Computational analysis of the characteristics of subsonic, transonic and supersonic base flows

The physics of cylindrical base flows at subsonic to supersonic speeds are computationally investigated with LES/RANS hybrid methodology. When the free-stream Mach number increases from subsonic to supersonic speeds, time-averaged base pressure experiences three different phases; (1) base pressure decreases in proportion to dynamic pressure at the subsonic speeds with Mach numbers less than 0.8, (2) base pressure begins to fall rapidly as the free-stream Mach number approaches to sonic speed, (3) base pressure gradually decreases toward an asymptotic value at supersonic speeds. The position of the high-pressure region behind the base which strongly depends on the characteristics of freeshear layers at each speed range is one of the dominant factors that determine the base pressure. The dominant factor that determine the time-averaged characteristics of base flows changes from subsonic to supersonic speeds; unsteady vortices shedding from the instability of free shear layers at subsonic speeds, oscillations of front and rear local shock waves along the free shear layer at transonic speeds, and oscillations of the free shear layer and the recompression shock wave at supersonic speeds.