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The phase relationship between the streamwise and the wall-normal velocity disturbances induced by a traveling-wave-like blowing or suction control [T. Min, J. Fluid Mech. 558, 309 (2006)] in a two-dimensional laminar Poiseuille flow is investigated. The investigation is done by solving the linearized Navier-Stokes equation and by using the identity equation between the skin-friction drag and the Reynolds shear stress [K. Fukagata, Phys. Fluids 14, L73 (2002)]. It has been known that a traveling wave creates a nonquadrature between the velocity disturbances and generates the positive phase shift of the streamwise velocity disturbance in the case of a skin-friction drag reduction. The present analysis further reveals that this nonquadrature consists of an inviscid base phase relationship and a near-wall phase shift induced by the viscosity. The analogy between the present control and Stokes' second problem is discussed. The thickness of the near-wall region in which the viscous phase shift takes place is found to be scaled similarly to the Stokes' second problem.

PurposeCoil embolization is a minimally invasive method used to treat cerebral aneurysms. Although this endovascular treatment has a high success rate, aneurysmal re-treatment due to recanalization remains a major problem of this method. The purpose of this study was to determine a combined parameter that can be useful for predicting aneurysmal re-treatment due to recanalization.MethodsPatient-specific geometries were used to retrospectively analyze the blood flow for 26 re-treated and 74 non-retreated aneurysms. Post-operatively aneurysms were evaluated at 12-month follow-up. The hemodynamic differences between the re-treatment and non-retreatment aneurysms were analyzed before and after coil embolization using computation fluid dynamics. Basic fluid characteristics, rates of change, morphological factors of aneurysms and patient-specific clinical information were examined. Multivariable analysis and logistic regression analysis were performed to determine a combined parameter—re-treatment predictor (RP).ResultsAmong examined hemodynamic, morphological, and clinical parameters, slight reduction of blood flow velocity rate in the aneurysm, slight increase of pressure rate at the aneurysmal neck and neck area, and hypertension were the main factors contributing to re-treatment. Notably, hemodynamic parameters between re-treatment and non-retreatment groups before embolization were similar: however, we observed significant differences between the groups in the post-embolization average velocity and the rate of reduction in this velocity in the aneurysmal dome.ConclusionsThe combined parameter, RP, which takes into consideration hemodynamic, morphological, and clinical parameters, accurately predicts aneurysm re-treatment. Calculation of RP before embolization may be able to predict the aneurysms that will require re-treatment.

A series of direct numerical simulations of a fully developed turbulent channel flow controlled by traveling waves induced by blowing and suction is performed. Relaminarization, i.e., the transition from turbulent flow to laminar flow, is observed for some sets of parameter when the wave is traveling in the downstream direction. Since the downstream traveling wave produces the drag, the drag of the flow is slightly larger than the corresponding laminar flow. A parametric study is performed, and reveals that the range of control parameters that produce relaminarization are the wave speed and amplitude of the wave which scale with the mean bulk flow rate corresponding to laminar flow and the wavelength which scales with the viscous scale. When relaminarization occurs, the amplitude of the wave, wavelength, and wave speed are in the range of \documentclass[12pt]{minimal}\begin{document}$a/\overline{u}_{\rm lam}>0.1$\end{document}a/u¯ lam >0.1, 200 < λ+ < 500, and \documentclass[12pt]{minimal}\begin{document}$c/\overline{u}_{\rm lam}>1.5$\end{document}c/u¯ lam >1.5, respectively. These ranges are organized by displacement thickness and are between 3 and 10 wall units when the relaminarization occurs. A three-component decomposition is used to observe the effects of the control parameters. The periodic component has the effect of decreasing the random component, resulting in relaminarization. When the displacement thickness is smaller, the periodic component does not have the effect of decreasing the random component, and the drag is virtually unchanged. When the displacement thickness is larger, the periodic component produces large drag, and the drag increases despite the decrease in the random component.

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BackgroundAlthough flow diversion is a promising procedure for the treatment of aneurysms, complications have been reported and it remains poorly understood. The occurrence of adverse outcomes is known to depend on both the mechanical properties and flow reduction effects of the flow diverter stent.ObjectiveTo clarify the possibility of designing a flow diverter stent considering both hemodynamic performance and mechanical properties.Materials and methodsComputational fluid dynamics (CFD) simulations were conducted based on an ideal aneurysm model with flow diverters. Structural analyses of two flow diverter models exhibiting similar flow reduction effects were performed, and the radial stiffness and longitudinal flexibility were compared.ResultsIn CFD simulations, two stents–Pore2-d35 (26.77° weave angle when fully expanded, 35 μm wire thickness) and Pore3-d50 (36.65°, 50 μm respectively)–demonstrated similar flow reduction rates (68.5% spatial-averaged velocity reduction rate, 85.0% area-averaged wall shear stress reduction rate for Pore2-d35, and 68.6%, 85.4%, respectively, for Pore3-d50). However, Pore3-d50 exhibited greater radial stiffness than Pore2-d35 (40.0 vs 21.0 mN/m at a 3.5 mm outer diameter) and less longitudinal flexibility (0.903 vs 0.104 N·mm bending moments at 90°). These measurements indicate that changing the wire thickness and weave angle allows adjustment of the mechanical properties while maintaining the same degree of flow reduction effects.ConclusionsThe combination of CFD and structural analysis can provide promising solutions for an optimized stent. Stents exhibiting different mechanical properties but the same flow reduction effects could be designed by varying both the weave angle and wire thickness.

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