'American Institute of Aeronautics and Astronautics (AIAA)'
Doi
Abstract
Passive flow control on aerofoils has largely been achieved through the use of protrusions
such as vane-type vortex generators. Consequently, innovative flow-control concepts should
be explored in an effort to improve current component performance. Therefore,
experimental research has been performed at The University of Manchester to evaluate the
flow-control effectiveness of a novel type of vortex generator made in the form of a surface
indentation. The surface indentation has a trapezoidal planform. A spanwise array of
indentations has been applied in a convergent orientation around the maximum-thickness
location of the upper surface of a NACA-0015 aerofoil. The aerofoil has been tested in a twodimensional
set-up in a low-speed wind tunnel at an angle of attack (AoA) of 3° and a chordbased
blockage-corrected Reynolds number (Recorr) of ~2.70 x 105
. The baseline model has
been found to suffer from a long laminar separation bubble (LSB) at low AoA. The
application of the indentations at low AoA has considerably shortened the separation
bubble. The indentations achieve this by shedding up-flow pairs of streamwise vortices.
Despite the considerable reduction in bubble length, the increase in leading-edge suction due
to the shorter bubble is limited by the removal of surface curvature and blockage (increase
in surface pressure) caused locally by the convergent indentations. Furthermore, the up-flow
region of the vortices, which locally weakens the pressure recovery around the trailing edge
of the aerofoil by thickening the boundary layer, also contributes to this limitation. Due to
the conflicting effects of the indentations, the changes in the pressure-lift and pressure-drag
coefficients, i.e., cl,p and cd,p, respectively, are small. Nevertheless, the indentations have
improved cl,p and cd,p beyond the uncertainty range, i.e., by ~1.3% and ~0.3%, respectively,
at 3° AoA. The wake measurements show that turbulence intensity and Reynolds stresses
have considerably increased in the indented case, thus implying that the indentations
increase the viscous drag on the model. In summary, the convergent indentations are able to
reduce the size of the LSB, but conversely, they are not highly effective in enhancing cl,p and
cd,p at the tested Re
