Mean Flowfield Evolution with Upsweep Angle in a Simulated Cargo Fuselage Aftbody

Abstract

The mean flowfield in the aft region of cargo aircraft configurations is analyzed using large-eddy simulations. A simplified surrogate fuselage is considered, consisting of a cylinder with axis parallel to the flow and a slanted (upswept) base. Emphasis is placed on understanding the major changes that occur in the flow as the upsweep angle of the base is increased in a broad 20°–45° range. Qualitative and quantitative comparisons with available experiments indicate that the simulations accurately capture the primary flowfield features, which are observed to be relatively insensitive to the Reynolds number. The simulated mean databases are then employed to elaborate on the evolution of the vortical field and its influence on the base surface. In the range of angles examined, the flow separates at the edge of the base and a horseshoe-shaped vortical structure is obtained, with a head region centered near the upstream apex of the base surface and an initial growth around its periphery. The vortical structure subsequently detaches from the base to form a streamwise vortex pair (legs) that persists for relatively long distances downstream. With an increasing upsweep angle, the size of the head region increases but the concomitant increase in vortex strength also results in a more rapid decay downstream. The angles formed by the vortex pair relative to the cylinder axis increase with upsweep; however, the trajectories collapse relatively well when scaled by the distance along the base. Secondary separation is observed at all upsweep angles, consistent with experimental observations. Changes in base surface pressure are related to those in the simulated oil flow, which shows significant changes between 32° and 45°. Some of these changes relate to the subsequent establishment of the anticipated wake regime at even higher angles, a brief discussion of which is presented by considering a 55° case.