The present paper introduces a novel approach for measuring near wall flow velocities by utilizing a sublayer surface fence probe. Hereby, a difference in static pressure builds up over a microscopic obstacle within the viscous sublayer. An analytical model of the angular dependent pressure difference is employed to derive information about the flow direction. Furthermore, a computational preston tube approach has been used to calibrate the surface fence probe with regard to a flow velocity to be assigned to half of the fence height. Through the use of a sophisticated analyzing algorithm the flow direction and its velocity can be determined as a function of time. As a proof of concept measurements were conducted within the radial gap of an annular compressor rig yielding both mean and time resolved near wall flow fields. Former are in very good compliance with oil flow visualizations proving the methods accuracy. The experimental results give unprecedented insights into an unsteady flow phenomenon that arises when an axial compressor is operating close to its stalling limit. The presented technique allows for investigating turbomachinery areas which formerly were observable only by computational means.

When approaching stall, rotating disturbances close to the blade tip clearance arise in many turbomachines. As a clear precursor to stalled operation, this regime imparts additional unsteady loading on the blades, possibly inducing noise and non-synchronous vibrations. Advancing beyond the current limited comprehension of this flow condition, this work employs full-annulus, scale-resolving simulations of a single-stage axial compressor to elucidate the initiation mechanisms of prestall disturbances and rotating stall. For that, transient throttling is implemented at the channel outlet. The numerical setup is validated by unsteady experimental results. The findings are contrasted with previous scale-resolving assessments with constant throttling levels. Testing the previously proposed hypothesis correlating time-average blockage and leading edge spillage with the onset of prestall disturbances, the present study confirms its validity under transient boundary conditions.