An Overview of Wake Breakdown in High-Fidelity Simulations of Rotor-in-Hover

Wake breakdown is a well-documented computational phenomenon associated with highly resolved computational hover predictions. As the computational state-of-the-art for hover predictions has progressed, allowing for higher resolution of the rotor wake, the formation of secondary braids in computed helical wake systems has manifested in various forms. The formation of 3D secondary braids between two parallel convecting vortex filaments, under the right conditions, is physical. Recent hi-definition rotor-hover experiments do confirm their presence. However, computed wake breakdown is more pervasive, and the question of whether high-fidelity methods exaggerate the extent of the secondary vortex production has been a topic of research in the past decade. In this paper, we survey the computational ingredients that make up a high-fidelity hover solver, highlight interesting recent developments, and try to summarize what we know (and what we do not know) about computed wake breakdown. Recent advances in the use of direct volume rendering to visualize the vortical content of the helical wake and the insights they provide into the wake breakdown vortical interplay are also highlighted. Future directions for unsteady, high fidelity hover simulation wake breakdown research are speculated to include an emphasis on temporal fidelity/convergence.