NEAR-WALL MEASUREMENTS IN A 3-DIMENSIONAL TURBULENT BOUNDARY-LAYER

An experiment was performed to measure near-wall velocity and Reynolds stress profiles in a pressure-driven three-dimensional turbulent boun dary layer. An initially two-dimensional boundary layer (Re-theta appr oximate to 4000) was exposed to a strong spanwise pressure gradient. A t the furthest downstream measurement locations there was also a fairl y strong favourable streamwise pressure gradient. Measurements were ma de using a specially designed near-wall laser-Doppler anemometer (LDA) , in addition to conventional methods. The LDA used short focal length optics, a mirror probe suspended in the flow and side-scatter collect ion to achieve a measuring volume 35 mu m in diameter and approximatel y 65 mu m long. The data presented include mean velocity measurements and Reynolds stresses, all extending well below y(+) = 10, at several profile locations. Terms of the turbulent kinetic energy transport equ ation are presented at two profile locations. The mean flow is nearly collateral (i.e. W is proportional to U) at the wall. Turbulent kineti c energy is mildly suppressed in the near-wall region and the shear st ress components are strongly affected by three-dimensionality. As a re sult, the ratio of shear stress to turbulent kinetic energy is suppres sed throughout most of the boundary layer. The angles of stress and st rain are misaligned, except very near the wall (around y(+) = 10) wher e the angles nearly coincide with the mean how angle. Three-dimensiona lity appears to mildly reduce the production of turbulent kinetic ener gy.