In:
Journal of Fluid Mechanics, Cambridge University Press (CUP), Vol. 371 ( 1998-09-25), p. 21-58
Abstract:
By using a Reynolds-averaged two-dimensional computation of a turbulent
flow over an airfoil at post-stall angles of attack, we show that the massively separated
and disordered unsteady flow can be effectively controlled by periodic blowing–suction near the leading edge with low-level power input. This unsteady forcing
can modulate the evolution of the separated shear layer to promote the formation of
concentrated lifting vortices, which in turn interact with trailing-edge vortices in
a favourable manner and thereby alter the global deep-stall flow field. In a certain
range of post-stall angles of attack and forcing frequencies, the unforced random separated
flow can become periodic or quasi-periodic, associated with a significant lift enhancement.
This opens a promising possibility for flight beyond the static stall to a much
higher angle of attack. The same local control also leads, in some situations, to a
reduction of drag. On a part of the airfoil the pressure fluctuation is suppressed as well,
which would be beneficial for high-α buffet control. The computations are in
qualitative agreement with several recent post-stall flow control experiments. The physical mechanisms responsible for post-stall flow control, as observed from the numerical
data, are explored in terms of nonlinear mode competition and resonance, as well
as vortex dynamics. The leading-edge shear layer and vortex shedding from the trailing
edge are two basic constituents of unsteady post-stall flow and its control.
Since the former has a rich spectrum of response to various disturbances, in a quite wide
range the natural frequency of both constituents can shift and lock-in to the forcing
frequency or its harmonics. Thus, most of the separated flow becomes resonant, associated
with much more organized flow patterns. During this nonlinear process the coalescence of small vortices from the disturbed leading-edge shear layer is enhanced,
causing a stronger entrainment and hence a stronger lifting vortex. Meanwhile, the
unfavourable trailing-edge vortex is pushed downstream. The wake pattern also has a
corresponding change: the shed vortices of alternate signs tend to be aligned, forming
a train of close vortex couples with stronger downwash, rather than a Kármán
street.
Type of Medium:
Online Resource
ISSN:
0022-1120
,
1469-7645
DOI:
10.1017/S0022112098002055
Language:
English
Publisher:
Cambridge University Press (CUP)
Publication Date:
1998
detail.hit.zdb_id:
1472346-3
detail.hit.zdb_id:
218334-1
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