BOUNDARY LAYER-TRIPPING STUDIES ON CHARACTERISTICS OF NEAR-WAKE BEHIND A CAMBERlED AIRFOIL

This paper presents an experimental study to the effects of boundary
layer-tripping on the aerodynamic performance and the near-wake which
generates immediately behind the trailing edge of NACA-2415 airfoil sections.
The airfoil was tripped by separately placing two different trip wires near the
leading edge as an artificial roughness to induce rapid transition. The tripped
and untripped airfoil were tested at different angles of attack to indicate the
effects of pressure gradient and streamline curvature on its aerodynamic
performance and the wake development behind the airfoil sections.
Measurements included lift and drag coefficients, static pressure distributions,
and mean velocity profiles in the wake region at Reynolds numbers between
3.5~10~ and 1.92~10~. The results of this experimental investigation showed
that the aerodynamic performance of the airfoil is sensitive to the tripping
mechanism. The drag coefficient of the airfoil increases due to the distorted
velocity profiles in the wake region caused by the boundary layer tripping. The
boundary layer tripping did not affect strongly the wake velocity profiles in the
case of negative angles of attack and the wake velocity distributions are
symmetric. Asymmetric velocity profiles are observed in the wake region as
the angle of attack increases, for both tripped and untripped airfoils. The
results indicated also that, the pressure gradient and the separation location on
airfoil surfaces significantly control the growth of the wake. The effect of the
boundary layer tripping on the growth of the wake width, the decay of the
maximum velocity defect and similarity of wake profiles are discussed.