Introduction of Multiscaled Longitudinal Vortices by Fractal-Patterned Surface Roughness

作     者：Manami Kosuda Yoshihiro Kubota Masao Yokoyama Osamu Mochizuki 

作者机构：Graduate School of Science and Engineering Toyo University Saitama Japan Department of Mechanical Engineering Toyo University Saitama Japan Department of Information Science Meisei University Tokyo Japan Department of Biomedical Engineering Toyo University Saitama Japan 

出 版 物：《Journal of Flow Control, Measurement & Visualization》 (流量控制、测量及可视化（英文）)

年 卷 期：2019年第7卷第2期

页      面：120-132页

学科分类：08[工学] 0801[工学-力学（可授工学、理学学位）] 

主　　题：Friction Drag Reduction Surface Roughness Fractal Pattern Vortex Flow Visualization 

摘      要：To determine the type of surface roughness pattern that is suitable for adaptive suppression of the drag of an obstacle, we observed flow structures introduced by such obstacles. Several roughness patterns were tested: geometric patterns, fractal patterns, reptile-skin patterns, and patterns of circular cylinders arranged in a lattice and in a zigzag manner. A suitable pattern for adaptive control of flow is one that generates longitudinal vortices with nonconstant distances. The preferred instability mode of a laminar boundary layer is expected to be selected automatically from fluctuations involving many frequencies and caused by fractal patterns. Snake- and reptile-skin patterns may have a similar ability as fractal patterns because they consist of multiscale patterns. The longitudinal vortices generated from peculiar positions and concave corners in patterns were observed. The distance between these vortices is not constant because the onset of vortices is at concave corners in fractal patterns. These vortices have differing strengths and easily cause nonlinear interactions, so they can disturb a laminar boundary layer with several higher-harmonic frequencies. The velocity profiles of the laminar boundary-layer flow over the fractal patterns were measured by using hydrogen bubbles. The results show the down-wash flow between the longitudinal vortices, which means that the vortices may effectively suppress the boundary layer separation in an adverse pressure gradient.