Hiroya Mamori

Effect of the parameters of traveling waves created by blowing and suction on the relaminarization phenomena in fully developed turbulent channel flow

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 a/u¯ lam >0.1, 200 1.5, respectively. These ranges are organized by displacement thickness and are between 3 and 10 wall units when the relamin...

Variability of hemodynamic parameters using the common viscosity assumption in a computational fluid dynamics analysis of intracranial aneurysms

BACKGROUNDnIn most simulations of intracranial aneurysm hemodynamics, blood is assumed to be a Newtonian fluid. However, it is a non-Newtonian fluid, and its viscosity profile differs among individuals. Therefore, the common viscosity assumption may not be valid for all patients.nnnOBJECTIVEnThis study aims to test the suitability of the common viscosity assumption.nnnMETHODSnBlood viscosity datasets were obtained from two healthy volunteers. Three simulations were performed for three different-sized aneurysms, two using measured value-based non-Newtonian models and one using a Newtonian model. The parameters proposed to predict an aneurysmal rupture obtained using the non-Newtonian models were compared with those obtained using the Newtonian model.nnnRESULTSnThe largest difference (25%) in the normalized wall shear stress (NWSS) was observed in the smallest aneurysm. Comparing the difference ratio to the NWSS with the Newtonian model between the two Non-Newtonian models, the difference of the ratio was 17.3%.nnnCONCLUSIONSnIrrespective of the aneurysmal size, computational fluid dynamics simulations with either the common Newtonian or non-Newtonian viscosity assumption could lead to values different from those of the patient-specific viscosity model for hemodynamic parameters such as NWSS.

Drag reduction by streamwise traveling wave-like Lorenz Force in channel flow

Skin-friction drag reduction effect of traveling wave-like wall-normal Lorenz force in a fully developed turbulent channel flow is investigated by means of direct numerical simulation. A sinusoidal profile of the wall-normal body force is assumed as the Lorenz force. While upstream traveling waves reduce the drag in the case of blowing/suction, standing waves reduce it in the case of present forcing. Visualization of vortical structure under the standing wave-like wall-normal Lorenz force reveals that the near-wall streamwise vortices, which increase the skin-friction drag, disappear and spanwise roller-like vortices are generated instead. Three component decomposition of the Reynolds shear stress indicates that the spanwise roller-like vortices contribute to the negative Reynolds shear stress in the region near the wall, similarly to the case of laminar flows. While the analogy between the wall-normal and streamwise forcings can be expected, the statistics are found to exhibit different behaviors due to the difference in the energy flow.