Numerical and experimental investigation of unsteady three-dimensional separation on axisymmetric bodies

• Flow visualizations from experiments and simulations for two axisymmetric bodies.
• Improved understanding of mean flow separation topology for hemisphere cylinder.
• Modal analysis of unsteady fluid dynamics.
• Discussion of possible hydrodynamic instabilities.

The understanding of unsteady three-dimensional separation is lagging behind the understanding of two-dimensional separation due to its greater complexity and the limited amount of data for universal or canonical flow problems. Three-dimensional separation occurs, for example, when low aspect ratio devices such as submarines and torpedoes are operated at large angles of attack, α  . Two low aspect ratio geometries, the DARPA Suboff bare hull geometry, which is a prototypical submarine shape, and a hemisphere-cylinder geometry were investigated. Numerical simulations and water tunnel experiments for the Suboff geometry for a Reynolds number based on diameter of Re=10,000Re=10,000 show little flow separation at α=30°α=30°. A hemisphere-cylinder geometry was derived by replacing the Suboff forebody with a hemisphere. Simulations and water tunnel experiments were carried out for Re=2000Re=2000 and 5000. For α=10°α=10° a large shedding separation bubble is observed on the leeward side. For α=30°α=30° two counter-rotating leeward vortices appear and shedding is reduced. Proper orthogonal decomposition and Fourier analysis in time are employed for investigating the unsteady fluid dynamics. Conclusions are drawn with respect to the mean flow topology and possibly relevant hydrodynamic instabilities.