Ching-Long Lin

The airflow in a subject-specific breathing human lung is simulated with a multiscale computational fluid dynamics (CFD) lung model. The three-dimensional (3D) airway geometry beginning from the mouth to about 7 generations of airways is reconstructed from the multi-detector row computed tomography (MDCT) image at the total lung capacity (TLC). Along with the segmented lobe surfaces, we can build an anatomically-consistent one-dimensional (1D) airway tree spanning over more than 20 generations down to the terminal bronchioles, which is specific to the CT resolved airways and lobes (J Biomech 43(11): 2159-2163, 2010). We then register two lung images at TLC and the functional residual capacity (FRC) to specify subject-specific CFD flow boundary conditions and deform the airway surface mesh for a breathing lung simulation (J Comput Phys 244:168-192, 2013). The 1D airway tree bridges the 3D CT-resolved airways and the registration-derived regional ventilation in the lung parenchyma, thus a multiscale model. Large eddy simulation (LES) is applied to simulate airflow in a breathing lung (Phys Fluids 21:101901, 2009). In this fluid dynamics video, we present the distributions of velocity, pressure, vortical structure, and wall shear stress in a breathing lung model of a normal human subject with a tidal volume of 500 ml and a period of 4.8 s. On exhalation, air streams from child branches merge in the parent branch, inducing oscillatory jets and elongated vortical tubes. On inhalation, the glottal constriction induces turbulent laryngeal jet. The sites where high wall shear stress tends to occur on the airway surface are identified for future investigation of mechanotransduction.

We present a registration algorithm that can handle the discontinuity of deformation with an ultimate goal to investigate how pulmonary lobes deform to accommodate chest wall shape changes. We first show that discontinuities can exist in both normal and tangent directions. Such discontinuities are accounted for by a spatially varying diffusive regularization which restricts smoothing inside objects. Meanwhile, a distance term is combined with the sum of squared intensity differences (SSD) to explicitly match corresponding interfaces and intensity patterns. The capability of this new method is demonstrated using two-dimensional (2-D) synthetic examples with complete or incomplete “fissures” and three-dimensional (3-D) computed tomography (CT) lung datasets.

In this paper, a new nonrigid image registration method is presented to align two volumetric lung CT datasets with an application to estimate regional ventilation. Instead of the sum of squared intensity difference (SSD), we introduce the sum of squared tissue volume difference (SSTVD) as the similarity criterion to take into account the variation of intensity due to respiration. This new criterion aims to minimize the local difference of tissue volume inside the lungs between two images scanned in the same session or over short periods of time, thus preserving the tissue weight of the lungs. Our approach is tested using a pair of volumetric lung datasets acquired at 15% and 85% of vital capacity (VC) in a single scanning session. The results show that the new SSTVD predicts a smaller registration error and also yields a better alignment of structures within the lungs than the normal SSD similarity measure. In addition, the regional ventilation derived from the new method exhibits a much more improved physiological pattern than that of SSD.

Stable Xenon (Xe) gas has been used as an imaging agent for decades in its radioactive form, is chemically inert, and has been used as a ventilation tracer in its non radioactive form during computerized tomography (CT) imaging. Magnetic resonance imaging (MRI) using hyperpolarized Helium (He) gas and Xe has also emerged as a powerful tool to study regional lung structure and function. However, the present state of knowledge regarding intra-bronchial Xe and He transport properties is incomplete. As the use of these gases rapidly advances, it has become critically important to understand the nature of their transport properties and to, in the process, better understand the role of gas density in general in determining regional distribution of respiratory gases. In this paper, we applied the custom developed characteristic-Galerkin finite element method, which solves the three-dimensional (3D) incompressible variable-density Navier-Stokes equations, to study the transport of Xe and He in the CT-based human lung geometries, especially emulating the washin and washout processes. The realistic lung geometries are segmented and reconstructed from CT images as part of an effort to build a normative atlas (NIH HL-064368) documenting airway geometry over 4 decades of age in healthy and disease-state adult humans. The simulation results show that the gas transport process depends on the gas density and the body posture. The implications of these results on the difference between washin and washout time constants are discussed.

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free surface motion is simulated using a level set method. Two subgrid scale models, namely dynamic Smagorinsky and dynamic two-parameter models, are employed. The present numerical predictions of mean flow field and turbulence statistics are in good agreement with experimental data. Streaky structures are observed in the wall layer after flow reattachment. Quadrant two events dominate near-wall and near-surface motions. Coherent structures are produced behind the dune crest by strong shear layer riding over the recirculation zone. These tube-like vortical structures are transported downstream with the mean flow and most are destroyed before arriving at the next crest.

Investigations of flow over sand dunes are too numerous to list here. However, Table 1 gives a summary of some recent experimental studies of flow over fixed dune-like disturbances in open channels. All of them have used LDV to explore various facets of the flow. The present investigation was motivated by the need for a more complete picture of the flow than is available from these previous studies.