The flow in a turbulent mixing layer resulting from two parallel different
velocity streams, that were brought together downstream of a jagged partition
was investigated experimentally. The trailing edge of the partition had a short
triangular ‘chevron’ shape that could also oscillate up
and down at a prescribed frequency, because it was hinged to the stationary part
of the partition to form a flap (fliperon). The results obtained from this
excitation were compared to the traditional results obtained by oscillating a
two-dimensional fliperon. Detailed measurements of the mean flow and the
coherent structures, in the periodically excited and spatially developing mixing
layer, and its random constituents were carried out using hot-wire anemometry
and stereo particle image velocimetry.
The prescribed spanwise wavelength of the chevron trailing edge generated
coherent streamwise vortices while the periodic oscillation of this fliperon
locked in-phase the large spanwise Kelvin–Helmholtz (K-H) rolls,
therefore enabling the study of the inter- action between the two. The
two-dimensional periodic excitation increases the strength of the spanwise rolls
by increasing their size and their circulation, which depends on the input
amplitude and frequency. The streamwise vortices generated by the jagged
trailing edge distort and bend the primary K-H rolls. The present investigation
endeavours to study the distortions of each mode as a consequence of their
mutual interaction. Even the mean flow provides evidence for the local bulging
of the large spanwise rolls because the integral width (the momentum thickness,
θ), undulates along the span. The lateral location of the centre of
the ensuing mixing layer (the location where the mean velocity is the arithmetic
average of the two streams, y0),
also suggests that these vortices are bent. Phase-locked and ensemble-averaged
measurements provide more detailed information about the bending and bulging of
the large eddies that ensue downstream of the oscillating chevron fliperon. The
experiments were carried out at low speeds, but at sufficiently high Reynolds
number to ensure naturally turbulent flow.