Flow-induced noise on a bluff body

Abstract

Flow noise measurements were made on inertial type pressure gradient hydrophones, configured as three-dimensional cylinders in cross flow, over a diameter-based Reynolds number range of 4×103 to 1.8×104. The measurements were made at frequencies from 4.2 to 50 Hz as the bodies were towed in a quiescent body of water. Systematic changes were made in the cylinder geometry as to affect the flow noise level and to aid in the identification of dominant flow noise sources. The cylinder aspect ratio was varied from 0.5 to 2.5, and the endcap geometry was altered by relieving the 90° edge with radii that ranged from 0.0315D to 0.5D, where D is the diameter of the cylinder. The data from these (and other flow visualization) experiments shows that the presence of a radius at the corner between the endcap and the cylinder results in a significant reduction of the separated flow over the endcap, and that the flow noise levels decrease accordingly. The flow noise levels are also observed to decrease as the body aspect ratio increases which suggests that when three-dimensional effects (endcap flow) are suppressed, unsteady forces will be reduced and the cylinder self-noise will decrease. Thus the dominant source of flow noise on short cylinders in cross flow is attributed to the unsteady separation and turbulent flow over the cylinder endcaps.