The Turbulent Boundary Layer on a Small Diameter Flexible Cylinder in the Wake of a Streamlined Towing Apparatus

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Long thin flexible cylinders instrumented with acoustic sensors are used by the Navy for surveillance and detection. Acoustic transducers are mounted in a long linear array surrounded by an insulated jacketing material resulting in a long flexible array of sensors with a smooth outer surface. These cylinders, called towed arrays, are towed behind ships and submarines. As a result of the towing speed, a turbulent boundary layer develops along the outer boundary of the flexible cylinder which generates noise in the near field. Flow noise is critical to the measured acoustic signatures and therefore, to system performance. By understanding the nature of turbulent boundary layers on long, thin flexible cylinders, the performance of towed acoustic sensors used for surveillance and tactical operations can be improved.

Towed array detection performance is a function of several factors including ambient acoustics, array gain, flow noise, and signal processing. To advance existing systems, the primary goal is to increase the system gain by introducing longer apertures, i.e. longer arrays. Longer arrays of conventional design lead to additional stowage requirements that are difficult to implement. The development of very small diameter towed array technology is required. Therefore, the results of this research will directly contribute to the work being done on small diameter arrays and multi-line array systems.

This is an experimental study to evaluate the development of the boundary layer thickness, δ, and momentum thickness, θ, along long thin flexible cylinders (L/a=1.510^5 and 3.010^5 where δ/a>>1, a=radius). The experiments use conventional test methods in conjunction with Stereo Particle Image Velocimetry (SPIV) measurement techniques to evaluate the flow in the boundary region of a small diameter flexible cylinder towed in the high speed towing basin at the Naval Surface Warfare Center Carderock Division (NSWCCD). The flexible cylinders are approximately neutrally buoyant and have an initial length of 152 m and radii of 0.45 mm and 1.25 mm. The first objective for this experiment is to evaluate the streamwise development of wall shear stress and momentum thickness in axisymmetric turbulent boundary layers using drag measurements at 3.1, 5.2, 9.3 and 14.4 m/s for comparison to existing data. The second and primary objective for this experiment is to determine the streamwise development of the axisymmetric boundary layer flow and to evaluate relevant boundary layer parameters at 3.8, 7.7, 12.9 and 15.4 m/sec using SPIV images acquired over the entire length of the cylinders. Drag measurements reveal that the wall shear stress is large and that the momentum thickness grows slowly when compared to flat plate boundary layers. The velocity field data shows that the boundary flow remains turbulent over the entire length of the flexible cylinder and that the turbulent profile is different from that of flat plate boundary layers.