Simultaneous wall-pressure and velocity measurements in the flow field downstream of an axisymmetric backward-facing step

Surface-pressure and planar, Particle Image Velocimetry measurements were obtained in the separating/reattaching flow region downstream of an axisymmetric, backward-facing step. Data were acquired for a two-dimensional (2D) separating boundary layer at four different Reynolds numbers based on step height (Re h), spanning 5900-18500. The experimental set-up consisted of an array of 32 flush-mounted microphones beneath the separation bubble. These microphones were used to detect the wall-pressure signature that was acquired simultaneously with the PIV measurements of the velocity field above the surface.; The surface-pressure and PIV measurements were used to characterize the wall-pressure distribution and the flow field in terms of Reynolds number dependencies. Statistics from these measurements were consistent with findings in the literature. In addition, the evolution of coherent structures in the flow field was investigated using proper orthogonal decomposition (POD) and multi-point, linear, stochastic estimation (LSE). POD was used to determine the dominant convective modes in the pressure signature. Two convective modes were found to be responsible for 60% of the energy of the pressure signature. Multi-point LSE was used to estimate the dominant flow structures above the wall from convective-mode, wall-pressure signatures over a series of time steps. It was found that a large-scale, coherent structure develops in place at approximately half the reattachment distance. Once this structure reaches a height equivalent to the step, it sheds and accelerates downstream. This growth in place and then shedding resembles a wake mode, which describes the flow field downstream of bluff bodies. The wake mode has been observed in numerical simulation studies of long cavities and backward-facing steps, where two dimensionality is controllable. The present study shows for the first time evidence for the existence of a wake mode in an experimental study of a backward-facing step. This is believed to be related to the good two dimensionality (i.e., axisymmetry) of the test geometry and the ability to track the temporal-evolution of structural features through LSE. Finally, it was observed that, unlike behind bluff bodies, the wake mode in the back-step flow is intermittent, switching on/off in an apparently random manner.