15th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
Paper ID:
ETC2023-176
Main Topic:
Axial Turbines
Authors
Abstract
The bursting phenomenon consists in the transition of a laminar separation bubble (LSB) from a short to a long configuration. In the former case, reduced effects on the profile pressure distribution are typically observed with respect to the attached condition. On the contrary, long bubbles provoke significant variations of the loading coefficient upstream of the separation position, with increased risk of stall. The present work presents an experimental database on separated-flow transitional boundary layers under different Reynolds numbers, two adverse pressure gradients and three turbulence intensity levels (Tu=1.5%, 2.5%, 3.5%). Overall, more than 60 flow conditions were tested concerning short and long type bubbles for the characterization of separated flows under turbine-like conditions. Measurements were performed on a flat plate geometry using a fast response particle image velocimetry (PIV) system. For each flow case, 6000 snapshots were acquired in the meridional plane of the plate at a sampling rate equal to 1kHz. Based on existing criteria for the identification of the bursting phenomenon, the flow cases were clustered in terms of short and long type bubbles. Additionally, the kind of instability (i.e. convective or absolute) involving the separated boundary layer was identified based on time-mean flow quantities. The present study clearly highlights the existing link between the bursting of a LSB and the onset of the absolute instability of the separated shear layer, with stationary vortices forming in the dead air region. The analysis of the instantaneous PIV data revealed also a sudden change of the size of the coherent vortical structures driving the boundary layer transition once scaled with the shear layer thickness at the separation position.
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