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A new high-frequency oscillating-hot-wire sensor for magnitude and direction measurements of the wall-shear stress in separated flows is presented. The stress direction is determined from the phase angle between the imposed and measured oscillation velocity, and the corresponding magnitude is obtained from the low-pass filtered signal of the sensor, after removal of the modulating influence of the oscillation. The sensor was used to conduct measurements downstream of an axisymmetric backward-facing step (BFS) at different streamwise locations ranging from 0.3 to 10 step heights downstream of the step. The results agreed qualitatively with existing one-point measurements, such as the mean/rms skin-friction distribution, forward flow probability, and power spectra, in planar BFS flows. However, some fundamental quantitative differences were found including a shorter reattachment length and a larger streamwise location for the peak rms wall-shear fluctuations. These differences were attributed to the axisymmetric nature of the present geometry, transverse curvature of the step, or differences in the measurement methods. Additionally, the power spectra of the fluctuating wall-shear stress revealed the existence of two characteristic frequencies: f* = 0.1 and 0.65. The former is associated with low-frequency shear-layer flapping, and the latter corresponds to the passage of the separated shear-layer vortex structures. [ABSTRACT FROM AUTHOR]

The article focuses on the two-point measurement of the shear stresses on walls beneath an axisymmetric separating/reattaching flow. It states that difficulties in measuring the direction-reversing shear stress beneath separation bubbles is responsible for the lack of information on surface shear stress. It mentions that traditional methods of measuring wall-shear-stress are wall hot wires, which cannot be used if there is flow reversal, and pulsed wire, which has a frequency response limited to tens of hertz.