FRACTAL ANALYSIS AND NUMERICAL SIMULATION ON PULSATING FLOW PATTERNS IN A THREE-DIMENSIONAL BRONCHIAL TREE

The Pebble Bed Modular Reactor (PBMR), a promising Generation IV nuclear reactor design, raises many novel technological issues for which new experience and techniques must be developed. This brief study explores a few of these issues, utilizes a computational fluid dynamics code to model some simple phenomena, and points out deficiencies in current knowledge that should be addressed by future research and experimentation. A highly simplified representation of the PBMR core is analyzed with FLUENT, a commercial computational fluid dynamics code. The applied models examine laminar and turbulent flow in the vicinity of a single spherical fuel pebble near the center of the core, accounting for the effects of the immediately adjacent fuel pebbles. Several important fluid flow and heat transfer parameters are examined, including heat transfer coefficient, Nusselt number, and pressure drop, as well as the temperature, pressure, and velocity profiles near the fuel pebble. The results of these “unit cell” calculations are also compared to empirical correlations available in the literature. As FLUENT is especially sensitive to geometry during the generation of a computational mesh, the sensitivity of code results to pebble spacing is also examined. The results of this study show that while a PBMR presents a novel and complex geometry, a code such as FLUENT is suitable for calculation of both local and global flow characteristics, and can be a valuable tool for the thermal-hydraulic study of this new reactor design. FLUENT results for pressure drop deviate from the Darcy correlation by several orders of magnitude in all cases. When determining the heat transfer coefficient, FLUENT is again much lower than Robinson’s correlation. Results for Nusselt number show better agreement, with FLUENT predicting results that are 10 or 20 times as large as those from the Robinson and Lancashire correlations. These differences may arise because the empirical correlations concern mainly integral parameters, while the FLUENT model focuses on local flow behaviors. Local phenomena are significant in the case of local heat transfer characteristics, fine temperature distribution calculations to identify hot spots, and fission product transport phenomena. All of these are important to a safety analysis of the PBMR reactor during normal operation, as well as during transient circumstances, and should be the focus of future research efforts.

Aircraft wing stall is a highly complex phenomenon as several mechanisms interact depending on global and local flow characteristics. Recent developments in numerical methods and aerodynamic modeling make possible the vision of an accurate prediction of stall in the near future. In order to reach this objective, there is a need for a high-quality experimental database for validation, including surface and flowfield measurements. To that purpose, a generic test model was specifically designed and built to provide force balance metrics as well as steady and unsteady pressure signals. In particular, the model takes into account forward, backward, and zero sweep angles while adding high-lift devices in order to modify the stall mechanism on the wing. The model was then tested at the ONERA-F2 wind tunnel, and experimental results validate the model’s intended stall characteristics and provide the needed flow data for future numerical validations. This paper describes the different design choices, the measurement techniques considered for this test campaign, and the first results obtained validating the stall processes expected for the different designed configurations.

Magnetic Resonance Imaging (MRI) techniques provide a non‐invasive method for the highly accurate anatomic depiction of the heart and vessels. Most MR‐sequences demonstrate more or less significant sensitivity to flow and motion, which can lead to artifacts in many applications. The intrinsic motion sensitivity of MRI can, however, also be used to image vessels like in phase contrast (PC) MR‐angiography or to quantify blood flow and motion of tissue. Such techniques offer the unique possibility to acquire spatially registered functional information simultaneously with the morphological data within a single experiment. Characterizations of the dynamic components of blood flow and cardiovascular function provide insight into normal and pathological physiology and have made considerable progress in recent yearsTo synchronize flow or motion sensitive measurements with periodic tissue motion or pulsatile flow, data acquisition is typically gated to the cardiac cycle and time resolved (CINE) anatomical images are collected to depict the dynamics of tissue motion and blood flow during the cardiac cycle. Visualization and quantification of blood flow and tissue motion using PC MRI has been widely used in a number of applications. In addition to analyzing tissue motion such as left ventricular function, time‐resolved 2D PC MRI techniques have proven to be useful tools for the assessment of blood flow within the cardiovascular system.Moreover, 3D spatial encoding offers the possibility of isotropic high spatial resolution and thus the ability to measure and visualize the temporal evolution of complex flow and motion patterns in a 3D‐volume. In this context, ECG synchronized and respiration controlled flow sensitive 3D MRI using 3‐directional velocity encoding (also termed ‘flow sensitive 4D MRI’) can be employed to detect and visualize global and local blood flow characteristics in targeted vascular regions (aorta, cranial arteries, carotid arteries, etc.). For the analysis and visualization of complex, three‐directional blood flow within a 3D volume, various visualization tools, including 2D vector‐fields, 3D streamlines and particle traces, have been reported. In addition more advanced data quantification strategies of directly measured (e.g. flow rates) or derived parameters (e.g. pressure difference maps, wall sheer stress, pulse wave velocity, etc.) are promising as new clinical markers for the characterization of cardiovascular disease.This lecture will provide an overview of the MR imaging principles and advanced acquisition methods, data processing, flow visualization and quantification strategies, and clinical applications of flow sensitive MRI imaging.Learning Objectives:1. Understand the basic and advanced methods for flow measurements using MRI2. Understand techniques for 3D flow visualization and quantification of blood flow and derived parameters3. Understand the issues related to clinical applications of flow‐sensitive MRI

This procedure was designed within the Hydraulic Laboratory of INCREST-INCAS, in the '80s, to provide a unique way of checking and qualification of local and global flow characteristics of the spool valve SMHR S.22.59.010, in order to be fitted to the servo SMHR assembly S 22.59.000.

uniformly highorder-accurate numerical methods for approximating the convective terms of the NavierStokes equations: the introduction of new, non-physical extrema near discontinuities. Extensive investigations have been conducted into myriad types of shock-capturing methods, which adaptively decrease their order of accuracy within shock-containing regions in order to maintain accuracy and stability over time. Inspired by such work, Grube et al. (AIAA 2007-4198) have proposed the conceptually similar idea of “shock-confining” filtering (SCF) for compressible large-eddy simulations (LES), in which a class of linear filters is subject to adaptive adjustments based on smoothness information provided by a weighted essentially non-oscillatory (WENO) method. In the current work, we utilize the turbulent flow fields produced by direct numerical simulations (DNS) of shock/isotropic-turbulence interaction to perform preliminary a priori testing of these shock-confining filters against the corresponding linear filters. Although additional study, both a priori and a posteriori, is required to verify the necessity and suitability of SCF for LES, we conclude that linear filtering can indeed create serious qualitative discrepancies in global flow characteristics, discrepancies that are avoided by the use of SCF.

Global hydrodynamic of hybrid external loop airlift reactor: Experiments and CFD modelling

Investigations in an external-loop airlift photobioreactor with annular light chambers and swirling flow

This paper aims to investigate the aerodynamics including the global performance and flow characteristics of a long-shrouded contra-rotating rotor by developing a full 3D RANS computation. Through validations by current experiments on the same shrouded contra-rotating rotor, the computation using sliding mesh method and the computational zone with an extended nozzle downstream flow field effectively works; the time-averaged solution of the unsteady computation reveals that more uniform flow presents after the downstream rotor, which implies that the rear rotor rotating at opposite direction greatly compensates and reduces the wake; the unsteady computations further explore the flow field throughout the whole system, along the span and around blade tips. Complex flow patterns including the vortices and their interactions are indicated around the blade roots and tips. For further identifying rotor configurations, the rotor–rotor distance and switching two rotor speeds were studied. The computation reveals that setting the second rotor backwards decreases the wake scale but increases its intensity in the downstream nozzle zone. However, for the effect of switching speeds, computations cannot precisely solve the flow when the rear rotor under the windmill because of the upstream rotor rotating much faster than the other one. All the phenomena from computations well implement the experimental observations.

The bronchial tree plays a main role in the human respiratory system because the air distribution throughout the lungs and gas exchange with blood occur in the airways whose dimensions vary from several centimeters to micrometers. Organization of about 60,000 conducting airways and 33 million respiratory airways in a limited space results in a complex structure. Due to this inherent complexity and a high number of airways, using target-oriented dimensional reduction is inevitable. In addition, there is no general reduced-order model for various types of problems. This necessitates coming up with an appropriate model from a variety of different reduced-order models to solve the desired problem. Lumped formulation, trumpet, or typical path model of whole or parts of bronchial tree are frequently used reduced-order models. On the other hand, using any of these models results in underestimation of flow heterogeneity leading to inaccurate prediction of the systems whose mechanisms depend on the fluid heterogeneity. In this study, a simple robust model combining mechanistic and non-mechanistic modeling approaches of the bronchial tree is proposed which overcomes the limitations of the previous reduced-order models and gives the same results of a detailed mechanistic model for the first time. This model starts from an accurate multi-branching model of conducting and respiratory airways (i.e., the base model) and suggests a proxy model of conducting airway and reduced-order model of respiratory airways based on the base model to significantly reduce computational cost while retaining the accuracy. The combination of these models suggests various reduced-order surrogate models of the human bronchial tree for different problems. The applications and limitations of each reduced-order model are also discussed. The accuracy of the proposed model in the prediction of fluid heterogeneity has been examined by the simulation of multi-breath inert gas washout because the alveolar slope is the reflection of fluid heterogeneity where the computational time decreases from 121h (using the base model) to 4.8s (using the reduced-order model). A parallel strategy for solving the equations is also proposed which decreases run time by 0.18s making the model suitable for real-time applications. Furthermore, the ability of the model has been evaluated in the modeling of asthmatic lung as an instance of abnormal lungs, and in the modeling of O2-CO2 exchange as an instance of nonlinear reacting systems. The results indicate that the proposed model outperforms previous models based on accuracy, robustness, and run time.

The goal of the study is the analytical design of the full asymmetric human bronchial tree (irregular dichotomy) for healthy patients and patients with obstructive pulmonary diseases. For this purpose, the author has derived the special analytical formulas. All surfaces of the bronchial tree are matched with the second-order smoothness (there are no acute angles or ribs). The geometric characteristics of the human bronchial tree in the pathological case are modeled by a “starry” shape of the inner structure of the bronchus; a level of the pathology is defined by two parameters: bronchus constriction level and level of distortion of the cylindrical shape of the bronchus. Closed analytical formulas allow a researcher to construct the human bronchial tree of an arbitrary complexity (up to alveoli); moreover, the parametric dependences make it possible to specify any desirable level of airway obstruction.

The thermal hydraulics simulation of the reactor coolant system for a typical three-loop pressurized water reactor was conducted based on numerical solution of Reynolds-averaged Navier-Stokes equations by commercial CFD software. This study aims to obtain the three-dimensional, global and localized flow features of the reactor coolant system. The completed model of the reactor coolant system is built including reactor pressure vessel and internals, core, steam generator, primary pump and linking pipe. The flow and the temperature have been investigated under normal steady operating condition with the full core thermal power and unbalanced operating condition with the failure of any primary pump during cold shutdown state. The local thermal hydraulic features of the reactor pressure vessel head dome, the thermal stratification in the reactor pressure vessel upper plenum and the hot legs, and the swirling flow of the primary pump are characterized to give reference to the reactor safety operation. This analysis practice provides an effective evaluation for the three-dimensional thermal hydraulics phenomena, and these encouraging results allow performing the comprehensive analysis for the reactor coolant system.

A numerical study was conducted to investigate the mechanism of laminar flow and heat transfer enhancement in an interrupted microchannel heat sink (IMCHS) with different shaped ribs at Reynolds number ranging from 100 to 900. The global flow features, heat transfer and friction for IMCHS with no ribs, rectangle ribs, triangle ribs and trapezoid ribs are detailed compared. The results show that the local heat transfer and friction performance of IMCHS with ribs show significant increase at the windward side of the ribs. Additionally, the smaller the chamfer of ribs, the larger average heat transfer and friction performance. For IMCHS with rectangle ribs, the maximum increment of Nu and f can reach to 1.81 and 2.59, respectively. Concerning the overall heat transfer performance (PEC), the trapezoid ribs show the best behavior with PEC = 1.65–1.38 at Re = 100–900.

A diameter-based reconstruction of the branching pattern of the human bronchial tree Part II. Mathematical formulation

To assess the distribution of gamma delta T-cells in the human bronchial tree; and to compare quantitatively the differences between gamma delta T-cell numbers in different parts of the airway wall in smokers and non-smokers. Full thickness bronchial wall sections were taken from 10 whole lung specimens from both smokers and non-smokers. Serial cryostat sections stained with the monoclonal antibodies CD3 and TCR delta-1 were examined with the aid of interactive image analysis to assess gamma delta T-cell numbers both in absolute terms and as a proportion of total T lymphocyte numbers. In all cases gamma delta T-cells were demonstrable throughout the airway wall. Although in absolute terms they tend to occur predominantly in the bronchial epithelium, this seems to reflect higher numbers of T lymphocytes in the epithelium in general compared with the submucosa. No genuine epitheliotropism is evident. Comparison by smoking status shows a significant increase in gamma delta T-cell numbers in the bronchial glands of smokers compared with non-smokers. gamma delta T-cells form an integral though variable component of the immunocompetent cell population of the human airway in both smokers and non-smokers. Although epitheliotropism does not exist in the bronchial tree, gamma delta T-cells seem to form a significant part of the bronchial gland inflammation associated with smoking.

Reynolds-averaged turbulence models have become one of the most important and popular techniques for practical engineering applications in aeronautics and astronautics. However, the poor performance in the prediction of flow separations restricts its application ranges due to the traditional linearity and equilibrium hypotheses that constitute the equation of Reynolds stress in turbulence modeling. In this study, an artificial neural network-based quadratic constitutive (ANN-QCR) Reynolds stress model is proposed for simulating turbulent flows with separations by using the field inversion and machine learning technique (FIML) with high-fidelity experimental data. In particular, the Reynolds stress is decomposed into linear and non-linear parts, respectively. The former is evaluated by the Spalart–Allmaras model with a correction factor imposed on the production term to account for the non-equilibrium effect, while the latter is a quadratic constitutive term with a self-calibrated factor. These correction factors are predicted by an artificial neural network (ANN) depending on the local flow features. The unified framework of FIML updates the weights of ANN-QCR directly by the gradient-based discrete adjoint method, ensuring consistency between the field inversion and the neural network training. The data-augmented ANN-QCR turbulence model is well validated through several separated turbulent flows induced by adverse pressure gradients, shock wave boundary interfaces, higher angles of attack, and higher Reynolds numbers (Re). With the optimization target at lift coefficients, the established model also improves the predictive performance in other flow quantities, such as drag coefficients and pressure distributions. In addition, this model captures the development of separation bubbles better with the increase in the angle of attack. Benefiting from the compatibility and convergence of forward simulation, the generalization capability of the present ANN-QCR model is successfully validated in various numerical simulations of separated turbulent flow problems across a wide range of attack angles and Reynolds numbers with good accuracy.