Passive flow control around a wall-mounted finite cylinder - Pegasus

(reaching a value greater than the base
one) and of C y (that becomes negative)
was noted. In the range (60±1)º< θ<
(90±1)º, a gradual reduction of the drag
and a signal variation of the derivative of
the C y were noted. Finally, for θ >
(90±1)º, both coefficients remain around
the base value. Thus, the existence of a
critical angle θ cr is supposed in [(56±1)º,
(60±1)º] for which substantial variations
of both coefficients are noted.
Nebres and Batill [9] found also a
critical angle θ c = 42º for which the lift
and drag coefficients vary in a similar
way to the present work. The cylinderperturbation
used by them had D/d=11,2
and the flow of Re= 30000. The
perturbation’s length was d/δ = 10,9,
where δ is the boundary layer thickness
on the cylinder. They observed that the
C pb had a maximal value for θ c such that
the smallest C x was generated,
accompanied by the maximal lift for this
angle. They obtained a drag reduction of
30% from the base value and lift
coefficients nearly the unit. They explained the drag reduction considering the boundary layer behavior. Until θ= θ c ,
the boundary layer reattaches, had a transition that delays the final separation and, consequently, generates a
reduction of the pressure drag.
Results from Matsumoto [8] show
also the existence of a critical angle of the
trip wire. The greatest variations of lift
and drag were observed for θ~ 50º. The
cylinder studied a diameter D of 50mm
and the perturbation was approximately
the same as the present work’s one. A
reduction of 20% in drag was obtained for
the critical angle and the maximum lift
coefficient was 0,4.
The value of C x for the twodimensional
reference case is presented in
Fig. 3 (Zdravkovich [11]). The same
behavior is noted for the drag’s curves.
The obvious difference is the base value
for C x . This coefficient is near to the twodimensional
case for the study of Nebres
and Batill [9] because in their
experiments the cylinder had a
considerable aspect ratio AR of 10,3 and
there were end-plates that reduced the
tridimensional effect of the flow.
Matsumoto [8] obtained a C x base value
greater than the cases of AR= 6 and 3 with
a cylinder of AR~ 22.
Figure 3. Curves of drag coefficients. (data from Nebres and
Batill [9] and Matsumoto [8] for comparisons)
Figure 4. Curves of lift coefficients. (data from Nebres and
Batill [9] and Matsumoto [8] for comparisons)
A possible explanation for the difference of drag reductions is related to the diameter d of the perturbations.
Zdravkovich [12] shows examples where its influence is perceived in the modification of the C p distribution. For
each fixed θ < θ cr , the greatest variations of C pb (its greatest values) were observed for the greater trip wires. When θ
was sufficiently big, the value of C pb is small (greater drag) for elevated diameters. The perturbations used by
Nebres and Batill [9] were the greatest ones such that they obtained a considerable reduction (30%) and increase
(40%) of drag.
4
American Institute of Aeronautics and Astronautics