Suppression of flexible feather-like structure on boundary layer transition

It is observed that when there is spanwise flow, the soft side edges of bird feathers are blown up, and thus the spanwise flow is resisted, i.e., the drag characteristics for streamwise direction and spanwise direction on the feather surface are quite different. In order to study the influence of this anisotropic characteristics of feathers on the laminar-turbulent transition in boundary layer flow, a phenomenological mechanical model, from the macroscopic view, is established to describe the anisotropic characteristics of bird feathers. By means of direct numerical simulation with finite difference method, the model is imposed to the transitional flow over a flat plate, where the freestream Mach number is 0.2 and the Reynolds number based on the displacement thickness of boundary layer at the inlet is 732. Numerical results show that when the spanwise flow is damped by the anisotropic model, the laminar-turbulent transition is significantly delayed, and even the friction in turbulent region is reduced to be almost the same level as that in laminar region. The stronger the damping for spanwise flow by the model, the more obvious the transition delay and friction reduction. Both the delay of transition and the reduction of friction drag in turbulent region are helpful to reduce the overall drag of the flat plate. When considering the influence of blown-up feathers on streamwise direction in the form of resistance, the resistance in streamwise direction results in the earlier onset of transition. Although friction drag is further reduced in this case, while the total drag of flat plate is increased. Overall, this study reveals some of the mysteries of bird flight, and also provides a new idea for the passive control of the boundary layer flow.