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A new methodology for CFD simulation of airflow in the human bronchopulmonary tree is presented. The new approach provides a means for detailed resolution of the flow features via three-dimensional Navier--Stokes CFD simulation without the need for full resolution of the entire flow geometry, which is well beyond the reach of available computing power now and in the foreseeable future. The method is based on a finite number of flow paths, each of which is fully resolved, to provide a detailed description of the entire complex small-scale flowfield. A stochastic coupling approach is used for the unresolved flow path boundary conditions, yielding a virtual flow geometry that allows accurate statistical resolution of the flow at all scales for any set of flow conditions. Results are presented for multigenerational lung models based on the Weibel morphology and the anatomical data of Hammersley and Olson (1992, 'Physical Models of the Smaller Pulmonary Airways,' J. Appl. Physiol., 72(6), pp. 2402-2414). Validation simulations are performed for a portion of the bronchiole region (generations 4-12) using the flow path ensemble method, and compared with simulations that are geometrically fully resolved. Results are obtained for three inspiratory flowrates and compared in terms of pressure drop, flow distribution characteristics, and flow structure. Results show excellent agreement with the fully resolved geometry, while reducing the mesh size and computational cost by up to an order of magnitude. [DOI: 10.1115/1.4001448]

Computational fluid dynamics results for synthetic turbulence generation of freestream turbulence by a proposed statistically targeted forcing (STF) method are presented. The STF method has been previously documented for homogeneous isotropic and anisotropic turbulence and formulated to introduce a fluctuating velocity field with a distribution of first and second moments that match a user-specified target mean velocity and Reynolds stress tensor, by incorporating deterministic time-dependent forcing terms into the momentum equation for the resolved flow. This study extends its applicability to generation of freestream turbulence in scale-resolving simulations in which flow is spatially developing. The method provides flexibility in regions where synthetic turbulence needs to be generated or damped, for use in engineering level scale-resolving simulations such as large eddy simulation (LES), and hybrid Reynolds-averaged Navier–Stokes (RANS)-LES. The objective of this study is to evaluate the performance of the proposed STF method in simulations that incorporate monotonically integrated LES (MILES), Smagorinsky (SMAG) LES subgrid stress model, and dynamic hybrid RANS-LES (DHRL) model in reproducing inflow freestream turbulent flow, and the capability of each model to reproduce proper energy decay characteristics downstream of forcing. Results are inter-rogated and compared to target statistical velocity and turbulent stress distributions for inflow turbulence and evaluated in terms of energy spectra. Analysis of the influence of STF model parameters, mesh resolution, and LES subgrid stress model on the results is investigated. Results show that the STF method can successfully reproduce desired statistical distributions in a nearly isotropic freestream turbulent flow. [ABSTRACT FROM AUTHOR]

This paper investigates the ability of computational fluid dynamics (CFD) simulations to accurately predict the turbulent flow separating from a three-dimensional (3D) axisymmetric hill using a recently developed four-equation eddy-viscosity model (EVM). The four-equation model, denoted as k-kL-ω-v2, was developed to demonstrate physically accurate responses to flow transition, streamline curvature, and system rotation effects. The model was previously tested on several two-dimensional cases with results showing improvement in predictions when compared to other popularly available EVMs. In this paper, we present a more complex 3D application of the model. The test case is turbulent boundary layer flow with thickness δ over a hill of height 2δ mounted in an enclosed channel. The flow Reynolds number based on the hill height (ReH) is 1.3×105. For validation purposes, CFD simulation results obtained using the k-kL-ω-v2 model are compared with two other Reynolds-averaged Navier-Stokes (RANS) models (fully turbulent shear stress transport k-ω and transition-sensitive k-kL-ω) and with experimental data. Results obtained from the simulations in terms of mean flow statistics, pressure distribution, and turbulence characteristics are presented and discussed in detail. The results indicate that both the complex physics of flow transition and streamline curvature should be taken into account to significantly improve RANS-based CFD predictions for applications involving blunt or curved bodies in a low Re turbulent regime. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) simulations were performed using large-scale models of the human lung airway and unsteady periodic breathing conditions. The computational domain included fully coupled representations of the orotracheal region and large conducting zone up to generation four (G4) obtained from patient-specific CT data, an the small conducting zone (to the 16th generation) obtained from a stochastically generated airway tree with statistically realistic morphological characteristics. A reduced geometry airway model was used, in which several airway branches in each generation were truncated, and only select flow paths were retained to the 16th generation. The inlet and outlet flow boundaries corresponded to the oral opening, the physical inlet/outlet boundaries at the terminal bronchioles, and the unresolved airway boundaries create from the truncation procedure. The total flow rate was specified according to the expected ventilation pattern for a healthy adult male, which was supplied by the whole body modeling software HumMod. The unsteady mass flow distribution at the distal boundaries was prescribed based on a preliminary steady-state simulation with an applied low rate equal to the average flow rate during the inhalation phase of the breathing cycle. In contrast to existing studies, this approach allows fully coupled simulation of the entire conducting zone, with no need to specify distal mass flow or pressure boundary conditions a priori, and without the use of impedance or one-dimensional (ID) flow models downstream of the truncated boundaries. The results show that: (1) physiologically realistic flow is obtained in the model, in terms of cyclic mass conservation an approximately uniform pressure distribution in the distal airways; (2) the predicted alveolar pressure is in good agreement with correlated experimental data; and (3) the use o reduced-order geometry modeling allows accurate and efficient simulation of large-scale breathing lung flow, provided care is taken to use a physiologically realistic geometry and to properly address the unsteady boundary conditions. [ABSTRACT FROM AUTHOR]

A physics-based modification to the kT-kL-ω transition-sensitive eddy-viscosity model is presented. The modification corrects an anomaly related to the physical mechanism of production of laminar kinetic energy for regions far from the wall in fully turbulent flows, by limiting the production of natural modes in the large-scale eddy-viscosity term by a rescale of the wall-limited turbulent length scale. Round jet and backward facing step test cases are used to reveal the relevant issue and to demonstrate that the new modification successfully addresses the problem. [ABSTRACT FROM AUTHOR]