Numerical investigation of natural transitions of bow boundary layers over underwater axisymmetric bodies

Abstract

The natural transitions of bow boundary layers over underwater axisymmetric bodies are investigated using numerical methods. The laminar flow fields over the underwater axisymmetric bodies are first calculated, and then the linear stability of the boundary layers is analyzed considering both the streamwise and circumferential curvatures of the wall. Based on the stability results, the eN method is employed to predict the transition locations. Numerical calculations are performed for seven forebody shapes under six oncoming flow velocities, allowing the influences of the forebody shapes and the oncoming flow velocities on the transition to be investigated. For the different forebody shapes, the boundary layer stability is generally the same behind the streamwise location of twice the forebody length, but varies within in the range of twice the forebody length. The transition locations are significantly different for the different forebody shapes. As the oncoming flow velocity increases, the dimensional unstable zone expands significantly, and the transition location moves upstream. The SUBOFF forebody shape proposed by Groves et al. [“Geometric characteristics of DARPA SUBOFF models (DTRC model numbers 5470 and 5471),” Report No. DTRC/SHD-1298-01 (David Taylor Research Center, West Bethesda, MD, 1989)] has a particularly late transition location and a large diameter close to the leading edge. This delayed transition location is caused by two separated unstable zones. Considering multiple factors, our analyses indicate that the SUBOFF forebody shape is quite valuable for practical engineering problems.