Gaku Tanaka

Spatial and temporal variation of secondary flow during oscillatory flow in model human central airways.

Axial and secondary velocity profiles were measured in a model human central airway to clarify the oscillatory flow structure during high-frequency oscillation. We used a rigid model of human airways consisting of asymmetrical bifurcations up to third generation. Velocities in each branch of the bifurcations were measured by two-color laser-Doppler velocimeter. The secondary velocity magnitudes and the deflection of axial velocity were dependent not only on the branching angle and curvature ratio of each bifurcation, but also strongly depended on the shape of the path generated by the cascade of branches. Secondary flow velocities were higher in the left bronchus than in the right bronchus. This spatial variation of secondary flow was well correlated with differing gas transport rates between the left and right main bronchus.

Murine pulmonary acinar mechanics during quasi-static inflation using synchrotron refraction-enhanced computed tomography

We visualized pulmonary acini in the core regions of the mouse lung in situ using synchrotron refraction-enhanced computed tomography (CT) and evaluated their kinematics during quasi-static inflation. This CT system (with a cube voxel of 2.8 μm) allows excellent visualization of not just the conducting airways, but also the alveolar ducts and sacs, and tracking of the acinar shape and its deformation during inflation. The kinematics of individual alveoli and alveolar clusters with a group of terminal alveoli is influenced not only by the connecting alveolar duct and alveoli, but also by the neighboring structures. Acinar volume was not a linear function of lung volume. The alveolar duct diameter changed dramatically during inflation at low pressures and remained relatively constant above an airway pressure of ∼8 cmH2O during inflation. The ratio of acinar surface area to acinar volume indicates that acinar distension during low-pressure inflation differed from that during inflation over a higher pressure range; in particular, acinar deformation was accordion-like during low-pressure inflation. These results indicated that the alveoli and duct expand differently as total acinar volume increases and that the alveolar duct may expand predominantly during low-pressure inflation. Our findings suggest that acinar deformation in the core regions of the lung is complex and heterogeneous.

Augmentation of Axial Dispersion by Intermittent Oscillatory Flow

The efficiency of axial gas dispersion during ventilation with high-frequency oscillation (HFO) is improved by manipulating the oscillatory flow waveform such that intermittent oscillatory flow occurs. We therefore measured the velocity profiles and effective axial gas diffusivity during intermittent oscillatory flow in a straight tube to verify the intermittency augmentation effect on axial gas transfer. The effective diffusivity was dependent on the flow patterns and significantly increased with an increase in the duration of the stationary phase. It was also found that the ratio of effective diffusivity to molecular diffusivity is two times greater than that in sinusoidal oscillatory flow. Moreover, turbulence during deceleration or at the beginning of the stationary phase further augments axial dispersion, with the effective diffusivity being over three times as large, thereby proving that the use of intermittent oscillatory flow effectively augments axial dispersion for ventilation with HFO.

Effect of elasticity on wall shear stress inside cerebral aneurysm at anterior cerebral artery

BACKGROUND Many numerical studies have been published with respect to about flow structures around cerebral aneurysm assuming to be rigid. Furthermore, there is little experimental research concerning aneurysm with elastic wall. Wall shear stress in elastic wall comparing with rigid wall should be clarified in experimental approach and verified in CFD. OBJECTIVE We have experimentally realized elastic aneurysm model accompanying with wall deformation. Wall shear stress was examined for both rigid and elastic aneurysm models in pulsatile flow. METHODS Effect of elasticity on wall shear stress inside aneurysm induced at the apex of anterior cerebral artery was experimentally examined by particle image velocimetry in vitro. In order to adjust the wall deformation, the pressure adjustment chamber was specially equipped outside the aneurysm wall. RESULTS Effect of elasticity on wall shear stress was noticed on the comparison with that of rigidity. Wall elasticity reduced the peak magnitude, the spatial and temporal averaged wall shear stress comparing with those of wall rigidity experimentally. These reductions were endorsed by fluid-structure interaction simulation. CONCLUSION Elastic wall comparing with rigid wall would reduce the peak magnitude, the spatial and temporal averaged wall shear stress acting on vascular wall.

Visualization of particle deposition in human nasal cavities

The location and concentration of particle deposition of pollen by filtration in the human nasal cavity were visualized in a transparent silicone nasal airway model using laser-induced fluorescence (LIF) to clarify the relationship between flow and particle deposition. The model was created from a water-soluble plaster mold fabricated by a 3D printer based on X-ray computed tomography images. The working fluid was air and the tracer particles as a substitute for cedar pollen were lycopodium powder doped with fluorescent dye (Rhodamine 6G). After particle deposition, the nasal airway model was filled with an aqueous solution of glycerin that had the same refractive index as silicone. Then, LIF was applied to illuminate the deposited particles with a YAG laser sheet. Results revealed that particle deposition in the right and left cavities was highly heterogeneous and was related to the complex flow structure in the nasal cavities.Graphical Abstract.

Repression of wall shear stress inside cerebral aneurysm at bifurcation of anterior cerebral artery by stents

The effect of a simple bare metal stent on repression of wall shear stress inside a model cerebral aneurysm was experimentally investigated by two-dimensional particle image velocimetry in vitro. The flow model simulated a cerebral aneurysm induced at the apex of bifurcation between the anterior cerebral artery and the anterior communicating artery. Wall shear stress was investigated using both stented and non-stented models to assess the simple stent characteristics. The flow behavior inside the stented aneurysm sac was unusual and wall shear stress was much smaller inside the aneurysm sac. Stent placement effectively repressed the temporal and spatial variations and the magnitude of wall shear stress. Hence, there is an effective possibility that would retard the progress of cerebral aneurysms by even simple stent.

Universality of Periodic Oscillation Induced in Side Branch of a T-Junction in Numerical Simulation

The flow instability through the side branch of a T-junction is analyzed in a numerical simulation. In a previous experimental study, the authors clarified the mechanism of fluid-induced vibration in the side branch of the T-junction in laminar steady flow through the trunk. However, in that approach there were restrictions with respect to extracting details of flow behavior such as the flow instability and the distribution of wall shear stress along the wall. Here the spatial growth of the velocity perturbation at the upstream boundary of the side branch is investigated. The simulation result indicates that a periodic velocity fluctuation introduced at the upstream boundary is amplified downstream, in good agreement with experimental result. The fluctuation in wall shear stress because of the flow instability shows local extrema in both the near and distal walls. From the numerical simulation, the downstream fluid oscillation under a typical condition has a Strouhal number of 1.05, which approximately agrees with the value obtained in experiments. Therefore, this periodic oscillation motion is a universal phenomenon in the side branch of a T-junction.

Suppression of Wall Shear Stress inside Intracranial Aneurysms by Simple Stents

The effect of a simple bare-metal stent on suppression of wall shear stress inside a spherical intracranial aneurysm model was experimentally investigated by two dimensional particle image velocimetry in vitro . The flow model simulated an intracranial aneurysm induced both at the side wall and at the apex of bifurcation between the anterior cerebral artery and the anterior communicating artery. Wall shear stress was investigated using both stented and non-stented models to assess the simple bare metal stent characteristics. The flow behavior inside the stented aneurysm sac was unusual and wall shear stress was much smaller inside the aneurysm sac. For both models, the maximum and the temporal and spatial averaged wall shear stress in the stented model is reduced by at least 50 % from those in the non-stented model. Stent placement effectively suppresses the temporal and spatial variations, and the magnitude of wall shear stress. Consequently, there is an effective possibility that would retard the progress of cerebral aneurysms by even simple stent.

Fluid Vibration Induced by High-Shear-Rate Flow in a T-Junction

For laminar flow in the side branch of a T-junction, periodic fluid vibrations occur with the Strouhal number independent of characteristic flow conditions. As the mechanics is unknown, an experiment was performed to establish the underlying cause in high-shear-rate flow. The fluid vibration appears along both the shearing separation layer and the boundary between two vortices immediately downstream of the side branch, where the shear rates are several orders larger than those further downstream. This vibration is caused by flow instability induced in two types of high-shear-rate flow confirming that is a universal phenomenon associated with the geometry of the T-junction.

Flow Pattern and Friction Coefficient of Water/Nonadecane-Particle Mixture Flow in Horizontal and Vertical Pipes

Cold heat storage utilizing night-time electricity is one of the relevant technologies for the electric load leveling. Latent heat storage system with a large number of small paraffin particles is one of the promising technologies for the cold heat storage system. Small paraffin particles are generated by nozzle injection of liquid paraffin into cold water. Direct heat transfer between the ascending particles and surrounding cold water enhances the storage of latent heat in a short time. Transportation of solid paraffin particles suspended in water should be the best way to transport cold heat, because the density of cold heat stored in water/paraffin-particle mixture is very high. The present paper aims at investigating flow patterns and pressure loss of water/nonadecane-particle mixture flowing in horizontal and vertical pipes. The inner diameter and the average diameter of the nonadecane particle were 20mm and 3.7mm, respectively. Reynolds number, Froude number and volumetric concentration of nonadecane particles were varied in the ranges of 5000 ≤ Re ≤ 80000, 1 ≤ Fr ≤ 260 and 0.02 ≤ Cv ≤ 0.25. We found the following main results: (1) Four flow patterns were observed in the horizontal flow, (a) flow with a stationary particle bed, (b) flow with a sliding particle layer (c) heterogeneous suspension flow and (d) homogeneous suspension flow. The flow pattern shifted from (a) to (d) with increasing Reynolds number. (2) Homogeneous suspension flow was observed in the vertical up-flow. (3) Homogeneous and heterogeneous suspension flow was observed in the vertical down-flow. (4) The pressure loss coefficients λ of the horizontal flow were correlated by a function of λ and Re (λ = 0.479 Re−0.311 ) for the heterogeneous and homogeneous suspension flows (Re ≥ about 25000) and by a function of the excess pressure loss coefficient Φ, Fr and Cv (φ/Cv 0.58 = 72.4Fr−1.25 ) for the flow with a sliding particle layer (Re ≤ about 20000). (5) The pressure loss coefficients of the vertical up-flow were correlated by a function of λ and Re (λ = 4.45 Re−0.501 ) in a large Reynolds number range of Re ≥ about 40000 and by a function of Φ, Fr and Cv (φ/Cv 0.47 = 282Fr−1.47 ) in a small Reynolds number range of Re ≤ about 40000. (6) The pressure loss coefficients of the vertical down-flow were correlated by a function of Φ and Fr (φ = 73.0Fr−0.765 ).© 2007 ASME