Flow visualization and wake analysis for standard and modified configurations of the AN/ALQ-78 antenna pod

Abstract

A low-speed wind tunnel investigation was conducted to compare the aerodynamics flow field characteristics for standard and modified configurations of a 20 percent scale model of the AN/ALQ-78 electronic support measures system antenna pod. The modification consisted of replacing the standard quasi-conically shaped radome with a cylindrical radome and adapter collar. The research was requested by the Naval Air Systems Command (NAVAIRSYSCOM) as part of a risk and feasibility assessment in preparation for full-scale flight tests of the modified configuration. Wake flow visualization was conducted to determine the general characteristics of the flow field. Base pressure measurements were made to assess three-dimensional effects. the presence of coherent vortex shedding the wake was investigated using a cross-probe constant temperature hot-film anemometer and spectrum analyzer. the Reynolds number varied from 4x10(4) to 6x10(5) with the upper value approximately equal to one-eighth that of the full-scale pod in flight. Asymmetric vortex shedding was visualized in the wakes of both configurations at subcritical Reynolds numbers. The modified configuration exhibited a pronounced region of quasi-two-dimensional flow during wake visualization and base pressure tests, indicating a significant increase in the aerodynamic forces acting on the pod structure. Spectral analysis of the hot-film output confirmed the presence of coherent vortex shedding in the wakes of both models at subcritical Reynolds numbers.