In-Pipe Aerodynamic Characteristics of a Projectile in Comparison with Free Flight for Transonic Mach Numbers Between 0.5 and 1.5
The transient shock dynamics and drag of a transonic projectile flying through a pipe 3.55 times larger than its diameter are analyzed by means of time-of-flight and pipe wall pressure measurements as well as computational fluid dynamics (CFD). In addition, free-flight drag of the 4.5-mm-pellet-type projectile was also measured in a Mach number range between 0.5 and 1.5, providing a means for comparison against in-pipe data and CFD. For nearly incompressible flow, the presence of the pipe has little influence on the drag. There is a strong increase, however, between Mach 0.3 and 0.8, to a value of about two times the free-flight drag. This is exactly where the nose-to-base pressure ratio of the projectile becomes critical, and henceforth drag can be estimated by supersonic nozzle theory. For even higher Mach numbers, the drag decreases again and finally drops below the free-flight drag. This behavior is explained by five different flow regimes that the in-pipe projectile experiences, as opposed to only two for the free-flying one (subsonic and supersonic, respectively). 2-D axisymmetric CFD simulations agree well with measured values for drag and shock speeds.