Experimental investigation of axisymmetric turbulent boundary layers

Measurements of the turbulent boundary layer over an experimental towed array were made at the David Taylor Model Basin, Naval Surface Warfare Center Carderock Division using a stationary stereo- particle image velocimetry (SPIV) system. Data were collected at discrete transverse planes along the length of the array at tow speeds between 6.2 and 15.4 m/s. The corresponding momentum thickness Reynolds numbers Re_θ varied from 4.8 × 10⁵ to 1.1 × 10⁶. This range is representative of values occurring in Navy applications and is much greater than those typically achieved in laboratory or with computational investigations. Instantaneous three-dimensional velocity fields and profiles have been published by the authors. However, the streamwise growth of the boundary layer and the variation with speed were not determined. The goal of this analysis is to provide additional insight into the development of axisymmetric boundary layers over long lengths and at moderate to high Reynolds numbers. A stationary stereo-particle image velocimetry (SPIV) system was used to obtain three-dimensional velocity measurements and evaluate the boundary layer flow development along full-scale fleet towed array modules. Measurements were collected at discrete transverse planes along the length at tow speeds between 6.2 and 15.4 m/s. Algorithms for image pre processing and filtering were applied to enhance the instantaneous images and maximize the data extracted from the images. Mean velocity profiles and relevant boundary layer parameters are extracted. This information is necessary to refine the scaling of the wall pressure measurements obtained simultaneously [1] and flow noise models. Independent load cell measurements of the total drag on the towed model provided the momentum thickness at the end of the model and the spatially averaged friction velocity u_t. In order to average multiple images and compute statistics of the bo- ndary layer parameters, new algorithms for identifying the array and its shadow are being developed. This facilitates the computation of relevant boundary layer parameters and the evaluation of the axisymmetry of the boundary layer. Trends in the data with Reynolds number will be also determined. The growth of the turbulent boundary layer over the length of the array is an important metric with regard to estimating the maximum turbulent boundary layer thickness which exists on a fleet towed array. The underlying structure of the axisymmetric boundary layer, which leads to significant increases in wall shear stress with respect to flat plate cases, is of primary importance. These new insights will facilitate efforts toward towed array reliability and an accurate prediction of drag and flow noise for any towed array application.