Oscillation of the vocal folds makes a sound source of the human voiced sound. Understanding of the oscillation mechanism, which is a complex flow-structure interaction problem in the airway, is crucial for considering clinical diagnosis of voice disorders. However, details of the oscillation mechanism are still unclear partly because, from a fluid mechanical viewpoint, the effect of oscillation of the vocal fold wall during the phonation on airflow behaviors remains elusive. In the present study, flow characteristics in a sinusoidally-oscillating constriction mimicking the vocal fold were investigated by numerical and experimental approaches. The numerical analyses focused in particular on the effect of constriction oscillation on flow separation demonstrated that the flow separation point moves continuously and periodically in a frequency-dependent manner. In the experimental study, an apparatus was newly designed, with a view to detect the oscillation-induced movement of the flow separation point, to enable detailed measurement of pressure distribution along the constriction with an interval of 2 mm that is synchronized with measurement of constriction displacement. Although movement of the separation point as seen in the numerical analyses was not detected by this limiting resolution of the apparatus, we obtained pressure-width relations that is partly contrary to the numerical results but is presumably dependent on the inlet boundary condition. These findings indicate that appropriate evaluations of separation point and inlet boundary conditions are key factors to characterize the flow in oscillating constriction, which is crucial for better understanding of the vocal fold mechanics.
Diffuse axonal injury (DAI) is a specific type of closed head injury often seen in automobile accidents, that directly leads to the morbidity and mortality, however, the injury mechanism of DAI has yet to be clarified. DAI is characterized by structural and functional damage in nerve fibers in the white matter, which may be caused by excessive tensile strain. While the white matter has a network-like structure of nerve fibers embedded in neuroglia and the extracellular matrix, the nerve fibers are undulated and the mechanical properties of these components are not necessarily equal. Thus, the strain in the white matter can be different from that in the fibers. In this study, we have measured stretch ratios of the nerve fibers running in various directions in porcine brain tissue subjected to uniaxial stretch and compared them with global strain. It was found that the fiber direction positively correlated with neural fiber strain whilst the fiber strain was not equal to global strain. Particularly, the maximum neural fiber strain was ∼25% of its surrounding tissue strain, indicating that the local strain in the neural fibers is not equal to global strain in the brain tissue. Consideration of neural fiber alignment in the white matter is important in studying the mechanical aspects of pathogenesis of DAI.
Since the cortical bone has higher elastic modulus compared to the cancellous bone, its geometry is very important for stress analysis of bone structure. During finite element modeling of bone structure, cortical bone is generally determined as the region having higher CT values with respect to specific threshold value. However, it is difficult to determine the thin cortical bone regions by considering a specific threshold value. This study proposes a method to select regions of cortical bone from clinical CT images by considering CT value distributions of cortical and cancellous bone. Applying the method to bovine proximal femur, the mean error in cortical thickness compared to the actual bone was found to be less than one pixel (0.39×0.39 mm). Hence, the proposed method could accurately determine the cortical bone regions from clinical CT images. The method was also applied to develop a finite element model with the precise cortical bone structure.
Cardiovascular disease is the one of most important diseases for human in the developed countries and is responsible for millions of deaths and disabilities every year. In cardiovascular biomechanics, the fluid-structure interaction within large blood vessel is required to understand the aortic wall tear, aortic dissection and so on. Strongly-coupled methods yield the resolution of a nonlinear problem on the fluid-structure interface, which may be very time-consuming. A loosely coupled method was used to study the complex mechanical interaction under steady flow and pulsatile flow in a three-layered aortic arch model. The results showed the impact of steady flow and pulsatile flow, the variations of wall stress along arch portion, and wall stress distribution in three-layered wall.