An experimental study of a turbulent wing-body junction and wake flow

Abstract

Extensive hot-wire measurements were conducted in an incompressible turbulent flow around a wing-body junction. The measurements were performed adjacent to the body and up to 11.56 chord lengths downstream of the body. The junction wake flow entered an adverse pressure gradient region approximately 6 chord lengths downstream. This region's geometry approximated the aft portion of an aircraft fuselage or a submersible's hull. The body geometry was formed by joining a 3:2 elliptic nose to a NACA 0020 tail section at their respective maximum thickness locations. The author's measurements were taken with approach flow conditions of Reθ = 6,300, and δ/T = .513, where T is the maximum body thickness.

The results clearly show the characteristic horseshoe vortex flow structure. The vortex flow structure is elliptically shaped, with â (W)/â Y forming the primary component of streamwise vorticity. Near wall measurements show a thin layer of highly concentrated vorticity, underneath and opposite in sign to the primary vortex, which is created by the wall no-slip condition. The development of the flow distortions and associated vorticity distributions are highly dependent on the geometry-induced pressure gradients and resulting flow skewing directions. A quantity known as the "distortion function" was used to separate the distortive effects of the secondary flow from those of the body and the local "2-D" boundary layer. The distortion function revealed that the adverse pressure gradient flow distortions grew primarily because of the increasing boundary layer thickness.

The author's results were compared to several other data sets obtained using the same body shape, enabling the determination of the approach boundary layer effects. The primary secondary flow structure was found to scale on T in the vertical and cross-stream directions, revealing that the juncture flow is driven by the appendage geometry and associated pressure gradients. A parameter known as the momentum deficit factor (MDF = (ReÏ ) 2 (θ/T) was found to correlate the observed trends in mean flow distortion magnitudes and vorticity distribution. Variations in flow skewing were observed to be comparable to changes in MDF, suggesting that this flow parameter changes the effective skewing magnitudes around a wing-body junction. Mean flow distortions were found to increase with decreasing values of MDF.

A numerical study was also performed to gain additional insights into the effects of appendage nose geometry. The velocity distributions around approximately 30 different appendage cross-sections were estimated using 2-D potential flow calculations. A correlation was found between the appendage nose bluntness and the average vortex stretching rate, and also between the invisicid velocity distribution and an experimentally determined non-dimensional circulation estimate.