates the related unsteady flow equation for the simulation of the swelling-drainage alternation characterizing the diastolic-systolic phases. When compared with the outcome of experimental in vivo observations in terms of left ventricle transmural perfusion ratio (T.P.R.), the analytical solution provided by the present study for the time-dependent blood pressure and flow rate across the ventricle wall proves to consistently reproduce the basic mechanisms of both healthy and ischemic perfusion. Therefore, it could constitute a useful interpretative support to improve the comprehension of the basic hemodynamic mechanisms leading to the most common cardiac diseases. Additionally, it could represent the mathematical basis for the application of inverse methods aimed at estimating the characteristic parameters of ischemic perfusion (i.e., location and severity of coronary stenoses) via downstream ventricular measurements, possibly inspiring their assessment via non-invasive myocardial imaging techniques. [ABSTRACT FROM AUTHOR]

Three-dimensional simulations of peripheral, deep venous flow during muscular exercise in limbs of healthy subjects and in those with venous dysfunction were carried out by a computational fluid-dynamics (CFD) approach using the STAR CCM + platform. The aim was to assess the effects of valvular incompetence on the venous calf pump efficiency. The model idealizes the lower limb circulation by a single artery, a capillary bed represented by a porous region and a single vein. The focus is on a segment of the circuit which mimics a typical deep vein at the level of the calf muscle, such as the right posterior tibial vein. Valves are idealized as ball valves, and periodic muscle contractions are given by imposing time-dependent boundary conditions to the calf segment wall. Flow measurements were performed in two cross-sections downstream and upstream of the calf pump. Model results demonstrate a reduced venous return for incompetent valves during calf exercise. Two different degrees of valvular incompetence are considered, by restricting the motion of one or both valves. Model results showed that only the proximal valve is critical, with a 30% reduction of venous return during calf exercise in case of valvular incompetence: the net flow volume ejected by the calf in central direction was 0.14 mL per working cycle, against 0.2 mL for simulated healthy limbs. This finding appeared to be consistent with a 25% reduction of the calf ejection fraction, experimentally observed in chronic venous disease limbs compared with healthy limbs.

Anaerobic digesters are extensively used for the treatment of wastewaters. The impact of nanoparticles, such as silver, zinc oxide, titanium dioxide, and copper oxide within the wastewater treatment systems is recently discussed in the literature. Furthermore, in some cases, different conclusions are drawn from studies investigating the impacts of nanoparticles on the microbial activity, during the anaerobic digestion. For a detailed analysis of this system, a computational fluid-dynamic analysis can be used. In fact, the fluid-dynamic analysis is generally used to evaluate the mixing of anaerobic digesters and different mixing systems can be used: mechanical pumping, mechanical agitation, recirculation of biogas or slurry, pneumatic mixing. An important parameter to evaluate mixing inside digesters is the uniformity index. In general, fermenters for bio-hydrogen production, anaerobic digesters with perfect mixing, photo-bioreactors, bio-film reactors are studied by fluid-dynamic analysis, as shown with the reported literature work of this review. However, more studies should be developed regarding the application of nanotechnologies in anaerobic digesters to evaluate the heat and mass transfer during mixing, using the computational fluid-dynamics analysis. This can be an important research for the next future. [ABSTRACT FROM AUTHOR]

Three-dimensional simulations of peripheral, deep venous flow during muscular exercise in limbs of healthy subjects and in those with venous dysfunction were carried out by a computational fluid-dynamics (CFD) approach using the STAR CCM + platform. The aim was to assess the effects of valvular incompetence on the venous calf pump efficiency. The model idealizes the lower limb circulation by a single artery, a capillary bed represented by a porous region and a single vein. The focus is on a segment of the circuit which mimics a typical deep vein at the level of the calf muscle, such as the right posterior tibial vein. Valves are idealized as ball valves, and periodic muscle contractions are given by imposing time-dependent boundary conditions to the calf segment wall. Flow measurements were performed in two cross-sections downstream and upstream of the calf pump. Model results demonstrate a reduced venous return for incompetent valves during calf exercise. Two different degrees of valvular incompetence are considered, by restricting the motion of one or both valves. Model results showed that only the proximal valve is critical, with a 30% reduction of venous return during calf exercise in case of valvular incompetence: the net flow volume ejected by the calf in central direction was 0.14 mL per working cycle, against 0.2 mL for simulated healthy limbs. This finding appeared to be consistent with a 25% reduction of the calf ejection fraction, experimentally observed in chronic venous disease limbs compared with healthy limbs. [ABSTRACT FROM AUTHOR]

Separation of particles (solid) from hot gases were carried out with the help of cyclone separators in most of the power generation industries (fluidized bed boilers) and process industries. Present research focuses on improving the performances of such devices by modifying the geometry. Conventional 2D2D Lapple type cyclone separator geometry was modified by attaching semi-circular cross-section helical fins along the height of barrel wall to make it as finned-cyclone separators. Semi-circular helical fins attached on the barrel wall varies (six-sets) with size (5 mm, 7.5 mm, and 10 mm) and pitch (30 mm and 50 mm). 3D computational fluid dynamics simulations were carried out using ANSYS 15.0 tool and validation were performed with experimental data available in the literature for conventional 2D2D Lapple type cyclone separator before proceeding towards simulations of semi-circular finned-cyclone separators. Improved performance was noticed from all selected six cyclone separators with semi-circular fins (cswf) when compared to conventional cyclone separators while studying the fluid mechanics characteristics, which directly influences on collection-efficiency of cyclone separators. Among selected cyclone separators, the cswf with size of fin 7.5 mm and fin-pitch 50 mm was noted to be performing improved collection efficiency for the particles size <3 µm. Around 5%–11% improved collection efficiency when compared to conventional cyclone separators were observed in the study by fixing the semi-circular fins along the height of barrel wall without disturbing the main role of cyclone separator, which is separating the very fine particles, which would otherwise causes pollution. [ABSTRACT FROM AUTHOR]

• CFD multiscale model can be used together to improve the subject specific response of a determined modelling approach. • CFD models can be integrated into a clinically workflow to assess non invasively the severity of aortic coarctation in a heterogeneous cohort of patients starting from retrospective imaging data. • Good agreement between CFD models and clinical derived measurements in case of aortic coarctation assessment. We present a multi-scale CFD-based study conducted in a cohort of 11 patients with coarctation of the aorta (CoA). The study explores the potential for implementation of a workflow using non-invasive routinely collected medical imaging data and clinical measurements to provide a more detailed insight into local aortic haemodynamics in order to support clinical decision making. Our approach is multi-scale, using a reduced-order model (1D/0D) and an optimization process for the personalization of patient-specific boundary conditions and aortic vessel wall parameters from non-invasive measurements, to inform a more complex model (3D/0D) representing 3D aortic patient-specific anatomy. The reliability of the modelling approach is investigated by comparing 3D/0D model pressure drop estimation with measured peak gradients recorded during diagnostic cardiac catheterization and 2D PC-MRI flow rate measurements in the descending aorta. The current study demonstrated that the proposed approach requires low levels of user interaction, making it suitable for the clinical setting. The agreement between computed blood pressure drop and catheter measurements is 10 ± 8 mmHg at the coarctation site. The comparison between CFD derived and catheter measured pressure gradients indicated that the model has to be improved, suggesting the use of time varying pressure waveforms to further optimize the tuning process and modelling assumptions. [ABSTRACT FROM AUTHOR]

A theoretical fluid mechanical model is proposed for the investigation of myocardial perfusion in healthy and stenotic conditions. The model hinges on Terzaghi’s consolidation theory and reformulates the related unsteady flow equation for the simulation of the swelling–drainage alternation characterizing the diastolic–systolic phases. When compared with the outcome of experimental in vivo observations in terms of left ventricle transmural perfusion ratio (T.P.R.), the analytical solution provided by the present study for the time-dependent blood pressure and flow rate across the ventricle wall proves to consistently reproduce the basic mechanisms of both healthy and ischemic perfusion. Therefore, it could constitute a useful interpretative support to improve the comprehension of the basic hemodynamic mechanisms leading to the most common cardiac diseases. Additionally, it could represent the mathematical basis for the application of inverse methods aimed at estimating the characteristic parameters of ischemic perfusion (i.e., location and severity of coronary stenoses) via downstream ventricular measurements, possibly inspiring their assessment via non-invasive myocardial imaging techniques.

In this paper, the problem of developing turbulent flow in rectangular duct is investigated by obtaining numerical results of the velocity profiles in duct by using large eddy simulation model in two dimensions with different Reynolds numbers, filter equations and mesh sizes. Reynolds numbers range from (11,000) to (110,000) for velocities (1 m/sec) to (50 m/sec) with (56×56), (76×76) and (96×96) mesh sizes with different filter equations. The numerical results of the large eddy simulation model are compared with k-ε model and analytic velocity distribution and validated with experimental data of other researcher. The large eddy simulation model has a good agreement with experimental data for high Reynolds number with the first, second and third mesh sizes and the agreement increase near the wall of the duct. The percentage error for the large eddy simulation model with experimental data of the (56×56) mesh size is less than 18 % and for the (76×76) mesh size is also less than 17% and for the (96×96) mesh size is less than 16 %. The large eddy simulation model show high stability and do not need extra differential equation like the k-ε model and a great saving in time and computer memory was achieved.

Natural ventilation can significantly reduce the energy consumption of modern buildings, but robust design is challenging because of the many uncertainties involved. The present work aims to predict the 4-hour nightflush in Stanford’s Y2E2 building during 4 different nights. We employ an uncertainty quantification (UQ) framework that combines an integral model and a computational fluid-dynamics (CFD) model and propagate the uncertainty in the inputs using a polynomial chaos expansion method. The integral model solves a one-dimensional equation for the thermal mass temperature and an equation for the volume-averaged air temperature inside the building. The CFD instead solves for the three-dimensional flow and temperature field in the building, hence enabling direct computation of the heat transfer and discharge coefficients. We first performed a UQ study of the integral model and found that the measured air temperature is inside the 95% confidence interval of the mean prediction ∼80% of the times. Subsequently we employed CFD to improve the accuracy of the probability distributions for the heat transfer and discharge coefficients. In the final step we updated the integral model with these new probability distributions and showed that the standard deviation of the indoor temperature could be reduced up to ∼40%. These results demonstrate that the proposed framework can effectively quantify and reduce uncertainty in models used for the design of natural ventilation systems. [ABSTRACT FROM AUTHOR]

This paper addresses a computational fluid–dynamics study of a multi–blade inlet damper for gas turbine power plants. This device is used in the aspiration duct of the aforementioned plants with the aim to control the air mass flow rate, to dump flow turbulence and to guarantee the plant security. Nowadays the flat plate is the standard shape for damper blades. Clearly the device is rather simple but the operating conditions may prove burdensome. This is related to the vortex shedding developing behind the damper blades. If the shedding frequency of the vortices is close to the resonance frequency of the structure it generates a synchronism that involves the oscillation growth. This can lead to the structure damage. In this research work we have devised a new airfoil shaped blade for inlet dampers in order to suppress the vortex shedding phenomenon behind the damper blades. A proper numerical analysis of the damper blades pitching stability requires to solve a two-way fluid–rigid body interaction problem. Moreover the overset mesh method is strictly needed to investigate the blades motion due to aerodynamic interactions; as a matter of fact other techniques cannot be advocated for this problem since blades closure requirements produce the overlap of moving zones. For this reason we have developed a specific approach, based on the above mentioned techniques, to investigate the dynamic behaviour of the blades of a inlet damper. The obtained results show as the new blades have a very good pitching stability. [ABSTRACT FROM AUTHOR]

• The urodynamic influence of catheter in the male lower urinary tract was discussed. • Four urethral configurations and three catheter sizes were analysed. • Each catheter configuration determined a specific decrease in flow rate. • Simulations revealed the negative impact of a catheter on uroflowmetry parameters. In the field of urology, the pressure-flow study (PFS) is an essential urodynamics practise which requires the patient's transurethral catheterization during the voiding phase of micturition to evaluate the functionality of the lower urinary tract (LUT) and reveal the pathophysiology of its dysfunctionality. However, the literature evidences confusion regarding the interference of the catheterization on the urethral pressure-flow behaviour. The present research study represents the first Computational Fluid-Dynamics (CFD) approach to this urodynamics issue, analysing the influence of a catheter in the male LUT through case studies which included the inter-individual and intra-individual dependence. A set of four three dimensional (3D) models of the male LUT, different in urethral diameters, and a set of three 3D models of the transurethral catheter, diverse in calibre, were developed leading to 16 CFD non-catheterized either catheterized configurations, to describe the typical micturition scenario considering both urethra and catheter characteristics. The developed CFD simulations showed that the urine flow field during micturition was influenced by the urethral cross-sectional area and each catheter determined a specific decrease in flow rate if compared to the relative free uroflow. In-silico methods allow to analyse relevant urodynamics aspects, which could not be investigated in vivo, and may support the clinical PFS to reduce uncertainty on urodynamic diagnosis. [ABSTRACT FROM AUTHOR]

This work describes the most complete and up-to-date thermal-hydraulics CFD model for studying the natural circulation regime within the entire TRIGA Mark II reactor at the University of Pavia. Building on preliminary works, this paper tackles the treatment and investigation of natural circulation in the reactor core and pool, highlighting the coupling between the two components. Despite the assumptions behind the model – such as the use of an unsteady RANS turbulence model and the treatment of the supporting grids as a porous media – the validation compared to experimental data shows quite a good agreement. The model is thus accurate enough to describe the natural convection regime within the core and the pool and highlight local phenomena, such as localised sub-cooled boiling and recirculation. • Development of a thermal-hydraulics CFD model for the TRIGA Mark II reactor. • Detailed study of the natural circulation regime in both reactor core and pool. • Sensitivity analysis over mesh resolution and presence of the measurement rod. • Validation of both core and pool using experimental data. • Identification of core recirculation zones and localised sub-cooled boiling phenomena. [ABSTRACT FROM AUTHOR]

This paper studies Kalman filtering applied to Reynolds-Averaged Navier⁻Stokes (RANS) equations for turbulent flow. The integration of the Kalman estimator is extended to an implicit segregated method and to the thermodynamic analysis of turbulent flow, adding a sub-stepping procedure that ensures mass conservation at each time step and the compatibility among the unknowns involved. The accuracy of the algorithm is verified with respect to the heated lid-driven cavity benchmark, incorporating also temperature observations, comparing the augmented prediction of the Kalman filter with the Computational Fluid-Dynamic solution found on a fine grid.

Steam explosion may occur in a nuclear power plant by molten core-coolant interactions when the external reactor vessel cooling strategy is failed. This phenomenon can threat the integrity of reactor cavity, penetration piping and support structures. Even though extensive researches have been performed to predict influences of the steam explosion, due to complexity of physical phenomena and environmental thermal-hydraulic conditions, it is remained as one of possible hazards. The present study is primarily to examine load carrying capacity of the reactor cavity under various steam explosion conditions through a series of numerical analyses; both hydro-dynamics and computational fluid-dynamics analyses were carried out in accordance with ten postulated conditions having different failure modes, explosion locations, corium conditions and turbulence models. Subsequent finite element analyses were conducted for the reactor cavity and penetration piping with support structures to calculate stresses and displacements. From the numerical analysis results, failure modes and corium conditions were identified as crucial parameters while the effect of explosion location was negligible. Moreover, structural assessment results showed that the reactor cavity had minor to medium potentials for localized damage but sustains the function of safety barrier to avoid release of radioactive substances under the steam explosion conditions. [ABSTRACT FROM AUTHOR]

The Savonius wind turbine appears to be particularly promising for low power applications, such as those in urban areas, and it can take advantage of a simple, reliable and cost-effective construction. Anyway, the Savonius wind turbine suffers from poor performance, when compared with other wind turbines having a major overall complexity and higher costs. For this reason, several studies have been carried-out in recent years in order to improve its energy performance. The aim of the present work is to enhance the aerodynamic performance of the Savonius wind rotor following two different approaches: the first one is based on the development of innovative airfoil-shaped blades, while the second one concerns the use of a new curtain system, self-orienting relative to the wind direction. For the purpose of the present work has been applied a customized numerical model, previously validated by means of experimental data obtained at the environmental wind tunnel of the Università Politecnica delle Marche. The obtained results show that both the approaches here proposed, following two distinct guidelines for the development of new and more efficient rotor geometries, are able to produce sensible enhancements in terms of the energy performance of the Savonius wind turbine. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) is frequently used to support the design of naturally ventilated buildings; however, the model accuracy should be thoroughly assessed, ideally through validation with full-scale measurements. The present study aims to (1) validate transient CFD simulations with uncertainty quantification (UQ) for buoyancy-driven natural ventilation against full-scale experiments in an operational atrium building, and (2) quantify the sensitivity of the CFD results to the thermal boundary conditions. The UQ and sensitivity analysis consider uncertainties in the initial and boundary conditions for the temperatures. Considering the volume-averaged air temperature on each floor, the predictions and measurements agree well with discrepancies less than 0.3 °C. When considering the temperature averaged over smaller zones on each floor, two trends can be observed. First, in zones not adjacent to windows, the discrepancies between the CFD and measurements can be explained by uncertainty in the boundary conditions and the measurements. Second, in zones adjacent to windows, higher discrepancies are observed due to oscillations in the inflow jets just downstream of the windows, and due to geometrical simplifications in the CFD model. The sensitivity analysis demonstrates that the boundary conditions for the thermal mass surface temperature and the outdoor temperature have a dominant effect on the indoor air temperature predictions, with their relative importance varying as a function of proximity to the windows. • URANS of buoyancy-driven natural ventilation with coupled indoor/outdoor environment. • URANS provides accurate predictions of floor-averaged air temperature. • Small errors in regions far from windows due to uncertainty in boundary conditions. • Larger errors near windows due to unsteady effects and geometrical simplifications. • Thermal boundary conditions are key parameters in buoyancy-driven ventilation. [ABSTRACT FROM AUTHOR]

Abstract: We develop a Lattice Boltzmann code for computational fluid-dynamics and optimize it for massively parallel systems based on multi-core processors. Our code describes 2D multi-phase compressible flows. We analyze the performance bottlenecks that we find as we gradually expose a larger fraction of the available parallelism, and derive appropriate solutions. We obtain a sustained performance for this ready-for-physics code that is a large fraction of peak. Our results can be easily applied to most present (or planned) HPC architectures, based on latest generation multi-core Intel processor architectures. [Copyright &y& Elsevier]

Abstract: When compared with of other wind turbine the Savonius wind rotor offers lower performance in terms of power coefficient, on the other hand it offers a number of advantages as it is extremely simple to built, it is self-starting and it has no need to be oriented in the wind direction. Although it is well suited to be integrated in urban environment as mini or micro wind turbine it is inappropriate when high power is requested. For this reason several studies have been carried-out in recent years in order to improve its aerodynamic performance. The aim of this research is to gain an insight into the complex flow field developing around a Savonius wind rotor and to evaluate its performance. A mathematical model of the interaction between the flow field and the rotor blades was developed and validated by comparing its results with data obtained at Environmental Wind Tunnel (EWT) laboratory of the “Polytechnic University of Marche”. [Copyright &y& Elsevier]