428 research outputs found

    Reports about 8 selected benchmark cases of model hierarchies : Deliverable number: D5.1 - Version 0.1

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    Based on the multitude of industrial applications, benchmarks for model hierarchies will be created that will form a basis for the interdisciplinary research and for the training programme. These will be equipped with publically available data and will be used for training in modelling, model testing, reduced order modelling, error estimation, efficiency optimization in algorithmic approaches, and testing of the generated MSO/MOR software. The present document includes the description about the selection of (at least) eight benchmark cases of model hierarchies.EC/H2020/765374/EU/Reduced Order Modelling, Simulation and Optimization of Coupled Systems/ROMSO

    Novel Algorithms for Merging Computational Fluid Dynamics and 4D Flow MRI

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    Time-resolved three-dimensional spatial encoding combined with three-directional velocity-encoded phase contrast magnetic resonance imaging (termed as 4D flow MRI), can provide valuable information for diagnosis, treatment, and monitoring of vascular diseases. The accuracy of this technique, however, is limited by errors in flow estimation due to acquisition noise as well as systematic errors. Furthermore, available spatial resolution is limited to 1.5mm - 3mm and temporal resolution is limited to 30-40ms. This is often grossly inadequate to resolve flow details in small arteries, such as those in cerebral circulation. Recently, there have been efforts to address the limitations of the spatial and temporal resolution of MR flow imaging through the use of computational fluid dynamics (CFD). While CFD is capable of providing essentially unlimited spatial and temporal resolution, numerical results are very sensitive to errors in estimation of the flow boundary conditions. In this work, we present three novel techniques that combine CFD with 4D flow MRI measurements in order to address the resolution and noise issues. The first technique is a variant of the Kalman Filter state estimator called the Ensemble Kalman Filter (EnKF). In this technique, an ensemble of patient-specific CFD solutions are used to compute filter gains. These gains are then used in a predictor-corrector scheme to not only denoise the data but also increase its temporal and spatial resolution. The second technique is based on proper orthogonal decomposition and ridge regression (POD-rr). The POD method is typically used to generate reduced order models (ROMs) in closed control applications of large degree of freedom systems that result from discretization of governing partial differential equations (PDE). The POD-rr process results in a set of basis functions (vectors), that capture the local space of solutions of the PDE in question. In our application, the basis functions are generated from an ensemble of patient-specific CFD solutions whose boundary conditions are estimated from 4D flow MRI data. The CFD solution that should be most closely representing the actual flow is generated by projecting 4D flow MRI data onto the basis vectors followed by reconstruction in both MRI and CFD resolution. The rr algorithm was used for between resolution mapping. Despite the accuracy of using rr as the mapping step, due to manual adjustment of a coefficient in the algorithm we developed the third algorithm. In this step, the rr algorithm was substituded with a dynamic mode decomposition algorithm to preserve the robustness. These algorithms have been implemented and tested using a numerical model of the flow in a cerebral aneurysm. Solutions at time intervals corresponding to the 4D flow MRI temporal resolution were collected and downsampled to the spatial resolution of the imaging data. A simulated acquisition noise was then added in k-space. Finally, the simulated data affected by noise were used as an input to the merging algorithms. Rigorous comparison to state-of-the-art techniques were conducted to assess the accuracy and performance of the proposed method. The results provided denoised flow fields with less than 1\% overall error for different signal-to-noise ratios. At the end, a small cohort of three patients were corrected and the data were reconstructed using different methods, the wall shear stress (WSS) was calculated using different reconstructed data and the results were compared. As it has been shown in chapter 5, the calculated WSS using different methods results in mutual high and low shear stress regions, however, the exact value and patterns are significantly different

    Intermittency and Self-Organisation in Turbulence and Statistical Mechanics

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    There is overwhelming evidence, from laboratory experiments, observations, and computational studies, that coherent structures can cause intermittent transport, dramatically enhancing transport. A proper description of this intermittent phenomenon, however, is extremely difficult, requiring a new non-perturbative theory, such as statistical description. Furthermore, multi-scale interactions are responsible for inevitably complex dynamics in strongly non-equilibrium systems, a proper understanding of which remains a main challenge in classical physics. As a remarkable consequence of multi-scale interaction, a quasi-equilibrium state (the so-called self-organisation) can however be maintained. This special issue aims to present different theories of statistical mechanics to understand this challenging multiscale problem in turbulence. The 14 contributions to this Special issue focus on the various aspects of intermittency, coherent structures, self-organisation, bifurcation and nonlocality. Given the ubiquity of turbulence, the contributions cover a broad range of systems covering laboratory fluids (channel flow, the Von Kármán flow), plasmas (magnetic fusion), laser cavity, wind turbine, air flow around a high-speed train, solar wind and industrial application

    Large Eddy Simulation of Transitional Separated-Reattached Flow over Geometries Characterized by Different Aspect Ratios and with Different Intensities of Free Stream Turbulence

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    In the current study, changes in the physics of transitional separated-reattached flow due to changes of a geometry nature and an increase of intensity of free stream turbulence have been investigated numerically using the large eddy simulation approach. Numerical simulations have been carried out using the Open FOAM tool box. Six case studies are selected and divided into two groups of the flows: a low level of intensity of free stream turbulence (< 0.2%) and a high level of intensity of free stream turbulence (3.7%). Each group involves three geometrical shapes: a two-dimensional flat plate, a three-dimensional geometry with an aspect ratio value of 1 and a three-dimensional geometry with an aspect ratio value of 2. To the best of the author’s knowledge, the current study is the first work to explore transitional separated-reattached flow over three-dimensional geometries. In a comparison among the case studies, the separation bubble that formed on the flat plate is longer than that on other geometries, leading to longer temporal and spatial evolution of the transition. In addition, maximum values of the Reynolds stresses in the flat plate are larger than that in other geometries. Furthermore, all case studies show that the transition in the free shear layer is driven by the Kelvin-Helmholtz instability mechanism. Spectral analysis is carried out to cover all the computational domains employing both Fourier transform and wavelet power transform methods. In the current geometries for both incoming flows (with high and low levels of intensity of free stream turbulence), the regular shedding frequencies are in a good agreement with that reported in the literature. In addition, these frequencies are compatible with the Kelvin-Helmholtz instability conditions. Moreover, the spectral analysis indicates that the low frequency of the free shear layer flapping is absent. The evolution of coherent structures is identified by performing flow visualisation techniques. Different evolution processes of transformation of large-scale structures from Kelvin-Helmholtz rolls to hairpin structures are observed depending on the geometry shapes and on the level of intensity of free stream turbulence. The development of the turbulent boundary layer after the reattachment is also examined. For all case studies used here, a dominant observation is that there is no apparent effect of the geometry nature on the delay in the recovery of the reattached turbulent boundary layer.Iraq Governmen

    Computational Aerodynamics and Anatomical Characterization of Laryngotracheal Stenosis in Children

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    Laryngotracheal stenosis (LTS) is a health condition in which an obstruction in the upper trachea can cause breathing difficulties and increased incidence of infection, among other symptoms. Occurring most commonly due to intubation in infants, LTS often requires corrective surgery. Currently, clinical methods of assessing the blockage region are simplistic and subjective, and it is challenging to determine the most effective surgical strategy for any given patient. In the present work, a comprehensive methodology is proposed for characterizing the stenosis region both in terms of its anatomical parameters and its corresponding aerodynamic properties. The combination of computational fluid dynamics (CFD) and medical imaging provides a non-invasive method for establishing these relationships between anatomy and aerodynamics. As software packages for such analyses have matured in recent years, computational approaches to solving medical problems have gained more widespread appeal. While patient-specific CFD has gained recent popularity, the approach presented in the present work aims to generate parametric correlations which may be applied to any member of a class of patients. These correlations may be used clinically to provide data-driven recommendations for surgical procedures

    Large eddy simulation of separated boundary layer transition under free-stream turbulence

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    Physics of laminar-to-turbulent transition in a separated-reattached flow subjected to two free-stream turbulence levels have been explored using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a flat plate with a semi-circular leading edge. A numerical trip has been used to generate the targeted free-stream turbulence levels. A dynamic Sub-grid-scale (SGS) model has been employed and excellent agreement has been achieved between the LES results and the experimental data. Detailed investigation of the LES data has been carried out to explore the primary instability mechanism at low (< 0.2%) and high free-stream turbulence (5.6%). The flow visualisations and spectral analysis of the separated shear layer reveal that the two-dimensional Kelvin-Helmholtz instability mode, well known to occur at low free-stream turbulence levels, is bypassed at a higher level leading to earlier breakdown to turbulence. The whole transition process leading to breakdown to turbulence has been revealed clearly by the flow visualisations and the differences between the low and high free-stream turbulence cases are clearly evident. Coherent structures are also visualised using iso-surfaces of the Q-criterion and for the high free-stream turbulence case the spanwise oriented two-dimensional rolls, which are clearly apparent in the low free-stream turbulence case, are not visible anymore. Detailed quantitative comparisons between the present LES results against experimental data and the previous LES results at low free-stream turbulence using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can predict transitional flows accurately. Comprehensive spectral analysis of the separated shear layer at two free-stream turbulence levels has been performed. Under very low free-stream turbulence condition, a distinct regular vortex shedding and trace of the low-frequency flapping phenomena were detected. Under the higher free-stream turbulence however, a mild high-frequency activity was observed. No low frequency oscillations could be detected
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