High-resolution numerical simulations for wake vortical flows have long been a challenge in rotor aerodynamics. A novel spectrum-optimized sixth-order Weighted Essentially Non-Oscillatory (WENO) scheme is proposed to discretize inviscid fluxes on moving overset grids, and the Improved Delayed Detached Eddy Simulation (IDDES) is employed to resolve turbulent vortices. The integration of these methods facilitates a comprehensive numerical investigation into the unsteady vortical flows over coaxial rotors in hover. The results highlight the substantial improvement in numerical resolution, in terms of both spatial structure and temporal evolution of unsteady multiscale wake vortices. Coaxial rotors in hover manifest three primary scales of wake vortex structures: (A) the helical evolution of primary blade tip vortices and the periodic occurrence of strong Blade-Vortex Interactions (BVI); (B) the continuous shedding of small-scale horseshoe-shaped vortices from the trailing edges of rotor blades, forming the vortex sheets; (C) the emergence of small-scale secondary vortex braids induced by interactions between rotor tip vortices and the vortex sheets. These vortex structures and their interactions cause high-frequency oscillations in rotor disk loads and induce unsteady perturbations in the local flow field. Interactions among these primary vortices, coupled with the generation of secondary vortices, result in the dissipation, distortion, and breakup of the rotor tip vortices, ultimately forming a vortex soup. Notably, a substantial quantity of seemingly weak small-scale secondary vortex braids significantly contribute to energy dissipation during the evolution of wake vortices for coaxial rotors in hover.

High-speed coaxial rotor systems generate complex unsteady vortical flow structures and strong shock waves at forward flight, which significantly impact the aerodynamics, noise, and vibration characteristics of high-speed helicopters. Capturing these flow phenomena with high resolution is one of the challenges in computational fluid dynamics research for high-speed helicopters. To address this challenge, a dynamic overset grid system capable of load balancing in large-scale parallel computations is developed. An efficient method for identifying overset relationships and an interpolation technique that maintains high-order accuracy at artificial boundaries are proposed. A novel fifth-order WENO-K scheme with adaptively optimized spectral characteristics has been introduced to enhance the numerical resolution and preserve the structures of rotor tip vortices. Additionally, an improved Delayed Detached Eddy Simulation (IDDES) method is used to capture the smaller-scale vortex structures and their unsteady fluctuations in the wake region. High-accuracy numerical simulations of unsteady vortical flow fields of a simple four-blade coaxial rotor and an eight-blade coaxial rotor similar to the X-2 configuration have been conducted. The generation, development, evolution, and vortex interference mechanisms of blade tip vortices of the high-speed rigid rotors are analyzed. The computational results demonstrate that the high-order schemes, when combined with the IDDES method, can effectively reduce numerical dissipation and enhance the resolution of multiscale turbulent structures, and successfully capture the rapid development and evolution of rotor tip vortices, as well as blade-vortex interactions. The sophisticated horseshoe-shaped vortex surface structures formed at the trailing edge of the rotor blades can be also discerned. These findings contribute to a deeper understanding of the vortex interference mechanisms and aerodynamic noise in high-speed coaxial rotor flow fields.