The book "New Results in Numerical and Experimental Fluid Mechanics IX" offers a comprehensive collection of contributions based on the 18th biannual symposium of the German Society for Aerodynamics in Aerospace (STAB). The individual chapters reflect the ongoing research work of STAB members in the fields of numerical and experimental fluid mechanics as well as aerodynamics, with a focus on applications in aerospace. The contributions cover both national and EU-funded projects and provide deep insights into essential research topics along with their physical and mathematical foundations. The book addresses current challenges and new directions in research, including topics such as high- and low-aspect ratio wings, bluff bodies, laminar flow control, active flow control, hypersonic flows, aeroelasticity, aeroacoustics, and biofluid mechanics.