Direct numerical simulation of a starting rotor at $$Re_c=15000$$

Abstract Rotors play a major role in various applications including ventilation and propulsion systems such as in helicopters, drones, gas turbines and wind turbines. This visualization of instantaneous vortical structures (identified by the $$\lambda _2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>λ</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math> criterion) shows complex flow structures emanating from a twisted drone rotor that is impulsively starting to rotate at 1600 rpm. Initially, a starting vortex is formed as a result of lift generation and shed as a connected vortex tube from the entire surface of the blade, which has a strong connection to the blade tip via the so-called tip vortex. Leading edge separation occurs at span positions of high twist, followed by wave-induced breakdown to turbulence along the whole wing span. This turbulence then sheds as small-scale vortices into the wake and dissipates. Understanding the behaviour of these vortices from such complex blades and how they interact with the other blade is critical to design more efficient and potentially more silent propellers. Graphical abstract