Toshio Kobayashi

Bio: Toshio Kobayashi is an academic researcher from University of Tokyo. The author has contributed to research in topics: Turbulence & Large eddy simulation. The author has an hindex of 33, co-authored 203 publications receiving 3819 citations.

Standard images for particle-image velocimetry

Abstract: Particle-image velocimetry (PIV) offers lots of advantages for studying fluid mechanics. Many PIV techniques and systems have been developed. However, no standard evaluation tool for evaluating the effectiveness and accuracy of the PIV systems has been established. To popularize PIV practically, for each PIV system there should be some means of evaluating the performance. PIV involves two processes, i.e. capturing the image for visualization and the image analysis. In order to evaluate the image analysis, the use of standard images has been proposed. Using these images, anybody can evaluate the effectiveness and accuracy of the PIV image analysis. The standard PIV images can be grouped into three categories, i.e. standard PIV images for two-dimensional, custom-made images with tunable parameters and images for a transient flow. The standard PIV images that we have developed are distributed via the web site http://www.vsj.or.jp/piv as part of a collaboration with the Visualization Society of Japan. They can be applied to investigate the performance of any PIV technique. The standard PIV images that we have developed have already been accessed by more than 3 000 researchers around the world.

Fluid–structure Interaction Modeling of Aneurysmal Conditions with High and Normal Blood Pressures

Abstract: Hemodynamic factors like the wall shear stress play an important role in cardiovascular diseases. To investigate the influence of hemodynamic factors in blood vessels, the authors have developed a numerical fluid–structure interaction (FSI) analysis technique. The objective is to use numerical simulation as an effective tool to predict phenomena in a living human body. We applied the technique to a patient-specific arterial model, and with that we showed the effect of wall deformation on the WSS distribution. In this paper, we compute the interaction between the blood flow and the arterial wall for a patient-specific cerebral aneurysm with various hemodynamic conditions, such as hypertension. We particularly focus on the effects of hypertensive blood pressure on the interaction and the WSS, because hypertension is reported to be a risk factor in rupture of aneurysms. We also aim to show the possibility of FSI computations with hemodynamic conditions representing those risk factors in cardiovascular disease. The simulations show that the transient behavior of the interaction under hypertensive blood pressure is significantly different from the interaction under normal blood pressure. The transient behavior of the blood-flow velocity, and the resulting WSS and the mechanical stress in the aneurysmal wall, are significantly affected by hypertension. The results imply that hypertension affects the growth of an aneurysm and the damage in arterial tissues.

Fluid–structure interaction modeling of a patient-specific cerebral aneurysm: influence of structural modeling

Abstract: Fluid–structure interaction (FSI) simulations of a cerebral aneurysm with the linearly elastic and hyper-elastic wall constitutive models are carried out to investigate the influence of the wall-structure model on patient-specific FSI simulations. The maximum displacement computed with the hyper-elastic model is 36% smaller compared to the linearly elastic material model, but the displacement patterns such as the site of local maxima are not sensitive to the wall models. The blood near the apex of an aneurysm is likely to be stagnant, which causes very low wall shear stress and is a factor in rupture by degrading the aneurysmal wall. In this study, however, relatively high flow velocities due to the interaction between the blood flow and aneurysmal wall are seen to be independent of the wall model. The present results indicate that both linearly elastic and hyper-elastic models can be useful to investigate aneurysm FSI.

PIVlab – Towards User-friendly, Affordable and Accurate Digital Particle Image Velocimetry in MATLAB

Abstract: Digital particle image velocimetry (DPIV) is a non-intrusive analysis technique that is very popular for mapping flows quantitatively. To get accurate results, in particular in complex flow fields, a number of challenges have to be faced and solved: The quality of the flow measurements is affected by computational details such as image pre-conditioning, sub-pixel peak estimators, data validation procedures, interpolation algorithms and smoothing methods. The accuracy of several algorithms was determined and the best performing methods were implemented in a user-friendly open-source tool for performing DPIV flow analysis in Matlab.

Spectral fingerprints of large-scale neuronal interactions

Abstract: Cognition results from interactions among functionally specialized but widely distributed brain regions; however, neuroscience has so far largely focused on characterizing the function of individual brain regions and neurons therein. Here we discuss recent studies that have instead investigated the interactions between brain regions during cognitive processes by assessing correlations between neuronal oscillations in different regions of the primate cerebral cortex. These studies have opened a new window onto the large-scale circuit mechanisms underlying sensorimotor decision-making and top-down attention. We propose that frequency-specific neuronal correlations in large-scale cortical networks may be 'fingerprints' of canonical neuronal computations underlying cognitive processes.

Stress and Heart Rate Variability: A Meta-Analysis and Review of the Literature

Abstract: Objective Physical or mental imbalance caused by harmful stimuli can induce stress to maintain homeostasis. During chronic stress, the sympathetic nervous system is hyperactivated, causing physical, psychological, and behavioral abnormalities. At present, there is no accepted standard for stress evaluation. This review aimed to survey studies providing a rationale for selecting heart rate variability (HRV) as a psychological stress indicator. Methods Term searches in the Web of Science®, National Library of Medicine (PubMed), and Google Scholar databases yielded 37 publications meeting our criteria. The inclusion criteria were involvement of human participants, HRV as an objective psychological stress measure, and measured HRV reactivity. Results In most studies, HRV variables changed in response to stress induced by various methods. The most frequently reported factor associated with variation in HRV variables was low parasympathetic activity, which is characterized by a decrease in the high-frequency band and an increase in the low-frequency band. Neuroimaging studies suggested that HRV may be linked to cortical regions (e.g., the ventromedial prefrontal cortex) that are involved in stressful situation appraisal. Conclusion In conclusion, the current neurobiological evidence suggests that HRV is impacted by stress and supports its use for the objective assessment of psychological health and stress.

Liquid Sloshing Dynamics

Abstract: Preface Introduction 1. Fluid field equations and modal analysis in rigid containers 2. Linear forced sloshing 3. Viscous damping and sloshing suppression devices 4. Weakly nonlinear lateral sloshing 5. Equivalent mechanical models 6. Parametric sloshing (Faraday's waves) 7. Dynamics of liquid sloshing impact 8. Linear interaction of liquid sloshing with elastic containers 9. Nonlinear interaction under external and parametric excitations 10. Interactions with support structures and tuned sloshing absorbers 11. Dynamics of rotating fluids 12. Microgravity sloshing dynamics Bibliography Index.

Magnitude and Role of Wall Shear Stress on Cerebral Aneurysm Computational Fluid Dynamic Study of 20 Middle Cerebral Artery Aneurysms

Abstract: Background and Purpose—Wall shear stress (WSS) is one of the main pathogenic factors in the development of saccular cerebral aneurysms. The magnitude and distribution of the WSS in and around human middle cerebral artery (MCA) aneurysms were analyzed using the method of computed fluid dynamics (CFD). Methods—Twenty mathematical models of MCA vessels with aneurysms were created by 3-dimensional computed tomographic angiography. CFD calculations were performed by using our original finite-element solver with the assumption of Newtonian fluid property for blood and the rigid wall property for the vessel and the aneurysm. Results—The maximum WSS in the calculated region tended to occur near the neck of the aneurysm, not in its tip or bleb. The magnitude of the maximum WSS was 14.396.21 N/m 2 , which was 4-times higher than the average WSS in the vessel region (3.641.25 N/m 2 ). The average WSS of the aneurysm region (1.641.16 N/m 2 ) was significantly lower than that of the vessel region (P0.05). The WSSs at the tip of ruptured aneurysms were markedly low. Conclusions—These results suggest that in contrast to the pathogenic effect of a high WSS in the initiating phase, a low WSS may facilitate the growing phase and may trigger the rupture of a cerebral aneurysm by causing degenerative changes in the aneurysm wall. The WSS of the aneurysm region may be of some help for the prediction of rupture. (Stroke. 2004;35:2500-2505.)