Drag reduction of up to 77.2% is achieved with air rings confined by alternating superhydrophobic and hydrophilic strips.

No abstract

This work proposes a comprehensive numerical dynamic model of a flexible-rotor bearing system based on the Hertzian and cubic polynomial nonlinear contact force methods. The model can consider the influences of the nonlinear bearing contact forces and unbalanced force caused by the rotor offset. The displacements and spectrums of the flexible-rotor bearing system from the Hertzian and cubic polynomial nonlinear contact force methods are discussed. The influences of the radial clearance, eccentricity, mass, and deformation of the rotor on the frequency–amplitude characteristics of the flexible-rotor bearing system considering a large speed range are analyzed. The results show that the dynamic and vibration characteristics of the flexible-rotor bearing system from the Hertzian and cubic polynomial nonlinear contact force methods are different. The differences of the frequency–amplitude characteristics between the flexible- and rigid-rotor bearing system are small in a lower speed stage; however, their differences are very large in a higher speed stage. This method can be applied to the nonlinear dynamic modeling and simulation of the flexible-rotor bearing system, which can predict the dynamics and prevent the system failures during the design processing of rotor system.

The effects of chordwise deformation and the half-amplitude asymmetry on the hydrodynamic performance and vortex dynamics of batoid fish have been numerically investigated, in which the two parameters were represented by the wavenumber ( $W$ ) and the ratio of the half-amplitude above the longitudinal axis to that below ( $HAR$ ). Fin kinematics were prescribed based on biological data. Simulations were conducted using the immersed boundary method. It was found that moderate chordwise deformation enhances the thrust, saves the power and increases the efficiency. A large $HAR$ can also increase thrust performance. By using the derivative-moment transformation theory at several subdomains to capture the local vortical structures and a force decomposition, it was shown that, at high Strouhal numbers ( $St$ ), the tip vortex is the main source of thrust, whereas the leading-edge vortex (LEV) and trailing-edge vortex weaken the thrust generation. However, at lower $St$ , the LEV would enhance the thrust. The least deformation ( $W=0$ ) leads to the largest effective angle of attack, and thus the strongest vortices. However, moderate deformation ( $W=0.4$ ) has an optimal balance between the performance enhancement and the opposite effect of different local structures. The performance enhancement of $HAR$ was also due to the increase of the vortical contributions. This work provides a new insight into the role of vortices and the force enhancement mechanism in aquatic swimming.

Research on fish locomotion has made extensive progress towards a better understanding of how fish control their flexible body and fin for propulsion and maneuvering. Although the biologically flexible fish fins are believed to be one of the most important features to achieve optimal swimming performance, due to the limitations of the existing numerical modeling tool, studies on a deformable fin with a non-uniformly distributed stiffness are rare. In this work, we present a