Onset of instability of a flag in uniform flow

This paper numerically and analytically studies the onset of instability of a flag in uniform flow. The three-dimensional （3D） simulation is performed by using an immersed-boundary method coupled with a nonlinear finite element method. The global stability, bistability and instability are identified in the 3D simulations. The Squire＇s theorem is extended to analyze the stability of the fluid-flag system with 3D initial perturbations. It is found that if a parallel flow around the flag admits an unstable 3D disturbance for a certain value of the flutter speed, then a two-dimensional （2D） disturbance at a lower flutter speed is also adnfitted. In addition, the growth rate of 2D disturbance is larger than that of the 3D disturbance.

Performance of a quasi-steady model for hovering hummingbirds

A quasi-steady model describing aerodynamics of hovering Ruby-throated hummingbirds is presented to study extent of the low-order model in representing the flow physics of the bird and also to separately quantify the forces from the translational, rotational, and acceleration effects. Realistic wing kinematics are adopted and the model is calibrated against computational fluid dynamics （CFD） simulations of a cor- responding revolving-wing model. The results show that the quasi-steady model is able to predict overall lift production reasonably well but fails to capture detailed force oscillations. The downstroke-upstroke asymmetry is consistent with that in the previous CFD study. Further analysis shows that significant ro- tational force is nroduced during mid-stroke rather than win~ reversal.

ULS-PBFT:An ultra-low storage overhead PBFT consensus for blockchain

Since the Practical Byzantine Fault Tolerance(PBFT)consensus mechanism can avoid the performance bottleneck of blockchain systems caused by Proof of Work(PoW),it has been widely used in many scenarios.However,in the blockchain system,each node is required to back up all transactions and block data of the system,which will waste a lot of storage resources.It is difficult to apply to scenarios with limited storage resources such as unmanned aerial vehicle networks and smart security protection;thus,it is often used in small-scale networks.In order to deploy PBFT-based blockchain systems in large-scale network scenarios,we propose an ultra-low storage overhead PBFT consensus(ULS-PBFT),which groups nodes hierarchically to limit the storage overhead within the group.In this paper,we first propose an optimal double-layer PBFT consensus from the perspective of minimizing the storage overhead,and prove that this consensus can significantly reduce the storage overhead.In addition,we also investigate the superiority of ULS-PBFT in terms of communication overhead while setting the security threshold in the presence of the possibility of Byzantine nodes.The simulation results demonstrate the advantages of ULS-PBFT.Then,we extend such grouping idea to the blockchain system with X-layer PBFT and analyze its storage and communication overhead.Finally,the node grouping strategy of double-layer PBFT is studied for four application scenarios when the performance of storage overhead,communication overhead,and security are considered comprehensively.