An experimental study of coherent structures in a three-dimensional turbulent boundary layer
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In the present study, measurements were performed in a three-dimensional, pressure-driven turbulent boundary layer (ReÎ¸ = 5936) in the flow around a wing-body junction with a variety of multiple-sensor probes, to examine the features of the coherent structures in the flow. This test flow has a number of practical applications and was selected because of its strong three-dimensional nature and the availability of an extensive set of mean-flow measurements from previous investigations. The measurements were carried out with a hot-wire rake with sixteen sensors spaced approximately logarithmically over 25.4 mm (1 inch), a parallel-sensor probe with two parallel sensors spaced approximately 4.8 mm apart, a rotatable wall-sensor probe with two wall-mounted hot-film sensors spaced 6.93 mm apart and a traversable wall-sensor probe with two variable-spacing wall-mounted hot-film sensors. The hot-wire rake was used to examine the structure of the flow in both the Y (normal to the wall) and Z (spanwise) directions. The parallel and rotatable wall-sensor probes were used to look at the angular characteristics of the coherent structures in the flow and at the wall, respectively, and the spanwise structure of the flow at the wall was examined through the traversable wall-sensor probe.
The results of the measurements show that the spectral characteristics of the flow are affected by three-dimensional effects. The direction of motion of the coherent structures lags behind the local mean-velocity vectors in the X-Z plane (parallel to the wall) with very little variation with frequency (structure size). Unlike two-dimensional boundary layers, the spectral variation of the convective wave speed does not collapse when normalized with the local mean velocity and friction velocity in the outer and inner regions, respectively. In the outer region of the boundary layer, the distribution of the intermittency with Y appears to agree quite closely with previously reported results for two-dimensional boundary layers. The mean ejection frequency in the near-wall flow and the frequency at the peak of the first moment of the wall shear-stress power spectrum show fairly close agreement, consistent with previously reported results for a two dimensional boundary layer. The measurements with the traversable wall-sensor probe indicate the presence of an organized structure, probably low-speed streaks in the near-wall region, with a preferred spanwise spacing. This spanwise spacing was found to be Î Î * = 85 and 135 at two different measurement stations. somewhat different from the well accepted value of Î Î * = 100 for two-dimensional boundary layers. Time-delayed correlations of the velocity signal over a range of Y locations reveal an inclined linear wavefront similar to previously reported results for a two-dimensional boundary layer.
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