An experimental study of the relationship between velocity and pressure fluctuations in a wing-body junction

Abstract

Velocity and pressure fluctuations were measured in a wind tunnel in the plane of symmetry in front of a wing-body junction at an approach free-stream velocity U_ref=28.3 m/s and momentum thickness Reynolds number Re_θ=6900. The cylindrical wing shape was a 3:2 elliptic nose attached to a NACA 0020 tail at maximum thickness. A two component laser Doppler anemometer was used to make the velocity measurements. Pressure measurements were made at two locations upstream of the wing by a pair of Sennheiser microphones. A relationship between the velocity and pressure was determined to reveal the bimodal structure of the flow field.

Histograms and power spectra of both quantities are presented along with cross-spectra and cross-correlations. The velocity power spectra revealed spectral slopes of -1 through out the entire flow field. Velocity and pressure autospectra show the bimodal region to be dominated by low frequency fluctuations centered at ft/U_ref=5x10⁻², where f is the average frequency and t is the maximum wing thickness.

Coherence was found between velocity fluctuations and pressure fluctuations in three frequency bands, each associated with a particular region of the wing-body junction flow field. Low frequency coherence between the velocity and pressure was found in the vicinity of the junction vortex where large scale unsteady bimodal structures are formed. High frequency coherence dominates closer to the juncture of the wing and body.

Conditionally-averaged velocity vectors were used to help identify the flow structure in the juncture. These vectors were used along with the other data to propose a model consisting of a single vortex rolling up and moving downstream. As the vortex moves downstream, it wraps around the wing which causes it to stretch and eventually dissipate. Preceding the vortex roll up, an intake of fluid down the wing occurs which is believed to be the cause of the bimodal unsteadiness.