The low-dissipation high-order accurate hybrid up-winding/central scheme based on fifth-order weighted essentially non-oscillatory (WENO) and sixth-order central schemes, along with the Spalart--Allmaras (SA)-based delayed detached eddy simulation (DDES) turbulence model, and the flow feature-based adaptive mesh refinement (AMR), are implemented into a dual-mesh overset grid infrastructure with parallel computing capabilities, for the purpose of simulating vortex-dominated unsteady detached wake flows with high spatial resolutions. The overset grid assembly (OGA) process based on collection detection theory and implicit hole-cutting algorithm achieves an automatic coupling for the near-body and off-body solvers, and the error-and-try method is used for obtaining a globally balanced load distribution among the composed multiple codes. The results of flows over high Reynolds cylinder and two-bladed helicopter rotor show that the combination of high-order hybrid scheme, advanced turbulence model, and overset adaptive mesh refinement can effectively enhance the spatial resolution for the simulation of turbulent wake eddies.

As a canonical problem, the impulsive starting of a circular disk contains the fundamental mechanisms of the force generation of the drag-based propulsions. In this paper, a circular disk is uniformly accelerated to a constant target velocity along a straight path, the instantaneous drag on and the flow fields around the disk are measured. A series of experiments were conducted by varying the two dimensionless numbers, i.e., the Reynolds number (Re) ranging from 40 000 to 80 000 and the acceleration number ($A^{*}$ ) (double normalized uniform-acceleration distance) ranging from 0.5 to 2. Based on the quasi-steady and the impulse-based ideas, two analytical models are proposed for predicting and accounting for the drag force on the disk. Moreover, the two models distinguish the generation of the drag force into three phases. In the acceleration phase, the growth rate and initial peak of the drag on the disk strongly depend on $A^{*}$ , which make the drag-force histories exhibit a good scaling law for a given $A^{*}$ , and the whole drag is generally contributed by the increased growth rate of the vortex ring circulation. In the transition phase, the drag decreases owing to the decrease in the circulation growth rate of the vortex ring. In the vortex pinch-off phase, the circulation of the vortex ring nearly no longer grows and the size growth rate of vortex ring gradually plays a dominant role in the drag generation. The present results suggest two implications. The peak of the drag in the accelerating phase implies an alternative perspective for understanding the high-lift generation in the reversal of wing stroke in flapping flight, and three-phase drag generation implies a controllable principle based on vortex formation for enhancing the force generation in drag-based propulsions.

Investigating the nucleation and freezing behavior of supercooled water under temperature gradients is of significance for theoretical research and practical application. This paper focuses on the freezing temperature of bulk supercooled water in a rectangular container for which the temperature at two sides can be controlled to present a temperature difference. The experimental results show that the nucleation temperature under temperature differences will be lower than that under isothermal conditions. In addition, an increase in the temperature difference is shown to linearly increase the supercooling needed for ice nucleation. The temperature gradient is capable of suppressing heterogeneous ice nucleation. Based on the classical nucleation theory, the free energy barrier for heterogeneous nucleation is attributed to the main factor to affect the nucleation rate. By comparing theoretical evaluations with experimental results, correction on the free energy barrier was conducted to predict the nucleation rate in the temperature gradient. Our experimental research and theoretical correction reveal the nucleation behavior of supercooled water under nonequilibrium conditions.