Modes of Base Pressure Fluctuations: Shape, Nature, and Origin

An experimental study was carried out to examine the unsteady pressure field on the base of a cylindrical after body at freestream Mach numbers ranging from high subsonic to low supersonic speeds. The objective was to gain insights into spatial modes of base pressure fluctuations by identifying their shape and nature, and the mechanism of their origin using spectral proper orthogonal decomposition. It is observed that 90-95% of the cumulative energy content resides within the first three modes at all Strouhal numbers for all freestream Mach numbers. Further, at subsonic Mach numbers and for Strouhal numbers O(10(-2)), the shape of the modes indicates that their nature is symmetric, and the mechanism for the origin of the dominant mode is the pulsing action of the recirculation bubble. However, for Strouhal numbers O(10(-1)), the shape of the modes is asymmetric, indicating that their nature is antisymmetric, and the mechanism associated with the dominant mode is the radial displacement of the recirculation bubble coupled with the motion of structures within the recirculation bubble. At supersonic Mach numbers and for all Strouhal numbers, the shape of the modes indicates that they are symmetric, and the dominant mechanism is the pulsing action of the recirculation bubble.