Momentum Thickness Measurements for Thick Axisymmetric Turbulent Boundary Layers

Experimental measurements of the mean wall shear stress and boundary layer momentum thickness on long, thin cylindrical bodies are presented. To date, the spatial growth of the boundary layer and the related boundary layer parameters have not been measured for cases where δ/a (a = cylinder radius) is of order one or greater. Moderate Reynolds numbers (10⁴ < Re_θ < 10⁵) encountered in hydrodynamic applications, are considered. Tow tests of cylinders with diameters of 0.89 mm and 2.5 mm and lengths ranging from approximately 30 meters to 150 meters were performed using the High-Speed Seawater Tow Tank at NASA Langley Research Center. The total drag (axial force) was measured at tow speeds ranging from 2.4 to 17.4 m/sec. These data were used to determine the tangential drag coefficients on each test specimen, which were found to be two to three times greater than the values for the corresponding hypothetical flat-plate cases. Using the drag measurements, the turbulent boundary layer momentum thickness at the end of the cylindrical bodies is determined, using a control volume analysis. The results show that for the smallest diameter cylinders, there is no indication of relaminarization, and a fully developed turbulent boundary layer exists. In addition, laser measurements showed no large scale transverse motions (snaking) existed during the tows, and the tow angle was less than 1 degree for all cases, confirming that the cylinders were neutrally buoyant.