High-accuracy numerical-simulation of unsteady flow over high-speed coaxial rigid rotors

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.