Yoko Takakura

Oscillation pattern of parachute and concave body

In this study, the experiment for the oscillation of a parachute after inflation process is done at low Reynolds number. Especially, effects of the payload motion to the canopy (parachute) motion are focused. A solid concave body is set in a vertical water tunnel which is driven by siphon’s low. Here, the uniform flow direction is opposed to the gravity direction in order to simulate the real parachute vertical drop. Four types of supporting system are applied to investigate the effects of the payload motion. There are differences about degree of freedom of payload between four type tests. The displacement of the object is measured by using CCD camera motion analyzer, and experimental data is processed by a short-time Fourier transform(STFT). Results are summarized as follows. The degree of freedom of payload affects on the concave body motion significantly. The oscillation pattern of the concave body changes by time, those are linear, arc, and circular motion.

In Vitro Study on Branch Flows with a Cerebral Aneurysm

The purpose of this study is to accumulate data to predict the ruptures of aneurysms on the bifurcation of the middle cerebral arteries at the base of the brain. Particular stress is laid on understanding the elemental nature of branch flows with/without an aneurysm. Therefore, “flow patterns” and “wall shear stress”, which are important factors for the causes of ruptures, are investigated by the three-dimensional experiments in vitro and the two-dimensional numerical simulations with simplified models. In the branch arteries without an aneurysm, there is a possibility of growing aneurysms at the location slightly away from a stagnation point. If an aneurysm forms into a centrosymmetric shape for the inlet axis, it is considered that they tend to grow further in a symmetrical plane. From the viewpoint of the risk of ruptures, recirculation flows become problematic with the lower Reynolds number, while the influence of wall shear stress becomes larger with the higher Reynolds number.

Numerical Simulation of Flows in a Pipe with an Aneurismal Sac (Effects of Aneurismal Models and Stents)

Recently, the intravascular therapy using microcoils and stents to treat aneurysms has attracted researcher’s interest. In this study, in order to evaluate the effects of the stents, a numerical simulation of two-dimensional flows has been carried out for a pipe with a model of an aneurismal sac. Using aneurismal models with different inclined angles to the pipe, inflow conditions with steady states or pulsations have been applied in the range of Reynolds number in human blood flows. First, the computational results are compared with experiments under the steady inflow condition, which has shown the reliability of the numerical simulation. Furthermore, the mechanism of flows with an aneurismal model is discussed in the case with or without a stent, and consequently the effect of the stent is clarified.

Flowfield around a Body with Elastic Walls

When a bluff body is located in a uniform flow, the flow is separated and vortices are formed. Consequently, the vortices cause “flow-induced vibrations”. Especially, if the Strouhal number and the frequency of the body oscillation coincide with the natural frequency, the lock-in regime will occur and we could find the large damages on it. Therefore, it is profitable, in engineering problems, to clarify this phenomenon and to suppress the vibration, in which the effect of elastic walls on the suppression is focused. Then, the aims of this article are to clarify the oscillatory characteristics of the elastic body and the flowfield around the body by numerical simulations, in which a square pillar with elastic walls is set in a uniform flow. Two dimensional incompressible flows are solved by the continuity equation, Navier-Stokes equation and the Poisson equation which are derived by taking divergence of Navier-Stokes equation. Results show that a small deformation of elastic walls has a large influence on the body motion. In particular, the effect is very distinct at the back.

Numerical Study on Flow-Adaptive Stents

Recently for the treatment of aneurysms, endovascular therapy with microcoils and stents has started. This study explores the design of better stents by means of numerical computations from the viewpoint of the fluid mechanics. Two-dimensional flows are numerically solved for a stented duct with a model of an aneurysmal sac by changing the distribution of stent filaments under the constraint of a constant porosity for the neck. Stents are assessed by whether the wall shear stress (WSS) on the aneurismal wall and the shear rate (SR) within the aneurysm are made lower. Barometers for the allocation of filaments are sought, and resultant optimized stents are those where filament(s) should be attached to both the distal and proximal wall of the neck, with more filaments to the distal wall, to make the WSS low, and filaments should be appropriately distributed in the off-wall portion of the neck to make the SR low.

Study on Dynamic Characteristics of the 3-Dimensional Parachute-Like Body with Consideration of Virtual Mass

In this article the freely oscillating 3-dimensional parachute-like body sustained at one point is focused. To catch the real phenomenon by numerical simulation, the virtual mass is considered in this study. The flow field is calculated by using the MAC method applying third order upwind scheme for the convex term, and applying central difference for the other terms. ALE method is applied in order to consider a moving boundary problem. The virtual mass is calculated by using a potential flow at every time step. As a result, it has been made clear that the virtual mass has influence on the freely oscillating rigid concave body motion. And the flow structure around the freely oscillating parachute-like body is made clear.

Flow in a tube with an aneurysmal sac: effect of aneurysm and stent.

Ruptured saccular aneurysms were severe condition for intracerebral Condition. And the cause for their rupture is not yet clear. In this study (I), since the configurations of aneurysms are considered to be a factor for the rupture of aneurysms, several shapes has been modeled using Aspect ratio AR and inclination angle of saccular aneurysms. In vitro the parametric study has been conducted on the range of Reynolds Number in human blood flow for aneurismal models of AR=2.1 and 1.3 and q =90° and 70°, As results, it was con firme d that there are characteristic flow patterns with Reynolds Number, And that the aneurismal configuration has effects on the shear stress and pressure losses. II) The object of this paper is to study the effects of STENTS. We made the model of aneurysm and performed in vitro study in range of Reynolds number of human blood flows using three kinds of STENTS. As results, it was confirmed that flow pattern and pressure loss changes with the kinds of STENTS. This study aims the accumulation of data to predict the hazard of aneurysmal rupture by their shapes and STENTS.