  The present study is a prelude to applying different flow control devices on
pitching and plunging airfoils with the intention of controlling the growth of
the leading edge vortex (LEV); hence, the lift under unsteady stall conditions.
As a pre-requisite, the parameters influencing the development of the LEV
topology must be fully understood, and this constitutes the main motivation of
the present experimental investigation. The aims of this study are twofold.
First, an approach is introduced to validate the comparability between flow
fields and LEV characteristics of two different facilities using water and air
as working media by making use of a common baseline case. The motivation behind
this comparison is that with two facilities the overall parameter range can be
greatly expanded. This comparison includes an overview of the respective
parameter ranges, control of the airfoil kinematics and careful scrutiny of how
post-processing procedures of velocity data from time-resolved particle image
velocimetry (PIV) influence the integral properties and topological features
used to characterise the LEV development. Second, and based on results coming
from both facilities, the appearance of secondary structures and their effect
on LEV detachment over an extended parameter range is studied. A Lagrangian
flow field analysis, based on finite-time Lyapunov Exponent (FTLE) ridges,
allows precise identification of secondary structures and reveals that their
emergence is closely correlated to a vortex Reynolds number threshold computed
from the LEV circulation. This threshold is used to model the temporal onset of
secondary structures. Further analysis indicates that the emergence of
secondary structures causes the LEV to stop accumulating circulation if the
shear layer angle at the leading edge of the flat plate has ceased to increase.
